arXiv subjects

Yohei Onuki

Publications and source records attributed to Yohei Onuki.

7 recordsLinked to original sources

arXiv2026

A reduced model for surface wave-current interactions without spatial scale separation

We propose a reduced asymptotic model for the mutual interaction between a weakly nonlinear surface gravity wave field and a slowly evolving incompressible current in a homogeneous rotating fluid. The formulation builds on the Craik-Leibovich theory for the wave-averaged momentum equation, but the Stokes drift is not prescribed externally. Instead, it is determined by a companion amplitude equation for a narrow-band wave field concentrated near the wavenumber circle associated with a prescribed carrier frequency. The derivation combines a multiple-time-scale expansion in wave steepness with a phenomenological closure that neglects quartic wave-wave interactions while retaining the third-order Stokes correction. Importantly, no spatial-scale separation is imposed on the wave-current interaction, allowing the wave equation to represent current-induced advection, refraction, and multidirectional scattering. The resulting equations conserve wave action and admit closed energy and momentum budgets for the coupled wave-current system. The model thus provides a tractable bidirectional extension of the classical Craik-Leibovich framework for regimes in which current-induced wave evolution feeds back significantly on the mean flow.

physics.flu-dyn↗

arXiv2025

Disentangling discrete and continuous spectra of tidally forced internal waves in shear flow

Generation of internal waves driven by barotropic tides over seafloor topography is a central issue in developing mixing and wave drag parameterizations for ocean circulation models. Traditional analytical approaches estimate the energy conversion rate from barotropic tides to internal waves using a modal expansion of the wave field. However, this framework becomes inadequate if a background shear flow is present, as singular solutions associated with critical levels emerge. To uncover the distinct roles of regular eigenmodes and singular solutions in tidal energy conversion, this study analytically investigates wave generation over a localized small topography in the presence of shear flow without Coriolis force. Applying horizontal Fourier and temporal Laplace transforms, we identify regions in the topographic wavenumber and forcing frequency space where unbounded energy growth occurs. These regions coincide with the spectrum of an operator governing free wave propagation and consist of discrete and continuous parts, which correspond to regular eigenmodes and singular solutions, respectively. Asymptotic evaluation of the Fourier integral reveals that the far-field response comprises standing wave trains linked to the discrete spectrum and evolving wave packets associated with the continuous spectrum. While the velocity amplitudes of the wave packets decay, their vertical velocity gradients grow during propagation, potentially leading to wave breaking. Finally, we derive a formula for the net barotropic-to-baroclinic energy conversion rate, extending the classical one by incorporating the contributions from both the discrete and continuous spectra.

physics.flu-dyn↗

arXiv2023

Bulk-Edge Correspondence Recovered in Incompressible Geophysical Flows

Bulk-edge correspondence is a cornerstone in topological physics, establishing a connection between the number of unidirectional edge modes in physical space and a Chern number, an integer that counts phase singularities of the eigenmodes in parameter space. In continuous media, violation of this correspondence has been reported when some of the frequency wavebands are unbounded, resulting in weak topological protection of chiral edge states. Here, we propose a strategy to reestablish strong bulk-edge correspondence in incompressible rotating stratified flows, taking advantage of a natural cutoff frequency provided by density stratification. The key idea involves the introduction of an auxiliary field to handle the divergence-free constraint. This approach highlights the resilience of internal coastal Kelvin waves near vertical walls under varying boundary conditions.

cond-mat.other↗

arXiv2023

Dynamical large deviations for an inhomogeneous wave kinetic theory: linear wave scattering by a random medium

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 equation is time-irreversible. A similar paradox appears whenever one makes a mesoscopic description of the evolution of a very large number of microscopic degrees of freedom. Recently, it has been understood that the kinetic description itself, at a mesoscopic level, should not break time-reversal symmetry. The proper theoretical or mathematical tool to derive a mesoscopic time-reversal stochastic process is large deviation theory, for which the deterministic wave kinetic equation appears as the most probable evolution. This paper follows Bouchet (2020) and a series of other works that derive the large deviation Hamiltonians of the classical kinetic theories. We propose a derivation of the large deviation principle for the linear scattering of waves by a weak random potential in an inhomogeneous situation. 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 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. We choose a generic model of wave scattering by weak disorder: the Schr\"odinger 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. (abridged)

cond-mat.stat-mech↗

arXiv2022

Breaking of internal waves parametrically excited by ageostrophic anticyclonic instability

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, $\lambda/N$. First, when $\lambda/N\gtrsim0.008$, 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 $\lambda/N\lesssim0.008$, 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.

physics.flu-dyn↗

arXiv2022

From ray tracing to waves of topological origin in continuous media

Inhomogeneous media commonly support a discrete number of wave modes that are trapped along interfaces defined by spatially varying parameters. When they are robust against continuous deformations of parameters, such waves are said to be of topological origin. It has been realized over the last decades that such waves of topological origin can be predicted by computing a single topological invariant, the first Chern number, in a dual bulk wave problem that is much simpler to solve than the original wave equation involving spatially varying coefficients. The correspondence between the simple bulk problem and the more complicated interface problem is usually justified by invoking an abstract index theorem. Here, by applying ray tracing machinery to the paradigmatic example of equatorial shallow water waves, we propose a physical interpretation of this correspondence. We first compute ray trajectories in a phase space given by position and wavenumber of the wave packet, using Wigner-Weyl transforms. We then apply a quantization condition to describe the spectral properties of the original wave operator. We show that the Chern number emerges naturally from this quantization relation.

physics.flu-dyn↗

arXiv2022

Irreversible energy extraction from negative temperature two-dimensional turbulence

The formation and transition of patterns of two-dimensional turbulent flows observed in various geophysical systems are commonly explained in terms of statistical mechanics. Different from ordinary systems, for a two-dimensional flow, the absolute temperature defined for a statistical equilibrium can take negative values. In a state of negative temperature, the second law of thermodynamics predicts that energy in microscopic fluctuations is irreversibly converted to a macroscopic form. This study explores the possibility of this one-way energy conversion in a two-dimensional flow using a basic conceptual model. We consider an inviscid incompressible fluid 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. Beginning from a state of Kraichnan's grand-canonical ensemble, when the domain shape is distorted from one shape to another in a finite time, the Jarzynski equality is established. This equality states that, on average, the direction of a net energy flow through the boundary during a cycle of domain distortion changes with the sign of the initial temperature of the system. Numerical experiments are carried out to verify this theoretical argument and to investigate the parameter dependence of the energy exchange rate.

physics.flu-dyn↗