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This paper aims at approximations of the collision operator in the Boltzmann equation. The developed framework guarantees the “normality” of the approximation, which means correct collision invariants, H-Theorem, and equilibrium solutions. It fits into the discrete velocity model framework, is given in such a way that it is understandable with undergraduate level mathematics and can be used to construct approximations with arbitrary high convergence orders. At last we give an example alongside a numerical verification. Here the convergence orders range up to 3 ( 2 ) and the time complexity is given by 3 + 1 2 ( 4 + 2 3 ) in 2 ( 3 ) dimensions.

We develop a family of second-order implicit-explicit (IMEX) schemes for the stiff Bhatnagar--Gross--Krook (BGK) kinetic equation. The method is asymptotic-preserving (can capture the Euler limit without numerically resolving the small Knudsen number) as well as positivity-preserving---a feature that is not possessed by any of the existing second- or high-order IMEX schemes. The method is based on the usual IMEX Runge--Kutta framework plus a key correction step utilizing the special structure of the BGK operator. Formal analysis is presented to demonstrate the property of the method and is supported by various numerical results. Moreover, we show that the method satisfies an entropy-decay property when coupled with suitable spatial discretizations. Additionally, we discuss the generalization of the method to some hyperbolic relaxation system and provide a strategy to extend the method to third order.

In order to solve the Boltzmann equation numerically, in the present work, we propose a new model equation to approximate the Boltzmann equation without angular cutoff. Here the approximate equation incorporates Boltzmann collision operator with angular cut-off and the Landau collision operator. As a first step, we prove the well-posedness theory for our approximate equation. Then in the next step we show the error estimate between the solutions to the approximate equation and the original equation. Compared to the standard angular cut-off approximation method, our method results in higher order of accuracy.

In this paper, we propose a novel finite volume lattice Boltzmann (FV-LB) scheme to simulate the neutron and radiative transfer problems governed by the linear Boltzmann transport equation (BTE). In the derivation of the FV-LB scheme, we utilize a multi-speed lattice Boltzmann model to reduce the linear BTE into a linear discrete velocity Boltzmann equation (DVBE). Considering the computational effectiveness and flexibility in complex geometries, we design a finite volume scheme by using the first order upwind numerical flux and solve the DVBE with the finite volume scheme on a unstructured tetrahedral or triangular mesh. The accuracy of the FV-LB scheme is verified by comparing our results with the benchmark values of three representative problems.

A lattice Boltzmann model for the Gross-Pitaevskii equation is proposed in this paper. Some numerical tests for one- and two-dimensional Gross-Pitaevskii equation have been conducted. The waves of the Gross-Pitaevskii equation are simulated. Numerical results show that the lattice Boltzmann method is an effective method for the wave of the Gross-Pitaevskii equation.

Direct numerical simulations of the transition process from steady laminar to chaotic flow are considered in this study with the relatively new incompressible lattice Boltzmann equation. Numerically, a multiple relaxation time fully incompressible lattice Boltzmann equation is implemented in a 2D driven cavity. Spatial discretization is 2nd-order accurate, and the Kolmogorov length scale estimation based on Reynolds number (Re) dictates grid resolution. Initial simulations show the method to be accurate for steady laminar flows, while higher Re simulations reveal periodic flow behavior consistent with an initial Hopf bifurcation at Re 7,988. Non-repeating flow behavior is observed in the phase space trajectories above Re 13,063, and is evidence of the transition to a chaotic flow regime. Finally, flows at Reynolds numbers above the chaotic transition point are simulated and found with statistical properties in good agreement with literature.

We present an effective asymptotic method for approximating the density of particles for kinetic equations with a Bhatnagar–Gross–Krook (BGK) relaxation operator in the large scale hyperbolic limit. The density of particles is transformed via a Hopf–Cole transformation, where the phase function is expanded as a power series with respect to the Knudsen number. The expansion terms can be determined by solving a sequence of equations. In particular, it has been proved in [3] that the leading order term is the viscosity solution of an effective Hamilton–Jacobi equation, and we show that the higher order terms can be formally determined by solving a sequence of transport equations. Both the effective Hamilton–Jacobi equation and the transport equations are independent of the Knudsen number, and are formulated in the physical space, where the effective Hamiltonian is obtained as the solution of a nonlinear equation that is given as an integral in the velocity variable, and the coefficients of the transport equations are given as integrals in the velocity variable. With appropriate Gauss quadrature rules for evaluating these integrals effectively, the effective Hamilton–Jacobi equation and the transport equations can be solved efficiently to obtain the expansion terms for approximating the density function. In this work, the zeroth, first and second order terms in the expansion are used to obtain second order accuracy with respect to the Knudsen number. The proposed method balances efficiency and accuracy, and has the potential to deal with kinetic equations with more general BGK models. Numerical experiments verify the effectiveness of the proposed method.

In this paper, we explore the possibility to derive an original lattice Boltzmann scheme for solving shallow water equations. Specifically, it is proposed to decompose the collision operator by means of a non-orthogonal basis of central moments which relax independently to a discrete equilibrium. Our method is strictly consistent with the BGK operator, as the latter is recovered exactly if all the moments relax with a common frequency. The methodology is validated against five well-consolidated established benchmark problems, showing very good agreement. Moreover, it possesses very high properties in terms of stability.

A variable property-based lattice Boltzmann flux solver (VPLBFS) is proposed in this paper for thermal flows with partial or total variation in fluid properties in the low Mach number limit. In the solver, one particle distribution function is introduced for pressure and momentum, and another for fluid temperature. The fluid properties are assumed to be functions of only the local fluid temperature. The macroscopic variables at cell centers are determined from the solution of macroscopic governing equations by the finite volume method. The fluxes at cell interfaces are evaluated by the local construction of the solution for the standard lattice Boltzmann equation. The additional terms due to fluid property variations and body forces are regarded as source terms and treated by the finite volume discretization. These features make its application on non-uniform grids with a fixed time interval more flexible in comparison with conventional lattice Boltzmann models. The VPLBFS is validated by several numerical examples, including fluid flows between two parallel plates, with one plate to be isothermally heated from a certain moment, the natural convection in a square cavity with a large temperature difference and the natural convection of supercritical carbon dioxide. Numerical results show the reliability of the VPLBFS for thermal flows with partial or total variation in fluid properties.

It was shown previously (Shan, 2010) that the lattice weights of the lattice Boltzmann method are the solutions to an optimization problem subjecting to a set of linear equality and inequality constraints. Here, a solution strategy is developed and complete solutions for such optimization problem are obtained in one-, two-, and three-dimensions up to 9th order. In addition to recovering the well-known lattices, a number of high-order lattices are also discovered for the first time. These lattices can be used to construct high-order lattice Boltzmann schemes for non-equilibrium flow simulation.

In this paper, a novel D2Q9 lattice Boltzmann model with BGK operator (LBGK) is proposed for the incompressible time-fractional Navier–Stokes equations with Caputo-type fractional derivative. First the fractional derivative is divided into the history part and the local part, in which the former is approximated using the efficient algorithm for the evaluation of the Caputo fractional derivative, while the latter is simply approximated by rectangle formula to keep the time-dependent characteristics of Navier–Stokes equations as the evolution equations. Then, a LBGK model is constructed for Navier–Stokes equations after approximation of the Caputo derivative. Through Chapman–Enskog analysis the macroscopic equations can be recovered from this model in the small Mach number limit. At the end of this paper, a numerical example with analytic solutions is carried out to show that the LBGK model is efficient.

In this paper, a simplified lattice Boltzmann method (SLBM) without evolution of the distribution function is developed for simulating incompressible viscous flows. This method is developed from the application of fractional step technique to the macroscopic Navier-Stokes (N-S) equations recovered from lattice Boltzmann equation by using Chapman-Enskog expansion analysis. In SLBM, the equilibrium distribution function is calculated from the macroscopic variables, while the non-equilibrium distribution function is simply evaluated from the difference of two equilibrium distribution functions. Therefore, SLBM tracks the evolution of the macroscopic variables rather than the distribution function. As a result, lower virtual memories are required and physical boundary conditions could be directly implemented. Through numerical test at high Reynolds number, the method shows very nice performance in numerical stability. An accuracy test for the 2D Taylor-Green flow shows that SLBM has the second-order of accuracy in space. More benchmark tests, including the Couette flow, the Poiseuille flow as well as the 2D lid-driven cavity flow, are conducted to further validate the present method; and the simulation results are in good agreement with available data in literatures.

In this work a lattice Boltzmann flux scheme for the convection-diffusion equation (CDE) is proposed. In this scheme, the fluxes across the cell interface are calculated by using the local solution of the multiple-relaxation-time lattice Boltzmann equation for CDE. The present method is suitable for simulating both isotopic and anisotropic diffusion processes. Meanwhile, through applying the midpoint time integration technique, the present method relaxes the time step constraint in the original lattice Boltzmann flux scheme. Four convection-diffusion problems are simulated to validate the present scheme. The obtained results agree well with the analytical or previous published solutions.

A novel lattice Boltzmann scheme is proposed for phonon transport based on the phonon Boltzmann equation. Through the Chapman-Enskog expansion, the phonon lattice Boltzmann equation under the gray relaxation time approximation recovers the classical Fourier's law in the diffusive limit. The numerical parameters in the lattice Boltzmann model are therefore rigorously correlated to the bulk material properties. The new scheme does not only eliminate the fictitious phonon speed in the diagonal direction of a square lattice system in the previous lattice Boltzmann models, but also displays very robust performances in predicting both temperature and heat flux distributions consistent with analytical solutions for diverse numerical cases, including steady-state and transient, macroscale and microscale, one-dimensional and multi-dimensional phonon heat transport. This method may provide a powerful numerical tool for deep studies of nonlinear and nonlocal heat transports in nanosystems.

A lattice Boltzmann model for solving the (2+1) dimensional cubic-quintic complex Ginzburg–Landau equation (CQCGLE) is proposed. Different from the classic lattice Boltzmann models, this lattice Boltzmann model is based on uniformly distributed lattice points in a two-dimensional space, and the evolution of the model is about a spatial axis rather than time. The algorithm provides advantages similar to the lattice Boltzmann method in that it is easily adapted to complex Ginzburg–Landau equations. Numerical results reproduce the phenomena of the fusion of necklace-ring pattern and the effect of non-linearity on the soliton in the CQCGLE.

A lattice Boltzmann model for spatiotemporal solitons is proposed.The model is in the cylindrical coordinate system.The vortex tori soliton in CQCGLE has been simulated. A lattice Boltzmann model for solving the three-dimensional cubic-quintic complex Ginzburg-Landau equation (CQCGLE) is proposed. Differently from the classic lattice Boltzmann models, this lattice Boltzmann model is based on uniformly distributed lattice points in a three-dimensional space, and the evolution of the model is about a spatial axis rather than time. The algorithm provides advantages similar to the lattice Boltzmann method in that it is easily adapted to complex Ginzburg-Landau equations. Examples show that the model accurately reproduces the vortex tori pattern in the CQCGLE.

A lattice Boltzmann model is developed for the simulation of the generalized nonlinear Boussinesq equation. Through adding a differential operator of the diffusion term as a source term to the evolution equation the macroscopic equation is recovered with higher-order truncation error. Detailed numerical simulations for the motion of the soliton solutions of the Boussinesq equation are performed and the numerical results agree well with the exact solutions. The results show that the lattice Boltzmann method is an efficient algorithm with excellent long-time numerical behaviors for the motion of the soliton solutions.

A new coercivity estimate on the spectral gap of the linearized Boltzmann collision operator for multiple species is proved. The assumptions on the collision kernels include hard and Maxwellian potentials under Grad's angular cutoff condition. Two proofs are given: a non-constructive one, based on the decomposition of the collision operator into a compact and a coercive part, and a constructive one, which exploits the “cross effects” coming from collisions between different species and which yields explicit constants. Furthermore, the essential spectra of the linearized collision operator and the linearized Boltzmann operator are calculated. Based on the spectral-gap estimate, the exponential convergence towards global equilibrium with explicit rate is shown for solutions to the linearized multispecies Boltzmann system on the torus. The convergence is achieved by the interplay between the dissipative collision operator and the conservative transport operator and is proved by using the hypocoercivity metho...

Lifting operators play an important role in starting a kinetic Boltzmann model from given macroscopic information. The macroscopic variables need to be mapped to the distribution functions, mesoscopic variables of the Boltzmann model. The Constrained Runs (CR) algorithm is used in the literature for the initialization of lattice Boltzmann models, special discretizations of the Boltzmann equation. It is based on the attraction of the dynamics toward the slow manifold and uses lattice Boltzmann steps to converge to the desired dynamics on the slow manifold. We focus on applying the CR algorithm to map density, average flow velocity, and temperature, the macroscopic variables, to distribution functions. Furthermore, we do not consider only lattice Boltzmann models. We want to perform the algorithm for different discretizations of the Boltzmann equation and consider a standard finite volume discretization of the one-dimensional in space Boltzmann equation with the Bhatnagar-Gross-Krook collision term.

A lattice Boltzmann model is proposed for solving the complex Ginzburg-Landau equation (CGLE) with curvilinear coordinates. The method maintains the algorithmic simplicity of the original lattice Boltzmann scheme, and does not require an interpolation or coarse-graining procedure. This lattice Boltzmann scheme is based on uniformly distributed lattice points in these curvilinear coordinate systems. The algorithm provides advantages similar to the previous lattice Boltzmann method in that it is easily adapted to CGLE. The numerical simulations show spiral wave on a disc, the surface of a sphere, and the inside of a sphere. Examples show that the model accurately reproduces the phenomena in the CGLE.

An accurate and robust lattice Boltzmann method for calculating turbulent flows by direct and large eddy simulation is developed. For the discrete velocity model, the D3Q27 model is proven to be desirable for turbulent flow simulation and then a multiple relaxation time (MRT) form is derived. To avoid unphysical kinks in turbulent quantities at grid interfaces by local mesh refinement, a correction method is developed. Coupling this imbalance correction (IBC) mesh refinement method with the D3Q27 MRT LBM, large scale turbulence simulation is effectively performed.

In this paper we present a Discontinuous Galerkin method for the Boltzmann equation. The distribution function f is approximated by a shifted Maxwellian times a polynomial in space and momentum, while the test functions are chosen as polynomials. The first property leads to consistency with the Euler limit, while the second property ensures conservation of mass, momentum and energy. The focus of the paper is on efficient algorithms for the Boltzmann collision operator. We transform between nodal, hierarchical and polar polynomial bases to reduce the inner integral operator to diagonal form.

The lattice Boltzmann method has become a standard for efficiently solving problems in fluid dynamics. While unstructured grids allow for a more efficient geometrical representation of complex boundaries, the lattice Boltzmann methods is often implemented using regular grids. Here we analyze two implementations of the lattice Boltzmann method on unstructured grids, the standard forward Euler method and the operator splitting method. We derive the evolution of the macroscopic variables by means of the Chapman-Enskog expansion, and we prove that it yields the Navier-Stokes equation and is first order accurate in terms of the temporal discretization and second order in terms of the spatial discretization. Relations between the kinetic viscosity and the integration time step are derived for both the Euler method and the operator splitting method. Finally we suggest an improved version of the bounce-back boundary condition. We test our implementations in both standard benchmark geometries and in the pore network of a real sample of a porous rock., Comment: 42 pages

A sequence of approximate linear collision models for hard-sphere and inverse-power-law gases is introduced. These models are obtained by expanding the linearized Boltzmann collision operator into series, and a practical algorithm is proposed for evaluating the coefficients in the series. The sequence is proven to be convergent to the linearized Boltzmann operator, and it established a connection between the Shakhov model and the linearized collision model. The convergence is demonstrated by solving the spatially homogeneous Boltzmann equation. By observing the magnitudes of the coefficients, simpler models are developed through removing small entries in the coefficient matrices.

We propose, analyze, and validate a lattice Boltzmann model with a cumulant collision operator. The new model is analytically and numerically shown to poses smaller errors than a moment based Multiple Relaxation Time lattice Boltzmann model. We demonstrate the usability of the cumulant lattice Boltzmann model by simulations of flow around a sphere for Reynolds numbers from 200 to 105.

The volume-averaged Navier-Stokes (VANS) equations are at the basis of numerous models used to investigate flows in porous media or systems containing multiple phases, one of which is made of solid particles. Although they are traditionally solved using the finite volume, finite difference or finite element method, the lattice Boltzmann method is an interesting alternative solver for these equations since it is explicit and highly parallelizable. In this work, we first show that the most common implementation of the VANS equations in the LBM, based on a redefined collision operator, is not valid in the case of spatially varying void fractions. This is illustrated through five test cases designed using the so-called method of manufactured solutions. We then present an LBM scheme for these equations based on a novel collision operator. Using the Chapman-Enskog expansion and the same five test cases, we show that this scheme is second-order accurate, explicit and stable for large void fraction gradients.

In this paper, we propose two initialization techniques for the lattice Boltzmann method. The first one is based on the theory of asymptotic analysis developed in [M. Junk and W.-A. Yong, Asymptotic Anal., 35(2003)]. By selecting consistent macroscopic quantities, this initialization leads to the second-order convergence for both velocity and pressure. Another one is an improvement of the consistent initial conditions proposed in [R. W. Mei, L.-S. Luo, P. Lallemand and D. d’Humières, Comput. Fluids, 35(2006)]. The improvement involves a modification of the collision term and a reconstruction step. Numerical examples confirm the accuracy and efficiency of our techniques.

The celebrated H-theorem of Boltzmann has important physical significance. The H-theorem states that entropy cannot diminish, and that the distribution function f(z, t) must tend towards its equilibrium state. In this paper, using the relationship between solutions of Boltzmann equation and the two-particle Boltzmann equation system, we obtain three forms of Two-Particle Boltzmann H-theorem from the two-particle Boltzmann equation system of BBGGKY hierarchy, and give an application example for the Two-Particle Boltzmann H-theorem. Also, the relation between entropy and the Two-Particle Boltzmann H-theorem is obtained.

Combining asymptotic analysis and weightedL2stability estimates, the convergence of lattice Boltzmann methods for the approximation of 1D convection-diffusion-reaction equations is proved. Unlike previous approaches, the proof does not require transformations to equivalent macroscopic equations.

An important result of statistical mechanics is the Boltzmann equation, which describes the evolution of the velocity distribution of a gas towards the equilibrium Maxwell distribution. We introduce the Boltzmann equation by considering a dynamical model of a two-dimensional gas consisting of hard disks. We derive the Boltzmann equation for the model and compare the behavior predicted by this equation against the actual behavior of the system as observed in computer simulations. A puzzling feature of the Boltzmann equation is that although the dynamical laws governing the gas are time-reversal invariant, the behavior predicted by the Boltzmann equation is time asymmetric. We show that this time asymmetry arises from assumptions made in the derivation of the Boltzmann equation, and we use computer simulations of the model system to investigate the circumstances under which these assumptions hold.

In this paper, a lattice Boltzmann model for cross-reaction-diffusion system is proposed. In order to recover the cross-diffusion equations by lattice Boltzmann method, we transform the equations into a complex reaction-diffusion equation. This complex reaction-diffusion equation is recovered with higher-order accuracy of the truncation error. Based on this model, two typical reaction-diffusion systems with cross-diffusion, predator-prey system and inhomogeneous Brusselator model are simulated. The numerical results show that this model can be used to simulate the cross-reaction-diffusion systems.

We propose a Bhatnagar–Gross–Krook (BGK) kinetic model in which the collision frequency is a linear combination of polynomials in the velocity variable. The coefficients of the linear combination are determined so as to enforce proper relaxation rates for a selected group of moments. The relaxation rates are obtained by a direct numerical evaluation of the full Boltzmann collision operator. The model is conservative by construction. Simulations of the problem of spatially homogeneous relaxation of hard spheres gas show improvement in accuracy of controlled moments as compared to solutions obtained by the classical BGK, ellipsoidal-statistical BGK and the Shakhov models in cases of strong deviations from continuum.

Predictor-corrector finite-difference lattice Boltzmann schemes are proposed. An approach with separate approximation of spatial derivatives in the advective term of kinetic equations and an approach when this term is replaced by a single finite difference are considered. Explicit finite-difference schemes are used at both the stages of the computation process. The lid-driven cavity flow problem and the Taylor — Green problem are solved numerically in a wide range of the Reynolds number. It is shown that the proposed schemes allow a larger time step compared to well-known upwind schemes. Mathematics Subject Classification: 76-04; 76M10; 65N30

In this study, the Lattice Boltzmann Method (LBM) is implemented through a finite-volume approach to perform 2-D, incompressible, and turbulent fluid flow analyses on structured grids. Even though the approach followed in this study necessitates more computational effort compared to the standard LBM (the so called stream and collide scheme), using the finite-volume method, the known limitations of the stream and collide scheme on lattice to be uniform and Courant-Friedrichs-Lewy (CFL) number to be one are removed. Moreover, the curved boundaries in the computational domain are handled more accurately with less effort. These improvements pave the way for the possibility of solving fluid flow problems with the LBM using coarser grids that are refined only where it is necessary and the boundary layers might be resolved better.

The exact solution to the hierarchy of nonlinear lattice Boltzmann kinetic equations, for the stationary planar Couette flow for any Knudsen number was presented by S. Ansumali et al. [Phys. Rev. Lett., 98 (2007), 124502]. In this paper, simulation results at a non-vanishing value of the Knudsen number are compared with the closed-form solutions for the higher-order moments. The order of convergence to the exact solution is also studied. The lattice Boltzmann simulations are in excellent agreement with the exact solution.

We introduce a lattice gas implementation that is based on coarse-graining a Molecular Dynamics (MD) simulation. Such a lattice gas is similar to standard lattice gases, but its collision operator is informed by an underlying MD simulation. This can be considered an optimal lattice gas implementation because it allows for the representation of any system that can be simulated with MD. We show here that equilibrium behavior of the popular lattice Boltzmann algorithm is consistent with this optimal lattice gas. This comparison allows us to make a more accurate identification of the expressions for temperature and pressure in lattice Boltzmann simulations which turn out to be related not only to the physical temperature and pressure but also to the lattice discretization. We show that for any spatial discretization we need to choose a particular temporal discretization to recover the lattice Boltzmann equilibrium., 13 pages, 11 figures

In this paper, different force treatments are analyzed in detail for a pseudopotential lattice Boltzmann equation (LBE), and the contribution of third-order error terms to pressure tensor with a force scheme is analyzed by a higher-order Chapman-Enskog expansion technique. From the theoretical analysis, the performance of the original force treatment of Shan-Chen (SC), Ladd, Guo et al., and the exact difference method (EDM) are ${\ensuremath{\varepsilon}}_{\mathrm{Ladd}}l{\ensuremath{\varepsilon}}_{\mathrm{Guo}}l{\ensuremath{\varepsilon}}_{\mathrm{EDM}}\ensuremath{\le}{\ensuremath{\varepsilon}}_{\mathrm{SC}}$ with the relaxation time $\ensuremath{\tau}\ensuremath{\ge}1$, while ${\ensuremath{\varepsilon}}_{\mathrm{Ladd}}l{\ensuremath{\varepsilon}}_{\mathrm{Guo}}l{\ensuremath{\varepsilon}}_{\mathrm{SC}}l{\ensuremath{\varepsilon}}_{\mathrm{EDM}}$ with $\ensuremath{\tau}l1$; here $\ensuremath{\varepsilon}$ is a parameter related to the mechanical stability and the subscripts are the corresponding force scheme. To be consistent with the thermodynamic theory, a force term is introduced to modify the coefficients in the pressure tensor. Some numerical simulations are conducted to show that the predictions of modified force treatment of the pseudopotential LBE are all in good agreement with the analytical solution and other predictions.

In this paper, a variant of the acoustic multipole source (AMS) method is proposed within the framework of the lattice Boltzmann method. A quadrupole term is directly included in the stress system (equilibrium momentum flux), and the dependency of the quadrupole source in the inviscid limit upon the fortuitous discretization error reported in the works of E. M. Viggen [Phys. Rev. E 87, 023306 (2013)PLEEE81539-375510.1103/PhysRevE.87.023306] is removed. The regularized lattice Boltzmann (RLB) method with this variant AMS method is presented for the 2D and 3D acoustic problems in the inviscid limit, and without loss of generality, the D3Q19 model is considered in this work. To assess the accuracy and the advantage of the RLB scheme with this AMS for acoustic point sources, the numerical investigations and comparisons with the multiple-relaxation-time (MRT) models and the analytical solutions are performed on the 2D and 3D acoustic multipole point sources in the inviscid limit, including monopoles, x dipoles, and xx quadrupoles. From the present results, the good precision of this AMS method is validated, and the RLB scheme exhibits some superconvergence properties for the monopole sources compared with the MRT models, and both the RLB and MRT models have the same accuracy for the simulations of acoustic dipole and quadrupole sources. To further validate the capability of the RLB scheme with AMS, another basic acoustic problem, the acoustic scattering from a solid cylinder presented at the Second Computational Aeroacoustics Workshop on Benchmark Problems, is numerically considered. The directivity pattern of the acoustic field is computed at r=7.5; the present results agree well with the exact solutions. Also, the effects of slip and no-slip wall treatments within the regularized boundary condition on this pure acoustic scattering problem are tested, and compared with the exact solution, the slip wall treatment can present a better result. All simulations demonstrate that the RLB scheme with the AMS method is capable of accurately simulating 2D and 3D acoustic generation, propagation, and scattering at zero viscosity.

In this paper, a novel model is proposed to investigate the neutron transport in scattering and absorbing medium. This solution to the linear Boltzmann equation is expanded from the idea of lattice Boltzmann method (LBM) with the collision and streaming process. The theoretical derivation of lattice Boltzmann model for transient neutron transport problem is proposed for the first time. The fully implicit backward difference scheme is used to ensure the numerical stability, and relaxation time and equilibrium particle distribution function are obtained. To validate the new lattice Boltzmann model, the LBM formulation is tested for a homogenous media with different sources, and both transient and steady-state LBM results get a good agreement with the benchmark solutions.

Applications of the transient Boltzmann transport equation (BTE) have undergone much investigation, such as radiative heat transfer and neutron transport. This paper provides a lattice Boltzmann model to efficiently resolve the multidimensional transient BTE. For a higher angular resolution, enough transport directions are considered while the transient BTE in each direction is treated as a conservation law equation and solved independently. Both macroscopic equations recovered from a Chapman-Enskog expansion and simulated results of typical benchmark problems show not only the second-order accuracy but also the flexibility and applicability of the proposed lattice Boltzmann model. This approach may contribute a powerful technique for the parallel simulation of large-scale engineering and some alternative perspectives for solving the nonlinear transport problem further.

In this work, we propose a new Galerkin-Petrov method for the numerical solution of the classical spatially homogeneous Boltzmann equation. This method is based on an approximation of the distribution function by associated Laguerre polynomials and spherical harmonics and test an a variational manner with globally defined three-dimensional polynomials. A numerical realization of the algorithm is presented. The algorithmic developments are illustrated with the help of several numerical tests., Comment: 44 pages, 13 figures

We present a high-order, fully explicit, asymptotic-preserving projective integration scheme for the nonlinear BGK equation. The method first takes a few small (inner) steps with a simple, explicit method (such as direct forward Euler) to damp out the stiff components of the solution. Then, the time derivative is estimated and used in an (outer) Runge-Kutta method of arbitrary order. Based on the spectrum of the linearized BGK operator, we deduce that, with an appropriate choice of inner step size, the time step restriction on the outer time step as well as the number of inner time steps is independent of the stiffness of the BGK source term. We illustrate the method with numerical results in one and two spatial dimensions., Comment: Proceedings FVCA 8