wiki:u/erica/GudonovMethodEuler

Version 29 (modified by Erica Kaminski, 12 years ago) ( diff )

The Godunov Method for Euler Equations, using EXACT Riemann Solver

Big Picture

  1. Define a piece-wise function of initial data
  1. Solve the IVP for the conservation laws, but with modified (discretized) initial data to get the solution for the next time level
  1. This produces local Riemann Problems (LRPs), centered on intercell boundaries.
  1. Solve these local RP's, and use solution to compute fluxes
  1. Use these fluxes to update cells.

Here is a picture of the situation:

In this picture, WL and WR denote the arrays of primitive fluid variables (namely, rho, u, p) designated as the values of those variables at the adjacent cell centers, the asterisk (*) indicates a star region that is bound by two outgoing arrows, the arrows indicate the wave pattern generated in the LRP, F is the numerical flux (described below), and the conservative formula is given by the Godunov scheme, also presented below.

If we were to blow the left star region up (on the intercell boundary between cell i-1 and i), what we would have is 3 non-linear waves (contact, shock(s) and/or rarefaction(s)) being generated at this boundary IF the left and right states (WL and WR) were different. If these states are the same, then NO non-linear waves would be generated. We would like to first find pstar, the value of pressure between the left and right waves, in the region so-called the "star region". Once this is known, we can determine the types of waves generated. Since they are propagating away from the intercell boundary, the exact values of the fluid variables along the intercell boundary depend on their relative speeds. We then would sample the solution to the LRP, along this boundary only. Once we have found the solution at this boundary, given by

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we compute the flux at this boundary. These fluxes are the fluxes of the Euler equations, in conservation form. That is,

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The method

Finding Pstar

Pstar is the solution for pressure in the "star region", which is set up at the intercell boundaries by the left and right cells involved in the local Riemann problems. This region is where the contact discontinuity lies, flanked on either side by a given combination of 2 shock or rarefaction waves. The first step at each intercell boundary is to solve for pstar. Pstar is found by Newton's iterative scheme, with the routine as it was in the Exact Riemann solver described before.

Sampling the solution

Once Pstar is found to within a certain degree of accuracy, ustar is given immediately by a simple formula in Toro, chapter 4.

In conservative form, the Godunov method is written:

where

is the numerical flux, which = the physical flux evaluated along the intercell boundary using the exact solution to the local Riemann Problem (LRP).

The time-step, is constrained by the CFL condition, which uses the max wave speed on the grid. The constraint used in the God. code here allows any given wave on the grid to travel a full delta_x in a time-step. In allowing this, we achieve a more efficient time-marching scheme, but with the assumption that wave interactions do not lead to wave accelerations.

The code

Here is an html version of the main program, with encompassing module, and problem.data file.

Debugging

Results

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