Generalized Riemann Problem Solver Research Articles

A cell-centered spatiotemporal coupled method for the compressible Euler equations

A cell-centered spatiotemporal coupled method is developed to solve the compressible Euler equations. The spatial discretization is performed using an improved weighted essentially non-oscillation scheme, where the Harten–Lax–van Leer–contact approximate Riemann solver is used for computing the numerical fluxes. A two-stage fourth-order scheme is adopted to carry out time advancement for unsteady problems. The proposed method is featured by spatiotemporal coupling time-stepping that can be generalized without using the case-dependent generalized Riemann problem solver. A number of one- and two-dimensional test cases are presented to demonstrate the performance of the proposed method for solving the compressible Euler equations on structured grids. The numerical results indicate that the novel method can achieve relatively large Courant–Friedrichs–Lewy (CFL) number compared to other studies that implement the two-stage fourth-order scheme, and that it is more capable of capturing small-scale flow structures than the Runge–Kutta (RK) method.

Multidimensional Generalized Riemann Problem Solver for Maxwell’s Equations

Approximate multidimensional Riemann solvers are essential building blocks in designing globally constraint-preserving finite volume time domain and discontinuous Galerkin time domain schemes for computational electrodynamics (CED). In those schemes, we can achieve high-order temporal accuracy with the help of Runge–Kutta or ADER time-stepping. This paper presents the design of a multidimensional approximate generalized Riemann problem (GRP) solver for the first time. The multidimensional Riemann solver accepts as its inputs the four states surrounding an edge on a structured mesh, and its output consists of a resolved state and its associated fluxes. In contrast, the multidimensional GRP solver accepts as its inputs the four states and their gradients in all directions; its output consists of the resolved state and its corresponding fluxes and the gradients of the resolved state. The gradients can then be used to extend the solution in time. As a result, we achieve second-order temporal accuracy in a single step. In this work, the formulation is optimized for linear hyperbolic systems with stiff, linear source terms because such a formulation will find maximal use in CED. Our formulation produces an overall constraint-preserving time-stepping strategy based on the GRP that is provably L-stable in the presence of stiff source terms. We present several stringent test problems, showing that the multidimensional GRP solver for CED meets its design accuracy and performs stably with optimal time steps. The test problems include cases with high conductivity, showing that the beneficial L-stability is indeed realized in practical applications.

A New Numerical Implementation for Solar Coronal Modeling by an HLL Generalized Riemann Problem Solver

In this paper, we employ a Harten–Lax–van Leer (HLL) generalized Riemann problem (HLL-GRP) solver within the framework of a finite volume method to model 3D solar coronal structures for the first time. Based on the rotational invariance of magnetohydrodynamics (MHD) equations, the HLL-GRP solver is successfully implemented into 3D MHD simulations. To constrain the divergence of the magnetic field, the locally divergence-free weighted-least-squares-based essentially nonoscillatory reconstruction and the properly discretized Godunov–Powell source term are applied. To keep density and pressure positive, a positivity-preserving limiter is added to the reconstructed polynomials of density and pressure. We first test a 3D blast wave problem to preliminarily validate the effectiveness of the proposed scheme on Cartesian structured grid. Then, we further run our code on a six-component grid to numerically study the steady-state coronal structures of Carrington rotation 2218 during the solar minimum phase. A comparison with the two-stage Runge–Kutta scheme is performed for both the 3D blast wave problem and solar coronal problem. Numerical results of large-scale solar coronal structures are basically consistent with the observational characteristics, indicating the robustness of the proposed model.

One-sided GRP solver and numerical boundary conditions for compressible fluid flows

In the computation of compressible fluid flows, numerical boundary conditions are always necessary for all physical variables at computational boundaries while just partial physical variables are often prescribed as physical boundary conditions. Certain extrapolation technique or ghost cells are often employed traditionally for this issue but spurious wave reflections often arise to cause numerical instability. In this paper, we associate this issue with the one-sided generalized Riemann problem (GRP) solver motivated by the accelerated piston problem in gas dynamics so that the extrapolation technique can be actually avoided. In fact, the compatibility arguments naturally require to formulate the one-sided generalized Riemann problem and incorporate it into the numerical procedure of boundary conditions. Once the corresponding solver is available, the boundary control volume is treated in the same way as the usual interior control volume. As far as the interaction of nonlinear waves with physical boundaries, such a one-sided GRP solver shows significant benefits, as numerical experiments demonstrate, on avoiding spurious wave reflections at the computational boundaries.

The simulation of compressible multi-fluid flows by a GRP-based energy-splitting method

This paper serves to simulate compressible multi-fluid flows using an energy-splitting method based on the generalized Riemann problem (GRP) solver. The emphasis is put on two-dimensional problems to address the fundamental role of transversal effect in the construction of numerical fluxes, which exhibits the genuine multi-dimensionality of the underlying numerical schemes. In contrast with the popularly-used methods of line which use exact or approximate Riemann solvers, the GRP solver, as the discontinuous version of the Lax–Wendroff method, takes the temporal-spatial coupling process to include the transversal effect. Combined with the energy-splitting technique developed earlier in Lei and Li [24], the resolution of flow fields around material interfaces is improved significantly.

A two-stage fourth-order discontinuous Galerkin method based on the GRP solver for the compressible euler equations

We develop a new fourth-order discontinuous Galerkin method using the generalized Riemann problem (GRP) solver based on the framework of the two-stage fourth-order accurate temporal discretization, with special application to compressible Euler equations. The appealing advantage of the two-stage fourth-order accurate temporal discretization is that it only takes a two-stage time-stepping to achieve the expected fourth-order accuracy. It turns out that the computational cost can be considerably reduced by more than 50% compared with the same order multi-stage SSP Runge–Kutta DG method. A number of test cases are presented to assess the accuracy and the efficiency of the two-stage fourth-order GRP-DG method, which demonstrates its appealing features to be an alternative approach for solving the unsteady compressible Euler equations.

An efficient, second order accurate, universal generalized Riemann problem solver based on the HLLI Riemann solver

The Riemann problem, and the associated generalized Riemann problem, are increasingly seen as the important building blocks for modern higher order Godunov-type schemes. In the past, building a generalized Riemann problem solver was seen as an intricately mathematical task because the associated Riemann problem is different for each hyperbolic system of interest. This paper changes that situation.The HLLI Riemann solver is a recently-proposed Riemann solver that is universal in that it is applicable to any hyperbolic system, whether in conservation form or with non-conservative products. The HLLI Riemann solver is also complete in the sense that if it is given a complete set of eigenvectors, it represents all waves with minimal dissipation. It is, therefore, very attractive to build a generalized Riemann problem solver version of the HLLI Riemann solver. This is the task that is accomplished in the present paper. We show that at second order, the generalized Riemann problem version of the HLLI Riemann solver is easy to design. Our GRP solver is also complete and universal because it inherits those good properties from original HLLI Riemann solver. We also show how our GRP solver can be adapted to the solution of hyperbolic systems with stiff source terms.Our generalized HLLI Riemann solver is easy to implement and performs robustly and well over a range of test problems. All implementation-related details are presented. Results from several stringent test problems are shown. These test problems are drawn from many different hyperbolic systems, and include hyperbolic systems in conservation form; with non-conservative products; and with stiff source terms. The present generalized Riemann problem solver performs well on all of them.

A non-oscillatory energy-splitting method for the computation of compressible multi-fluid flows

This paper proposes a new non-oscillatory energy-splitting conservative algorithm for computing multi-fluid flows in the Eulerian framework. In comparison with existing multi-fluid algorithms in the literature, it is shown that the mass fraction model with isobaric hypothesis is a plausible choice for designing numerical methods for multi-fluid flows. Then we construct a conservative Godunov-based scheme with the high order accurate extension by using the generalized Riemann problem solver, through the detailed analysis of kinetic energy exchange when fluids are mixed under the hypothesis of isobaric equilibrium. Numerical experiments are carried out for the shock-interface interaction and shock-bubble interaction problems, which display the excellent performance of this type of schemes and demonstrate that nonphysical oscillations are suppressed around material interfaces substantially.

A Hermite WENO reconstruction for fourth order temporal accurate schemes based on the GRP solver for hyperbolic conservation laws

This paper develops a new fifth order accurate Hermite WENO (HWENO) reconstruction method for hyperbolic conservation schemes in the framework of the two-stage fourth order accurate temporal discretization in Li and Du (2016) [13]. Instead of computing the first moment of the solution additionally in the conventional HWENO or DG approach, we can directly take the interface values, which are already available in the numerical flux construction using the generalized Riemann problem (GRP) solver, to approximate the first moment. The resulting scheme is fourth order temporal accurate by only invoking the HWENO reconstruction twice so that it becomes more compact. Numerical experiments show that such compactness makes significant impact on the resolution of nonlinear waves.

A fully discrete ALE method over untwisted time–space control volumes

SummaryIn this paper, a fully discrete high‐resolution arbitrary Lagrangian–Eulerian (ALE) method is developed over untwisted time–space control volumes. In the framework of the finite volume method, 2D Euler equations are discretized over untwisted moving control volumes, and the resulting numerical flux is computed using the generalized Riemann problem solver. Then, the fluid flows between meshes at two successive time steps can be updated without a remapping process in the classic ALE method. This remapping‐free ALE method directly couples the mesh motion into a physical variable update to reflect the temporal evolution in the whole process. An untwisted moving mesh is generated in terms of the vorticity‐free part of the fluid velocity according to the Helmholtz theorem. Some typical numerical tests show the competitive performance of the current method. Copyright © 2016 John Wiley & Sons, Ltd.

A Direct Eulerian GRP Scheme for Spherically Symmetric General Relativistic Hydrodynamics

The paper proposes a second-order accurate direct Eulerian generalized Riemann problem (GRP) scheme for the spherically symmetric general relativistic hydrodynamical (RHD) equations and a second-order accurate discretization for the spherically symmetric Einstein (SSE) equations. The former is directly using the Riemann invariants and the Runkine-Hugoniot jump conditions to analytically resolve the left and right nonlinear waves of the local GRP in the Eulerian formulation together with the local change of the metrics to obtain the limiting values of the time derivatives of the conservative variables along the cell interface and the numerical flux for the GRP scheme. While the latter utilizes the energy-momentum tensor obtained in the GRP solver to evaluate the fluid variables in the SSE equations and keeps the continuity of the metrics at the cell interfaces. Several numerical experiments show that the GRP scheme can achieve second-order accuracy and high resolution, and is effective for spherically symmetric general RHD problems.

Implicit, semi-analytical solution of the generalized Riemann problem for stiff hyperbolic balance laws

We present a semi-analytical, implicit solution to the generalized Riemann problem (GRP) for non-linear systems of hyperbolic balance laws with stiff source terms. The solution method is based on an implicit, time Taylor series expansion and the Cauchy–Kowalewskaya procedure, along with the solution of a sequence of classical Riemann problems. Our new GRP solver is then used to construct locally implicit ADER methods of arbitrary accuracy in space and time for solving the general initial–boundary value problem for non-linear systems of hyperbolic balance laws with stiff source terms. Analysis of the method for model problems is carried out and empirical convergence rate studies for suitable tests problems are performed, confirming the theoretically expected high order of accuracy.

A well‐balanced stable generalized Riemann problem scheme for shallow water equations using adaptive moving unstructured triangular meshes

SUMMARYWe propose a well‐balanced stable generalized Riemann problem (GRP) scheme for the shallow water equations with irregular bottom topography based on moving, adaptive, unstructured, triangular meshes. In order to stabilize the computations near equilibria, we use the Rankine–Hugoniot condition to remove a singularity from the GRP solver. Moreover, we develop a remapping onto the new mesh (after grid movement) based on equilibrium variables. This, together with the already established techniques, guarantees the well‐balancing. Numerical tests show the accuracy, efficiency, and robustness of the GRP moving mesh method: lake at rest solutions are preserved even when the underlying mesh is moving (e.g., mesh points are moved to regions of steep gradients), and various comparisons with fixed coarse and fine meshes demonstrate high resolution at relatively low cost. Copyright © 2013 John Wiley & Sons, Ltd.

The adaptive GRP scheme for compressible fluid flows over unstructured meshes

Unstructured mesh methods have attracted much attention in CFD community due to the flexibility for dealing with complex geometries and the ability to easily incorporate adaptive (moving) mesh strategies. When the finite volume framework is applied, a reliable solver is crucial for the construction of numerical fluxes, for which the generalized Riemann problem (GRP) scheme undertakes such a task in the sense of second order accuracy. Combining these techniques yields a second order accurate adaptive generalized Riemann problem (AGRP) scheme for two dimensional compressible fluid flows over unstructured triangular meshes. Besides the generation of meshes, the main process of this combination consists of two ingredients: Fluid dynamical evolution and mesh redistribution. The fluid dynamical evolution ingredient serves to evolve the compressible fluid flows on a fixed nonuniform triangular mesh with the direct Eulerian GRP solver. The role of the mesh redistribution is to redistribute mesh points on which a conservative interpolation formula is adopted to calculate the cell-averages for the conservative variables, and the gradients of primitive variables are reconstructed using the least squares method. Several examples are taken from various contexts to demonstrate the performance of such a program.