D. M. Causon

On the Choice of Wavespeeds for the HLLC Riemann Solver

This paper considers a class of approximate Riemann solver devised by Harten, Lax, and van Leer (denoted HLL) for the Euler equations of inviscid gas dynamics. In their 1983 paper, Harten, Lax, and van Leer showed how, with a priori knowledge of the signal velocities, a single-state approximate Riemann solver could be constructed so as to automatically satisfy the entropy condition and yield exact resolution of isolated shock waves. Harten, Lax, and van Leer further showed that a two-state approximation could be devised, such that both shock and contact waves would be resolved exactly. However, the full implementation of this two-state approximation was never given. We show that with an appropriate choice of acoustic and contact wave velocities, the two-state so-called HLLC construction of Toro, Spruce, and Speares will yield this exact resolution of isolated shock and contact waves. We further demonstrate that the resulting scheme is positively conservative. This property, which cannot be guaranteed by any linearized approximate Riemann solver, forces the numerical method to preserve initially positive pressures and densities. Numerical examples are given to demonstrate that the solutions generated are comparable to those produced with an exact Riemann solver, only with a stronger enforcement of the entropy condition across expansion waves.

Numerical simulation of wave overtopping of coastal structures using the non-linear shallow water equations

A one-dimensional high-resolution finite volume model capable of simulating storm waves propagating in the coastal surf zone and overtopping a sea wall is presented. The model (AMAZON) is based on solving the non-linear shallow water (NLSW) equations. A modern upwind scheme of the Godunov-type using an HLL approximate Riemann solver is described which captures bore waves in both transcritical and supercritical flows. By employing a finite volume formulation, the method can be implemented on an irregular, structured, boundary-fitted computational mesh. The use of the NLSW equations to model wave overtopping is computationally efficient and practically flexible, though the detailed structure of wave breaking is of course ignored. It is shown that wave overtopping at a vertical wall may also be approximately modelled by representing the wall as a steep bed slope. The AMAZON model solutions have been compared with analytical solutions and laboratory data for wave overtopping at sloping and vertical seawalls and good agreement has been found. The model requires more verification tests for irregular waves before its application as a generic design tool.

Developments in Cartesian cut cell methods

This paper describes the Cartesian cut cell method, which provides a flexible and efficient alternative to traditional boundary fitted grid methods. The Cartesian cut cell approach uses a background Cartesian grid for the majority of the flow domain with special treatments being applied to cells which are cut by solid bodies, thus retaining a boundary conforming grid. The development of the method is described with applications to problems involving both moving bodies and moving material interfaces.

Calculation of shallow water flows using a Cartesian cut cell approach

A new grid generation method for the computation of shallow water flows is presented. The procedure, based on the use of cut cells on a Cartesian background mesh, can cope with shallow water problems having arbitrarily complex geometries. Although the method provides a fully boundary-fitted capability, no mesh generation in the conventional sense is required. Solid regions are simply cut out of a background Cartesian mesh with their boundaries represented by different types of cut cell. For the flow calculations a multi-dimensional high resolution upwind finite volume scheme is used in conjunction with an efficient approximate Riemann solver to deal with complex shallow water problems involving steady or unsteady hydraulic discontinuities. The method is validated for several test problems involving unsteady shallow water flows.

A bore-capturing finite volume method for open-channel flows

A high-resolution finite volume hydrodynamic solver is presented for open-channel flows based on the 2D shallow water equations. This Godunov-type upwind scheme uses an efficient Harten-Lax-van Leer (HLL) approximate Riemann solver capable of capturing bore waves and simulating supercritical flows. Second-order accuracy is achieved by means of MUSCL reconstruction in conjunction with a Hancock two-stage scheme for the time integration. By using a finite volume approach, the computational grid can be irregular which allows for easy boundary fitting. The method can be applied directly to model 1D flows in an open channel with a rectangular cross-section without the need to modify the scheme. Such a modification is normally required for solving the 1D St Venant equations to take account of the variation of channel width. The numerical scheme and results of three test problems are presented in this paper.

POSITIVELY CONSERVATIVE HIGH-RESOLUTION CONVECTION SCHEMES FOR UNSTRUCTURED ELEMENTS

Despite their geometric flexibility, unstructured mesh schemes for compressible gas dynamics do not usually resolve captured shocks and contact discontinuities as well as corresponding structured mesh schemes. The main reason for this appears to be the difficulty in constructing analogous extensions to higher-order accuracy. This issue is addressed in some detail and a new, compact stencil, Maximum Limited Gradient (MLG) reconstruction technique is presented for unstructured elements. The MLG reconstruction turns out to be a multidimensional analogue of the one-dimensional Superbee slope. We then describe a simple and robust extension to systems of equations, which does not require any diagonalization of flux Jacobian matrices. An application to a blast wave hazard prediction problem is presented using the wave-by-wave extension of the MLG limiter to the Euler equations.

A free-surface capturing method for two fluid flows with moving bodies

A two-fluid solver has been developed for flow problems with both moving solid bodies and free surfaces. The underlying scheme is based on the solution of the incompressible Navier–Stokes equations for a variable density fluid system with a free surface. The cut cell method is used for tracking moving solid boundaries across a stationary background Cartesian grid. The computational domain encompasses fully both fluid regions and the fluid interface is treated as a contact discontinuity in the density field, which is captured automatically without special provision as part of the numerical solution using a time-accurate artificial compressibility method and high resolution Godunov-type scheme. A pressure-splitting algorithm is proposed for the accurate treatment of the normal pressure gradient at the interface in the presence of a gravity term. The Cartesian cut cell technique provides a highly efficient and fully automated process for generating body fitted meshes, which is particularly useful for moving boundary problems. Several test cases have been calculated using the present approach including a moving paddle as a wave generator and the initial stages of entry into still water of rigid wedges. The results compare well with other theoretical results and experimental data. Finally, test cases involving the entry into water and subsequent total immersion of a two-dimensional rigid wedge-shaped body as well as the inverse problem of wedge egress have been calculated to demonstrate the ability of the current method to tackle more general two fluid flows with interface break-up, reconnection, entrapment of one fluid into the other, as well as handling moving bodies of complex geometry.

On Riemann solvers for compressible liquids

A number of Riemann solvers are proposed for the solution of the Riemann problem in a compressible liquid. Both the Tait and Tammann equations of state are used to describe the liquid. Along with exact Riemann solvers. a detailed description of a primitive variable Riemann solver, a two-shock Riemann solver, a two-rarefaction Riemann solver and an extension to the HLL Riemann solver, namely the HLLC Riemann solver, are presented. It is shown how these Riemann solvers may be implemented into Godunov-type numerical methods. The appropriateness of each of the Riemann solvers for a number of flow situations is demonstrated by applying Godunovs method to some revealing shock tube test problems

A Cartesian cut cell method for shallow water flows with moving boundaries

A new computational method for the calculation of shallow water flows with moving physical boundaries is presented. The procedure can cope with shallow water problems having arbitrarily complex geometries and moving boundary elements. Although the method provides a fully boundary-fitted capability, no mesh generation is required in the conventional sense. Solid regions are simply cut out of a background Cartesian mesh with their boundaries represented by different types of cut cell. Moving boundaries are accommodated by up-dating the local cut cell information on a stationary background mesh as the boundaries move. No large-scale re-meshing is required. For the flow calculations, a multi-dimensional high resolution upwind finite volume scheme is used in conjunction with an efficient approximate Riemann solver at flow interfaces, and an exact Riemann solution for a moving piston at moving boundary elements. The method is validated for test problems that include a ships hull moving at supercritical velocity and two hypothetical landslide events where material plunges laterally into a quiescent shallow lake and a fiord.

Calculation of compressible flows about complex moving geometries using a three-dimensional Cartesian cut cell method

A three-dimensional Cartesian cut cell method is described for modelling compressible flows around complex geometries, which may be either static or in relative motion. A background Cartesian mesh is generated and any solid bodies cut out of it. Accurate representation of the geometry is achieved by employing different types of cut cell. A modified finite volume solver is used to deal with boundaries that are moving with respect to the stationary background mesh. The current flow solver is an unsplit MUSCL–Hancock method of the Godunov type, which is implemented in conjunction with a cell-merging technique to maintain numerical stability in the presence of arbitrarily small cut cells and to retain strict conservation at moving boundaries. The method is applied to some steady and unsteady compressible flows involving both static and moving bodies in three dimensions. Copyright