記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Results in Engineering  

Here, it is presented a possible methodology and experimental model for benchmarking of air quality in cities. The concept behind the methodology is that a city's inherent structure affects the potential for contaminant removal due to the resistance it poses to inflow. The approach is based on homogenous emission across the street surface network, representing a worst-case situation. Different levels of complexity can be used for benchmarking, making it valuable for evaluating different layouts. Additionally, an urban ventilation index suitable for these kinds of experimental studies has been suggested.

DOI： 10.1016/j.rineng.2023.101617

Scopus

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Building and Environment  

The Reynolds number independence (Re independence) criterion of Re > 11,000 is widely adopted to fulfill the dynamic similarity between the urban flow modeling and the down-scale measurements. However, for 2D street canyons with H/W ≥ 1.5, experiments, numerical simulations, and in-situ observations have reported different vortex-flow regimes with similar building configurations but at different scales. This study uses both wind tunnel experiments and numerical simulations to revisit the Re-independent flow regimes and Re independence criteria with an extensive Re range for idealized 2D street canyons with various aspect ratios (H/W = 1.1, 2.4, 3, 4, and 5). We introduced an optimized ratio of relative changes (RRCs) to evaluate the flow regimes’ similarity. The wind tunnel experiment confirms that the cavity flow with H/W = 1.1 meets the Re independence when reference building Re (Re<inf>ref</inf>) exceeds 11,000. Simulations validated by the experiment results are conducted to investigate detailed flow regimes and the critical Re (Re<inf>c</inf>) range for each aspect ratio. The canyons with H/W = 2.4, 3, and 4 are dominated by a single asymmetric vortex when the Re independence is satisfied, while there are two vertically-stacked counter-rotating vortices in the canyon with H/W = 5. The value range of Re<inf>c</inf> increases with aspect ratio from 1.9 × 10⁴–2.6 × 10⁴ (H/W = 2.4) to 1.3 × 10⁵–2.1 × 10⁵ (H/W = 3), and 2.1 × 10⁶–6.4 × 10⁶ (H/W = 4 and 5). Our results indicate that the fully Re-independent flow regimes in deep canyons have fewer vortices than the literature value with down-scale experiments and simulations. The variant Re<inf>c</inf> with different aspect ratios suggests the requirement to conduct the Re-independence test for different model configurations.

DOI： 10.1016/j.buildenv.2021.107965

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Building and Environment  

Optimizing urban ventilation is an effective way to improve urban air quality and thermal environment. For this purpose, under the validation of wind-tunnel experiments, flow regimes and micro thermal environment in typical reduced-canyon models with aspect ratios (AR) of 1.1, 2.4, 4 and 5.67 were investigated by CFD simulations using periodic boundary condition. ANSYS Fluent 15.0 with a solar ray tracing model and radiation model was performed to numerically study turbulence characteristics with wind-driven force and solar-heating conditions. Results revealed that, with wind-driven condition, a clockwise vortex existed in normal and deep street canyon (AR = 1.1 and 2.4) while two counter-rotating vortices appeared in extremely deep canyon (AR = 4 and 5.67). Moreover, different turbulence structures and air temperature distribution existed in canyons with different solar-heating conditions. When the leeward wall or ground was heated, the pedestrian-level velocity increased and street ventilation was strengthened compared to wind-driven condition for all AR values. Particularly, the single main vortex was strengthened (AR = 1.1 and 2.4), and the two-vortex structure in extremely deep canyons (AR = 4 and 5.67) changed to single-vortex structure. When the windward wall was heated, the clockwise main vortex at AR = 1.1 and 2.4 was deformed, and a new sub vortex gradually appeared near street bottom. Furthermore, at AR = 4 and 5.67, windward solar heating destroyed the two-vortex structure and slightly improved pollutant dilution capacity. This work implied that extremely deep street design with weak pedestrian-level ventilation should be avoided. It also provides a meaningful reference for urban planning.

DOI： 10.1016/j.buildenv.2020.107525

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Building and Environment  

Validated by wind tunnel data, this study numerically investigates the integrated impacts of wind and thermal buoyancy on urban turbulence, ventilation and pollutant dispersion in full-scale 2D deep street canyons (aspect ratio AR = H/W = 3 and 5, W = 24 m). Isothermal urban airflows for such deep street canyons can be Reynolds-number-independent when reference Reynolds number (Re) exceeds the critical Re (Re<inf>c</inf>~10⁶,10⁷ when AR = 3, 5), i.e. AR = 5 experiences two main vortices and one-order smaller NEV* (~10⁻³, the normalized net escape velocity) than AR = 3 with a single main vortex (NEV*~10⁻²). With sufficiently large Re (Re > Re<inf>c</inf>) and the same air-wall temperature difference (Ri = 2.62, 4.36 when AR = 3, 5), four uniform wall heating patterns were considered, including leeward-wall heating (L-H), windward-wall heating (W–H), ground heating (G-H), and all-wall heating (A-H). Various indicators were adopted to evaluate street ventilation and pollutant dilution capacity (e.g. age of air (τ,s), NEV*, pollutant transport rates (PTR)). Full-scale wall heating produces a strong upward near-wall buoyancy force, which significantly influences flow patterns and improves street ventilation for most cases. When AR = 3, L-H strengthens the single-vortex airflow. When AR = 5, L-H converts the isothermal double vortices into a single-clockwise vortex. For both ARs, W–H reverses the main clockwise vortex to an enhanced counterclockwise one, moreover G-H and A-H cause a more complicated multi-vortex pattern than isothermal cases. Overall, when AR = 3, L-H and W–H increase NEV* by 68% and 40% than the isothermal case. When AR = 5, four wall heating patterns all raise NEV* considerably (by 150%–556%). For both ARs, the L-H, W–H and A-H amplify the contribution of mean flows on removing pollutants but reduce that by turbulent diffusion compared with isothermal cases.

DOI： 10.1016/j.buildenv.2020.107510

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Building and Environment  

Wind-driven dynamic force and buoyancy force are two main driving forces in urban ventilation. As a novelty, this paper performs scaled outdoor measurements to examine the relative role of these two driving forces by non-dimensional buoyancy parameter B within two-dimensional (2D) street canyons in SOMUCH (Scaled Outdoor Measurement of Urban Climate and Health). Four aspect ratios (building height/street width, H/W = 1, 2, 3, 6; H = 1.2 m) were considered to mimic different morphologies. The temperature difference between east and west walls (ΔT<inf>east−west</inf>) was emphasized to reveal the flow characteristics affected by such thermal effects. The results show that the wall temperature varies with street-wall orientation. ΔT<inf>east−west</inf> becomes negative in the morning but positive in the afternoon, and maximum |ΔT<inf>east−west</inf>| could reach 12.7 °C. Street aspect ratio largely determines the thermal structure and flow characteristics inside street canyons. As aspect ratio increases, ΔT<inf>east−west</inf> and normalized horizontal velocity U<inf>0.25H</inf>/U<inf>2H</inf> (i.e. ratio of horizontal velocity at height of z = 0.25H to that at z = 2H) decrease. Besides, two flow regimes are observed: For H/W = 1, 2, 3, U<inf>0.25H</inf>/U<inf>2H</inf> is approximately constant (i.e. 0.26, 0.17, 0.12) when B < B<inf>c</inf> (critical value, B<inf>c</inf> ≈ 0.006, 0.002, 0.0008) and dynamic force dominates urban airflow; However, when B > B<inf>c</inf> (i.e. buoyancy force cannot be negligible), U<inf>0.25H</inf>/U<inf>2H</inf> rises non-linearly with B, and such increase is faster as H/W = 2, 3 than H/W = 1. It indicates that narrower streets attain more evident impact of buoyancy effect on street ventilation. Overall, this study directly investigates the influence of solar heating on turbulent flow and provides high-quality experimental data for validation of further numerical simulations.

DOI： 10.1016/j.buildenv.2020.107159

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Building and Environment  

Validated by experimental data, this paper performs computational fluid dynamics (CFD) simulations to investigate the influence of tree plantings on urban airflow and vehicular CO exposure in two-dimensional (2D) street canyons with various aspect ratios (building height/street width, AR = H/W = 0.5, 1, 3, 5) and ground-level source. The impacts of tree canopy bottom height (Htb = 2 m, 6 m), tree stand density (y-density = 0.33, 0.67, 1) and leaf area density (LAD = 0.5, 1, 2 m²/m³) are considered. Personal intake fraction (P_IF) and its spatial mean value in leeward and windward sides (<P_IF><inf>lee</inf>, <P_IF><inf>wind</inf>) and for entire streets (street intake fraction, <P_IF>) are adopted to assess overall pollutant exposure. For cases without trees, only one main vortex exists in shallow streets with AR = 0.5-3 and <P_IF> as AR = 3 (5.80 ppm) slightly exceeds AR = 0.5-1 (3.98-3.84 ppm). However, two counter-rotating vortexes appear in deep streets (AR = 5), inducing 1-2 orders smaller pedestrian-level velocity (U/U<inf>ref</inf>~10⁻⁴-10⁻³) and one-order greater <P_IF> (46.80 ppm) than shallow streets. Trees always weaken wind in streets and raise <P_IF> more in shallower streets by 46.0% as AR = 0.5 (3.98 ppm-5.81 ppm), 26.0-45.9% as AR = 1 (3.84 ppm to 4.84-5.60 ppm), 16.2-50.3% as AR = 3 (5.80 ppm to 6.74-8.72 ppm), but only 8.5-23.4% as AR = 5 (46.80 ppm to 50.78-57.73 ppm). Particularly, as AR = 1, trees raise <P_IF><inf>lee</inf> (5.87 ppm) by 27.1-57.2%, while <P_IF><inf>wind</inf> (1.80 ppm) only by 0%-23.3%. Higher Htb, smaller y-density or LAD produce less increase of <P_IF>. As AR = 3, vegetation increases <P_IF><inf>lee</inf> (8.84 ppm) by 21.2%-66.4% but little affects <P_IF><inf>wind</inf> (2.76 ppm). Lower Htb produces smaller <P_IF> differing from AR = 1. As AR = 5, vegetation increases <P_IF><inf>wind</inf> (63.97 ppm) by 15.1-36.6% but reduces <P_IF><inf>lee</inf> (29.63 ppm) by 5.2-8.5%. Although further investigations are still required for design purpose, this paper provides effective methodologies to quantify how vegetation influences street-scale pollutant exposure.

DOI： 10.1016/j.buildenv.2019.106529

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Building and Environment  

The integrated assessments of indoor and outdoor ventilation are still rare so far. As a novelty, this paper aims to quantify the influence of street aspect ratios (building height/street width, H/W = 0.5–5) and window sizes (1 m × 1 m, 1.5 m × 1.5 m) on indoor-outdoor ventilation in two-dimensional streets with single-sided naturally-ventilated buildings. Numerical simulations with RNG k-ε model are validated against experimental data and the grid independence are tested as well. Air change rates per hour (ACH, h⁻¹) are adopted for assessing indoor-outdoor ventilation by mean flows (ACH<inf>mean</inf>) and turbulent fluctuations (ACH<inf>turb</inf>) respectively. Age of air(τ), purging flow rate (PFR) and its corresponding ACH<inf>PFR</inf> are used to evaluate overall ventilation capacities. Shallower streets experience better indoor-outdoor ventilation. Outdoor ACH<inf>PFR</inf> drop from 14.69 to 17.55 h⁻¹ to 3.96–3.97 h⁻¹ as H/W rises from 0.5 to 3. In extremely deep canyon (H/W = 5), two-counter-rotating vortices produce much smaller velocity at low-level regions (U/U<inf>ref</inf>~10⁻³-10⁻⁵), resulting in small ACH<inf>PFR</inf> for outdoor (~0.76–0.91 h⁻¹) and indoor in 1–13th floors (~0.03–0.61 h⁻¹). When H/W = 0.5–1, leeward 5–6th floors experience smaller ACH<inf>PFR</inf>(e.g.~1.13–1.40 h⁻¹ as H/W = 1) than the other floors (e.g. ~1.54–9.52 h⁻¹ as H/W = 1). Particularly, as H/W = 2–3, leeward-side indoor ACH<inf>PFR</inf> in the middle floors (except the first and top two floors) are nearly constants (~1.02–1.69 h⁻¹) and much smaller than windward-side ACH<inf>PFR</inf>(~1.41–4.35 h⁻¹) which increase toward upper floors. Besides, the smaller window size reduces indoor ACH<inf>PFR</inf> by 19.38%~88.28%, but hardly influences outdoor ventilation. Moreover, both outdoor and indoor ACH<inf>PFR</inf> are greater than ACH<inf>mean</inf> but smaller than ACH<inf>mean</inf> + ACH<inf>turb</inf>. Although further investigations are still required, this paper provides an insight and scientific foundation on integrated indoor-outdoor ventilation evaluation with various effective ventilation indexes.

DOI： 10.1016/j.buildenv.2019.106528

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Building and Environment  

Deep street canyons and unfavourable meteorological conditions usually induce high pollutant exposure. Validated by experimental data, this paper employs computational fluid dynamic simulations with RNG k-ε model to investigate the flow, and passive pollutant dispersion within scale-model two-dimensional street canyons(H = 3 m). As a novelty, this paper quantifies the impacts of various wall heating scenarios(bottom, leeward/windward wall and all-wall heating), ambient velocity(U<inf>ref</inf> = 0.5–2 m s⁻¹, Froude numbers Fr = 0.25–4.08, Reynolds numbers Re = 95602–382409) and aspect ratios(building height/street width, AR = 0.5, 0.67, 1, 2, 3) on personal intake fraction for entire streets(<P_IF>). The governing equations are implicitly discretized by a finite volume method (FVM) and the second-order upwind scheme with Boussinesq model for quantifying buoyancy effects. The SIMPLE scheme is adopted for the pressure and velocity coupling. In most isothermal cases, one-main-vortex structure exists as AR = 0.5–3(<P_IF> = 0.43–3.96 ppm and 1.66–27.51 ppm with U<inf>ref</inf> = 2 and 0.5 m s⁻¹). For non-isothermal cases with Fr = 4.08(U<inf>ref</inf> = 2 m s⁻¹), wind-driven force dominates urban airflow as AR = 0.5–1 and four heating conditions attain similar <P_IF>(0.39–0.43 ppm, 0.57–0.60 ppm, 0.91–0.98 ppm). As AR = 2, windward and all-wall heating get two-vortex structures with greater <P_IF>(3.18–3.33 ppm) than others(<P_IF> = 2.13–2.21 ppm). As AR = 3, leeward-wall heating slightly reduces <P_IF>(~3.72–3.96 ppm), but the other three produce two-vortex structures with greater <P_IF>(6.13–10.32 ppm). As Fr = 0.25(U<inf>ref</inf> = 0.5 m s⁻¹), leeward-wall heating always attains smaller <P_IF>(1.20–7.10 ppm) than isothermal cases(1.66–27.51 ppm) as AR = 0.5–3, however the influence of the other three is complicated which sometimes raises or reduces <P_IF>. Overall, smaller background wind speed (Fr = 0.25) with two-vortex structures attains much larger <P_IF>. Special attention is required at night(all-wall heating), noon(bottom-heating) and cloudy period(no-wall heating) as AR = 2–3, while it is during windward-wall heating and cloudy period for AR = 0.5–1.

DOI： 10.1016/j.buildenv.2019.106536

Scopus

担当区分：最終著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）   出版者・発行元：16th Conference of the International Society of Indoor Air Quality and Climate Creative and Smart Solutions for Better Built Environments Indoor Air 2020  

Scopus

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Building and Environment  

Heavy traffic flows commonly result in large vehicular pollutant exposure in near-road buildings. Street layouts and pollutant source settings are key factors. Advertisement boards are sometimes adopted for business purpose, but their impacts on pollutant dispersion and exposure are still unclear. Thus, this paper numerically investigates the influence of aspect ratios (building height/street width, H/Ws = 1 or 2; H = 30 m), source locations and advertisement-board settings on the flow, carbon monoxide (CO) dispersion and exposure within three-dimensional urban-like models under the parallel approaching wind to the main streets. Personal intake fraction (P_iF) represents the fraction of total vehicular emissions inhaled averagely by each person of a population. Spatial mean P_iF is named as <P_iF> and that for the entire building as building intake fraction (<P_iF> <inf>B</inf> ). With span-wise CO source fixed in target secondary streets of No 2 or 13 (S2 or S13), <P_iF> is particularly large in target streets. <P_iF> <inf>B</inf> decreases exponentially toward downstream from the target street and S13 cases attain greater <P_iF> <inf>B</inf> and larger exponential decreasing rates. Cases with H/Ws = 2 experiences more limited upward dispersion and subsequently smaller <P_iF> (0.155–0.339 ppm) of entire target street than cases with H/Ws = 1 (0.375–0.731 ppm). For cases with stream-wise CO source along the main street (Smain), <P_iF> <inf>B</inf> first rises quickly toward downstream, then adjusts to equilibrium values (0.051–0.063 ppm). Finally, with span-wise source, vertical and double-layer advertisement boards produce stronger upward CO transportation and greater <P_iF> <inf>B</inf> than lateral and single-layer types, while with Smain source, the double-layer and lateral types produce larger <P_iF> <inf>B</inf> and shorter exposure adjustment distance.

DOI： 10.1016/j.buildenv.2019.01.012

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Urban Climate  

Semi-open settings (or hanged walls) above the pedestrian level in street canyons can provide protection to pedestrians from heavy-rain and strong sunshine, but may also influence street ventilation and thus pollutant dispersion. By performing computational fluid dynamics (CFD) simulations and employing ventilation concepts of net escape velocity (i.e. diluting rate for exhale effect) and the age of air (i.e. freshness of air for inhale effect), this paper investigates the effects of low-level semi-open settings on ventilation in idealized block arrays with rectangular or square form (building height/street width H/W = 1.25) under neutral atmospheric conditions. Four upstream wind directions of 0° (parallel), 15° 30° 45° between the approaching wind and the main street axis and four widths of semi-open settings (Ws = 0 m, 2 m, 4 m, 6 m) are investigated. Results indicate that the increasing width of semi-open settings produce greater age of air (i.e. older air) and smaller net escape velocity (i.e. less diluting rate) than those without semi-open settings. The wider semi-open settings experience the worst ventilation. In addition, the approaching wind directions of 0° and 15° lead to worse ventilation than 30° and 45° for both rectangular and square forms.

DOI： 10.1016/j.uclim.2018.07.003

Scopus

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Science of the Total Environment  

Sustainable urban design is an effective way to improve urban ventilation and reduce vehicular pollutant exposure to urban residents. This paper investigated the impacts of urban open space and ‘lift-up’ building design on vehicular CO (carbon monoxide) exposure in typical three-dimensional (3D) urban canopy layer (UCL) models under neutral atmospheric conditions. The building intake fraction (IF) represents the fraction of total vehicular pollutant emissions inhaled by residents when they stay at home. The building daily CO exposure (E<inf>t</inf>) means the extent of human beings' contact with CO within one day indoor at home. Computational fluid dynamics (CFD) simulations integrating with these two concepts were performed to solve turbulent flow and assess vehicular CO exposure to urban residents. CFD technique with the standard k-ε model was successfully validated by wind tunnel data. The initial numerical UCL model consists of 5-row and 5-column (5 × 5) cubic buildings (building height H = street width W = 30 m) with four approaching wind directions (θ = 0°, 15°, 30°, 45°). In Group I, one of the 25 building models is removed to attain urban open space settings. In Group II, the first floor (Lift-up1), or second floor (Lift-up2), or third floor (Lift-up3) of all buildings is elevated respectively to create wind pathways through buildings. Compared to the initial case, urban open space can slightly or significantly reduce pollutant exposure for urban residents. As θ = 30° and 45°, open space settings are more effective to reduce pollutant exposure than θ = 0° and 15°.The pollutant dilution near or surrounding open space and in its adjacent downstream regions is usually enhanced. Lift-up1 and Lift-up2 experience much greater pollutant exposure reduction in all wind directions than Lift-up3 and open space. Although further investigations are still required to provide practical guidelines, this study is one of the first attempts for reducing urban pollutant exposure by improving urban design.

DOI： 10.1016/j.scitotenv.2018.03.194

Scopus

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）   出版者・発行元：15th Conference of the International Society of Indoor Air Quality and Climate Indoor AIR 2018  

Urban street aspect ratio and urban thermal storage are significant to influence urban ventilation, urban thermal environment and subsequently building energy consumption in cities. We performed scale-model field measurement of urban thermal environment in summer of 2016-2017 in Guangzhou, China. Two types of north-south street canyons (building height H=1.2m, street widths are W=1.2m, 0.6m, 0.4m respectively; Aspect ratio H/W=1, 2, 3) were investigated. The 'empty' model consisted of hollow buildings with wall thickness of 1.5cm and the 'sand' model were made up of buildings filled with sand producing bigger thermal storage. The velocity and turbulence distribution, air temperature and basic characteristics of radiation in and above the idealized street canyon were measured by using ultrasonic anemometer, temperature and humidity sensors, four component radiometers, infrared camera. Especially the impacts of different building heat storage and street aspect ratio on urban thermal environment were emphasized.

Scopus

担当区分：筆頭著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）   出版者・発行元：15th Conference of the International Society of Indoor Air Quality and Climate Indoor AIR 2018  

Previous studies of flow patterns in idealized two-dimensional (2D) deep street canyons with various aspect ratios and Reynolds number have different findings. To further verify the Re independence and the correlation between aspect ratios and the flow pattern, this paper conducts both wind tunnel experiments and CFD simulations on urban turbulence in 2D deep street canyons (H/W=2 to 6). RNG k-ε model applying scalable wall functions performs well for the wind-tunnel-scale simulation, and simulation results can agree with experimental data generally well. Results show that, as H/W=2.4, relative low Re number (Re<5000) can produce two main vortices, and the single main vortex appears when Re rises to be much greater than 11000 with the increasing wind speed. For street canyons with higher aspect ratio (H/W>4), two main vortices start to appear, however in the literature two main vortexes exist as H/W=2 and more vortexes show up as H/W=3 to 10.

Scopus