Kyushu University Academic Staff Educational and Research Activities Database
Researcher information (To researchers) Need Help? How to update
Yohei Onuki Last modified date:2024.06.03

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, Jules Guioth, Freddy Bouchet, Dynamical large deviations for an inhomogeneous wave kinetic theory: linear wave scattering by a random medium, Annales Henri Poincaré: A Journal of Theoretical and Mathematical Physics,, 25, 1215-1259, 45 pages, 2024.01, The wave kinetic equation predicts the averaged temporal evolution of a continuous spectral density of waves either randomly interacting or scattered by the fine structure of a medium. In a wide range of systems, the wave kinetic equation is derived from a fundamental equation of wave motion, which is symmetric through time reversal. By contrast, the corresponding wave kinetic equations is time irreversible: Its solutions monotonically increase an entropy-like quantity. A similar paradox appears whenever one makes a mesoscopic description of the evolution of a very large number of microscopic degrees of freedom, the paradigmatic example being the kinetic theory of dilute gas molecules leading to the Boltzmann equation. Since Boltzmann, it has been understood that a probabilistic understanding solves the apparent paradox. More recently, it has been understood that the kinetic description itself, at a mesoscopic level, should not break time reversal symmetry (Bouchet in J Stat Phys 181(2):515-550, 2020). The time reversal symmetry remains a fundamental property of the mesoscopic stochastic process: Without external forcing, the path probabilities obey a detailed balance relation with respect to an equilibrium quasipotential. The proper theoretical or mathematical tool to derive fully this mesoscopic time reversal stochastic process is large deviation theory: A large deviation principle uncovers a time reversible field theory, characterized by a large deviation Hamiltonian, for which the deterministic wave kinetic equation appears as the most probable evolution. Its irreversibility appears as a consequence of an incomplete description, rather than as a consequence of the kinetic limit itself, or some related chaotic hypothesis. This paper follows Bouchet (J Stat Phys 181(2):515-550, 2020) and a series of other works that derive the large deviation Hamiltonians of the main classical kinetic theories, for instance, Guioth et al. (J Stat Phys 189(20):20, 2022) for homogeneous wave kinetics. We propose here a derivation of the large deviation principle in an inhomogeneous situation, for the linear scattering of waves by a weak random potential. This problem involves microscopic scales corresponding to the typical wavelengths and periods of the waves and mesoscopic ones which are the scales of spatial inhomogeneities in the spectral density of both the random scatterers and the wave spectrum, and the time needed for the random scatterers to alter the wave spectrum. The main assumption of the kinetic regime is a large separation of these microscopic and mesoscopic scales. For the sake of simplicity, we consider a generic model of wave scattering by weak disorder: the Schrödinger equation with a random potential. We derive the path large deviation principle for the local spectral density and discuss its main properties. We show that the mesoscopic process obeys a time reversal symmetry at the level of large deviations..
2. Yohei Onuki, Sylvain Joubaud, Thierry Dauxois, Breaking of internal waves parametrically excited by ageostrophic anticyclonic instability, Journal of Physical Oceanography,, 53, 6, 1591-1613, 2023.06, A gradient-wind balanced flow with an elliptic streamline parametrically excites internal inertia-gravity waves through ageostrophic anticyclonic instability (AAI). This study numerically investigates the breaking of internal waves and the following turbulence generation resulting from the AAI. In our simulation, we periodically distort the calculation domain following the streamlines of an elliptic vortex and integrate the equations of motion using a Fourier spectral method. This technique enables us to exclude the overall structure of the large-scale vortex from the computation and concentrate on resolving the small-scale waves and turbulence. From a series of experiments, we identify two different scenarios of wave breaking conditioned on the magnitude of the instability growth rate scaled by the buoyancy frequency, λ/N. First, when λ/N ≳ 0.008, the primary wave amplitude excited by AAI quickly goes far beyond the overturning threshold and directly breaks. The resulting state is thus strongly nonlinear turbulence. Second, if λ/N ≲ 0.008, weak wave-wave interactions begin to redistribute energy across frequency space before the primary wave reaches a breaking limit. Then, after a sufficiently long time, the system approaches a Garrett-Munk-like stationary spectrum, in which wave breaking occurs at finer vertical scales. Throughout the experimental conditions, the growth and decay time scales of the primary wave energy are well correlated. However, since the primary wave amplitude reaches a prescribed limit in one scenario but not in the other, the energy dissipation rates exhibit two types of scaling properties. This scaling classification has similarities and differences with D’Asaro and Lien’s (2000) wave-turbulence transition model..
3. Antoine Venaille, Yohei Onuki, Nicolas Perez, Armand Leclerc, From ray tracing to waves of topological origin in continuous media, SciPost Physics, 10.21468/SciPostPhys.14.4.062, 14, 4, 36 pages, 2023.04.
4. Yohei Onuki, Irreversible energy extraction from negative temperature two-dimensional turbulence, Physical Review E, 10.1103/PhysRevE.106.064131, 106, 064131, 17 pages, 2022.12.
5. 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, 14 pages, 2021.05.
6. Yohei Onuki, Quasi-local method of wave decomposition in a slowly varying medium, Journal of Fluid Mechanics, 10.1017/jfm.2019.825, 883, A56, 27 pages, 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..
7. Yohei Onuki, Yuki Tanaka, Instabilities of Finite-Amplitude Internal Wave Beams, Geophysical Research Letters, 10.1029/2019GL082570, 46, 13, 7527-7535, 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..
8. 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].
9. 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。.
10. 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, Sylvain Joubaud, Thierry Dauxois, Breaking of internal waves parametrically excited by ageostrophic anticyclonic instability, 14th European Fluid Mechanics Conference, 2022.09.
2. Yohei Onuki, Sylvain Joubaud, Thierry Dauxois, Simulating turbulent mixing caused by local instability of internal gravity waves, IX International Symposium on Stratified Flows, 2022.08.
3. Yohei Onuki, Sylvain Joubaud, Thierry Dauxois, Breaking of internal waves parametrically excited by ageostrophic anticyclonic instability, Japan Geoscience Union Meeting 2022, 2022.05.
4. Yohei Onuki, Sylvain Joubaud, Thierry Dauxois, Breaking of internal waves parametrically excited by ageostrophic anticyclonic instability, Ocean Sciences Meeting, 2022.03.
5. Yohei Onuki, Irreversible energy extraction from negative-temperature two-dimensional turbulence, Japan Geoscience Union Meeting 2021, 2021.06, The formation and transition of patterns observed in various geophysical turbulent flows are commonly explained in terms of statistical mechanics. This study aims to extend the range of such a statistical mechanics theory by introducing a perhaps novel conceptual model. We consider an inviscid two-dimensional flow contained in a bounded domain, the shape of which is distorted by an externally imposed force. Unlike the usual fixed boundary cases, the flow energy within the domain is exchanged with the external system via pressure work through the moving lateral boundary. Concurrently, the flow field remains constrained by vorticity conservation. The vorticity equation expanded onto time-dependent Laplacian eigenfunctions and subsequently truncated at a finite number satisfies Liouville's theorem. Consequently, based on energy and enstrophy constraints, a grand-canonical ensemble (GCE) is defined with a negative or positive temperature. Beginning from the GCE, when the domain shape is distorted from one shape to another in a finite time, the Jarzynski equality is established. The pressure work performed on the system is thus related to the difference in the Helmholtz free energy. The direction of the inequality between the mean work and the free energy difference derived from the Jarzynski equality depends on the sign of the initial temperature. If the temperature is negative, the pressure work irreversibly extracts flow energy from the system. This result provides new insights into the unique thermodynamic characteristics of geophysical flows..
6. 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..
7. Yohei Onuki, Yuki Tanaka, Instabilities of finite-amplitude internal wave beams, Ocean Sciences Meeting 2020, 2020.02.
8. Yohei Onuki, Toshiyuki Hibiya, Decay rates of internal tides estimated by an improved wave-wave interaction analysis, Ocean Mixing Gordon Research Conference, 2018.06.
9. Yohei Onuki, Toshiyuki Hibiya, Decay rates of internal tides estimated by an improved wave-wave interaction analysis, Japan Geoscience Union Meeting 2018, 2018.05.
10. Yohei Onuki, Toshiyuki Hibiya, Theoretical analysis of resonant interaction between internal tides and an internal wave continuum, Ocean Sciences Meeting, 2018.02.
11. 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.
12. 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.
13. 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.
14. 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.
15. 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