31 research outputs found
Analysis of an asymptotic preserving scheme for linear kinetic equations in the diffusion limit
We present a mathematical analysis of the asymptotic preserving scheme
proposed in [M. Lemou and L. Mieussens, SIAM J. Sci. Comput., 31, pp. 334-368,
2008] for linear transport equations in kinetic and diffusive regimes. We prove
that the scheme is uniformly stable and accurate with respect to the mean free
path of the particles. This property is satisfied under an explicitly given CFL
condition. This condition tends to a parabolic CFL condition for small mean
free paths, and is close to a convection CFL condition for large mean free
paths. Ou r analysis is based on very simple energy estimates
Time--Splitting Schemes and Measure Source Terms for a Quasilinear Relaxing System
Several singular limits are investigated in the context of a
system arising for instance in the modeling of chromatographic processes. In
particular, we focus on the case where the relaxation term and a
projection operator are concentrated on a discrete lattice by means of Dirac
measures. This formulation allows to study more easily some time-splitting
numerical schemes
Space-time FLAVORS: finite difference, multisymlectic, and pseudospectral integrators for multiscale PDEs
We present a new class of integrators for stiff PDEs. These integrators are
generalizations of FLow AVeraging integratORS (FLAVORS) for stiff ODEs and SDEs
introduced in [Tao, Owhadi and Marsden 2010] with the following properties: (i)
Multiscale: they are based on flow averaging and have a computational cost
determined by mesoscopic steps in space and time instead of microscopic steps
in space and time; (ii) Versatile: the method is based on averaging the flows
of the given PDEs (which may have hidden slow and fast processes). This
bypasses the need for identifying explicitly (or numerically) the slow
variables or reduced effective PDEs; (iii) Nonintrusive: A pre-existing
numerical scheme resolving the microscopic time scale can be used as a black
box and easily turned into one of the integrators in this paper by turning the
large coefficients on over a microscopic timescale and off during a mesoscopic
timescale; (iv) Convergent over two scales: strongly over slow processes and in
the sense of measures over fast ones; (v) Structure-preserving: for stiff
Hamiltonian PDEs (possibly on manifolds), they can be made to be
multi-symplectic, symmetry-preserving (symmetries are group actions that leave
the system invariant) in all variables and variational
A radiation-hydrodynamics scheme valid from the transport to the diffusion limit
We present in this paper the numerical treatment of the coupling between
hydrodynamics and radiative transfer. The fluid is modeled by classical
conservation laws (mass, momentum and energy) and the radiation by the grey
moment system. The scheme introduced is able to compute accurate
numerical solution over a broad class of regimes from the transport to the
diffusive limits. We propose an asymptotic preserving modification of the HLLE
scheme in order to treat correctly the diffusion limit. Several numerical
results are presented, which show that this approach is robust and have the
correct behavior in both the diffusive and free-streaming limits. In the last
numerical example we test this approach on a complex physical case by
considering the collapse of a gas cloud leading to a proto-stellar structure
which, among other features, exhibits very steep opacity gradients.Comment: 29 pages, submitted to Journal of Computational physic
Asymptotic-preserving projective integration schemes for kinetic equations in the diffusion limit
We investigate a projective integration scheme for a kinetic equation in the
limit of vanishing mean free path, in which the kinetic description approaches
a diffusion phenomenon. The scheme first takes a few small steps with a simple,
explicit method, such as a spatial centered flux/forward Euler time
integration, and subsequently projects the results forward in time over a large
time step on the diffusion time scale. We show that, with an appropriate choice
of the inner step size, the time-step restriction on the outer time step is
similar to the stability condition for the diffusion equation, whereas the
required number of inner steps does not depend on the mean free path. We also
provide a consistency result. The presented method is asymptotic-preserving, in
the sense that the method converges to a standard finite volume scheme for the
diffusion equation in the limit of vanishing mean free path. The analysis is
illustrated with numerical results, and we present an application to the
Su-Olson test
High Order Asymptotic Preserving DG-IMEX Schemes for Discrete-Velocity Kinetic Equations in a Diffusive Scaling
In this paper, we develop a family of high order asymptotic preserving
schemes for some discrete-velocity kinetic equations under a diffusive scaling,
that in the asymptotic limit lead to macroscopic models such as the heat
equation, the porous media equation, the advection-diffusion equation, and the
viscous Burgers equation. Our approach is based on the micro-macro
reformulation of the kinetic equation which involves a natural decomposition of
the equation to the equilibrium and non-equilibrium parts. To achieve high
order accuracy and uniform stability as well as to capture the correct
asymptotic limit, two new ingredients are employed in the proposed methods:
discontinuous Galerkin spatial discretization of arbitrary order of accuracy
with suitable numerical fluxes; high order globally stiffly accurate
implicit-explicit Runge-Kutta scheme in time equipped with a properly chosen
implicit-explicit strategy. Formal asymptotic analysis shows that the proposed
scheme in the limit of epsilon -> 0 is an explicit, consistent and high order
discretization for the limiting equation. Numerical results are presented to
demonstrate the stability and high order accuracy of the proposed schemes
together with their performance in the limit
Numerical methods for one-dimensional aggregation equations
We focus in this work on the numerical discretization of the one dimensional
aggregation equation \pa_t\rho + \pa_x (v\rho)=0, , in the
attractive case. Finite time blow up of smooth initial data occurs for
potential having a Lipschitz singularity at the origin. A numerical
discretization is proposed for which the convergence towards duality solutions
of the aggregation equation is proved. It relies on a careful choice of the
discretized macroscopic velocity in order to give a sense to the product . Moreover, using the same idea, we propose an asymptotic preserving
scheme for a kinetic system in hyperbolic scaling converging towards the
aggregation equation in hydrodynamical limit. Finally numerical simulations are
provided to illustrate the results