A semi-Lagrangian finite difference WENO scheme for scalar nonlinear conservation laws
Introduction
We develop a semi-Lagrangian finite difference WENO (SL-WENO) scheme to approximate the scalar conservation law with the possibly nonlinear flux . Moreover, multiple space dimensions will be treated using operator splitting.
The semi-Lagrangian computational scheme uses a mixture of Eulerian and Lagrangian reference frames in the sense that it has a fixed (Eulerian) numerical grid but advances each time step by evolving the partial differential equations by propagating information along (Lagrangian) characteristic curves. A semi-Lagrangian scheme does not inherently suffer the CFL time step restriction of an Eulerian approach. In consequence, longer time step evolution can be achieved, potentially resulting in less computational effort and less numerical diffusion.
In 2011, Qiu and Shu [1] (see also [2]) developed a semi-Lagrangian finite difference WENO method for linear scalar conservation laws for which for some advection velocity . In fact, they could develop the linear weights needed in the WENO reconstruction [3], [4] only when the velocity a is constant. For variable advection velocity, they developed weights based on heuristic empirical criteria.
In this paper we develop a locally mass conservative SL-WENO scheme for nonlinear scalar conservation laws. In the linear case, it is an extension of the scheme of Qiu and Shu [1]. It reduces to their scheme if the advection velocity a is constant and linear WENO weights are used; however, our scheme has exact WENO linear weights in the case of variable advection problems. Our scheme is new in the case of nonlinear problems, and it can be viewed as an extension of related work on finite volume Eulerian–Lagrangian WENO schemes [5], [6], in which characteristic tracing is incorporated only in an approximate way. For simplicity of exposition, we will describe only the third order SL-WENO scheme (SL-WENO3), but in principle any order scheme can be implemented.
Our finite difference scheme is based on the standard sliding average formulation of Shu and Osher [7], [8], but the flux function is defined using WENO reconstructions of (semi-Lagrangian) characteristic tracings of grid points [1]. To handle nonlinear problems for which the trace velocity in unknown, we use a fixed, approximate trace velocity (as is done in, e.g., [9], [6] and in arbitrary Lagrangian–Eulerian (ALE) schemes [9]). This approximate trace velocity is locally frozen to avoid numerical difficulties [6]. The introduction of an approximate trace velocity means that we do not trace the exact characteristic curves, and necessitates the use of a flux correction step [6], which is implemented using the approach of Levy, Puppo, and Russo [10] developed for CWENO schemes.
Unlike the direct WENO reconstructions used by Qiu and Shu [1], we develop a two-stage WENO reconstruction procedure. We first reconstruct cell averages of the numerical flux function, and then we reconstruct the flux function at the needed point. In our procedure, the linear WENO weights always exist. We nonlinearly weight at each stage using a smoothness indicator that is based on the underlying smoothness of the solution u; that is, rather than using the smoothness of the cell average fluxes in the second stage, we use the smoothness indicator for u in both stages.
For nonlinear problems, the scheme requires a special upstream bias in the computation of the flux function between two grid points, since nonsmooth shocks may develop. The Rankine–Hugoniot shock speed is computed assuming a shock based on the solution u between the two grid points (i.e., the Roe speed [11]). The direction of this assumed shock determines the wave direction and consequent upstream biasing in our scheme.
The flux correction step requires a relaxed CFL constraint, given later in (42) [12], [6]. The constraint is based on the speed of the difference between the true value of and the fixed approximate trace velocity. If these are approximately equal, the constraint is very mild.
For multi-dimensional problems, we use a standard, usually second order Strang splitting which decouples the differential equation into a sequence of one-dimensional problems (see (53)). The advantage of a finite difference scheme is that the splitting error manifests itself as temporal error, because the splitting does not induce a shearing of the variable coefficients, and so the scheme maintains its formal high order spatial accuracy [2].
We present a detailed description of our formally third order SL-WENO3 scheme in the next section. Sections 3 and 4 are devoted to numerical results of SL-WENO3 and SL-WENO5 in one and two space dimensions, respectively. In Section 5 we apply our new scheme to two models of plasma transport: (1) the Vlasov–Poisson system, which has a constant advection within the Strang split scheme, and (2) the guiding-center model [13], [14], which has a nonconstant advection. We conclude briefly in the last section.
Section snippets
The semi-Lagrangian finite difference WENO scheme
The development of our semi-Lagrangian finite difference WENO (SL-WENO) scheme for nonlinear problems parallels and extends the development of the finite difference scheme given by Qiu and Shu [1] for linear problems. For simplicity of exposition, we describe the third order SL-WENO scheme.
Partition time and space as and , respectively, where is constant. We approximate as . Integrate (1) over and evaluate at a grid point to obtain
Some numerical results in one space dimension
We present several examples of our numerical scheme to test its accuracy and performance. Some of the examples are for the linear equation for which we specify only the velocity . In that case, unless stated otherwise, we use the simple trace velocity .
Some numerical results in two space dimensions
There are standard ways to apply our finite difference scheme to two-space dimensional problems. For we might simply compute the analogue of (7), which involves a sliding average function in the y-variable . The conservative scheme is based on However, we prefer to view the treatment of space as a Strang splitting. For a first order Strang splitting, we compute the two-stage scheme
The Vlasov–Poisson (VP) system in one space dimension
The well-known nondimensionalized Vlasov–Poisson (VP) system in one space dimension is where x and v are the coordinates in the phase space , E is the electric field, ϕ is the self-consistent electrostatic potential, and is the probability distribution function describing the probability of finding a particle with velocity v at position x at time t. The probability distribution function couples to long ranges fields via the
Conclusions
We developed a new semi-Lagrangian finite difference WENO scheme for nonlinear scalar conservation laws in one or more space dimensions. The scheme is locally mass conservative, formally high-order accurate in space, has small time truncation error, and is essentially non-oscillatory. It is subject to a relaxed CFL time step stability condition (42). The scheme can be considered as an extension of the SL-WENO scheme of Qiu and Shu [1] for linear problems, and as a finite difference version of
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Supported in part by Taiwan National Science Council grant NSC 102-2115-M-110-010-MY3.
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Supported in part by U.S.National Science Foundation grant DMS-1418752.