Generalized Riemann Problem Scheme Research Articles

A Random Choice Method based on the Generalized Riemann Problem for the Euler equations in gas dynamics

This paper proposes and implements the Random Choice Method (RCM) based on a direct Eulerian generalized Riemann problem (GRP) scheme for one-dimensional Euler equations in gas dynamics. It is an application of the second order accurate GRP scheme proposed by Ben-Artzi et al. (2006) [6]. Since the RCM was introduced as a numerical tool in the gas dynamics and it has the advantage of capturing discontinuities with sharp resolution, we here implement the RCM by choosing the GRP scheme as the “building block”. The initial data are assumed to be piecewise linear function, and the local GRP is resolved directly and analytically at each interface. Special attention is paid to the treatment of resolving smooth wave. Numerical simulations on some typical problems show that the proposed method achieves good performance.

Second-order direct Eulerian GRP schemes for radiation hydrodynamical equations

The paper proposes second-order accurate direct Eulerian generalized Riemann problem (GRP) schemes for the radiation hydrodynamical equations (RHE) in the zero diffusion limit. The difficulty comes from no explicit expression of the flux in terms of the conservative vector. The characteristic fields and the relations between the left and right states across the elementary-waves are first studied, and the exact solution of the 1D Riemann problem is then gotten. After that, the direct Eulerian GRP scheme is derived by directly using the generalized Riemann invariants and the Rankine–Hugoniot jump conditions to analytically resolve the left and right nonlinear waves of the local GRP in the Eulerian formulation. Several numerical examples show that the GRP schemes can achieve second-order accuracy and high resolution of strong discontinuity.

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.

A third-order accurate direct Eulerian GRP scheme for the Euler equations in gas dynamics

The paper proposes and implements a third-order accurate direct Eulerian generalized Riemann problem (GRP) scheme for one- and two-dimensional (1D & 2D) Euler equations in gas dynamics. It is an extension of the second-order accurate GRP scheme proposed in Ben-Artzi et al. (2006) [5]. The approximate states in numerical fluxes of the third-order accurate GRP scheme are derived by using the higher-order WENO reconstruction of the initial data, the limiting values of the time derivatives of the solutions at the singularity point, and the Jacobian matrix. Besides the limiting values of the first-order time derivatives of fluid variables, the second-order time derivatives are also needed in developing the present GRP scheme and obtained by directly and analytically resolving the local GRP in the Eulerian formulation via two main ingredients, i.e. the Riemann invariants and Rankine–Hugoniot jump conditions. Unfortunately, for the sonic case that the transonic rarefaction wave appears in the GRP, the Jacobian matrix is singular on the sonic line. To this end, those approximate states are given in a different way that is based on the analytical resolution of the transonic rarefaction wave and the local quadratic polynomial interpolation. The 2D GRP scheme is implemented by using the third-order accurate time-splitting method. Several numerical examples are given to demonstrate the accuracy and effectiveness of the proposed GRP scheme, in comparison to the second-order accurate GRP scheme.

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.

A Direct Eulerian GRP Scheme for the Prediction of Gas-Liquid Two-Phase Flow in HTHP Transient Wells

A coupled system model of partial differential equations is presented in this paper, which concerns the variation of the pressure and temperature, velocity, and density at different times and depths in high temperature-high pressure (HTHP) gas-liquid two-phase flow wells. A new dimensional splitting technique with Eulerian generalized riemann problem (GRP) scheme is applied to solve this set of conservation equations, where Riemann invariants are introduced as the main ingredient to resolve the generalized Riemann problem. The basic data of “X well” (HTHP well), 7100 m deep, located in Southwest China, is used for the case history calculations. Curve graphs of pressures and temperatures along the depth of the well are plotted at different times. The comparison with the results of Lax Friedrichs (LxF) method shows that the calculating results are more fitting to the values of real measurement and the new method is of high accuracy.

A direct Eulerian GRP scheme for relativistic hydrodynamics: Two-dimensional case

This paper develops a direct Eulerian generalized Riemann problem (GRP) scheme for two-dimensional (2D) relativistic hydrodynamics (RHD). It is an extension of the GRP scheme for one-dimensional (1D) RHDs [Z.C. Yang, P. He, H.Z. Tang, J. Comput. Phys. 230 (2011) 7964–7987] and the GRP scheme for the non-relativistic hydrodynamics [M. Ben-Artzi, J.Q. Li, G. Warnecke, J. Comput. Phys. 218 (2006) 19–43]. In order to derive the direct Eulerian GRP scheme, the (local) GRP of the split 2D RHD equations in the Eulerian formulation has to be directly resolved by using corresponding Riemann invariants and Rankine–Hugoniot jump conditions so that the crucial and delicate Lagrangian treatment in the original GRP scheme [M. Ben-Artzi, J. Falcovitz, J. Comput. Phys. 55 (1984) 1–32] may be avoided. An important difference of resolving the GRP of the split 2D RHD equations from the GRP of the 1D RHD equations or the non-relativistic hydrodynamical equations is coming from the fact that the flow regions across the shock or rarefaction wave in the GRP of the split 2D RHD equations are nonlinearly coupled through the Lorentz factor which is also built in terms of the tangential velocities. It is a purely multi-dimensional relativistic feature. Several numerical examples are given to demonstrate the accuracy and effectiveness of the proposed 2D GRP scheme.

Accuracy of the Adaptive GRP Scheme and the Simulation of 2-D Riemann Problems for Compressible Euler Equations

AbstractThe adaptive generalized Riemann problem (GRP) scheme for 2-D compressible fluid flows has been proposed in [J. Comput. Phys., 229 (2010), 1448-1466] and it displays the capability in overcoming difficulties such as the start-up error for a single shock, and the numerical instability of the almost stationary shock. In this paper, we will provide the accuracy study and particularly show the performance in simulating 2-D complex wave configurations formulated with the 2-D Riemann problems for compressible Euler equations. For this purpose, we will first review the GRP scheme briefly when combined with the adaptive moving mesh technique and consider the accuracy of the adaptive GRP scheme via the comparison with the explicit formulae of analytic solutions of planar rarefaction waves, planar shock waves, the collapse problem of a wedge-shaped dam and the spiral formation problem. Then we simulate the full set of wave configurations in the 2-D four-wave Riemann problems for compressible Euler equations [SIAM J. Math. Anal., 21 (1990), 593-630], including the interactions of strong shocks (shock reflections), vortex-vortex and shock-vortex etc. This study combines the theoretical results with the numerical simulations, and thus demonstrates what Ami Harten observed “for computational scientists there are two kinds of truth: the truth that you prove, and the truth you see when you compute” [J. Sci. Comput., 31 (2007), 185-193].

A direct Eulerian GRP scheme for relativistic hydrodynamics: One-dimensional case

The paper proposes a direct Eulerian generalized Riemann problem (GRP) scheme for one-dimensional relativistic hydrodynamics. It is an extension of the Eulerian GRP scheme for compressible non-relativistic hydrodynamics proposed in [M. Ben-Artzi, J.Q. Li, G. Warnecke, A direct Eulerian GRP scheme for compressible fluid flows, J. Comput. Phys. 218 (2006) 19–43]. Two main ingredients, the Riemann invariant and the Rankine–Hugoniot jump condition, are directly used to resolve the local GRP in the Eulerian formulation, and thus the crucial and delicate Lagrangian treatment in the original GRP scheme [3] can be avoided. Several numerical examples are given to demonstrate the accuracy and effectiveness of the proposed GRP scheme.

An adaptive GRP scheme for compressible fluid flows

This paper presents a second-order accurate adaptive generalized Riemann problem (GRP) scheme for one and two dimensional compressible fluid flows. The current scheme consists of two independent parts: Mesh redistribution and PDE evolution. The first part is an iterative procedure. In each iteration, mesh points are first redistributed, and then a conservative interpolation formula is used to calculate the cell-averages and the slopes of conservative variables on the resulting new mesh. The second part is to evolve the compressible fluid flows on a fixed nonuniform mesh with the Eulerian GRP scheme, which is directly extended to two-dimensional arbitrary quadrilateral meshes. Several numerical examples show that the current adaptive GRP scheme does not only improve the resolution as well as accuracy of numerical solutions with a few mesh points, but also reduces possible errors or oscillations effectively.

Implementation of the GRP scheme for computing radially symmetric compressible fluid flows

The study of radially symmetric compressible fluid flows is interesting both from the theoretical and numerical points of view. Spherical explosion and implosion in air, water and other media are well-known problems in application. Typical difficulties lie in the treatment of singularity in the geometrical source and the imposition of boundary conditions at the symmetric center, in addition to the resolution of classical discontinuities (shocks and contact discontinuities). In the present paper we present the implementation of direct generalized Riemann problem (GRP) scheme to resolve this issue. The scheme is obtained directly by the time integration of the fluid flows. Our new contribution is to show rigorously that the singularity is removable and derive the updating formulae for mass and energy at the center. Together with the vanishing of the momentum, we obtain new numerical boundary conditions at the center, which are then incorporated into the GRP scheme. The main ingredient is the passage from the Cartesian coordinates to the radially symmetric coordinates.

The convergence of the GRP scheme

This paper deals with the convergence of the second-order GRP(Generalized Riemann Problem) numerical scheme to the entropysolution for scalar conservation laws with strictly convex fluxes.The approximate profiles at each time step are linear in each cell,with possible jump discontinuities (of functional values and slopes)across cell boundaries. The basic observation is that the discretevalues produced by the scheme are exact averages of an approximate conservation law, which enables the use of propertiesof such solutions in the proof. In particular, the“total-variation" of the scheme can be controlled, using analyticproperties. In practice, the GRP code allows “sawteeth" profiles(i.e., the piecewise linear approximation is not monotone even ifthe sequences of averages is such). The “reconstruction" procedureconsidered here also allows the formation of “sawteeth" profiles,with an hypothesis of “Godunov Compatibility", which limits theslopes in cases of non-monotone profiles. The scheme is proved toconverge to a weak solution of the conservation law. In the case ofa monotone initial profile it is shown (under a further hypothesison the slopes) that the limit solution is indeed the entropysolution. The constructed solution satisfies the “finitepropagation speed", so that no rarefaction shocks can appear inintervals such that the initial function is monotone in their domainof dependence. However, the characterization of the limit solutionas the unique entropy solution, for general initial data, is stillan open problem.

Hyperbolic balance laws: Riemann invariants and the generalized Riemann problem

The Generalized Riemann Problem (GRP) for a nonlinear hyperbolic system of m balance laws (or alternatively “quasi-conservative” laws) in one space dimension is now well-known and can be formulated as follows: Given initial-data which are analytic on two sides of a discontinuity, determine the time evolution of the solution at the discontinuity. In particular, the GRP numerical scheme (second-order high resolution) is based on an analytical evaluation of the first time derivative. It turns out that this derivative depends only on the first-order spatial derivatives, hence the initial data can be taken as piecewise linear. The analytical solution is readily obtained for a single equation (m = 1) and, more generally, if the system is endowed with a complete (coordinate) set of Riemann invariants. In this case it can be “diagonalized” and reduced to the scalar case. However, most systems with m > 2 do not admit such a set of Riemann invariants. This paper introduces a generalization of this concept: weakly coupled systems (WCS). Such systems have only “partial set” of Riemann invariants, but these sets are weakly coupled in a way which enables a “diagonalized” treatment of the GRP. An important example of a WCS is the Euler system of compressible, nonisentropic fluid flow (m = 3). The solution of the GRP discussed here is based on a careful analysis of rarefaction waves. A “propagation of singularities” argument is applied to appropriate Riemann invariants across the rarefaction fan. It serves to “rotate” initial spatial slopes into “time derivative”. In particular, the case of a “sonic point” is incorporated easily into the general treatment. A GRP scheme based on this solution is derived, and several numerical examples are presented. Special attention is given to the “acoustic approximation” of the analytical solution. It can be viewed as a proper linearization (different from the approach of Roe) of the nonlinear system. The resulting numerical scheme is the simplest (second-order, high-resolution) generalization of the Godunov scheme.

Remark on the generalized Riemann problem method for compressible fluid flows

The generalized Riemann problem (GRP) method was proposed for compressible fluid flows based on the Lagrangian formulation [M. Ben-Artzi, J. Falcovitz, A second-order Godunov-type scheme for compressible fluid dynamics, J. Comput. Phys., 55(1) (1984) 1–32], and a direct Eulerian version was developed in [M. Ben-Artzi, J. Li, G. Warnecke, A direct Eulerian GRP scheme for compressible fluid flows, J. Comput. Phys., 28 (2006) 19–43] by using the concept of Riemann invariants. The central feature of the GRP method is the resolution of centered rarefaction waves. In this note we show how to use the concept of Riemann invariants in order to resolve the rarefaction waves in the Lagrangian coordinate system and result in the GRP scheme.

Application of the GRP scheme to open channel flow equations

The GRP (generalized Riemann problem) scheme, originally conceived for gasdynamics, is reformulated for the numerical integration of the shallow water equations in channels of rectangular cross-section, variable width and bed profile, including a friction model for the fluid–channel shear stress. This scheme is a second-order analytic extension of the first-order Godunov-scheme, based on time-derivatives of flow variables at cell-interfaces resulting from piecewise-linear data reconstruction in cells. The second-order time-integration is based on solutions to generalized Riemann problems at cell-interfaces, thus accounting for the full governing equations, including source terms. The source term due to variable bed elevation is treated in a well-balanced way so that quiescent flow is exactly replicated; this is done by adopting the Surface Gradient Method (SGM). Several problems of steady or unsteady open channel flow are considered, including the terms corresponding to variable channel width and bed elevation, as well as to shear stress at the fluid–channel interface (using the Manning friction model). In all these examples remarkable agreement is obtained between the numerical integration and the exact or accurate solutions.

A direct Eulerian GRP scheme for compressible fluid flows

A direct Eulerian generalized Riemann problem (GRP) scheme is derived for compressible fluid flows. Riemann invariants are introduced as the main ingredient to resolve the generalized Riemann problem (GRP) directly for the Eulerian formulation. The crucial auxiliary Lagrangian scheme in the original GRP scheme is not necessary in the present framework. The delicate sonic cases can be easily treated and the extension to multidimensional cases is obtained using the dimensional splitting technique.

Diaphragm rupture. Impingement by a conically-nosed, ram-accelerator projectile

The effect of diaphragm rupture by a conically-nosed projectile on the gasdynamics related to ram accelerator operation was experimentally and numerically studied. The experiments were conducted using a 25-mm-bore ram accelerator. Either air or nitrogen was used as the test gas. Using a high-speed image converter camera, it was observed that during the process of the diaphragm rupture a region with strong radiation developed between the diaphragm and the approaching projectile/sabot. This radiating region corresponds to the shock-heated gas which is originated from a precursory shock wave driven by the accelerating projectile/sabot. The flow around the projectile upon entering the test section by rupturing the diaphragm was visualized by holographic interferometry. During the diaphragm rupture, the system of oblique shock waves around the conical nose of the projectile was seen undisturbed on the downstream side of the diaphragm. Under the same condition as the experiment, numerical simulation was conducted using GRP (Generalized Riemann Problem) scheme which was extended to the computation of compressible flow fields bounded by moving surfaces. Two diaphragm rupture models were examined: (1) the diaphragm deformed by wrapping tightly around the moving projectile; (2) the diaphragm was ruptured instantly at the moment the projectile touched the diaphragm. Comparing these models with the experimentally visualized flow, the former was found to express the diaphragm rupture process much better than the latter.

Computation of reactive duct flows in external fields

A GRP-scheme is introduced for the numerical integration of the Euler system of equations of compressible reactive flow in a duct of variable cross section, subject to an external potential. The GRP (generalized Riemann problem) scheme is based on an analytic solution of the GRP at jump discontinuities. It is a second-order scheme generalizing the first-order Godunov scheme, having the property of high resolution of shocks and other discontinuities. Some numerical examples are considered, including an infinite spherical reflected shock, a spherical blast wave and gas collapse under an external potential.