A simple robust and accurate a posteriori sub-cell finite volume limiter for the discontinuous Galerkin method on unstructured meshes

In this paper we propose a simple, robust and accurate nonlinear a posteriori stabilization of the Discontinuous Galerkin (DG) finite element method for the solution of nonlinear hyperbolic PDE systems on unstructured triangular and tetrahedral meshes in two and three space dimensions. This novel a posteriori limiter, which has been recently proposed for the simple Cartesian grid case in [62], is able to resolve discontinuities at a sub-grid scale and is substantially extended here to general unstructured simplex meshes in 2D and 3D. It can be summarized as follows: At the beginning of each time step, an approximation of the local minimum and maximum of the discrete solution is computed for each cell, taking into account also the vertex neighbors of an element. Then, an unlimited discontinuous Galerkin scheme of approximation degree N is run for one time step to produce a so-called candidate solution. Subsequently, an a posteriori detection step checks the unlimited candidate solution at time t(n+1) for positivity, absence of floating point errors and whether the discrete solution has remained within or at least very close to the bounds given by the local minimum and maximum computed in the first step. Elements that do not satisfy all the previously mentioned detection criteria are flagged as troubled cells. For these troubled cells, the candidate solution is discarded as inappropriate and consequently needs to be recomputed. Within these troubled cells the old discrete solution at the previous time t(n) is scattered onto small sub-cells (N-s = 2N + 1 sub-cells per element edge), in order to obtain a set of sub-cell averages at time t(n). Then, a more robust second order TVD finite volume scheme is applied to update the sub-cell averages within the troubled DG cells from time t(n) to time t(n+1). The new sub-grid data at time t(n+1) are finally gathered back into a valid cell-centered DG polynomial of degree Nby using a classical conservative and higher order accurate finite volume reconstruction technique. Consequently, if the number N-s is sufficiently large (N-s >= N + 1), the subscale resolution capability of the DG scheme is fully maintained, while preserving at the same time an essentially non-oscillatorybehavior of the solution at discontinuities. Many standard DG limiters only adjust the discrete solution in troubled cells, based on the limiting of higher order moments or by applying a nonlinear WENO/HWENO reconstructionon the data at the new time t(n+1). Instead, our new DG limiter entirely recomputes the troubled cells by solving the governing PDE system again starting from valid data at the old time level t(n), but using this time a more robust scheme on the sub-grid level. In other words, the piecewise polynomials produced by the new limiter are the result of a more robust solution of the PDE system itself, while most standard DG limiters are simply based on a mere nonlinear data post-processing of the discrete solution. Technically speaking, the new method corresponds to an element-wise checkpointing and restarting of the solver, using a lower order scheme on the sub-grid. As a result, the present DG limiter is even able to curefloating point errors like NaN values that have occurred after divisions by zero or after the computation of roots from negative numbers. This is a unique feature of our new algorithm among existing DG limiters. The new a posteriori sub-cell stabilization approach is developed within a high order accurate one-step ADER-DG framework on multidimensional unstructured meshes for hyperbolic systems of conservation laws as well as for hyperbolicPDE with non-conservative products. The method is applied to the Euler equations of compressible gas dynamics, to the ideal magneto-hydrodynamics equations (MHD) as well as to the seven-equation Baer-Nunziato model of compressible multi-phase flows. A large set of standard test problems is solved in order to assess the accuracy and robustness of the new limiter. (C) 2016 The Authors. Published by Elsevier Inc.