Unified gas-kinetic scheme for multi-species non-equilibrium flow

2014 ◽  
Author(s):  
Ruijie Wang ◽  
Kun Xu
Keyword(s):  
Kinetic Scheme ◽  
Equilibrium Flow ◽  
Unified Gas Kinetic Scheme ◽  
Non Equilibrium

An Implicit Unified Gas Kinetic Scheme for Radiative Transfer with Equilibrium and Non-Equilibrium Diffusive Limits

2017 ◽  
Vol 22 (4) ◽  
pp. 889-912 ◽  
Author(s):  
Wenjun Sun ◽  
Song Jiang ◽  
Kun Xu
Keyword(s):  
Radiative Transfer ◽  
Nonlinear Diffusion ◽  
Kinetic Scheme ◽  
Diffusion Limit ◽  
Radiative Transport ◽  
Time Step ◽  
Equilibrium Limit ◽  
Unified Gas Kinetic Scheme ◽  
Material Temperature ◽  
Non Equilibrium

AbstractThis paper is about the construction of a unified gas-kinetic scheme (UGKS) for a coupled system of radiative transport and material heat conduction with different diffusive limits. Different from the previous approach, instead of including absorption/emission only, the current method takes both scattering and absorption/emission mechanism into account in the radiative transport process. As a result, two asymptotic limiting solutions will appear in the diffusive regime. In the strong absorption/emission case, an equilibrium diffusion limit is obtained, where the system is mainly driven by a nonlinear diffusion equation for the equilibrium radiation and material temperature. However, in the strong scattering case, a non-equilibrium limit can be obtained, where coupled nonlinear diffusion system with different radiation and material temperature is obtained. In addition to including the scattering term in the transport equation, an implicit UGKS (IUGKS) will be developed in this paper as well. In the IUGKS, the numerical flux for the radiation intensity is constructed implicitly. Therefore, the conventional CFL constraint for the time step is released. With the use of a large time step for the radiative transport, it becomes possible to couple the IUGKS with the gas dynamic equations to develop an efficient numerical method for radiative hydrodynamics. The IUGKS is a valid method for all radiative transfer regimes. A few numerical examples will be presented to validate the current implicit method for both optical thin to optical thick cases.

A Unified Gas-Kinetic Scheme for Continuum and Rarefied Flows II: Multi-Dimensional Cases

2012 ◽  
Vol 12 (3) ◽  
pp. 662-690 ◽  
Author(s):  
Juan-Chen Huang ◽  
Kun Xu ◽  
Pubing Yu
Keyword(s):  
Kinetic Equation ◽  
Kinetic Scheme ◽  
Current Method ◽  
Flow Regimes ◽  
Navier Stokes ◽  
Rarefied Flow ◽  
One Dimensional ◽  
Flow Physics ◽  
Unified Gas Kinetic Scheme ◽  
Non Equilibrium

AbstractWith discretized particle velocity space, a multi-scale unified gas-kinetic scheme for entire Knudsen number flows has been constructed based on the kinetic model in one-dimensional case [J. Comput. Phys., vol. 229 (2010), pp. 7747-7764]. For the kinetic equation, to extend a one-dimensional scheme to multidimensional flow is not so straightforward. The major factor is that addition of one dimension in physical space causes the distribution function to become two-dimensional, rather than axially symmetric, in velocity space. In this paper, a unified gas-kinetic scheme based on the Shakhov model in two-dimensional space will be presented. Instead of particle-based modeling for the rarefied flow, such as the direct simulation Monte Carlo (DSMC) method, the philosophical principal underlying the current study is a partial-differential-equation (PDE)-based modeling. Since the valid scale of the kinetic equation and the scale of mesh size and time step may be significantly different, the gas evolution in a discretized space is modeled with the help of kinetic equation, instead of directly solving the partial differential equation. Due to the use of both hydrodynamic and kinetic scales flow physics in a gas evolution model at the cell interface, the unified scheme can basically present accurate solution in all flow regimes from the free molecule to the Navier-Stokes solutions. In comparison with the DSMC and Navier-Stokes flow solvers, the current method is much more efficient than DSMC in low speed transition and continuum flow regimes, and it has better capability than NS solver in capturing of non-equilibrium flow physics in the transition and rarefied flow regimes. As a result, the current method can be useful in the flow simulation where both continuum and rarefied flow physics needs to be resolved in a single computation. This paper will extensively evaluate the performance of the unified scheme from free molecule to continuum NS solutions, and from low speed micro-flow to high speed non-equilibrium aerodynamics. The test cases clearly demonstrate that the unified scheme is a reliable method for the rarefied flow computations, and the scheme provides an important tool in the study of non-equilibrium flow.

A Unified Gas-Kinetic Scheme for Continuum and Rarefied Flows III: Microflow Simulations

2013 ◽  
Vol 14 (5) ◽  
pp. 1147-1173 ◽  
Author(s):  
Juan-Chen Huang ◽  
Kun Xu ◽  
Pubing Yu
Keyword(s):  
Distribution Function ◽  
Kinetic Scheme ◽  
Particle Collision ◽  
Gas Distribution ◽  
Time Step ◽  
Rarefied Flow ◽  
Low Speed ◽  
Flow Computation ◽  
Unified Gas Kinetic Scheme ◽  
Non Equilibrium

AbstractDue to the rapid advances in micro-electro-mechanical systems (MEMS), the study of microflows becomes increasingly important. Currently, the molecular-based simulation techniques are the most reliable methods for rarefied flow computation, even though these methods face statistical scattering problem in the low speed limit. With discretized particle velocity space, a unified gas-kinetic scheme (UGKS) for entire Knudsen number flow has been constructed recently for flow computation. Contrary to the particle-based direct simulation Monte Carlo (DSMC) method, the unified scheme is a partial differential equation-based modeling method, where the statistical noise is totally removed. But, the common point between the DSMC and UGKS is that both methods are constructed through direct modeling in the discretized space. Due to the multiscale modeling in the unified method, i.e., the update of both macroscopic flow variables and microscopic gas distribution function, the conventional constraint of time step being less than the particle collision time in many direct Boltzmann solvers is released here. The numerical tests show that the unified scheme is more efficient than the particle-based methods in the low speed rarefied flow computation. The main purpose of the current study is to validate the accuracy of the unified scheme in the capturing of non-equilibrium flow phenomena. In the continuum and free molecular limits, the gas distribution function used in the unified scheme for the flux evaluation at a cell interface goes to the corresponding Navier-Stokes and free molecular solutions. In the transition regime, the DSMC solution will be used for the validation of UGKS results. This study shows that the unified scheme is indeed a reliable and accurate flow solver for low speed non-equilibrium flows. It not only recovers the DSMC results whenever available, but also provides high resolution results in cases where the DSMC can hardly afford the computational cost. In thermal creep flow simulation, surprising solution, such as the gas flowing from hot to cold regions along the wall surface, is observed for the first time by the unified scheme, which is confirmed later through intensive DSMC computation.

Thermal non-equilibrium effect of small-scale structures in compressible turbulence

2018 ◽  
Vol 32 (12n13) ◽  
pp. 1840013 ◽  
Author(s):  
Shi-Yi Li ◽  
Qi-Bing Li
Keyword(s):  
Isotropic Turbulence ◽  
Kinetic Scheme ◽  
Compressible Turbulence ◽  
Navier Stokes ◽  
Small Scale ◽  
Unified Gas Kinetic Scheme ◽  
Scale Structures ◽  
Equilibrium Effect ◽  
Small Scale Structures ◽  
Non Equilibrium

The thermal non-equilibrium effect of the small-scale structures in the canonical two-dimensional turbulence is studied. Comparative studies of Unified Gas Kinetic Scheme (UGKS) and GKS-Navier–Stokes (NS) for Taylor–Green flow with initial Ma = 1, Kn = 0.01 and decaying isotropic turbulence with initial [Formula: see text], [Formula: see text] show that the discrepancy exists both in small and large scales, even beyond the dissipation range to 10[Formula: see text] with accuracy to 8% in the SGS energy transfer of the decaying isotropic turbulence, illustrating the necessity for resolving the kinetic scales even at moderated [Formula: see text].

scholarly journals Large-eddy simulation of wall-bounded turbulent flow with high-order discrete unified gas-kinetic scheme

2020 ◽  
Vol 2 (1) ◽  
Author(s):  
Rui Zhang ◽  
Chengwen Zhong ◽  
Sha Liu ◽  
Congshan Zhuo
Keyword(s):  
Turbulent Flow ◽  
Turbulent Flows ◽  
Parallel Implementation ◽  
Kinetic Scheme ◽  
Cavity Flow ◽  
Equilibrium Distribution Function ◽  
Third Order ◽  
Two Stage ◽  
Large Eddy ◽  
Unified Gas Kinetic Scheme

AbstractIn this paper, we introduce the discrete Maxwellian equilibrium distribution function for incompressible flow and force term into the two-stage third-order Discrete Unified Gas-Kinetic Scheme (DUGKS) for simulating low-speed turbulent flows. The Wall-Adapting Local Eddy-viscosity (WALE) and Vreman sub-grid models for Large-Eddy Simulations (LES) of turbulent flows are coupled within the present framework. Meanwhile, the implicit LES are also presented to verify the effect of LES models. A parallel implementation strategy for the present framework is developed, and three canonical wall-bounded turbulent flow cases are investigated, including the fully developed turbulent channel flow at a friction Reynolds number (Re) about 180, the turbulent plane Couette flow at a friction Re number about 93 and lid-driven cubical cavity flow at a Re number of 12000. The turbulence statistics, including mean velocity, the r.m.s. fluctuations velocity, Reynolds stress, etc. are computed by the present approach. Their predictions match precisely with each other, and they are both in reasonable agreement with the benchmark data of DNS. Especially, the predicted flow physics of three-dimensional lid-driven cavity flow are consistent with the description from abundant literature. The present numerical results verify that the present two-stage third-order DUGKS-based LES method is capable for simulating inhomogeneous wall-bounded turbulent flows and getting reliable results with relatively coarse grids.

scholarly journals Discrete unified gas kinetic scheme on unstructured meshes

2016 ◽  
Vol 127 ◽  
pp. 211-225 ◽  
Author(s):  
Lianhua Zhu ◽  
Zhaoli Guo ◽  
Kun Xu
Keyword(s):  
Kinetic Scheme ◽  
Unstructured Meshes ◽  
Unified Gas Kinetic Scheme
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