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PRECOND - Pre-conditioning for low Mach number flows

Unstructured Grids


This keyword may be used to apply matrix pre-conditioning to improve (or allow) convergence for flows at low Mach numbers. While it theoretically may be used at all Mach numbers, it increases the CPU time required for each time step, and is therefore recommended for use only when the Mach number is less than about 0.05.

The input parameter εpre is an artificial compressibility factor. The recommended value is

εpre = max (sqrt(3) M , 0.2)

where M is the freestream Mach number. Very small values for εpre will lead to very large values for artificial compressibility, and convergence problems, especially when stagnation regions are present. Values of εpre that are too large will decrease the amount of pre-conditioning, leading to slow (or no) convergence.

Additional details on the implementation of the pre-conditioning scheme, and results for various test cases, are presented by:

This keyword may only be used when a physical time is specified with the CFL# keyword, not a CFL number. It also may not be used with the HLLE scheme (RHS HLLE).

Last updated 1 Apr 2016