Samuel Kokh

Samuel Kokh
Atomic Energy and Alternative Energies Commission | CEA · Centre d'Etudes de Saclay

PhD

About

56
Publications
7,237
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1,122
Citations

Publications

Publications (56)
Preprint
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In this paper, we propose a flux splitting finite volume method for the approximation of the Euler equations with source terms derived from a potential. The flux splitting strategy that we adopt here relies on a separate treatment of the terms related to pressure effects from the terms related to transport. We show that this approach can be recast...
Article
Full-text available
Aims. Clouds are expected to form in a broad range of conditions in the atmosphere of exoplanets given the variety of possible condensible species. This diversity, however, might lead to very different small-scale dynamics depending on radiative transfer in various thermal conditions. Here, we aim to provide some insight into these dynamical regime...
Article
Full-text available
We present a sharp interface approach for compressible two-phase that preserves its accuracy in the low Mach regime. The interface between both fluids is captured by a Level Set function via the Ghost Fluid method. In the low Mach regime, classic Finite Volume compressible solvers lose accuracy on quadrangle or hexahedral meshes and a low Mach corr...
Preprint
Full-text available
Clouds are expected to form in a wide range of conditions in the atmosphere of exoplanets given the large range of possible condensible species. However this diversity might lead to very different small-scale dynamics depending on radiative transfer in various thermal conditions: we aim at providing some insights into these dynamical regimes. We pe...
Article
Full-text available
The purpose of this contribution is to derive a reduced-order two-phase flow model including interface subscale modeling through geometrical variables based on Stationary Action Principle (SAP) and Second Principle of Thermodynamics in the spirit of [1, 2]. The derivation is conducted in the disperse phase regime for the sake of clarity but the res...
Chapter
We are interested in the numerical approximation of the shallow water equations in two space dimensions. We propose a well-balanced, all-regime, and positive scheme. Our approach is based on a Lagrange-projection decomposition which allows to naturally decouple the acoustic and transport terms.
Preprint
Full-text available
The present paper proposes a two-phase flow model that is able to account for two-scale kinematics and two-scale surface tension effects based on geometric variables at small scale. At large scale, the flow and the full geometry of the interface may be retrieved thanks to the bulk variables, while at small scale the interface is accurately describe...
Article
We propose here to investigate the impact of small-scale effects on the bulk evolution of a two-phase flow system. More precisely, we choose to examine the sole influence of a small-scale (with respect to the bulk velocity) off-equilibrium velocity on the system. In order to narrow our analysis and avoid complex well-posedness issues, we choose to...
Article
By generalizing the theory of convection to any type of thermal and compositional source terms (diabatic processes), we show that thermohaline convection in Earth's oceans, fingering convection in stellar atmospheres, and moist convection in Earth's atmosphere are derived from the same general diabatic convective instability. We also show that "rad...
Preprint
Full-text available
Convection is an important physical process in astrophysics well-studied using numerical simulations under the Boussinesq and/or anelastic approximations. However these approaches reach their limits when compressible effects are important in the high Mach flow regime, e.g. in stellar atmospheres or in the presence of accretion shocks. In order to t...
Preprint
Full-text available
By generalizing the theory of convection to any type of thermal and compositional source terms (diabatic processes), we show that thermohaline convection in Earth oceans, fingering convection in stellar atmospheres, and moist convection in Earth atmosphere are deriving from the same general diabatic convective instability. We show also that "radiat...
Preprint
Full-text available
In this work, we focus on the numerical approximation of the shallow water equations in two space dimensions. Our aim is to propose a well-balanced, all-regime and positive scheme. By well-balanced, it is meant that the scheme is able to preserve the so-called lake at rest smooth equilibrium solutions. By all-regime, we mean that the scheme is able...
Poster
Full-text available
Revisiting LBM algorithms and numerical implementations and optimisations using the Kokkos library. Performance measurements on multiple architectures (Intel Skylake, Intel KNL, ARM ThunderX2, Nvidia P100). Applications to multiphase flow by coupling LBM Navier-Stkokes with phase field model (Allen-Cahn).
Presentation
Full-text available
- a short overview of exascale hardware trends - overview of performance portability - LBM (Lattice Boltzmann) kernels implemented with C++/kokkos library - LBM kernels optmizations and performance measuments on multiple architectures (Skylake, KNL, Nvidia GPUs, ...)
Article
In high-elevation, boreal and arctic regions, hydrological processes and associated water bodies can be strongly influenced by the distribution of permafrost. Recent field and modeling studies indicate that a fully-coupled multidimensional thermo-hydraulic approach is required to accurately model the evolution of these permafrost-impacted landscape...
Article
Full-text available
We propose an all regime Lagrange-Projection like numerical scheme for 2D homogeneous models for two-phase flows. By all regime, we mean that the numerical scheme is able to compute accurate approximate solutions with an under-resolved discretization, i.e. a mesh size and time step much bigger than the Mach number M of the mixture. The key idea is...
Chapter
We review sharpening methods for finite volume schemes, with an emphasis on the basic structure of sharpening methods. It covers high-order methods and nonlinear techniques for linear advection, Glimm's method, antidiffusion techniques, and the interaction of these techniques with the PDE structures. Additional approaches like level sets, interface...
Article
Full-text available
We review sharpening methods for finite volume schemes, with an emphasis on the basic structure of sharpening methods. It covers high order methods and non linear techniques for linear advection, Glimm's method, anti-diffusion techniques, the interaction of these techniques with the PDE structures. Additional approaches like level sets, interface r...
Article
Full-text available
With the objective of modeling both separate and disperse two-phase flows, we use in this paper a methodology for deriving two-fluid models that do not assume any flow topology. This methodology is based on a variational principle and on entropy dissipation requirement. Some of the models that are such derived and studied are already known in the c...
Article
Full-text available
This work focuses on the numerical approximation of the Shallow Water Equations (SWE) using a Lagrange-Projection type approach. We propose to extend to this context recent implicit-explicit schemes developed in the framework of compressibleflows, with or without stiff source terms. These methods enable the use of time steps that are no longer cons...
Article
Many physical problems involve spatial and temporal inhomogeneities that require a very fine discretization in order to be accurately simulated. Using an adaptive mesh, a high level of resolution is used in the appropriate areas while keeping a coarse mesh elsewhere. This idea allows to save time and computations, but represents a challenge for dis...
Conference Paper
Full-text available
The impacts of climate change in boreal regions has received considerable attention recently due to the warming trends that have been experienced in recent decades and are expected to intensify in the future. Large portions of these regions, corresponding to permafrost areas, are covered by water bodies (lakes, rivers) that interact with the surrou...
Article
Full-text available
In this paper, we present an anti-diffusive method dedicated to the simulation of interface flows on Cartesian grids involving an arbitrary number m of compress- ible components. Our work is two folds. First, we introduce a m-component flow model that generalizes a classic two material five-equation model. In that way, interfaces are localized than...
Article
Full-text available
We propose an all regime Lagrange-Projection like numerical scheme for the gas dynamics equations. By all regime , we mean that the numerical scheme is able to compute accurate approximate solutions with an under-resolved discretization with respect to the Mach number M, i.e. such that the ratio between the Mach number M and the mesh size or the ti...
Article
The present paper is dedicated to the simulation of liquid-gas flows with interfaces in the framework of fast transient fluid-structure dynamics. The two-fluid interface is modelled as a discontinuity surface in the fluid property. We use an anti-dissipative Finite-Volume discretization strategy for unstructured meshes in order to capture the posit...
Article
Full-text available
We propose a large time step and asymptotic preserving scheme for the gas dynamics equations with external forces and friction terms. By asymptotic preserving, we mean that the numerical scheme is able to reproduce at the discrete level the parabolic-type asymptotic behaviour satisfied by the continuous equations. By large time-step, we mean that t...
Article
Full-text available
In the present work we investigate the numerical simulation of liquid-vapor phase change in compressible flows. Each phase is modeled as a compressible fluid equipped with its own Equation of State (EOS). We suppose that inter-phase equilibrium processes in the medium operate at a short time-scale compared to the other physical phenomena such as co...
Article
Full-text available
We propose a method dedicated to the simulation of interface flows involving an arbitrary number m of compressible components. Our task is two-fold: we first introduce a m-component flow model that generalizes the two-material five-equation model of [2,3]. Then, we present a discretization strategy by means of a Lagrange-Remap [8,10] approach follo...
Article
Full-text available
We build a non-dissipative second order algorithm for the approximate resolution of the one-dimensional Euler system of compressible gas dynamics with two components. The considered model was proposed in [1]. The algorithm is based on [8] which deals with a non-dissipative first order resolution in Lagrange-remap formalism. In the present paper we...
Chapter
Full-text available
We consider the seven-equation model for compressible two-phase flows and propose a large time-step numerical scheme based on a time implicit-explicit Lagrange-Projection strategy introduced in Coquel et al. [6] for Euler equations. The main objective is to get a Courant-Friedrichs-Lewy (CFL) condition driven by (slow) contact waves instead of (fas...
Article
We propose a discretization method of a five-equation model with isobaric closure for the simulation of interfaces between compressible fluids. This numerical solver is a Lagrange–Remap scheme that aims at controlling the numerical diffusion of the interface between both fluids. This method does not involve any interface reconstruction procedure. T...
Article
This Note investigates the approximation of phase change in compressible fluids with complex equation of state (EOS). Assuming a local and instantaneous equilibrium with respect to phasic pressures, temperatures and chemical potentials when both phases are present leads to the classical definition of an equilibrium EOS for the two-phase medium. Unf...
Article
Full-text available
The present work is dedicated to the simulation of compressible two-phase flows with phase change for pool boiling type problems. The model we are concerned with involves scales that allow to distinguish the interface between both phases. The mass transfer is driven by assuming local and instantaneous equilibria with respect to phasic pressures, te...
Conference Paper
Full-text available
The present work investigates the simulation of phase transition in compressible fluids. We postulate a local and instantaneous equilibrium with respect to phasic pressures, temperatures and chemical potentials when both phases are present. This hypothesis leads to the definition of an equilibrium equation of state (EOS) for the two-phase medium. W...
Article
This Note is concerned with the strict hyperbolicity of the compressible Euler equations equipped with an equation of state that describes the thermodynamical equilibrium between the liquid phase and the vapour phase of a fluid. The proof is valid for a very wide class of fluids. The argument only relies on smoothness assumptions and on the classic...
Article
Full-text available
We present a simple method for simulating isothermal compressible two-phase flows with mass transfer. The convective part of the model is compatible with the Least Action Principle and the system is endowed with an entropy inequality which accounts for phase change terms and phasic pressure unbalance. A study of the system as a relaxed model of two...
Article
Full-text available
We will try to present herein the main issues of our investigation in numerical methods for two-phase flow modeling, within the framework of the NEPTUNE project, which benefits from both contributions of CEA and EDF. These may be recast in five work packages. The first two are devoted to the mathematical and numerical modeling of two-phase flows wi...
Conference Paper
Full-text available
We present an isothermal model for liquid-phase transition. We use an Hamilton principle to derive the conservative part of the system and we propose additionalsource stiff phase change source terms that are compatible with the second thermodynamic principle. These terms are considered as relaxation terms and defined an equlibrium manifold for the...
Conference Paper
Full-text available
We present a numerical scheme based on a two-step convection-relaxation strategy for the simulation of compressible two-phase flows with phase change. The core system used here is a simple isothermal model where stiff source terms account for mass transfer.
Article
This test case concerns the propagation of a shock through a helium gas bubble in air. The shape of the helium bubble and the pressure downstream from its position should be compared with available experimental data.
Article
Four shock tube test cases are proposed: three for gas-gas flows and one for a gas-liquid flow. Both fluids are inviscid and separated by a flat contact discontinuity. The flow is one-dimensional. The analytical solutions of the corresponding Riemann problems are given.
Article
A diffuse-interface method is proposed for the simulation of interfaces between compressible fluids with general equations of state, including tabulated laws. The interface is allowed to diffuse on a small number of computational cells and a mixture model is given for this transition region. We write conservation equations for the mass of each flui...
Article
This paper is devoted to the direct numerical simulation of compressible two-phase flows, i.e. including material interfaces, in an Eulerian framework. Eulerian methods, such as Volume Of Fluid, are easy to handle but suffer from numerical diffusion which spreads out the precise localization of the interface. We discuss some remedies to this loss o...
Article
In the Eulerian approach for simulating interfaces in two-phase flows, the main difficulties arise from the fixed character of the mesh which does not follow the interface. Therefore, near the interface there are computational cells containing both fluids which require a suitable modelling of the mixture. Furthermore, most numerical algorithms, suc...
Article
this paper we address this modelling issue. Following the lead of [1], [20], [22] (among others) we derive a so-called kinetic relation which gives the mass ux at the phase interface. In a second part we introduce mixing rules for the two phases since any eulerian scheme will necessary spread out the interface and one has to handle cells containing...
Article
Full-text available
The accurate prediction of the so-called Departure from Nucleate Boiling is a cru-cial point in the safety analysis of nuclear reactor cores. Nowadays, the DNB evaluation is based on experimental correlations using numerical results of averaged two-phase flow models. A natural goal is therefore to replace this rough procedure by a direct numerical...
Article
One of the key technical challenges of the NEPTUNE project (7) is to improve and validate the two-phase flow models in order to pave the road for upcoming models and methods of two-phase flow simulation. We develop a methodology which consists in testing these models against series of simulations at finer scales. The present work deals with the sim...
Article
Résumé Ce travail porte sur la modélisation mathématique et la simulation numérique directe du changement de phase liquide-vapeur pour des écoulements de fluides compressibles. Nous utilisons un modèle diphasique à interface diffuse et nous montrons que par un processus de relaxation on peut obtenir une loi d'état complète pour le mélange liquide-v...

Projects

Project (1)
Project
LBM_Saclay is a software code providing parallel implementation of LBM (Lattice Boltzmann Methods) models for multiphase flow applications. Parallelization is done using both MPI and Kokkos (https://github.com/kokkos/kokkos) so that the code can run on almost any current HPC architectures (Intel Skylake, KNL, Nvidia GPU, ARM ThunderX2, etc...) Code available upon gentle request: pierre.kestener@cea.fr