3,554 research outputs found
A class of residual distribution schemes and their relation to relaxation systems
Residual distributions (RD) schemes are a class of of high-resolution finite
volume methods for unstructured grids. A key feature of these schemes is that
they make use of genuinely multidimensional (approximate) Riemann solvers as
opposed to the piecemeal 1D Riemann solvers usually employed by finite volume
methods. In 1D, LeVeque and Pelanti [J. Comp. Phys. 172, 572 (2001)] showed
that many of the standard approximate Riemann solver methods (e.g., the Roe
solver, HLL, Lax-Friedrichs) can be obtained from applying an exact Riemann
solver to relaxation systems of the type introduced by Jin and Xin [Comm. Pure
Appl. Math. 48, 235 (1995)]. In this work we extend LeVeque and Pelanti's
results and obtain a multidimensional relaxation system from which
multidimensional approximate Riemann solvers can be obtained. In particular, we
show that with one choice of parameters the relaxation system yields the
standard N-scheme. With another choice, the relaxation system yields a new
Riemann solver, which can be viewed as a genuinely multidimensional extension
of the local Lax-Friedrichs scheme. This new Riemann solver does not require
the use Roe-Struijs-Deconinck averages, nor does it require the inversion of an
m-by-m matrix in each computational grid cell, where is the number of
conserved variables. Once this new scheme is established, we apply it on a few
standard cases for the 2D compressible Euler equations of gas dynamics. We show
that through the use of linear-preserving limiters, the new approach produces
numerical solutions that are comparable in accuracy to the N-scheme, despite
being computationally less expensive.Comment: 46 pages, 14 figure
Solving the Boltzmann equation in N log N
In [C. Mouhot and L. Pareschi, "Fast algorithms for computing the Boltzmann
collision operator," Math. Comp., to appear; C. Mouhot and L. Pareschi, C. R.
Math. Acad. Sci. Paris, 339 (2004), pp. 71-76], fast deterministic algorithms
based on spectral methods were derived for the Boltzmann collision operator for
a class of interactions including the hard spheres model in dimension three.
These algorithms are implemented for the solution of the Boltzmann equation in
two and three dimension, first for homogeneous solutions, then for general non
homogeneous solutions. The results are compared to explicit solutions, when
available, and to Monte-Carlo methods. In particular, the computational cost
and accuracy are compared to those of Monte-Carlo methods as well as to those
of previous spectral methods. Finally, for inhomogeneous solutions, we take
advantage of the great computational efficiency of the method to show an
oscillation phenomenon of the entropy functional in the trend to equilibrium,
which was suggested in the work [L. Desvillettes and C. Villani, Invent. Math.,
159 (2005), pp. 245-316].Comment: 32 page
Meshfree finite differences for vector Poisson and pressure Poisson equations with electric boundary conditions
We demonstrate how meshfree finite difference methods can be applied to solve
vector Poisson problems with electric boundary conditions. In these, the
tangential velocity and the incompressibility of the vector field are
prescribed at the boundary. Even on irregular domains with only convex corners,
canonical nodal-based finite elements may converge to the wrong solution due to
a version of the Babuska paradox. In turn, straightforward meshfree finite
differences converge to the true solution, and even high-order accuracy can be
achieved in a simple fashion. The methodology is then extended to a specific
pressure Poisson equation reformulation of the Navier-Stokes equations that
possesses the same type of boundary conditions. The resulting numerical
approach is second order accurate and allows for a simple switching between an
explicit and implicit treatment of the viscosity terms.Comment: 19 pages, 7 figure
Pointwise Green's function bounds and stability of relaxation shocks
We establish sharp pointwise Green's function bounds and consequent
linearized and nonlinear stability for smooth traveling front solutions, or
relaxation shocks, of general hyperbolic relaxation systems of dissipative
type, under the necessary assumptions ([G,Z.1,Z.4]) of spectral stability,
i.e., stable point spectrum of the linearized operator about the wave, and
hyperbolic stability of the corresponding ideal shock of the associated
equilibrium system. This yields, in particular, nonlinear stability of weak
relaxation shocks of the discrete kinetic Jin--Xin and Broadwell models. The
techniques of this paper should have further application in the closely related
case of traveling waves of systems with partial viscosity, for example in
compressible gas dynamics or MHD.Comment: 120 pages. Changes since original submission. Corrected typos, esp.
energy estimates of Section 7, corrected bad forward references, expanded
Remark 1.17, end of introductio
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