Kyushu University Academic Staff Educational and Research Activities Database
Researcher information (To researchers) Need Help? How to update
Onuki Yohei Last modified date:2021.05.28

E-Mail *Since the e-mail address is not displayed in Internet Explorer, please use another web browser:Google Chrome, safari.
 Reseacher Profiling Tool Kyushu University Pure
Academic Degree
Ph. D.
Country of degree conferring institution (Overseas)
Field of Specialization
Physical Oceanography
ORCID(Open Researcher and Contributor ID)
Total Priod of education and research career in the foreign country
Research Interests
  • Mathematical modeling of the deep ocean environment: basic comprehension and prediction
    keyword : Geophysical fluid dynamics, Ocean dynamics, Nonlinear mechanics, Internal wave, Turbulence
Academic Activities
1. Yohei Onuki, Sylvain Joubaud, Thierry Dauxois, Simulating turbulent mixing caused by local instability of internal gravity waves, Journal of Fluid Mechanics, 10.1017/jfm.2021.119, 915, A77, 2021.05.
2. Yohei Onuki, Quasi-local method of wave decomposition in a slowly varying medium, Journal of Fluid Mechanics, 10.1017/jfm.2019.825, 883, A56, 2020.01, [URL], The general asymptotic theory for wave propagation in a slowly varying medium, classically known as the Wentzel–Kramers–Brillouin–Jeffreys (WKBJ) approximation, is revisited here with the aim of constructing a new data diagnostic technique useful in atmospheric and oceanic sciences. Using the Wigner transform, a kind of mapping that associates a linear operator with a function, we analytically decompose a flow field into mutually independent wave signals. This method takes account of the variations in the polarisation relations, an eigenvector that represents the kinematic characteristics of each wave component, so as to project the variables onto their eigenspace quasi-locally. The temporal evolution of a specific mode signal obeys a single wave equation characterised by the dispersion relation that also incorporates the effect from the local gradient in the medium. Combining this method with transport theory and applying them to numerical simulation data, we can detect the transfer of energy or other conserved quantities associated with the propagation of each wave signal in a wide variety of situations..
3. Yohei Onuki, Yuki Tanaka, Instabilities of finite‐amplitude internal wave beams, Geophysical Research Letters, 10.1029/2019GL082570, 2019.07, [URL], Beam-like internal waves are commonly generated by tides in the ocean, but their dissipation processes that cause vertical mixing remain poorly understood. Previous studies examined small-amplitude beams to find that parametric subharmonic instability (PSI) induces latitude-dependent wave dissipation. Using a novel approach based on Floquet theory, this study analyzes the stability of finite-amplitude beams over a wide range of parameters. If beam amplitude is small, PSI is indeed the principal mode under the condition f∕𝜎 ≤ 0.5, where f is the Coriolis parameter and 𝜎 is the beam frequency, and the growth rate is maximum when equality holds. However, as beam amplitude is increased, instability arises even when f∕𝜎 > 0.5, but the location of maximum instability shifts toward lower f∕𝜎; thus, the latitudinal dependence of instability is significantly altered. Furthermore, the resulting energy spectrum is strongly Doppler shifted to higher frequencies, which therefore distinguishes this configuration from the common cases of PSI..
4. Yohei Onuki, Toshiyuki Hibiya, Parametric subharmonic instability in a narrow-band wave spectrum, Journal of Fluid Mechanics, 10.1017/jfm.2019.44, 865, 247-280, 2019.04, [URL].
5. Yohei Onuki, Toshiyuki Hibiya, Decay Rates of Internal Tides Estimated by an Improved Wave-Wave Interaction Analysis, Journal of Physical Oceanography, 10.1175/JPO-D-17-0278.1, 48, 11, 2689-2701, 2018.11, [URL], 回転成層流体における波数空間内のエネルギーカスケード過程について、密度成層構造の空間変化や上下境界の影響を取り入れた形で、弱非線形乱流理論に基づく運動学的方程式の定式化を行った。これを用いて、海洋内部で潮汐によって励起された波動が非線形共鳴を経てエネルギーを失う割合を全球的に計算したところ、観測研究に整合する形で、中緯度海域に卓越したエネルギー損失率のピークを再現することに成功した。.
6. Yohei Onuki, Toshiyuki Hibiya, Excitation mechanism of near-inertial waves in baroclinic tidal flow caused by parametric subharmonic instability, Ocean Dynamics, 10.1007/s10236-014-0789-3, 65, 1, 107-113, 2015.01.
1. Yohei Onuki, Irreversible energy extraction from negative-temperature two-dimensional turbulence
, Japan Geoscience Union Meeting 2021, 2021.06.
2. Yohei Onuki, Sylvain Joubaud, Thierry Dauxois, Direct numerical simulation of stratified turbulence generated by local instability of internal gravity waves, JpGU - AGU Joint Meeting 2020, 2020.07, With the aim of assessing internal wave-driven mixing in the ocean, we develop a new technique for direct numerical simulations of stratified turbulence. Since the spatial scale of oceanic internal gravity waves is typically much larger than that of turbulence, fully incorporating both in a model would require a high computational cost, and is therefore out of our scope. Alternatively, we cut out a small domain periodically distorted by an unresolved large-scale internal wave and locally simulate the energy cascade to the smallest scales. In this model, even though the Froude number of the outer wave, Fr, is small such that density overturn or shear instability does not occur, a striped pattern of disturbance is exponentially amplified through a parametric subharmonic instability. When the disturbance amplitude grows sufficiently large, secondary instabilities arise and produce much smaller-scale fluctuations. Passing through these two stages, wave energy is transferred into turbulence energy and will be eventually dissipated. Different from the conventional scenarios of vertical shear-induced instabilities, a large part of turbulent potential energy is supplied from the outer wave and directly used for mixing. The mixing coefficient, a ratio between the dissipation rate of kinetic energy and that of available potential energy, is always greater than 0.5 and tends to increase with Fr. Although our results are mostly consistent with the recently proposed scaling relationship between the mixing coefficient and the turbulent Froude number, the values of mixing coefficient obtained here are larger by a factor of about two than previously reported..
3. Yohei Onuki, Yuki Tanaka, Instabilities of finite-amplitude internal wave beams, Ocean Sciences Meeting 2020, 2020.02.
4. Yohei Onuki, Toshiyuki Hibiya, Decay rates of internal tides estimated by an improved wave-wave interaction analysis, Ocean Mixing Gordon Research Conference, 2018.06.
5. Yohei Onuki, Toshiyuki Hibiya, Decay rates of internal tides estimated by an improved wave-wave interaction analysis, Japan Geoscience Union Meeting 2018, 2018.05.
6. Yohei Onuki, Toshiyuki Hibiya, Theoretical analysis of resonant interaction between internal tides and an internal wave continuum, Ocean Sciences Meeting, 2018.02.
7. Yohei Onuki, Toshiyuki Hibiya, Theoretical estimates of the intensity of resonant coupling between internal tides and internal waves in the western Pacific, The 19th Pacific-Asian Marginal Seas (PAMS) Meeting, 2017.04.
8. Yohei Onuki, Toshiyuki Hibiya, Geography of the attenuation rates of baroclinic tidal energy estimated using wave-wave interaction theory, Japan Geoscience Union Meeting 2016, 2016.05.
9. Yohei Onuki, Toshiyuki Hibiya, Estimates of the attenuation rates of baroclinic tidal energy caused by resonant interactions among internal waves based on the weak turbulence theory, Ocean Sciences Meeting, 2016.02.
10. Yohei Onuki, Toshiyuki Hibiya, Estimates of the attenuation rates of baroclinic tidal waves caused by resonant interactions with the background internal wave continuum, International Union of Geodesy and Geophysics 26th General Assembly, 2015.06.
11. Yohei Onuki, Toshiyuki Hibiya, Excitation Mechanism of Near-inertial Currents in Baroclinic Tidal Flow Caused by Parametric Subharmonic Instability, Asia Oceania Geosciences Society Annual 11th Meeting, 2014.08.
Membership in Academic Society
  • Japan Geoscience Union