Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Takanori Uchida Last modified date:2024.04.11

Professor / Renewable Energy Center / Research Institute for Applied Mechanics


Papers
1. Koichiro Shibuya, Takanori Uchida, Wake asymmetry of yaw state wind turbines induced by interference with wind towers, Energy, https://doi.org/10.1016/j.energy.2023.128091, 280, 2023.10, Wind turbine wakes are known to cause significant reductions in power generation and increased load on downwind wind turbines. Therefore, controlling wind turbine wakes is crucial, and yaw steering is considered one of the most effective methods of doing so. In this study, wind tunnel experiments using a wind turbine model and Large-eddy simulations (LES) were conducted to gain a better understanding of the wake of a yawed wind turbine. The results showed a clear difference in the horizontal distribution of wake velocity at hub height for positive and negative yaw angles, as well as both lateral and vertical wake deflections. Further investigation revealed that wind towers have a significant effect on the wakes of yawing wind turbines. For positive yaw angles, the velocity deficit is larger above the rotor, resulting in a vertical wake shape caused by the interference between the blade wakes and detached flow from the tower. Conversely, for negative yaw angles, the velocity deficit is larger below the rotor, resulting in a horizontal wake shape. These findings will facilitate the development of more accurate wake models and control methods..
2. @Koichi Watanabe, Megumi Matsumoto, Thandar Nwe, Yuji Ohya, Takashi Karasudani, Takanori Uchida , Power Output Enhancement of Straight-Bladed Vertical-Axis Wind Turbines with Surrounding Structures, Energies, https://doi.org/10.3390/en16186719, 16, 18, 6719, 2023.09, Wind tunnel experiments were conducted by installing wind-acceleration structures on both sides of a straight-bladed vertical-axis wind turbine (VAWT) to improve the output performance of the turbine. In the case of Venturi-shape structures, a curved shape with a large outlet opening produced a higher power output than straight or brimmed Venturi shapes. More importantly, two simple flat plates installed upstream of the wind turbine achieved the highest power enhancement of 2.4 times the power of the bare wind turbine. From the analysis of the flow visualization results, the power enhancement was attributed to the increase in lift force on the blades in the upstream region due to the acceleration of the gap flow between the flat plates, and the decrease in drag force on the blades toward the upstream region due to stagnation of the flow behind the plates..
3. @Tran The Hung, #Masato Hijikuro, @Masayuki Anyoji, @Takanori Uchida, @Takuji Nakashima, @Keigo Shimizu, Surface flow and aerodynamic drag of Ahmed body with deflectors, Experimental Thermal and Fluid Science, 145, 110887(18pages), 2023.07, This study investigated the surface flow and the aerodynamic characteristics of a 7/10 scale Ahmed model equipped with deflectors at several angles. The experiments were conducted at Reynolds number of 2.45 × 105 based on the model height. A global luminescent oil film skin-friction measurement technique was applied for the flow fields on the slant and base surfaces. The aerodynamic forces of the model were measured to understand the effect of the deflection angle on the aerodynamic characteristics. The pressure was measured and pressure drags acting on the slant and base surfaces were also calculated. The results showed that the drag and lift of the model suddenly dropped at the deflection angle above −5°. The pressure distribution on the slant and base surfaces became flat with a maximum drag reduction of 8 for the model with a deflection angle of 0°. The skin-friction measurement indicated that the drag reduction was connected to the breakdown of the separation bubble and longitudinal vortices on the slant. At the fully separated flow state, the skin-friction results illustrated two small vortices and a counter-clockwise vortex on the slant. Additionally, the wake vortices shifted from horizontal to vertical structures. The relation among the flow fields on the slant surface, flow on the symmetric plan, pressure distribution, and drag was illustrated for the model with different deflection angles..
4. @Takanori Uchida, Koichiro Shibuya, Gustavo Richmond-Navarro, Williams R. Calderón-Muñoz, A wind tunnel investigation of yawed wind turbine wake impacts on downwind wind turbine performances and wind loads, Wind Engineering, https://doi.org/10.1177/0309524X221150219, 47, 3, 0309524X221150219, 2023.06, In the current work we experimentally explored yawed wind turbine wake impacts on downwind wind turbine performances and wind loads. The lab-scale wind turbine model with a rotor diameter (D) of 0.442 m and a height of 1 m (=2.26D) was installed in a closed-circuit boundary layer wind tunnel (test section: 15 m long × 3.6 m wide × 2.0 m high) of the Research Institute for Applied Mechanics (RIAM) of Kyushu University. Power performance tests were initially conducted with a single rotor in isolation in order to characterize a rotor’s power output in stand-alone conditions. A detailed comparison of the tests revealed that the power output decreased rapidly as the yaw angle (γ) increased. It is presumed that the power output decrease in yawed cases is mainly due to the decrease in the effective rotor area and the change in the angle of the incoming wind flow with respect to the wind turbine blade. Next, using two wind turbine models aligned with the dominant inflow direction, the merging wakes behaviors caused by three different lateral separation distances were tested: (a) Case 1 (y = 0), (b) Case 2 (y = 0.5D), and (c) Case 3 (y = 1D). Here, the separation distance between the two wind turbine models was fixed at 6D in all cases. Extremely large power output deficits of 46%–76% were seen in the Case 1 configuration. This is mainly due to the significant wake velocity deficits induced by the upwind wind turbine model. In the Case 2 configuration with γ values of 20° and 30°, a significant increase in the power output of the downwind wind turbines was observed. Similar to Case 1 configuration, these results are considered to be mainly due to the upwind turbine-induced wake velocity deficits and wake deflection. Finally, in the Case 3 configuration, no significant difference was found in all of the results, and the tendency was almost the same. We show that the wake velocity deficits induced by the upwind wind turbine model had almost no effect on the power output of the downwind wind turbine model. We evaluated the total power output of the two turbines. As a result, in the Case 2 configuration with 20° yaw angle, the total power output of the two wind turbine models was the highest due to the increase in the power output of the downwind wind turbine model. In order to investigate the main cause of the significant increase in the power output of the downwind wind turbine model at 20° and 30° yaw angles in the Case 2 configuration, we measured the lateral wind speed distribution at the 6D position on the downwind side of the upwind wind turbine model by using the ultrasonic anemometer. As a results, it was clarified that the peak of the wake velocity deficits induced by the upwind wind turbine model is clearly shifted away from the downwind turbine such that it experiences a smaller deficit due to wake steering. Also, with wake steering the upwind turbine-induced wake velocity deficits may be smaller due to the reduction in rotor area. Finally, it is extremely important to understand the wind load acting on the downwind wind turbine model operating within the wake region induced by the upwind yawed wind turbine model when the maximum power output is generated. It can be seen that as the yaw angle of the upwind wind turbine model increased, the power output generated by the downwind wind turbine model and the streamwise wind load acting on it also increased. However, it was also clarified that the streamwise wind load acting on the downwind wind turbine model in this situation did not exceed the stand-alone value..
5. Takanori UCHIDA, Teppei TANAKA, Ryuta SHIZUI, Hiroto ICHIKAWA, Ryo TAKAYAMA, Kazuomi YAHAGI, Ryoya OKUBO, GPU simulation of wake effects at the Horns Rev 1 offshore wind farm using the CFD porous disk wake model, Wind Engineering, https://doi.org/10.1177/0309524X221132003, 47, 2, 408-421, 0309524X221132003, 2023.04, To verify the effectiveness of the GPU simulation of wake effects at a large-scale offshore wind farm, we ran an in-house large-eddy simulation (LES) solver with a CFD porous disk wake model for the Horns Rev 1 wind farm. For this numerical research, we prepared the latest workstation equipped with a Xeon W-2265 CPU and an NVIDIA RTX A6000 GPU. We clarified that the calculation speed of the single GPU of the NVIDIA RTX A6000 is approximately 10 times faster than the calculation speed of the Xeon W-2265. Careful data analysis and visualization of the unsteady turbulent flow fields obtained in the current LES study suggest that the mutual interference of the wakes developed by wind turbines may frequently form a local speed-up region around wind turbines, located on the downstream side of large offshore wind farms..
6. @Tran The Hung, #Masato Hijikuro, @Masayuki Anyoji, @Takanori Uchida, @Takuji Nakashima, @Keigo Shimizu, Effect of a short, bio-mimetic control device on aerodynamic drag of Ahmed body, Journal of Fluid Enginerring, https://doi.org/10.1115/1.4056341, 145, 3, 031206 (11 pages), 2023.03, The effects of short automatic moving deflectors (AMD) on the aerodynamic characteristics of Ahmed body are considered in this study. AMDs, known as biomimetic control devices, were added to the leading edge of the slanted surface. Its position was automatically adjusted in a separation flow. The aerodynamic drag, the pressure, and the skin-friction distribution on the slanted surface were measured for the model with three deflectors with lengths of 9%, 18%, and 30% of the slant. Particle image velocimetry was also utilized to assess the flow on the vertical symmetric plane. The Reynolds number based on the height of the model is between 1.44 × 105 and 2.80 × 105. The results showed that at a low Reynolds number, a short deflector increases the drag of the model. The effectiveness of the deflector in reducing the drag arises at a high velocity, where a maximum drag reduction of 11% was observed. The deflectors also reduced the lift coefficient by as much as 89%. Global luminescent oil-film skin-friction measurements showed that in the low drag state, the structure of the longitudinal vortexes and the separation bubble disappear on the surface. A complex flow structure is classified for the baseline model and the model with deflectors. The relationship between the surface flow, pressure distribution, and flow on the symmetric vertical plane is discussed in detail..
7. Takanori UCHIDA, Susumu TAKAKUWA, Keiichiro WATANABE, Seiya HASEGAWA, Yoshitaka BABA Reo MURAKAMI, Masahide YAMASAKI, Kunihiko HIDAKA, Numerical visualization of wind turbine wakes using passive scalar advection-diffusion equation and its application for wake management, Wind Engineering, 10.1177/0309524X221113011, 46, 6, 1870-1887, 0309524X221113011, 2022.12.
8. @Tran The Hung, #Masato Hijikuro, @Masayuki Anyoji, @Takanori Uchida, @Takuji Nakashima, @Keigo Shimizu, Deflector effect on flow behavior and drag of an Ahmed body under crosswind conditions, Journal of Wind Engineering and Industrial Aerodynamics, https://doi.org/10.1016/j.jweia.2022.105238, 231, 105238, 2022.12, This study investigates the effect of a deflector on Ahmed model drag and the flow on the slant during steady crosswind conditions by experimental methods at a based-height Reynolds number of 2.45 × 105. The length deflector is 0.09 times the length of the slant. The deflector was fixed at the leading edge of the slant with an upward angle of 5° to the horizontal axis. Force, pressure, and global skin-friction measurements were conducted to understand the relation between drag, pressure distribution, and flow fields on the slant surface. The results showed that the deflector reduces aerodynamic drag at low yaw angles. This result is due to the breakdown structure of longitudinal vortexes and the separation bubble, which leads to fully separated flow on the leading edge of the slant. However, at yaw angles above 8°, a large reversed flow region forms on the slant and the drag of the model increases. A similar flow structure on the slant is observed for yaw angles of 12° and 15°. Conversely, the model lift is reduced with the deflector at all yaw angles tested. Detailed pressure distribution and complex flow structure on the slant provide insight in this study..
9. Susumu TAKAKUWA, Takanori UCHIDA, Improvement of Airflow Simulation by Refining the Inflow Wind Direction and Applying Atmospheric Stability for Onshore and Offshore Wind Farms Affected by Topography, Energies, 10.3390/en15145050, 15, 14, 5050, 2022.07.
10. Gustavo Richmond-Navarro, Takanori Uchida, Williams R. Calderón-Muñoz, Shrouded wind turbine performance in yawed turbulent flow conditions, Wind Engineering, 10.1177/0309524X211036041, 46, 2, 518-528, Aritical Number 0309524X211036041, 2022.04, Wind turbines represent a growing energy source worldwide. In many cases, operating in turbulent and changing wind direction spots. In this work, we use a wind tunnel to analyze the effect of the turbulence in a wind turbine provided with a Wind Lens flow concentrator, under yaw conditions, for turbulence intensity values of 10% and 15%. Measurements are made of the power coefficient as a function of the Tip Speed Ratio using two types of Wind Lens, CiiB5 and CiiB10, at yaw angles from 0° to 30°. In general, for the turbine with Wind Lens, an increase of the yaw angle causes a reduction of the power coefficient. If the turbine operates with the CiiB10, the stronger the turbulence, the greater performance is obtained. In conclusion, for the case of turbulent flow and yaw = 20° or less, the Wind Lens turbine offers better performance than without the flow concentrator..
11. @Takanori UCHIDA, @Yves GAGNON, Effects of continuously changing inlet wind direction on near-to-far wake characteristics behind wind turbines over flat terrain, Journal of Wind Engineering and Industrial Aerodynamics, 10.1016/j.jweia.2021.104869, 220, 104869, 1-17, 2022.01, The wake characteristics of a utility-scale wind turbine under realistic atmospheric boundary layer conditions are affected by the continuously changing wind direction arriving at the wind turbine. In the present study, the effects of continuous changes in the incoming wind direction were studied for a wind turbine on flat terrain, with the objective of understanding the wake characteristics of the wind turbine. Thus, understanding the effects of continuously changing incoming wind direction on the wake characteristics of wind turbines over flat terrain is important in the design of wind farm layouts, including in the design of offshore wind power plants. For this purpose, a computational fluid dynamics (CFD) approach using large-eddy simulations (LES) was adopted in the present study. An in-house LES-solver based on the actuator line (AL) aerodynamics technique was constructed in order to successfully capture the wake structure behind the wind turbine. First, experimental investigations on both a blade-only wind turbine scale model and a full 3D wind turbine scale model (isolated wind turbine) were conducted for a fixed inlet wind condition, the latter including the nacelle and the tower. Through a detailed comparison of the wind tunnel experimental and numerical results, the prediction accuracy of the in-house LES-solver was verified and validated for fixed inlet wind conditions. On the basis of the validation results obtained, and using the full 3D wind turbine scale model, the effects of the continuously changing inlet wind conditions on the wake characteristics in the near- and far-wake regions were numerically investigated. In addition, the effects of the wind turbine nacelle and tower on the wake characteristics were also investigated. The numerical results show that the most significant impact of the effects of the continuously changing wind direction was the rapid recovery of the mean velocity deficits in the wind turbine wake region. Further, at the x = 10D position (D is the rotor diameter) downstream of the wind turbine, the non-dimensional streamwise mean velocity was 0.93, which nearly matches the approaching flow speed, under an optimal tip speed ratio of 4.0, compared to the fixed wind direction scenario..
12. @Tadasuke YOSHIDA, @Takanori UCHIDA, @Masaki INUI, @Yoshihiro TANIYAMA, @Yuki FUKATANI, Wind turbine wake evaluation using a vertical profiling Lidar and new CFD porous disk model, 第15回動力エネルギー際会議(ICOPE-2021) The Japan Society of Mechanical Engineers, https://doi.org/10.1299/jsmeicope.2021.15.2021-0175, セッションID: 2021-0175, 2021.11, Wind turbine wakes reduce the power generation and life of the downstream wind turbines. Accurately predicting the impact of wind turbine wakes is very important for evaluating the feasibility of large wind farms. We have recently proposed a computational fluid dynamics (CFD) porous disk (PD) wake model as an intermediate method between engineering wake models and CFD wake models in order to predict accurately the time-averaged wind speed deficits in the wind turbine wakes. In this study, to further evaluate the validity of the CFD PD wake model, we additionally measured the wind turbine wakes of 2 MW‐class downwind turbines installed on the coastal area using a vertical profiling lidar (ZephIR ZX300), and considered them including the data in the previous report to clarify its airflow characteristics in detail. Based on the measurement results by the lidar, we reproduced the wind turbine wake using the CFD PD wake model. The simulated vertical distribution of wind speed by the CFD PD wake model corresponded with the measurement result of the lidar..
13. Yuki FUKATANI, Takanori UCHIDA, Yoshihiro TANIYAMA, Zhiren BAI, Tadasuke YOSHIDA, Masaki INU, Wind turbine wake evaluation considering atmospheric stability using vertical profiling LiDARs, 第15回動力エネルギー際会議(ICOPE-2021) The Japan Society of Mechanical Engineers, https://doi.org/10.1299/jsmeicope.2021.15.2021-0175, セッションID: 2021-0175, 2021.11, In wind farms, an upstream turbine causes the turbine wake which leads to a decrease in the power output and an unsteady loading to the downstream turbine due to the velocity deficit and the increased level of turbulence. Therefore, accurate prediction of the wake flow is important for evaluating a feasibility of large scale wind farms. In this research, we report results of analysis of seaward and landward wake data measured using three vertical profiling LiDARs. In addition, the effect of atmospheric stability on the wake is investigated using atmospheric reanalysis data. The results show that considering atmospheric stability for seaward wake is important especially when a wide range of atmospheric stability can be observed..
14. Takanori Uchida, Ryo Araya, Applications of the Atmospheric Transport and Diffusion of LES Modeling to the Spread and Dissipation of COVID-19 Aerosol Particles inside and outside the Japan National Stadium (Tokyo Olympic Stadium), Modelling and Simulation in Engineering, 10.1155/2021/8822548, 2021, Article ID 8822548, 1-19, 2021.06, In this paper, we use an analysis function for gas diffusion known as the Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain (RIAM-COMPACT), which was developed for complex terrain, in Airflow Analyst software, and apply it to the spread and dissipation of a fluid layer (assuming the fluid layer contains COVID-19 particles). First, to verify the prediction accuracy of the gas diffusion using RIAM-COMPACT, comparisons with past wind tunnel test results conducted on simple and complex terrains are presented under neutral atmospheric stability. The results of the numerical simulations carried out in this study show good agreement with the wind tunnel experiments for both simple and complex terrains. Next, a model of the Japan National Stadium (Tokyo Olympic Stadium) was constructed using 3D detailed topographic Advanced World 3D Map (AW3D) data generated by combining high-resolution satellite images. We tried to reproduce the hypothetical spread and dissipation of the fluid layer (assuming the fluid layer contains COVID-19 particles) inside and outside of the Japan National Stadium using Airflow Analyst implemented with the RIAM-COMPACT analysis function for gas diffusion. We paid special attention to the effect of wind ventilation driven by natural wind. The numerical results under various scenarios show that ventilation driven by natural wind is very effective for the Japan National Stadium..
15. @Takanori Uchida, Tadasuke Yoshida, Masaki Inui, Yoshihiro Taniyama, Doppler Lidar Investigations of Wind Turbine Near-Wakes and LES Modeling with New Porous Disc Approach, Energies, 10.3390/en14082101, 14, 8, 1-33, Article Number 21012101, 2021.04.
16. Amr M. Halawa, Takanori Uchida, Koichi Watanabe, Yuji Ohya, Validation Study of Multi-Rotor Systems Using Two Shrouded Wind Turbines, Journal of Physics: Conference Series, 10.1088/1742-6596/1618/3/032017, 1618, 1-15, The Science of Making Torque from Wind (TORQUE 2020), 2020.09, Multi rotor systems (MRS) have shown a great potential as a future application of wind energy. In this study, the aim is simulating the aerodynamic performance of a an MRS using fully-resolved shrouded wind turbine blades then validating with experimental data. MRS for wind turbine configurations have been studied using both numerical and experimental approaches. Different case studies have been studied and the power output comparison have been reported. The wind lens turbine (WLT) obviously shows a large increase in power output compared to the bare rotor case. Besides, the twin side-by-side (SBS) WLTs shows even larger power increase compared to the case of single WLT by around 20% for computational fluid dynamics (CFD) calculations at the optimum tip speed ratio s/D of around 0.2. The increase in power coefficient in close proximity can be explained by flow interference and gap flow behaviors. Previously, we used simplified models for blade modeling like actuator line method (ALM) and actuator disk method (ADM). However, currently we are improving the accuracy using CFD with full-scale blades with higher grid resolutions. As the number of units for an MRS is increased, the increase in power output becomes larger and larger. This is because that the gap flows between brimmed diffuser-augmented wind turbines (B-DAWT) in a MRS are accelerated and cause lowered pressure regions due to vortex interaction behind the brimmed diffusers. Thus, an MRS with more B-DAWTs can draw more wind into turbines showing higher power output..
17. Takanori Uchida, Effects of Inflow Shear on Wake Characteristics of Wind-Turbines over Flat Terrain, Energies, 10.3390/en13143745, 13, 14, 3745, 2020.07, The scope of the present study was to understand the wake characteristics of wind-turbines under various inflow shears. First, in order to verify the prediction accuracy of the in-house large-eddy simulation (LES) solver, called RIAM-COMPACT, based on a Cartesian staggered grid, we conducted a wind-tunnel experiment using a wind-turbine scale model and compared the numerical and experimental results. The total number of grid points in the computational domain was about 235 million. Parallel computation based on a hybrid LES/actuator line (AL) model approach was performed with a new SX-Aurora TSUBASA vector supercomputer. The comparison between wind-tunnel experiment and high-resolution LES results showed that the AL model implemented in the in-house LES solver in this study could accurately reproduce both performances of the wind-turbine scale model and flow characteristics in the wake region. Next, with the LES solver developed in-house, flow past the entire wind-turbine, including the nacelle and the tower, was simulated for a tip-speed ratio (TSR) of 4, the optimal TSR. Three types of inflow shear, N = 4, N = 10, and uniform flow, were set at the inflow boundary. In these calculations, the calculation domain in the streamwise direction was very long, 30.0 D (D being the wind-turbine rotor diameter) from the center of the wind-turbine hub. Long-term integration of t = 0 to 400 R/Uin was performed. Various turbulence statistics were calculated at t = 200 to 400 R/Uin. Here, R is the wind-turbine rotor radius, and Uin is the wind speed at the hub-center height. On the basis of the obtained results, we numerically investigated the effects of inflow shear on the wake characteristics of wind-turbines over a flat terrain. Focusing on the center of the wind-turbine hub, all results showed almost the same behavior regardless of the difference in the three types of inflow shear..
18. Takanori UCHIDA, Susumu TAKAKUWA, Numerical Investigation of Stable Stratification Effects on Wind Resource Assessment in Complex Terrain, Energies, 10.3390/en13153896, 6638, 2020.07.
19. Takanori Uchida, Yoshihiro Taniyama, Yuki Fukatani, Michiko Nakano, Zhiren Bai, Tadasuke Yoshida, Masaki Inui, A new wind turbine CFD modeling method based on a porous disk approach for practical wind farm design, Energies, 10.3390/en13123197, 13, 12, 3197, 2020.06, In this study, the new computational fluid dynamics (CFD) porous disk (PD) wake model was proposed in order to accurately predict the time-averaged wind speed deficits in the wind turbine wake region formed on the downstream side by the 2-MW wind turbine operating at a wind speed of 10 m/s. We use the concept of forest canopy model as a new CFD PD wake model, which has many research results in the meteorological field. In the forest canopy model, an aerodynamic resistance is added as an external force term to all governing equations (Navier-Stokes equations) in the streamwise, spanwise, and vertical directions. Therefore, like the forest model, the aerodynamic resistance is added to the governing equations in the three directions as an external force term in the CFD PD wake model. In addition, we have positioned the newly proposed the LES using the CFD PD wake model approach as an intermediate method between the engineering wake model (empirical/analytical wake model) and the LES combined with actuator disk (AD) or actuator line (AL) models. The newly proposed model is intended for use in large-scale offshore wind farms (WFs) consisting of multiple wind turbines. In order to verify the validity of the new method, the optimal model parameter CRC was estimated by comparison with the time-averaged wind speed database in the wind turbine wake region with fully resolved geometries, combined with unsteady Reynolds-averaged Navier-Stokes (RANS) equations, implemented using the ANSYS(R) CFX(R) software. Here, product names (mentioned herein) may be trademarks of their respective companies. As a result, in the range from x = 5D of the near wake region to x = 10D of the far wake region, by selecting model parameter CRC, it was clarified that it is possible to accurately evaluate the time-averaged wind speed deficits at those separation distances. We also examined the effect of the spatial grid resolution using the CFD PD wake model that is proposed in the present study, clarifying that the spatial grid resolution has little effect on the simulation results shown here..
20. Koichi WATANABE, Sho FUKUTOMI, Yuji OHYA, Takanori UCHIDA, An Ignored Wind Generates More Electricity A Solar Updraft Tower to a Wind Solar Tower, International Journal of Photoenergy, 10.1155/2020/4065359, 2020, 2020.01, A solar updraft tower is one of the wind power generation plants which utilizes solar energy. The purpose of this study was to ascertain whether the tower was also able to utilize crosswind energy. Wind tunnel experiments and numerical simulations were conducted simulating the crosswind. The results showed that suctioned updraft speed in the tower was proportional to the crosswind speed, and its conversion rate depended on the tower configuration. A diffuser-shaped tower with a vortex generator achieved to produce the updraft whose speed exceeded the crosswind speed. It was due to the low pressure created by the vortex atop the tower and to the diffuser effect. The crosswind utilization enables the simple power generation device to generate electricity during the night, and the hybrid utilization of renewable energies contributes to the increasing wind energy market..
21. Takanori Uchida, Kenichiro Sugitani, Numerical and Experimental Study of Topographic Speed-Up Effects in Complex Terrain, Energies, 13, 15, 3896, 2020.01, Our research group is developing computational fluid dynamics (CFD)-based software for wind resource and energy production assessments in complex terrain called RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University (RIAM)-Computational Prediction of Airflow over Complex Terrain), based on large eddy simulation (LES). In order to verify the prediction accuracy of RIAM-COMPACT, we conduct a wind tunnel experiment that uses a two-dimensional steep ridge model with a smooth surface. In the wind tunnel experiments, airflow measurements are performed using an I-type hot-wire probe and a split film probe that can detect forward and reverse flows. The results of the numerical simulation by LES are in better agreement with the wind tunnel experiment using the split film probe than the results of the wind tunnel experiment using the I-type hot wire probe. Furthermore, we calculate that the two-dimensional ridge model by changing the length in the spanwise direction, and discussed the instantaneous flow field and the time-averaged flow field for the three-dimensional structure of the flow behind the model. It was shown that the eddies in the downwind flow-separated region formed behind the two-dimensional ridge model were almost the same size in all cases, regardless of the difference in the length in the spanwise direction. In this study, we also perform a calculation with a varying inflow shear at the inflow boundary. It was clear that the size in the vortex region behind the model was almost the same in all the calculation results, regardless of the difference in the inflow shear. Next, we conduct wind tunnel ….
22. Takanori UCHIDA, Keiji ARAKI, Reproduction of Local Strong Wind Area Induced in the Downstream of Small-scale Terrain by Computational Fluid Dynamic (CFD) Approach, Modelling and Simulation in Engineering, 10.1155/2019/7378315, Volume 2019, Article ID 7378315, 12 pages, 2019.12, In this research, the computational fluid dynamic (CFD) approach that has been used in wind power generation In this research, the computational fluid dynamic (CFD) approach that has been used in wind power generation In this research, the computational fluid dynamic (CFD) approach that has been used in wind power generation In this research, the computational fluid dynamic (CFD) approach that has been used in wind power generation In this research, the computational fluid dynamic (CFD) approach that has been used in wind power generation.
23. Takanori UCHIDA, Numerical Investigation of Terrain-Induced Turbulence in Complex Terrain Using High-Resolution Elevation Data and Surface Roughness Data Constructed with a Drone, Energies, 10.3390/en12193766, 12(19), 3766, 2019.10, Using the method based on unmanned aerial vehicle (UAV) imagery, two kinds of data can be obtained: the digital elevation model (DEM) for the digital expression of terrain, and the digital surface model (DSM) for the digital expression of the surface of the ground, including trees. In this research, a 3D topography model with a horizontal spatial resolution of 1 m was reproduced using DEM. In addition, using the differences between the DEM and DSM data, we were able to obtain further detailed information, such as the heights of trees covering the surface of the ground and their spatial distribution. Therefore, the surface roughness model and the UAV imagery data were directly linked. Based on the above data as input data, a high-resolution 3D numerical flow simulation was conducted. By using the numerical results obtained, we discussed the effect of the existence of surface roughness on the wind speed at the height of the hub of the wind turbine. We also discussed the effect of the differences in the spatial resolution in the horizontal direction of the computational grid on the reproductive precision of terrain-induced turbulence. As a result, the existence and the vortex structure of terrain-induced turbulence occurring near the target wind turbine was clearly revealed. It was shown that a horizontal grid resolution of about 5 m was required to reproduce terrain-induced turbulence formed from topography with an altitude of about 127 m. By the simulation using the surface roughness model, turbulence intensity higher than class A in the International Electrotechnical Commission (IEC) turbulence category was confirmed at the present study site, as well as the measured data..
24. Takanori UCHIDA, Ryo ARAYA, Practical Applications of the Large-Eddy Simulation Technique for Wind Environment Assessment around New National Stadium, Japan (Tokyo Olympic Stadium), Open Journal of Fluid Dynamics, 10.4236/ojfd.2019.94018, 09, 04, Article ID:96061, 2019.10.
25. Koichi WATANABE, Yuji OHYA, Takanori UCHIDA, Power Output Enhancement of a Ducted Wind Turbine by Stabilizing Vortices around the Duct, Energies, 10.3390/en12163171, 12(16), 3171, 2019.08, brimmed-diffuser augmented turbine (called a wind lens turbine: WLT) actively uses vortices around the brim to enhance its power output. However, the vortices are usually unstable
and asymmetric. This study attempted to stabilize the vortices to enhance the power output of a WLT. Then, we investigated new approaches using vortex stabilization plates and polygonal brims in wind tunnel experiments and numerical simulations. Both approaches achieved a 1.5–3.8% increase in power output compared with a standard WLT. Our numerical simulations revealed a periodicity existing in a fluctuating vortex structure on the circular brim. Importantly, vortex stabilization plates and polygonal brims must be the same periodic scale to suppress the vortex fluctuation and stabilize the vortices effectively. In addition, a larger brim tended to enhance the stabilizing effects. We believe that this discovery rovides an easy way to increase the power output of existing wind turbines. It is particularly important in light of advances in wind energy technology and the increasing wind energy market..
26. Takanori UCHIDA, Numerical Prediction of Strong Wind Induced by Topographic Effect, Open Journal of Fluid Dynamics, 10.4236/ojfd.2019.93015, 09, 03, ISSN Online: 2165-3860, ISSN Print: 2165-3852, 2019.08.
27. Takanori Uchida, Yasushi Kawashima, New assessment scales for evaluating the degree of risk of wind turbine blade damage caused by terrain-induced turbulence, Energies, 10.3390/en12132624, 12, 13, 2019.07, The present study scrutinized the impacts of terrain-induced turbulence on wind turbine blades, examining measurement data regarding wind conditions and the strains of wind turbine blades. Furthermore, we performed a high-resolution large-eddy simulation (LES) and identified the three-dimensional airflow structures of terrain-induced turbulence. Based on the LES results, we defined the Uchida-Kawashima Scale_1 (the U-K scale_1), which is a turbulence evaluation index, and clarified the existence of the terrain-induced turbulence quantitatively. The threshold value of the U-K scale_1 was determined as 0.2, and this index was confirmed to not be dependent on the inflow profile, the influence of the horizontal grid resolution, and the influence of the computed azimuth. In addition, we defined the Uchida-Kawashima Scale_2 (the U-K scale_2), which is a fatigue damage evaluation index based on the measurement data and the design value obtained by DNV GL’s Bladed. DNV GL (Det Norske Veritas Germanischer Lloyed) is a third party certification body in Norway, and Bladed has been the industry standard aero-elastic wind turbine modeling software. Using the U-K scale_2, the following results were revealed: the U-K scale_2 was 0.86 1.0 (exceeding the designed value) in the case of easterly wind. As a result, it was revealed that the blades of the target wind turbine were directly and strongly affected by terrain-induced turbulence when easterly winds occurred..
28. Takanori Uchida, Susumu Takakuwa, A Large-Eddy Simulation-Based Assessment of the Risk of Wind Turbine Failures Due to Terrain -Induced Turbulence over a Wind Farim inComplex Terrain, Energies, 10.3390/en12101925, 19pages, 2019.05, The first part of the present study investigated the relationship among the number of yaw gear and motor failures and turbulence intensity (TI) at all the wind turbines under investigation with the use of in situ data. The investigation revealed that wind turbine #7 (T7), which experienced a large number of failures, was affected by terrain-induced turbulence with TI that exceeded the TI presumed for the wind turbine design class to which T7 belongs. Subsequently, a computational fluid dynamics (CFD) simulation was performed to examine if the abovementioned observed wind flow characteristics could be successfully simulated. The CFD software package that was used in the present study was RIAM-COMPACT, which was developed by the first author of the present paper. RIAM-COMPACT is a nonlinear, unsteady wind prediction model that uses large-eddy simulation (LES) for the turbulence model. RIAM-COMPACT is capable of simulating flow collision, separation, and reattachment and also various unsteady turbulence–eddy phenomena that are caused by flow collision, separation, and reattachment. A close examination of computer animations of the streamwise (x) wind velocity revealed the following findings: As we predicted, wind flow that was separated from a micro-topographical feature (micro-scale terrain undulations) upstream of T7 generated large vortices. These vortices were shed downstream in a nearly periodic manner, which in turn generated terrain-induced turbulence, affecting T7 directly. Finally, the temporal change of the streamwise (x) wind velocity (a non-dimensional quantity) at the hub-height of T7 in the period from 600 to 800 in non-dimensional time was re-scaled in such a way that the average value of the streamwise (x) wind velocity for this period was 8.0 m/s, and the results of the analysis of the re-scaled data were discussed. With the re-scaled full-scale streamwise wind velocity (m/s) data (total number of data points: approximately 50,000; time interval: 0.3 s), the time-averaged streamwise (x) wind velocity and TI were evaluated using a common statistical processing procedure adopted for in situ data. Specifically, 10-min moving averaging (number of sample data points: 1932) was performed on the re-scaled data. Comparisons of the evaluated TI values to the TI values from the normal turbulence model in IEC61400-1 Ed.3 (2005) revealed the following: Although the evaluated TI values were not as large as those observed in situ, some of the evaluated TI values exceeded the values for turbulence class A, suggesting that the influence of terrain-induced turbulence on the wind turbine was well simulated..
29. Kenji Ono, Takanori Uchida, High-Performance Parallel Simulation of Airflow for Complex Terrain Surface, Modelling and Simulation in Engineering, 10.1155/2019/5231839, 2019, Article ID 5231839, 10 pages, 2019.02, It is important to develop a reliable and high-throughput simulation method for predicting airflows in the installation planning phase of windmill power plants. This study proposes a two-stage mesh generation approach to reduce the meshing cost and introduces a hybrid parallelization scheme for atmospheric fluid simulations. The meshing approach splits mesh generation into two stages: in the first stage, the meshing parameters that uniquely determine the mesh distribution are extracted, and in the second stage, a mesh system is generated in parallel via an in situ approach using the parameters obtained in the initialization phase of the simulation. The proposed two-stage approach is flexible since an arbitrary number of processes can be selected at run time. An efficient OpenMP-MPI hybrid parallelization scheme using a middleware that provides a framework of parallel codes based on the domain decomposition method is also developed. The preliminary results of the meshing and computing performance show excellent scalability in the strong scaling test..
30. Takanori Uchida, Large-Eddy Simulation of Airflow over a Steep,Three-Dimensional Isolated Hill with Multi-GPUs Computing, Open Journal of Fluid Dynamics, 10.4236/ojfd.2018.84027, 8, 4, 416-434, 2018.11, The present research attempted a Large-Eddy Simulation (LES) of airflow over a steep, three-dimensional isolated hill by using the latest multi-cores multi-CPUs systems. As a result, it was found that 1) turbulence simulations using approximately 50 million grid points are feasible and 2) the use of this system resulted in the achievement of a high computation speed, which exceeded the speed of parallel computation attained by a single CPU on one of the latest supercomputers. Furthermore, LES was conducted by using the multi-GPUs systems. The results of these simulations revealed the following findings: 1) the multi-GPUs environment which used the NVDIA Tesla M2090 or the M2075 could simulate turbulence in a model with as many as approximately 50 million grid points. 2) The computation speed achieved by the multi-GPUs environments exceeded that by parallel computation which
used four to six CPUs of one of the latest supercomputers..
31. Takanori Uchida, Numerical Investigation of Terrain-induced Turbulence in Complex Terrain by Large-eddy Simulation (LES) Technique, Energies, 10.3390/en11061530, 11(10), 2638, 15pages, 2018.10, [URL], In the present study, field observation wind data from the time of the wind turbine blade damage accident on Shiratakiyama Wind Farm were analyzed in detail. In parallel, high-resolution large-eddy simulation (LES) turbulence simulations were performed in order to examine the model’s ability to numerically reproduce terrain-induced turbulence (turbulence intensity) under strong wind conditions (8.0–9.0 m/s at wind turbine hub height). Since the wind velocity and time acquired from the numerical simulation are dimensionless, they are converted to full scale. As a consequence, both the standard deviation of the horizontal wind speed (m/s) and turbulence intensity evaluated from the field observation and simulated wind data are successfully in close agreement. To investigate the cause of the wind turbine blade damage accident on Shiratakiyama Wind Farm, a power spectral analysis was performed on the fluctuating components of the observed time series data of wind speed (1 s average values) for a 10 min period (total of 600 data) by using a fast Fourier transform (FFT). It was suggested that the terrain-induced turbulence which caused the wind turbine blade damage accident on Shiratakiyama Wind Farm was attributable to rapid wind speed and direction fluctuations which were caused by vortex shedding from Tenjogadake (elevation: 691.1 m) located upstream of the wind farm..
32. Takanori UCHIDA, Graham Li, Comparison of RANS and LES in the Prediction of Airflow Field over Steep Complex Terrain, Open Journal of Fluid Dynamics, 10.4236/ojfd.2018.83018, 08, 286-307, Article ID:87086,22 pages, 2018.09, The present study compared the prediction accuracy of the three CFD software packages for simulating airflow around a three-dimensional, isolated hill with a steep slope: 1) WindSim (turbulence model: RNG k-ε RANS), 2) Meteodyn WT (turbulence model: k-L RANS), which are the leading commercially available CFD software packages in the wind power industry and 3) RIAM-COMPACT (turbulence model: standard Smagorinsky LES), which has been developed by the lead author of the present paper. Distinct differences in the airflow patterns were identified in the vicinity of the isolated hill (especially downstream of the hill) between the RANS results and the LES results. No reverse flow region (vortex region) characterized by negative wind velocities was identified downstream of the isolated hill in the result from the simulation with WindSim (RNG k-ε RANS) and Meteodyn WT (k-L RANS). In the case of the simulation with RIAM-COMPACT natural terrain version (standard Smagorinsky LES), a reverse flow region (vortex region) characterized by negative wind velocities clearly forms. Next, an example of wind risk (terrain-induced turbulence) diagnostics was presented for a large-scale wind farm in China. The vertical profiles of the streamwise (x) wind velocity do not follow the so-called power law wind profile; a large velocity deficit can be seen between the hub center and the lower end of the swept area in the case of the LES calculation (RIAM-COMPACT)..
33. Takanori Uchida, Computational Investigation of the Causes of Wind Turbine Blade Damage at Japan's Wind Farm in Complex Terrain, Journal of Flow Control, Measurement & Visualization, 10.4236/jfcmv.2018.63013, 6, 3, 152-167, 2018.07, [URL], During the passage of Typhoon 0918 (Melor) over southern Honshu in Japan on 7 and 8 October 2009, strong winds with extremely high turbulence fluctuations were observed over Shirataki Mountain and the surrounding mountains in Shimonoseki, Yamaguchi Prefecture, Japan. These strong winds caused damage to wind turbine blades at the Shiratakiyama Wind Farm owned by Kinden Corporation. In order to investigate the causes of the blade damage, the airflow characteristics from the time of the incidences are first simulated in detail with the combined use of the WRF-ARW mesoscale meteorological model and the RIAM-COMPACT LES turbulence model (CFD model). Subsequently, in order to evaluate the wind pressure acting on the wind turbine blades, an airflow analysis is separately performed for the vicinity of the blades with the RANS turbulence model. Finally, the stress on the blades is investigated using the FEM with the RANS analysis results as the boundary conditions..
34. Takanori Uchida, Designed Wind Speed Evaluation Technique in Wind Turbine Installation Point by Using the Meteorological Model and CFD Model, Journal of Flow Control, Measurement & Visualization, 10.4236/jfcmv.2018.63014, 06, 03(2018), Article ID:85916,17 pages, 2018.07, It is highly important in Japan to choose a good site for wind turbines, becausethe spatial distribution of wind speed is quite complicated over steepcomplex terrain. We have been developing the unsteady numerical model called the RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain).The RIAM-COMPACT is based on the LES (Large-Eddy Simulation).The object domain of the RIAM-COMPACT is from several m to several km,and can predict the airflow and gas diffusion over complex terrain with highprecision. In the present paper, the design wind speed evaluation technique inwind turbine installation point by using the mesoscale meteorological modeland RIAM-COMPACT CFD model was proposed. The design wind speed tobe used for designing WTGs can be calculated by multiplying the ratio of the mean wind speed at the hub-height to the mean upper-air wind speed at the inflow boundary, i.e. , the fractional increase of the mean hub-height wind speed, by the reduction ratio, R. The fractional increase of the mean hub-height wind speed was evaluated using the CFD simulation results. This method was proposed as Approach 1 in the present paper. A value of 61.9 m/s was obtained for the final design wind speed, Uh, in Approach 1. In the evaluation procedure of the design wind speed in Approach 2, neither the above-mentioned reduction rate, R, nor an upper-air wind speed of 1.7 Vo, where Vo is the reference wind speed, was used. Instead, the value of the maximum wind speed which was obtained from the typhoon simulation for
each of the investigated wind directions was adopted. When the design wind speed was evaluated using the 50-year recurrence value, the design wind speed was 48.3 m/s. When a somewhat conservative safety factor was applied, that is, when the 100 year recurrence value was used instead, the design wind speed was 52.9 m/s..
35. Takanori Uchida, LES Investigation of Terrain-Induced Turbulence in Complex Terrain and Economic Effects of Wind Turbine Control, Energies, 10.3390/en11061530, 11(6), 1530, 15pages, 2018.06, [URL], In the present study, numerical wind simulation was conducted by reproducing the realistic topography near wind turbine sites with high spatial resolutions and using the Large-Eddy Simulation (LES) technique. The topography near wind turbine sites serves as the origin of the terrain-induced turbulence. The obtained numerical simulation results showed that the terrain-induced turbulence is generated at a small terrain feature located upstream of the wind turbine. The generated terrain-induced turbulence affects the wind turbine directly. The wind speed and wind direction at the wind turbine site are significantly changed with time. In the present study, a combination of the series of wind simulation results and on-site operation experience led to a decision to adopt an “automatic shutdown program”. Here, an “automatic shutdown program” means the automatic suspension of wind turbine operation based on the wind speed and wind direction meeting the conditions associated with significant effects of terrain-induced turbulence at a wind turbine site. The adoption of the “automatic shutdown program” has successfully led to a large reduction in the number of occurrences of wind turbine damage, thus, creating major positive economic effects..
36. Takanori Uchida, Computational Fluid Dynamics Approach to Predict the Actual Wind Speed over Complex Terrain, Energies, 10.3390/en11071694, 11(7), 1694, 13pages, 2018.06, [URL], This paper proposes a procedure for predicting the actual wind speed for flow over complex terrain with CFD. It converts a time-series of wind speed data acquired from field observations into a time-series data of actual scalar wind speed by using non-dimensional wind speed parameters, which are determined beforehand with the use of CFD output. The accuracy and reproducibility of the prediction procedure were investigated by simulating the flow with CFD with the use of high spatial resolution (5 m) surface elevation data for the Noma Wind Park in Kagoshima Prefecture, Japan. The errors of the predicted average monthly wind speeds relative to the observed values were less than approximately 20%..
37. Takanori Uchida, Computational Fluid Dynamics (CFD) Investigation of Wind Turbine Nacelle Separation Accident over Complex Terrain in Japan, Energies, 10.3390/en11061485, 11(6), 1485, 13pages, 2018.06, [URL], We have developed an unsteady and non-linear wind synopsis simulator called RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, COMputational Prediction of Airflow over Complex Terrain) to simulate the airflow on a micro scale, i.e., a few tens of km or less. In RIAM-COMPACT, the large-eddy simulation (LES) has been adopted for turbulence modeling. LES is a technique in which the structures of relatively large eddies are directly simulated and smaller eddies are modeled using a sub-grid scale model. In the present study, we conducted numerical wind diagnoses for the Taikoyama Wind Farm nacelle separation accident in Japan. The simulation results suggest that all six wind turbines at Taikoyama Wind Farm are subject to significant influence from separated flow (terrain-induced turbulence) which is generated due to the topographic irregularities in the vicinity of the wind turbines. A proposal was also made on reconstruction of the wind farm..
38. Takanori UCHIDA, A New Proposal for Vertical Extrapolation of Offshore Wind Speed and an Assessment of Offshore Wind Energy Potential for the Hibikinada Area, Kitakyushu, Japan, Energy and Power Engineering, 10.4236/epe.2018.104011, 10, 04, 11pages, 2018.04, [URL], The author’s research group has been conducting research on applications of various meteorological Grid Point Value (GPV) data offered by the Japan Meteorological Agency (JMA) to the field of wind power generation. In particular, the group’s research has been focusing on the following areas: 1) the use of GPV data from the JMA Meso-Scale Model (MSM-S; horizontal resolution: 5 km) and the JMA Local Forecast Model (LFM-S; horizontal resolution: 2 km), and 2) examinations of the prediction accuracy of local wind assessment with the use of these data. In both the MSM-S and the LFM-S, grid points are fixed at 10 m above the sea (ground) surface. The purpose of the present study is to establish a method in which the values of the MSM-S and LFM-S wind speed data from the 10 m height are used as the reference wind speed and are, using a power law, vertically extrapolated to 80 to 90 m heights, typical hub-heights of offshore wind turbines. For this purpose, the present study examined time-averaged vertical profiles of wind speed over the ocean based on the MSM-S data and in-situ data in the Hibikinada area, Kitakyushu City, Fukuoka Prefecture, Japan. As a result, it was revealed that a strong wind shear existed close to the sea surface, between the 10 and 30 m heights. In order to address the above-mentioned wind shear, a two-step vertical extrapolation method was proposed in the present study. In this method, two values of N, specifically for low and high altitudes (below and above approximately 30 m, respectively), were calculated and used. The data were created for the five years between 2012 and 2016. Similarly to previous analyses, the analysis of the created data set indicated that the potential of offshore wind power generation in the Hibikinada area, Kitakyushu City is quite high..
39. Takanori UCHIDA, Large-Eddy Simulation and Wind Tunnel Experiment of Airflow over Bolund Hill, Open Journal of Fluid Dynamics, 10.4236/ojfd.2018.81003, 8, 30-43, 2018.03, [URL].
40. Yasushi KAWASHIMA, Takanori UCHIDA, Effects of Terrain-Induced Turbulence on Wind Turbine Blade Fatigue Loads, Energy and Power Engineering, 10.4236/epe.2017.913053, 9, 843-857, 2017.12, [URL].
41. Takanori UCHIDA, Three-Dimensional Numerical Simulation of Stably Stratified Flows over a Two-Dimensional Hill, Open Journal of Fluid Dynamics, 10.4236/ojfd.2017.74039, 7, 579-595, 2017.12.
42. Takanori UCHIDA, High-Resolution LES of Terrain-Induced Turbulence around Wind Turbine Generators, by Using Turbulent Inflow Boundary Conditions, Open Journal of Fluid Dynamics, 10.4236/ojfd.2017.74035, 7, 511-524, 2017.12.
43. Takanori UCHIDA, CFD Prediction of the Airflow at a Large-Scale Wind Farm above a Steep, Three-Dimensional Escarpment, Energy and Power Engineering, 10.4236/epe.2017.913052, 9, 829-842, 2017.12.
44. Takanori UCHIDA, High-Resolution Micro-Siting Technique for Large Scale Wind Farm Outside of Japan Using LES Turbulence Model, Energy and Power Engineering, 10.4236/epe.2017.912050, 9, 802-813, 2017.12.
45. Koichi Watanabe, Yuji Ohya, Takanori Uchida, Tomoyuki Nagai, Numerical Prediction and Field Verification Test of Wind-Power Generation Potential in Nearshore Area Using a Moored Floating Platform, Journal of Flow Control, Measurement & Visualization, 10.4236/jfcmv.2017.52002, 5, 2, 2017.04.
46. Yuji Ohya, Masaki Wataka, Koichi Watanabe, Takanori Uchida, Laboratory experiment and numerical analysis of a new type of solar tower efficiently generating a thermal updraft, energies, 10.3390/en9121077, 9, 12, 2016.12, A new type of solar tower was developed through laboratory experiments and numerical analyses. The solar tower mainly consists of three components. The transparent collector area is an aboveground glass roof, with increasing height toward the center. Attached to the center of the inside of the collector is a vertical tower within which a wind turbine is mounted at the lower entry to the tower. When solar radiation heats the ground through the glass roof, ascending warm air is guided to the center and into the tower. A solar tower that can generate electricity using a simple structure that enables easy and less costly maintenance has considerable advantages. However, conversion efficiency from sunshine energy to mechanical turbine energy is very low. Aiming to improve this efficiency, the research project developed a diffuser-type tower instead of a cylindrical tower, and investigated a suitable diffuser shape for practical use. After changing the tower height and diffuser open angle, with a temperature difference between the ambient air aloft and within the collector, various diffuser tower shapes were tested by laboratory experiments and numerical analyses. As a result, it was found that a diffuser tower with a semi-open angle of 4° is an optimal shape, producing the fastest updraft at each temperature difference in both the laboratory experiments and numerical analyses. The relationships between thermal updraft speed and temperature difference and/or tower height were confirmed. It was found that the thermal updraft velocity is proportional to the square root of the tower height and/or temperature difference..
47. Yuji Ohya, Masaki Wataka, Koichi Watanabe, Takanori Uchida, Laboratory Experiment and Numerical Analysis of a New Type of Solar Tower Efficiently Generating a Thermal Updraft, ENERGIES, 10.3390/en9121077, 9, 12, 2016.12, A new type of solar tower was developed through laboratory experiments and numerical analyses. The solar tower mainly consists of three components. The transparent collector area is an aboveground glass roof, with increasing height toward the center. Attached to the center of the inside of the collector is a vertical tower within which a wind turbine is mounted at the lower entry to the tower. When solar radiation heats the ground through the glass roof, ascending warm air is guided to the center and into the tower. A solar tower that can generate electricity using a simple structure that enables easy and less costly maintenance has considerable advantages. However, conversion efficiency from sunshine energy to mechanical turbine energy is very low. Aiming to improve this efficiency, the research project developed a diffuser-type tower instead of a cylindrical tower, and investigated a suitable diffuser shape for practical use. After changing the tower height and diffuser open angle, with a temperature difference between the ambient air aloft and within the collector, various diffuser tower shapes were tested by laboratory experiments and numerical analyses. As a result, it was found that a diffuser tower with a semi-open angle of 4 degrees is an optimal shape, producing the fastest updraft at each temperature difference in both the laboratory experiments and numerical analyses. The relationships between thermal updraft speed and temperature difference and/or tower height were confirmed. It was found that the thermal updraft velocity is proportional to the square root of the tower height and/or temperature difference..
48. Masahiro Konishi, Shigeyuki Sako, Takanori Uchida, Ryou Araya, Koui Kim, Yuzuru Yoshii, Mamoru Doi, Kotaro Kohno, Takashi Miyata, Kentaro Motohara, Masuo Tanaka, Takeo Minezaki, Tomoki Morokuma, Yoichi Tamura, Toshihiko Tanabé, Natsuko Kato, Takafumi Kamizuka, Hidenori Takahashi, Tsutomu Aoki, Takao Soyano, Ken'ichi Tarusawa, The University of Tokyo Atacama Observatory 6.5m Telescope: enclosure design and wind analysis, Ground-based and Airborne Telescopes VI, https://doi.org/10.1117/12.2231385, 9906, 2016.07, We present results on the computational fluid dynamics (CFD) numerical simulations as well as the wind tunnel experiments for the observation facilities of the University of Tokyo Atacama Observatory 6.5m Telescope being constructed at the summit of Co. Chajnantor in northern Chile. Main purpose of this study starting with the baseline design reported in 2014 is to analyze topographic effect on the wind behavior, and to evaluate the wind pressure, the air turbulence, and the air change (ventilation) efficiency in the enclosure. The wind velocity is found to be accelerated by a factor of ~ 1.2 to reach the summit (78 m sec-1 expected at a maximum), and the resulting wind pressure (3,750 N m-2) is used for the framework design of the facilities. The CFD data reveals that the open space below the floor of the facilities works efficiently to drift away the air turbulence near the ground level which could significantly affect the dome seeing. From comparisons of the wind velocity field obtained from the CFD simulation for three configurations of the ventilation windows, we find that the windows at a level of the telescope secondary mirror have less efficiency of the air change than those at lower levels. Considering the construction and maintenance costs, and operation procedures, we finally decide to allocate 13 windows at a level of the observing floor, 12 at a level of the primary mirror, and 2 at the level of the secondary mirror. The opening area by those windows accounts for about 14% of the total interior surface of the enclosure. Typical air change rate of 20-30 per hour is expected at the wind velocity of 1 m sec-1..
49. Takanori Uchida, Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD Software ―Report 1: For the Case of a Three-Dimensional Isolated-Hill With Steep Slopes―, Reports of Research Institute for Applied Mechanics, Kyushu University, 150, 47-59, 2016.03.
50. Takanori Uchida, Reproducibility of Complex Turbulent Flow Using Commercially-Available CFD Software ―Report 3: For the Case of a Three-dimensional Cube―, Reports of Research Institute for Applied Mechanics, Kyushu University, 150, 71-83, 2016.03.
51. Shinsuke Okada, Takanori Uchida, Takashi Karasudani, Yuji Ohya, Improvement in Solar Chimney Power Generation by Using a Diffuser Tower, Journal of Solar Energy Engineering, Transactions of the ASME, 138, 1, 2016.02.
52. Takanori Uchida, Fumihito Watanabe, Shin Mikami, Analysis of the Airflow Field around a Steep, Three-dimensional Isolated Hill with Commercially Available CFD Software, Reports of Research Institute for Applied Mechanics, Kyushu University, 149, 91-98, 2015.09.
53. Shinsuke Okada, Takanori Uchida, Takashi Karasudani, Yuji Ohya, Improvement in Solar Chimney Power Generation by Using a Diffuser Tower, JOURNAL OF SOLAR ENERGY ENGINEERING-TRANSACTIONS OF THE ASME, 10.1115/1.4029377, 137, 3, 2015.06, The solar chimney prototype, operated in Spain from 1982 to 1989, verified the concept of the solar chimney. The power generation mechanism in this system is to turn the wind turbine placed inside a high rise cylindrical hollow tower by an induced thermal updraft. As long as the thermal updraft is induced inside the tower by the solar radiation, this system can produce electricity. The disadvantage of this system is the low power generation efficiency compared to other solar energy power generation systems. To overcome this disadvantage, we improved the mechanism in order to augment the velocity of the air which flows into the wind turbine. By applying a diffuser tower instead of a cylindrical one, the efficiency of the systems power generation is increased. The mechanism that we investigated was the effect of the diffuser on the solar chimney structure. The inner diameter of the tower expands as the height increases so that the static pressure recovery effect of the diffuser causes a low static pressure region to form at the bottom of the tower. This effect induces greater airflow within the tower. The laboratory experiment, as does the computational fluid dynamics (CFD) analysis of the laboratory sized model, shows that the proposed diffuser type tower induces a velocity approximately 1.38-1.44 times greater than the conventional cylindrical type. The wind power generation output is proportional to the cube of the incoming wind velocity into the wind turbine; therefore, approximately 2.6-3.0 times greater power output can be expected from using the diffuser type tower..
54. Kenji Kitagawa, Shintaro Iwama, Sho Fukui, Yuuki Sunaoka, Hayato Yazawa, Atsushi Usami, Masaaki Naramoto, Takanori Uchida, Satoshi Saito, Hiromi Mizunaga, Effects of Components of the Leaf Area Distribution on Drag Relations for Cryptomeria Japonica and Chamaecyparis Obtusa, European Journal of Forest Research, 10.1007/s10342-014-0859-6, 134, 3, 403-414, 2015.05, The objectives of this study were to clarify the effects of components of the leaf area distribution on the drag coefficient of crowns and streamlining (e.g., leaf area index; LAI, outline of the crown shape, and clumpiness) and to contribute to the accumulation of data on drag relations by quantifying data for Chamaecyparis obtusa and Cryptomeria japonica. We conducted drag experiments while simultaneously capturing dynamic crown images for 28 Ch. obtusa crowns and 13 Cr. japonica crowns to analyze the relationships between the leaf area distribution components and drag coefficient or streamlining. The static drag coefficient increased with the LAI for Ch. obtusa and with decreasing clumpiness for Cr. japonica. The reduction rate of the static drag coefficient decreased with increasing clumpiness for Ch. obtusa and with a combination of increasing LAI and decreasing clumpiness for Cr. japonica. The reduction rate of the static drag coefficient had a clear relationship with the decreasing rate of the dynamic crown projected area of obstacles (foliage elements, branches, and stems) calculated from captured video images under windy conditions for Cr. japonica, while Ch. obtusa did not show clear relationship between them. The drag coefficients assuming non-porous crown; Cmax estimated by simple model combining LAI and clumpiness were approximately 1.0 in Ch. obtusa and 0.5 in Cr. japonica and were equivalent to the dynamic drag coefficients from video image under windy condition. The combination of LAI and clumpiness provided simple estimation for drag relations and enable to link crown structure to wind damage easier..
55. Takanori Uchida, An Examination of the Taikoyama Wind Farm Nacelle Separation Accident Using a CFD Approach, Reports of Research Institute for Applied Mechanics, Kyushu University, 148.0, 15.0-24.0, 2015.03, Because a significant portion of the topography of Japan is characterized by steep, complex terrain, which results in a complex spatial distribution of wind speed, great care is necessary for selecting a site for the construction of wind turbines. We have developed a computational fluid dynamics (CFD) model for unsteady flow called Research Institute for Applied Mechanics, Kyushu University, COMputational Prediction of Airflow over Complex Terrain (RIAM-COMPACT®). The RIAM-COMPACT® CFD model is based on the large-eddy simulation (LES) technique. In this paper, a numerical wind simulation for the Taikoyama Wind Farm is performed using high-resolution terrain elevation data. The results suggest that all six wind turbines at the Taikoyama Wind Farm are subject to significant influence from separated flow (terrain-induced turbulence) which is generated due to the topographical irregularities in the vicinity of the wind turbines. A proposal has been also made on reconstruction of the wind farm..
56. Shinsuke Okada, Takanori Uchida, Takashi Karasudani, Yuji Ohya, Improvement in solar chimney power generation by using a diffuser tower, Journal of Solar Energy Engineering, Transactions of the ASME, 10.1115/1.4029377, 137, 3, 2015.01, The solar chimney prototype, operated in Spain from 1982 to 1989, verified the concept of the solar chimney. The power generation mechanism in this system is to turn the wind turbine placed inside a high rise cylindrical hollow tower by an induced thermal updraft. As long as the thermal updraft is induced inside the tower by the solar radiation, this system can produce electricity. The disadvantage of this system is the low power generation efficiency compared to other solar energy power generation systems. To overcome this disadvantage, we improved the mechanism in order to augment the velocity of the air which flows into the wind turbine. By applying a diffuser tower instead of a cylindrical one, the efficiency of the systems power generation is increased. The mechanism that we investigated was the effect of the diffuser on the solar chimney structure. The inner diameter of the tower expands as the height increases so that the static pressure recovery effect of the diffuser causes a low static pressure region to form at the bottom of the tower. This effect induces greater airflow within the tower. The laboratory experiment, as does the computational fluid dynamics (CFD) analysis of the laboratory sized model, shows that the proposed diffuser type tower induces a velocity approximately 1.38-1.44 times greater than the conventional cylindrical type. The wind power generation output is proportional to the cube of the incoming wind velocity into the wind turbine; therefore, approximately 2.6-3.0 times greater power output can be expected from using the diffuser type tower..
57. Fumihito Watanabe, Takanori Uchida, Micro-siting of wind turbine in complex terrain Simplified fatigue life prediction of main bearing in direct drive wind turbines, Wind Engineering, 10.1260/0309-524X.39.4.349, 39, 4, 349-368, 2015.01, In Japan, 1,516 wind turbine accidents have been recorded between year 2004 and 2012, and 84% of them were for turbines in complex terrains. The longest downtime was associated with damage to main shafts or bearings with an average downtime of 5.7 months. Careful micro-siting in complex terrains can prevent these accidents from happening. The objective of the paper is to provide an intermediate step that allows consultants, developers, and wind farm owners to further evaluate micro-siting of wind turbines in complex terrains, prior to load simulations by manufacturers. The author developed a simplified method to predict fatigue life of a main rear bearing in direct drive wind turbines. The method uses hub-height 10 minutes wind data as an input. The validation with an actual accident showed practically good agreement of 12.0 years of the predicted life against 12.7 years of the actual life. The method was also applied to quantify the effect of a curtailment. The proposed curtailment increased the predicted life to 35.5 years with a relatively small range of wind speed at a direction of only I % frequency distribution. With the proposed method, it is possible to layout turbines where fatigue life of a main rear bearing is longer than its design life. The method can also be applied to existing turbines in order to spot turbines that require careful maintenance, and to consider an installation of condition monitoring system on a bearing..
58. Takanori Uchida, Validation Testing of the Prediction Accuracy of the Numerical Wind Synopsis Prediction Technique RIAM-COMPACT for the Case of the Bolund Experiment-Comparison against a Wind-Tunnel Experiment-, Reports of Research Institute for Applied Mechanics, Kyushu University, 147, 7-14, 2014.09.
59. Yuji Ohya, Takanori Uchida, Tomoyuki Nagai, Near Wake of a Horizontal Circular Cylinder in Stably Stratified Flows, Open Journal of Fluid Dynamics, Vol.3, 311-320, 2013.12.
60. Kana Kamimura, Satoshi Saito, Hiroko Kinoshita, Kenji Kitagawa, Takanori Uchida, Hiromi Mizunaga, Analysis of wind damage caused by multiple tropical storm events in Japanese Cryptomeria japonica forests, Forestry, 10.1093/forestry/cpt011, 86, 4, 411-420, 2013.10, This study analyzed wind damage caused by tropical storms from 1991 to 2007 to Japanese forests mainly consisting of Cryptomeria japonica. Statistical analyses based on logistic regression and Cox regression models were conducted in relation to conditions at the forest and stand level. Known damage to forests managed by Kyusyu Rinsan Corporation (KR forests), located on the Kyushu Island, was analyzed at the forest level, using tropical storm characteristics such as air pressure, precipitation and periods when the forests were within the storm zone as predictors. Wind damage was also examined at the stand level (150 analysis points) using Cox regression models, according to stand age, site index, terrain conditions, management practices and wind velocity indicators (horizontal and vertical velocity vectors). The results indicated that at the forest level, higher maximum hourly wind speed and longer periods of >15 m s-1 of wind speed were significantly correlated to damage occurrence. At the stand level, indicators of upward vertical velocity, thinning treatments and site index were positively associated with the probability of wind damage. For instance, stands receiving higher upward vertical velocities and thinning treatment within 2 years were more likely to have reduced stability against tropical storms. Stands with higher and lower site index than average also showed lower stability in our analysis..
61. Kana Kamimura, Satoshi Saito, Hiroko Kinoshita, Kenji Kitagawa, 内田 孝紀, Hiromi Mizunaga, Analysis of wind damage caused by multiple tropical storm events in Japanese Cryptomeria japonica forests, Forestry, 10.1093/forestry/cpt011, 0, pp.1-10, 2013.04.
62. Large-Eddy Simulation of Wake Flow behind the Wind Turbine Generator in Complex Terrain
In the present numerical study, we carried out the large-eddy simulation (LES) of the wake generated behind the wind turbine under optimal tip speed ratio over complex terrain. Although existence of tip vortex was seen, it was shown that the structure  has collapsed quickly, due to the separated flow from topography..
63. Yuji Ohya, Takanori Uchida, Takashi Karasudani, Masaru Hasegawa, Hiroshi Kume, Numerical studies of flow around a wind turbine equipped with a flanged-diffuser shroud using an actuator-disk model, Wind Engineering, 10.1260/0309-524X.36.4.455, 36, 4, 455-472, 2012.08, Unsteady 3-D direct numerical simulations based on the finite-difference method (FDM) were performed for flow fields around a wind turbine equipped with a flanged-diffuser shroud. Generally, it is difficult to numerically simulate the flow around rotating bodies such as the blades of wind turbines because of the unsteady flow generated by moving bodies with complex geometry. Therefore, we have devised an actuator-disk model for a wind turbine for simulating the drag and rotational forces exerted on the fluid by the wind turbine. By incorporating the body forces derived from the actuator-disk model into the external terms in the Navier-Stokes equations, the unsteady flow around a wind turbine can be simulated. The results of numerical simulations were compared with the results from wind tunnel experiments and showed good agreement for the velocity and pressure fields..
64. Shuhei Takahashi, Yuya Hata, Yuji Ohya, Takashi Karasudani, Takanori Uchida, Behavior of the blade tip vortices of a wind turbine equipped with a brimmed-diffuser shroud, Energies, 10.3390/en5125229, 5, 12, 5229-5242, 2012.01, To clarify the behavior of the blade tip vortices of a wind turbine equipped with a brimmed-diffuser shroud, called a "Wind-Lens turbine", we conducted a three-dimensional numerical simulation using a large eddy simulation (LES). Since this unique wind turbine consists of not only rotating blades but also a diffuser shroud with a broad-ring brim at the exit periphery, the flow field around the turbine is highly complex and unsteady. Previously, our research group conducted numerical simulations using an actuator-disc approximation, in which the rotating blades were simply modeled as an externalforce on the fluid. Therefore, the detailed flow patterns around the rotating blades and the shroud, including the blade tip vortices, could not be simulated. Instead of an actuator-disc approximation, we used a moving boundary technique in the present CFD simulation to simulate the flow around a rotating blade in order to focus especially on blade tip vortices. The simulation results showed a pair ofvortices consisting of a blade tip vortex and a counter-rotating vortex which was generated between the blade tip and the inner surface of the diffuser. Since these vortices interacted with each other, the blade tip vortex was weakened by the counter-rotating vortex. The results showed good agreement with past wind tunnel experiments..
65. Takanori UCHIDA, Takashi MARUYAMA, Tetsuya TAKEMI, Yuichiro OKU, Yuji OHYA and Graham Li, Proposal of Designed Wind Speed Evaluation Technique in WTG Installation Point by Using the Meteorological Model and CFD Model, 九州大学応用力学研究所所報, 第141号, pp.1-12, 2011.10.
66. Takanori UCHIDA, Takashi MARUYAMA, Hirohiko ISHIKAWA, Masaru ZAKO and Akira DEGUCHI, Investigation of the Causes of Wind Turbine Blade Damage at Shiratakiyama Wind Farm in Japan-A Computer Simulation Based Approach-, 九州大学応用力学研究所所報, 第141号, pp.13-25, 2011.10.
67. 内田孝紀, 大屋裕二, LES技術を用いたウインドファーム風況診断―熊本県阿蘇車帰風力発電所を例として―, 土木学会論文集A2(応用力学)Vol.67 特集号, 2011.09.
68. Takanori Uchida, Takashi Maruyama, Yuji Ohya, New evaluation technique for WTG design wind speed using a CFD-model-based unsteady flow simulation with wind direction changes, Modelling and Simulation in Engineering, 10.1155/2011/941870, 2011, 2011.09, Because a significant portion of the topography in Japan is characterized by steep, complex terrain, which results in a complex spatial distribution of wind speed, great care is necessary for selecting a site for the construction of wind turbine generators (WTG). We have developed a CFD model for unsteady flow called RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, computational prediction of airflow over complex terrain). The RIAM-COMPACT CFD model is based on large eddy simulation (LES). The computational domain of RIAM-COMPACT can extend from several meters to several kilometers, and RIAM-COMPACT can predict airflow and gas diffusion over complex terrain with high accuracy. The present paper proposes a technique for evaluating the deployment location of a WTG. The proposed technique employs the RIAM-COMPACT CFD model and simulates a continuous wind direction change over 360 degrees..
69. Takanori Uchida, Yuji Ohya, Kenichiro Sugitani, Comparisons between the wake of a wind turbine generator operated at optimal tip speed ratio and the wake of a stationary disk, Modelling and Simulation in Engineering, 10.1155/2011/749421, 2011, 1-7, 2011.04, The wake of a wind turbine generator (WTG) operated at the optimal tip speed ratio is compared to the wake of a WTG with its rotor replaced by a stationary disk. Numerical simulations are conducted with a large eddy simulation (LES) model using a nonuniform staggered Cartesian grid. The results from the numerical simulations are compared to those from wind-tunnel experiments. The characteristics of the wake of the stationary disk are significantly different from those of the WTG. The velocity deficit at a downstream distance of 10 D (D: rotor diameter) behind the WTG is approximately 30 to 40 of the inflow velocity. In contrast, flow separation is observed immediately behind the stationary disk (≤2D), and the velocity deficit in the far wake (10 D) of the stationary disk is smaller than that of the WTG. Copyright © 2011 Takanori Uchida et al..
70. Takanori Uchida, Takashi Maruyama and Yuji Ohya, New Evaluation Technique for WTG Design Wind Speed using a CFD-model-based Unsteady Flow Simulation with Wind Direction Changes, Modelling and Simulation in Engineering, Volume 2011 (2011), 2011.03.
71. Takanori Uchida, Yuji Ohya and Kenichiro Sugitani, Comparisons Between The Wake Of A Wind Turbine Generator Operated At Optimal Tip Speed Ratio And The Wake Of A Stationary Disk, Modelling and Simulation in Engineering, Volume 2011 (2011), 2011.03.
72. Takanori Uchida, Yuji Ohya, Latest developments in numerical wind synopsis prediction using the RIAM-COMPACT® CFD model-design wind speed evaluation and wind risk (terrain-induced turbulence) diagnostics in Japan, Energies, 10.3390/en4030458, 4, 3, 458-474, 2011.01, Because a significant portion of the topography in Japan is characterized by steep, complex terrain, which results in a complex spatial distribution of wind speed, great care is necessary for selecting a site for the construction of Wind Turbine Generators (WTGs). We have developed a CFD model for unsteady flow called Research Institute for Applied Mechanics, Kyushu University, COMputational Prediction of Airflow over Complex Terrain (RIAM-COMPACT®). The RIAM-COMPACT® CFD model is based on Large-Eddy Simulation (LES) technique. The computational domain of RIAM-COMPACT® can extend from several meters to several kilometers, and RIAM-COMPACT® can predict airflow and gas diffusion over complex terrains with high accuracy. First, the present paper proposes a technique for evaluating the deployment location of WTGs. Next, wind simulation of an actual wind farm was executed using the high resolution elevation data. As a result, an appropriate point and an inappropriate point for locating WTGs were shown based on the numerical results obtained. This cause was found to be a topographical irregularity in front of WTGs..
73. Takanori UCHIDA, Yuji OHYA, Challenge to Huge Computation of Airflow around Urban Area by using RIAM-COMPACT® CFD Model, Proceedings of EAEP2010/The 4th International Symposium on the Asian Environmental Problems, pp.191-194, 2010.09.
74. Hirotaka HANO, Takanori UCHIDA, Yuji OHYA, Wake Structure Behind Wind Turbine Generator in Turbulent Boundary Layer, Proceedings of EAEP2010/The 4th International Symposium on the Asian Environmental Problems, pp.195-200, 2010.09.
75. Takanori Uchida and Yuji Ohya, Large-Eddy Simulation of Topography-Induced Turbulence around WTG by using the RIAM-COMPACT® CFD Model, Proceedings of RENEWABLE ENERGY 2010 (RE2010), 2010.06.
76. Takanori Uchida and Yuji Ohya, HIGH RESOLUTION LES OF TURBULENT AIRFLOW OVER COMPLEX TERRAIN, Proceedings of Seventh Asia-Pacific Conference on Wind Engineering (APCWE-VII), pp.405-408, 2009.11.
77. Tomohiro Hara, Yuji Ohya, Takanori Uchida, Ryohji Ohba, Wind-tunnel and numerical simulations of the coastal thermal internal boundary layer, Boundary-Layer Meteorology, 10.1007/s10546-008-9343-5, 130, 3, 365-381, 2009.02, Wind-tunnel experiments in a thermally stratified wind tunnel and direct numerical simulations were performed to simulate the thermal internal boundary layer (TIBL) that developed over a coastal area in a sea-breeze flow. The results of the simulations were analyzed to investigate turbulence structure in the TIBL. To study the effects of the atmospheric stability over the sea on the TIBL, two vertical profiles of temperature were created in the upstream portion of the wind-tunnel experiment and the direct numerical simulation. Turbulence statistics of the TIBL changed significantly according to the temperature profile over the sea, indicating that the stability of the flow over the sea has a significant effect on the structure and turbulence characteristics of the TIBL. Furthermore, the TIBL heights were estimated from the vertical profiles of the local Richardson number. The estimated TIBL heights agreed with those predicted by a pre-existing relation, suggesting that both the wind-tunnel experiment and the direct numerical simulation accurately reproduced the growth of the TIBL..
78. Takanori Uchida, Yuji Ohya, The wind risk management in the wind farm by using the RIAM-COMPACT CFD code, Proceedings of China Wind Power 2008 & Global Wind Power 2008, 2008.10.
79. Yuji Ohya, Takanori Uchida, Laboratory and numerical studies of the atmospheric stable boundary layers, Journal of Industrial Aerodynamics, 10.1016/j.jweia.2008.02.037, 96, 10-11, 2150-2160, 2008.10, We have investigated the turbulence phenomena of SBLs for a wide range of stability, particularly focusing on the effects of strong stratification on turbulent boundary layers, using a thermally stratified wind tunnel. In parallel with wind tunnel experiments, to understand the turbulence features and fluid dynamics in detail, we have also performed numerical simulations of SBLs under the boundary conditions similar to those in the wind tunnel experiments. The numerical studies based on a finite-difference method (FDM) are direct Navier-Stokes simulations without any turbulence model (DNS). Under the Boussinesq approximation, the governing equations consist of the Navier-Stokes, continuity and energy equations for 3D incompressible stratified flows. Stable stratification rapidly suppresses the fluctuations of streamwise velocity and temperature as well as the vertical velocity fluctuation. Momentum and heat fluxes are also significantly decreased with increasing stability and become nearly zero over the whole boundary depth of the boundary layer with very strong stability. From the flow visualization in both wind tunnel experiment and DNS, wave-like motions driven by buoyancy and waves due to the Kelvin-Helmholtz instability can be observed locally and intermittently in a SBL flow with strong stability..
80. Takanori Uchida, Yuji Ohya, Micro-siting technique for wind turbine generators by using large-eddy simulation, Journal of Industrial Aerodynamics, 10.1016/j.jweia.2008.02.047, 96, 10-11, 2121-2138, 2008.10, It is highly important in Japan to choose a good site for wind turbines, because the spatial distribution of wind speed is quite complicated over steep complex terrain. We are developing the unsteady numerical model called the Research Institute for Applied Mechanics, Kyushu University, COMputational Prediction of Airflow over Complex Terrain (RIAM-COMPACT). The RIAM-COMPACT is based on the large-eddy simulation (LES). The object domain of the RIAM-COMPACT is from several m to several km. First, to test the accuracy of the RIAM-COMPACT we have performed an experimental and a numerical simulation of a non-stratified airflow past a two-dimensional ridge and a three-dimensional hill in a uniform flow. Attention is focused on airflow characteristics in a wake region. For this purpose, the velocity components were measured with a split-film probe (SFP) in the wind tunnel experiment. Through comparison of the experimental and numerical results, they showed a good agreement. The accuracy of both of the wind tunnel experiment by the SFP and also numerical simulation by the RIAM-COMPACT were confirmed. Next, we have applied the RIAM-COMPACT to the airflow over real complex terrain. The numerical results obtained by the RIAM-COMPACT demonstrated that the changes induced on the wind field by the topographic effect, such as the local wind acceleration and the flow separation, were successfully simulated. Furthermore, the forecast accuracy of an actual wind speed is examined..
81. Yuji Ohya, Takanori Uchida, Laboratory and numerical studies of the atmospheric stable boundary layers, JOURNAL OF WIND ENGINEERING AND INDUSTRIAL AERODYNAMICS, 10.1016/j.jweia.2008.02.037, 96, 10-11, 2150-2160, 2008.10, We have investigated the turbulence phenomena of SBLs for a wide range of stability, particularly focusing on the effects of strong stratification on turbulent boundary layers, using a thermally stratified wind tunnel. In parallel with wind tunnel experiments, to understand the turbulence features and fluid dynamics in detail, we have also performed numerical simulations of SBLs under the boundary conditions similar to those in the wind tunnel experiments. The numerical studies based on a finite-difference method (FDM) are direct Navier-Stokes simulations without any turbulence model (DNS). Under the Boussinesq approximation, the governing equations consist of the Navier-Stokes, continuity and energy equations for 3D incompressible stratified flows. Stable stratification rapidly suppresses the fluctuations of streamwise velocity and temperature as well as the vertical velocity fluctuation. Momentum and heat fluxes are also significantly decreased with increasing stability and become nearly zero over the whole boundary depth of the boundary layer with very strong stability. From the flow visualization in both wind tunnel experiment and DNS, wave-like motions driven by buoyancy and waves due to the Kelvin-Helmholtz instability can be observed locally and intermittently in a SBL flow with strong stability. (C) 2008 Elsevier Ltd. All rights reserved..
82. Takanori Uchida, Yuji Ohya, Numerical Simulation of Airflow around Urban Area by using the RIAM-COMPACT CFD Model, Proceedings of Sino-Japan International Symposium on The East Asian Environmental Problems (EAEP2008), pp.51-53, 2008.08.
83. Yuji Ohya, Takanori Uchida, Laboratory and numerical studies of the atmospheric stable boundary layers, Journal of Wind Engineering & Industrial Aerodynamics, Vol.96, pp.2150-2160, 2008.07.
84. Takanori Uchida and Yuji Ohya, Micro-siting Technique for Wind Turbine Generators by Using Large-Eddy Simulation, Journal of Wind Engineering & Industrial Aerodynamics, Vol.96, pp.2121-2138, 2008.07, 風力業界で未解決課題であった風車に対する風の乱れ(ウィンドリスク)に対して,NEDO技術開発機構の産業技術研究助成事業(若手研究グラント)に採択され,研究代表者として3年間の研究開発を実施した.本研究を通じ,先端的数値風況予測モデル「RIAM-COMPACT®(リアムコンパクト)」を駆使し,世界で初めてウィンドリスクの存在を視覚的に特定することに成功し,その力学的機構を解明した.一連の成果に対し,2010年科学技術分野の文部科学大臣表彰・若手科学者賞を受賞した..
85. T.Uchida and Y.Ohya, Verification of the Prediction Accuracy of Annual Energy Output at Noma Wind Park by the Non-Stationary and Non-Linear Wind Synopsis Simulator, RIAM-COMPACT, Journal of Fluid Science and Technology, Vol.3, No.3, pp.344-358, 2008.06.
86. Yuji Ohya, Reina Nakamura, Takanori Uchida, Intermittent bursting of turbulence in a stable boundary layer with low-level jet, Boundary-Layer Meteorology, 10.1007/s10546-007-9245-y, 126, 3, 349-363, 2008.03, The atmospheric stable boundary layer (SBL) with a low-level jet is simulated experimentally using a thermally stratified wind tunnel. The turbulence structure and flow characteristics are investigated by simultaneous measurements of velocity and temperature fluctuations and by flow visualization. Attention is focused on the effect of strong wind shear due to a low-level jet on stratified boundary layers with strong stability. Occasional bursting of turbulence in the lower portion of the boundary layer can be found in the SBL with strong stability. This bursting originates aloft away from the surface and transports fluid with relatively low velocity and temperature upward and fluid with relatively high velocity and temperature downward. Furthermore, the relationship between the occurrence of turbulence bursting and the local gradient Richardson number (Ri) is investigated. The Ri becomes larger than the critical Ri, Ricr = 0.25, in quiescent periods. On the other hand, the Ri number becomes smaller than Ricr during bursting events..
87. Yuji Ohya, Reina Nakamura, Takanori Uchida, Intermittent Bursting of Turbulence in a Stable Boundary Layer with Low-Level Jet,Boundary-Layer Meteorology, Boundary-Layer Meteorology, vol.26, No.3, pp.349-363, 2008.01.
88. Takanori Uchida, High Resolution LES of Airflow over Complex Terrain, Proceedings of APCOM'07-EPMESC XI, 2007.12.
89. Takanori Uchida, Yuji Ohya, Application of LES technique to diagnosis of wind farm by using high resolution elevation data, JSME International Journal, Series B: Fluids and Thermal Engineering, 10.1299/jsmeb.49.567, 49, 3, 567-575, 2006.12, We are developing the numerical model called the RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain). The object domain of this numerical model is from several m to several km, and can predict the airflow and the gas diffusion over complex terrain with high precision. The RIAM-COMPACT has already been marketed by certain tie-up companies. The estimation of the annual electrical power output is also possible now based on the field observation data. In the present study, wind simulation of an actual wind farm was executed using the high resolution elevation data. As a result, an appropriate point and an inappropriate point for locating a wind turbine generator were shown based on the numerical results obtained. This cause was found to be a topographical irregularity in front of the wind turbine generator..
90. Takanori Uchida, Yuji Ohya, Diagnosis of Airflow Characteristics in Wind Farm by Using the Unsteady Numerical Model RIAM-COMPACT, Proceedings of Renewable Energy 2006, 2006.10.
91. T.Uchida and Y.Ohya, Application of LES Technique to Diagnosis of Wind Farm by Using High Resolution Elevation Data, JSME International Journal, 「Environmental Flows」, Series B, Vol.49, No.3, pp.567-575, 2006.09.
92. Takanori Uchida, Yuji Ohya, Application of LES technique to diagnosis of wind farm by using high resolution elevation data, JSME INTERNATIONAL JOURNAL SERIES B-FLUIDS AND THERMAL ENGINEERING, 49, 3, 567-575, 2006.08, We are developing the numerical model called the RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain). The object domain of this numerical model is from several m to several km, and can predict the airflow and the gas diffusion over complex terrain with high precision. The RIAM-COMPACT has already been marketed by certain tie-up companies. The estimation of the annual electrical power output is also possible now based on the field observation data. In the present study, wind simulation of an actual wind farm was executed using the high resolution elevation data. As a result, an appropriate point and an inappropriate point for locating a wind turbine generator were shown based on the numerical results obtained. This cause was found to be a topographical irregularity in front of the wind turbine generator..
93. Takashi Maruyama, Hirohiko Ishikawa, Takanori Uchida, Hiromasa Kawai, Yuji Ohya, Numerical simulation of strong wind fields around Miyajima during Typhoon Songda in 2004, Journal of Wind Engineering, 10.5359/jwe.31.95, 31, 3, 95-104, 2006.07, The strong wind fields around Miyajima in Hiroshima Bay were simulated numerically during Typhoon Songda in 2004. The PSU/MM5 model was used for the calculation of meso-scale regions. The strong wind that passed over the sea and blew into the Hiroshima Bay was simulated. The predicted wind fields were examined comparing with the observed records and the field investigation of damage to the houses and buildings. The calculated maximum wind speed map was well correlated to the distribution of damage rate by strong wind in Hiroshima Prefecture. Unsteady wind fields were also computed by Large Eddy Simulation in the fine region around Miyajima. The local wind characteristics caused by the topography around Itsukushima Shrine were investigated by using the calculated results. The down flow along the valley to the south of the shrine was simulated in the neutral atmospheric condition. The local wind flows around the shrine were discussed..
94. Takanori Uchida, Kenichirou Sugitani, Yuji Ohya, Evaluation on wind characteristics around a steep simple terrain in a uniform flow-case of a three-dimensional isolated-hill model-, Journal of Wind Engineering, 10.5359/jwe.31.63, 31, 2, 63-74, 2006.04, The purpose of this research is to construct a database of a non-stratified airflow past a steep simple terrain under an imposition of a uniform flow, and, in addition, is to do the accuracy inspection of the numerical model under development at present. This numerical model is referred to as the RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain), and is for the purpose of the prediction of airflow over complex terrain with several m to several km. This paper describes the experimental and numerical study of a non-stratified airflow past a three-dimensional isolated-hill in a uniform flow as the second phase. The Reynolds number, based on the uniform flow and the height of the hill, is about 104. Airflows around the hill include the unsteady vortex shedding. Attention is focused on airflow characteristics in a wake region. For this purpose, the velocity components in the streamwise direction were measured with a SFP (Split-Film Probe) in the wind tunnel experiment. In addition, the flow visualization was performed by using the smoke-wire technique. Through comparison of the experimental and numerical results, they showed a good agreement. The accuracy of both of the wind tunnel experiment by the SFP and also numerical simulation by the RIAM-COMPACT were confirmed as the result..
95. Toshiyuki SANADA, Masatoshi FUJINO, Daisuke MATSUSHITA, Takanori UCHIDA, Hikaru MATSUMIYA, Masao WATANABE, Yoshinori HARA, Minori SHIROTA, Numerical Site Calibration on a Complex Terrain and its Application for Wind Turbine Performance Measurements, 2006 European Wind Energy Conference Proceedings, 2006.02.
96. T. Uchida and Y. Ohya, Micro-siting Technique for Wind Turbine Generator by Using One PC -Introduction of the RIAM-COMPACT based on LES-, EXPO WCWRF 2005(CD-ROM), 2005.09.
97. Yuji Ohya, Takanori Uchida, Laboratory and numerical studies of the convective boundary layer capped by a strong inversion, Boundary-Layer Meteorology, 10.1023/B:BOUN.0000027913.22130.73, 112, 2, 223-240, 2004.08, The convective boundary layer (CBL) with a wide range of stability is simulated experimentally using a thermally stratified wind tunnel, and numerically by direct numerical simulation (DNS). The turbulence structures and flow characteristics of various CBL flows, capped by a strong temperature inversion and affected by surface shear, are investigated. The various vertical profiles of turbulence statistics similar to those from the observed CBL in the field are successfully simulated in both the wind-tunnel experiment and in DNS. The comparison of the wind-tunnel data and DNS results with those of atmospheric observations and water-tank studies shows the crucial dependence of the turbulence statistics in the upper part of the layer on the strength of the inversion layer, as well as the modification of the CBL turbulence regime by the surface shear..
98. Yuji Ohya, Takanori Uchida, Laboratory and numerical studies of the convective boundary layer capped by a strong inversion, Boundary-Layer Meteorology, 10.1023/B:BOUN.0000027913.22130.73, 112, 2, 223-240, 2004.08, The convective boundary layer (CBL) with a wide range of stability is simulated experimentally using a thermally stratified wind tunnel, and numerically by direct numerical simulation (DNS). The turbulence structures and flow characteristics of various CBL flows, capped by a strong temperature inversion and affected by surface shear, are investigated. The various vertical profiles of turbulence statistics similar to those from the observed CBL in the field are successfully simulated in both the wind-tunnel experiment and in DNS. The comparison of the wind-tunnel data and DNS results with those of atmospheric observations and water-tank studies shows the crucial dependence of the turbulence statistics in the upper part of the layer on the strength of the inversion layer, as well as the modification of the CBL turbulence regime by the surface shear. © 2004 Kluwer Academic Publishers..
99. Takanori Uchida, Kenichirou Sugitani, Yuji Ohya, Evaluation on wind characteristics around a steep simple terrain in a uniform flow-case of a two-dimensional ridge model, Journal of Wind Engineering, 10.5359/jwe.29.35, 29, 3, 35-43, 2004.07, The purpose of this research is to construct a database of a non-stratified airflow past a steep simple terrain under an imposition of a uniform flow, and, in addition, is to do the accuracy inspection of the numerical model under development at present. This numerical model is referred to as the RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain), and is for the purpose of the prediction of airflow over complex terrain with several m to several km. This paper describes the experimental and numerical study of a non-stratified airflow past a two-dimensional ridge in a uniform flow as the first phase. The Reynolds number, based on the uniform flow and the height of the ridge, is about 10 4. Airflows around the ridge include the unsteady vortex shedding. Attention is focused on airflow characteristics in a wake region. For this purpose, the velocity components in the streamwise direction were measured with a SFP (Split-Film Probe) in the wind tunnel experiment. In addition, the flow visualization was performed by using the smoke-wire technique. Through comparison of the experimental and numerical results, they showed a good agreement. The accuracy of both of the wind tunnel experiment by the SFP and also numerical simulation by the RIAM-COMPACT were confirmed as the result..
100. Yuji Ohya, Takanori Uchida, Turbulence structure of stable boundary layers with a near-linear temperature profile, Boundary-Layer Meteorology, 10.1023/A:1023069316164, 108, 1, 19-38, 2003.07, By using a thermally stratified wind tunnel, we have successfully simulated stably stratified boundary layers (SBL), in which the mean temperature increases upward almost linearly. We have investigated the flow structure and the effects of near-linear stable stratification on the transfer of momentum and heat. The vertical profiles of turbulence quantities exhibit different behaviour in two distinct stability regimes of the SBL flows with weak and strong stability. For weak stability cases, the turbulent transfer of momentum and heat is basically similar to that for neutral turbulent boundary layers, although it is weakened with increasing stability. For strong stability cases, on the other hand, the time-mean transfer is almost zero over the whole boundary-layer depth. However, the instantaneous turbulent transfer frequently occurs in both gradient and counter-gradient directions in the lower part of the boundary layer. This is due to the Kelvin-Helmholtz (K-H) shear instability and the rolling up and breaking of K-H waves. Moreover, the internal gravity waves are observed in the middle and upper parts of all stable boundary layers..
101. Takanori Uchida, Kenichiro Sugitani, Yuji Ohya, Experimental study on wind characteristics around a two-dimension escapement in a uniform flow, Journal of Wind Engineering, 28, 2, 233-244, 2003.04, This paper describes the experimental study of a non-stratified airflow past a two-dimensional escarpment in a uniform flow. The Reynolds number, based on the uniform flow and the height of the escarpment, is about 104. The slope gradient of the escarpment is 25, 35 and 45 degree. Airflows around the escarpment include the unsteady separated and reattaching flow (hereafter called a "separation bubble"), where the separation occurs from a sharp comer. Attention is focused on an influence of a surface roughness on airflow characteristics in a wake region. For this purpose, the velocity components were measured with the X-wire probe and the split-film probe. In addition, the flow visualization was performed by using the smoke-wire technique. Through comparison between the experimental results with the surface roughness and those without it, the significant difference in the airflow characteristics is confirmed in the separation bubble. This is mainly due to the size in the separation bubble. The size in the separation bubble with the surface roughness is much larger than the one without it. In the case under an imposition of the surface roughness, the velocity is strongly defected near the slope surface. As a result, the production of the vorticity in the separated shear layer is also inhibited, leading to the elongation in the separation bubble..
102. T. Uchida, Y. Ohya, Stable stratification effect on the separated and reattaching flow behind two-dimensional topography, 九州大学応用力学研究所所報, 第124号,pp.17-24, 2003.03.
103. Takanori Uchida, Yuji Ohya, Large-eddy simulation of turbulent airflow over complex terrain, Journal of Industrial Aerodynamics, 10.1016/S0167-6105(02)00347-1, 91, 1-2, 219-229, 2003.01, In order to develop an overall efficient and accurate method of predicting an unsteady three-dimensional airflow over a complex terrain with characteristic length scales on the order of kilometers, we recently developed the CFD codes referred to as the RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain). In this paper, we carried out the calculation of turbulent airflow over a real complex terrain in a horizontal region of 9.5km × 5km with a relatively fine spatial resolution of 50 m. This area covers the new campus of Kyushu University. In order to generate instantaneous velocity fluctuations in an approaching flow, an unsteady flow field in the driver unit is directly calculated. The numerical results obtained by RIAM-COMPACT demonstrated that the changes induced on the wind field by the topographic effect, such as the local wind acceleration and the flow separation, were successfully simulated. We emphasized the following features as regards the wind field over the new campus area under the influence of the west wind. The wind field in the central region of the new campus area is strongly influenced by the wake region generated behind Mt. Hiyama (244m). The airflows moving around the side of Mt. Hiyama exhibit relatively small fluctuations. When these winds reach Mt. Ishigateke (99m), they are locally accelerated at the peak. Consequently, a local speed-up effect is confirmed in the southern part of the new campus area..
104. Takanori Uchida, Yuji Ohya, Large-eddy simulation of turbulent airflow over complex terrain, Journal of Wind Engineering and Industrial Aerodynamics, 10.1016/S0167-6105(02)00347-1, 91, 1-2, 219-229, 2003.01, In order to develop an overall efficient and accurate method of predicting an unsteady three-dimensional airflow over a complex terrain with characteristic length scales on the order of kilometers, we recently developed the CFD codes referred to as the RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain). In this paper, we carried out the calculation of turbulent airflow over a real complex terrain in a horizontal region of 9.5km × 5km with a relatively fine spatial resolution of 50 m. This area covers the new campus of Kyushu University. In order to generate instantaneous velocity fluctuations in an approaching flow, an unsteady flow field in the driver unit is directly calculated. The numerical results obtained by RIAM-COMPACT demonstrated that the changes induced on the wind field by the topographic effect, such as the local wind acceleration and the flow separation, were successfully simulated. We emphasized the following features as regards the wind field over the new campus area under the influence of the west wind. The wind field in the central region of the new campus area is strongly influenced by the wake region generated behind Mt. Hiyama (244m). The airflows moving around the side of Mt. Hiyama exhibit relatively small fluctuations. When these winds reach Mt. Ishigateke (99m), they are locally accelerated at the peak. Consequently, a local speed-up effect is confirmed in the southern part of the new campus area. © 2002 Elsevier Science Ltd. All rights reserved..
105. R. Ohba, T. Hara, S. Nakamura, Y. Ohya, Takanori Uchida, Gas diffusion over an isolated hill under neutral, stable and unstable conditions, Atmospheric Environment, 10.1016/S1352-2310(02)00642-8, 36, 36-37, 5697-5707, 2002.12, Wind tunnel experiments of gas diffusion were performed over flat terrain and over an isolated three-dimensional hill under neutral, stable and unstable (sea breeze) conditions. Conditions of airflow in the wind tunnel were determined so as to satisfy the similarity rule for the bulk Richardson number, by controlling temperature profiles and wind velocity of the thermally stratified wind tunnel. Typical characteristics were observed under each condition of atmospheric stability; reversed vortex behind the hill in neutral condition, downward slope wind in stable one and convective motion in unstable one. We compared these experiments with the results of a Direct Numerical Simulation (DNS) model for the wind velocity over the hill under neutral conditions. The numerical results showed good agreement with the experimental results..
106. Takanori Uchida, Yuji Ohya, Numerical simulation on wind fields over complex terrain using nested grids, Journal of Wind Engineering, 27, 3, 135-144, 2002.07, In order to develop an overall efficient and accurate model of simulating an unsteady three-dimensional airflow over complex terrain with characteristic length scales on the order of kilometers, we have been examining the large-eddy simulation (LES) technique using a finite-difference method (FDM). These LES codes are referred to as the RIAM-COMPACT (Research institute for Applied Mechanics, Kyushu University, Computational JPrediction of Airflow over Complex Terrain), and are based on two grid systems and corresponding variable arrangement: one is an orthogonal staggered grid; the other is a generalized curvilinear collocated grid. In this paper, using the RIAM-COMPACT based on a generalized curvilinear collocated grid, we have performed the calculations of a non-stratified airflow over real complex terrain. This area covers the new campus of Kyushu University. To investigate more clearly the influence of topography on the airflow over the new campus area, we employed nested grids: the outer grid domain is 9.75km × 4.65km × 1.46km with horizontal grid spacing of 50m and the inner grid domain is 5km × 2.7km × 1.46km with horizontal grid spacing of 25m. The numerical results were compared with the wind tunnel experiment, and airflow characteristics due to the topographic effect, such as the wind speed-up and the separated flow, were successively simulated..
107. Takanori Uchida, Yuji Ohya, Numerical study of stably stratified flows over a two-dimensional hill in a channel of finite depth, Fluid Dynamics Research, 10.1016/S0169-5983(01)00025-9 , 29, 4, 227-250, 2001.10, Stably stratified flows over a two-dimensional hill in a channel of finite depth are investigated numerically at a Reynolds number of 2000, which is based on the uniform upstream velocity U and the hill height h. As a first step, we assume a free-slip condition on the ground, both upstream and downstream of the hill, and impose a no-slip condition only on the hill surface. Such a configuration corresponds to that of the previous towing tank experiments and numerical studies. For strong stratification (1 d on the hill, and emphasize the following features, where N is the buoyancy frequency and H is the channel depth. For 1.1 ≤ K ≤ 1.7, columnar disturbances with mode n = 1 are dominant so that the flow around the hill shows a persistent periodic unsteadiness. This flow unsteadiness is mainly due to the periodic shedding of upstream advancing columnar disturbances with mode n = 1 with a clockwise circulation. For 1.8 ≤ K ≤ 2, as columnar disturbances with mode n = 2 become dominant, the flow around the hill rapidly reaches an almost steady state. In addition, through the calculations with the blockage ratio H/h = 6, 10 and corresponding Re=20, 100 and 2000, it is found that the normalized periods of Cd oscillations have a strong dependence on both the H/h and Re. As a next step, to investigate the flow around the hill under real atmospheric situations, we have performed calculations under imposition of a no-slip condition on the ground, particularly focusing on the effect of stable stratifications on the unsteady separated-reattached flow behind the hill. The flow around the hill exhibits different behavior over the whole range 0 ≤ K ≤ 2, corresponding to the difference in the boundary condition on the ground. For 0 ≤ K ≤ 0.9, the vortex shedding from the separation bubble behind the hill occurs. For K = 1 and 1.1, the vortex shedding is strongly suppressed so that the flow around the hill rapidly reaches an almost steady state. Under strong stratification (1
108. Takanori Uchida, Yuji Ohya, Numerical simulation of atmospheric flow over complex terrain, Journal of Industrial Aerodynamics, 10.1016/S0167-6105(99)00024-0, 81, 283-293, 1999.05, In order to develop an overall efficient and accurate method of simulating an unsteady three-dimensional atmospheric flow over topography, we examined two grid systems and corresponding variable arrangements: one is a body-fitted coordinate (BFC) grid system based on a collocated variable arrangement; the other is an orthogonal grid system based on a staggered variable arrangement. Using these codes, we calculated the wind system over topography such as an isolated hill and real complex terrain. Both codes remarkably removed the numerical difficulties such as the convergence of the SOR method in solving the pressure Poisson equation, resulting in numerical results with much higher accuracy. Despite the differences in the grid system and in variable arrangement, no significant differences in the flow pattern between the both numerical results were found. Compared with the previous studies, the numerical results obtained are very satisfactory in the sense that overall characteristic flows are successfully simulated irrespective of the simulation codes..
109. Takanori Uchida, Y. Ohya, A numerical study of stably stratified flows over a two-dimensional hill - Part I. Free-slip condition on the ground, Journal of Industrial Aerodynamics, 10.1016/S0167-6105(97)00096-2, 67-68, 493-506, 1997.01, Stably stratified flows over a two-dimensional hill in a channel of finite depth are analyzed numerically by using a newly-developed multi-directional finite-difference method at a Reynolds number Re = 2000. To simplify the phenomena occurring in the flow around the hill, the free-slip condition for the velocity is assumed on the ground, and the nonslip condition is imposed only on the hill surface. Attention is focused on the unsteadiness in the flow around the hill for the cases of K( = NH/πU) > 1 where N and U are the buoyancy frequency and free-stream velocity and H is the domain depth. The flow unsteadiness is discussed, being associated with shedding of the upstream advancing columnar disturbance..