arXiv subjects

Thomas Dubos

Publications and source records attributed to Thomas Dubos.

7 recordsLinked to original sources

arXiv2026

Thermodynamically consistent large-eddy simulation models

Filtered budgets for anelastic turbulence and a general expression of the turbulent sensible heat flux are derived for a multicomponent fluid with an arbitrary equation of state. A family of subgrid-scale closures is then found under the constraint of consistency with (i) the first and second laws of thermodynamics and (ii) invariance with respect to irrelevant thermodynamic constants. A similar family of fully compressible models is also constructed heuristically. These models predict turbulent kinetic energy, assume down-gradient closures for three-dimensional turbulent fluxes and impose certain relationships between the closures for the turbulent fluxes of heat, matter, entropy, and the work of buoyancy forces. A key finding is the explicit derivation of the local rate of entropy production in the filtered model. Positive entropy production is guaranteed whenever the turbulent diffusions of heat and composition are positive and no cross-diffusion occurs. Cross-diffusivities are admissible provided their magnitude is within the bounds of an explicit criterion. The filtered model is invariant under a wider class of transformations than the unfiltered model. Furthermore, in the special case of a single turbulent diffusivity, an arbitrary conservative variable can be prognosed while ignoring its precise relationship to entropy. These findings show that down-gradient closures are consistent with the first and second law of thermodynamics even when they lead to a turbulent sensible heat flux up the temperature gradient. Indeed, while molecular conduction/diffusion is spontaneous and energy-conserving, stratified turbulent mixing is driven by mechanical turbulence and enabled by the consumption of turbulent kinetic energy.

physics.flu-dyn↗

arXiv2023

On the thermodynamic invariance of fine-grain and coarse-grain fluid models

In models of oceanic and atmospheric flows, thermodynamic functions and conservative variables may be defined up to a certain degree of arbitrariness, in the sense that, for a given set of observable quantities such as pressure and temperature, the predictions of the model are insensitive to, e.g. some reference enthalpies, entropies or pressures.Since the compressible Navier-Stokes-Fourier model, regarded as a "mother" fine-grain model, is invariant with respect to arbitrary changes in reference enthalpies and entropies, restricted only by phase change, any coarse-grain model obtained, even conceptually, from it must be invariant at least to the same extent. Upon examination, it is found that the dependence of usual conservative variables to a reference pressure propagates to their fluxes and gradients, and to down-gradient closures based on them.Conversely, closure relationships between adequately defined "reduced" gradients and fluxes of enthalpy and entropy, are guaranteed to lead to invariant models, even with multiple turbulent diffusivities and cross-diffusivities. More work is required to address the invariance of more sophisticated closures, especially shallow and deep convective closures.

physics.flu-dyn↗

arXiv2023

A Simple and Transparent Method for Improving the Energetics and Thermodynamics of Seawater Approximations: Static Energy Asymptotics (SEA)

The static energy encodes all possible information about the thermodynamics and potential energy (and all related forces) of stratified geophysical fluids. In this paper, we develop a systematic methodology, called static energy asymptotics, that exploits this property for constructing energetically and thermodynamically consistent sound-proof approximations of the equations of motion. By approximating the static energy to various orders of accuracy, two main families of approximations are (re-)derived and discussed: the pseudo-incompressible (PI) approximation and the anelastic (AN) approximation. For all approximations, the background and available potential energies (in Lorenz sense) can be constructed to match their exact counterparts as closely as feasible and to be expressible in terms of the exact (as opposed to ad-hoc) thermodynamic potentials. For hydrostatic motions, the AN approximation (of which the Boussinesq approximation is a special case) has the same structure as that of legacy Seawater Boussinesq primitive equations. The energetics of such models could therefore be made transparently traceable to that of the full Navier-Stokes equations at little to no additional cost, thus allowing them to take full advantage of the Gibbs Sea Water (GSW) library developed as part of the new thermodynamic standard for seawater TEOS-10

physics.flu-dyn↗

arXiv2019

Global climate modeling of Saturn's atmosphere. Part II: multi-annual high-resolution dynamical simulations

The Cassini mission unveiled the intense and diverse activity in Saturn's atmosphere: banded jets, waves, vortices, equatorial oscillations. To set the path towards a better understanding of those phenomena, we performed high-resolution multi-annual numerical simulations of Saturn's atmospheric dynamics. We built a new Global Climate Model [GCM] for Saturn, named the Saturn DYNAMICO GCM, by combining a radiative-seasonal model tailored for Saturn to a hydrodynamical solver based on an icosahedral grid suitable for massively-parallel architectures. The impact of numerical dissipation, and the conservation of angular momentum, are examined in the model before a reference simulation employing the Saturn DYNAMICO GCM with a $1/2^{\circ}$ latitude-longitude resolution is considered for analysis. Mid-latitude banded jets showing similarity with observations are reproduced by our model. Those jets are accelerated and maintained by eddy momentum transfers to the mean flow, with the magnitude of momentum fluxes compliant with the observed values. The eddy activity is not regularly distributed with time, but appears as bursts; both barotropic and baroclinic instabilities could play a role in the eddy activity. The steady-state latitude of occurrence of jets is controlled by poleward migration during the spin-up of our model. At the equator, a weakly-superrotating tropospheric jet and vertically-stacked alternating stratospheric jets are obtained in our GCM simulations. The model produces Yanai (Rossby-gravity), Rossby and Kelvin waves at the equator, as well as extratropical Rossby waves, and large-scale vortices in polar regions. Challenges remain to reproduce Saturn's powerful superrotating jet and hexagon-shaped circumpolar jet in the troposphere, and downward-propagating equatorial oscillation in the stratosphere.

astro-ph.EP↗

arXiv2018

Simple geometric approximations for global atmospheres on moderately oblate planets

Certain geometric approximations such as the widely used traditional shallow-atmosphere, spherical-geoid (TSA-SG) and the deep-atmosphere, spherical-geoid (DA-SG) approximations boil down to the specification of a spatial metric tensor. In order to eliminate the leading-order errors due to the SG and TSA approximations, a sequence of three metric geometric approximations of increasing accuracy at high altitudes is obtained. Their metric tensors possess a simple, closed-form analytical expression. The approximations capture to leading order the oblateness of the planet, the widening of atmospheric columns with height, the horizontal and vertical variations of gravity and the non-traditional part of the Coriolis force. Furthermore, for the first two approximations, the horizontal metric is conformal (proportional) to the spherical metric, which simplifies analytical and numerical formulations of the equations of motion.

physics.ao-ph↗

arXiv2018

Short-wavelength secondary instabilities in homogeneous and stably stratified shear flows

We present a numerical investigation of three-dimensional, short-wavelength linear instabilities in Kelvin-Helmholtz (KH) vortices in homogeneous and stratified environments. The base flow, generated using two-dimensional numerical simulations, is characterized by the Reynolds number and the Richardson number defined based on the initial one-dimensional velocity and buoyancy profiles. The local stability equations are then solved on closed streamlines in the vortical base flow, which is assumed quasi-steady. For the unstratified case, the elliptic instability at the vortex core dominates at early times, before being taken over by the hyperbolic instability at the vortex edge. For the stratified case, the early time instabilities comprise a dominant elliptic instability at the core and a hyperbolic instability strongly influenced by stratification at the vortex edge. At intermediate times, the local approach shows a new branch of instability (convective branch) that emerges at the vortex core and subsequently moves towards the vortex edge. A few more convective instability branches appear at the vortex core and move away, before coalescing to form the most unstable region inside the vortex periphery at large times. The dominant instability characteristics from the local approach are shown to be in good qualitative agreement with results from global instability studies for both homogeneous and stratified cases. Compartmentalized analyses are then used to elucidate the role of shear and stratification on the identified instabilities. The role of buoyancy is shown to be critical after the primary KH instability saturates, with the dominant convective instability shown to occur in regions with the strongest statically unstable layering. We conclude by highlighting the potentially insightful role that the local approach may offer in understanding the secondary instabilities in other flows.

physics.flu-dyn↗

arXiv2014

A conservative adaptive wavelet method for the shallow water equations on the sphere

We introduce an innovative wavelet-based approach to dynamically adjust the local grid resolution to maintain a uniform specified error tolerance. Extending the work of Dubos and Kevlahan (2013), a wavelet multi-scale approximation is used to make dynamically adaptive the TRiSK model (Ringler et al. 2010) for the rotating shallow water equations on the sphere. This paper focuses on the challenges encountered when extending the adaptive wavelet method to the sphere and ensuring an efficient parallel implementation using MPI. The wavelet method is implemented in Fortran95 with an emphasis on computational efficiency and scales well up to O(10^2) processors for load-unbalanced scenarios and up to at least O(10^3) processors for load-balanced scenarios. The method is verified using standard smooth test cases (Williamson et al. 1992) and a nonlinear test case proposed by (Galewsky te al. 2004). The dynamical grid adaption provides compression ratios of up to 50 times in a challenging homogenous turbulence test case. The adaptive code is about three times slower per active grid point than the equivalent non-adaptive TRiSK code and about four times slower per active grid point than an equivalent spectral code. This computationally efficient adaptive dynamical core could serve as the foundation on which to build a complete climate or weather model.

physics.geo-ph↗