Wind-US User's Guide - Keyword Reference, FIXER

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FIXER - Instability smoothing

Structured Grids

FIXER [PRINT] [zone_selector]

Unstructured Grids

FIXER [FIRST ORDER] [MAX max_pts AFTER iter] [zone_selector]

Wind-US's FIXER capability searches for computational points at which numerical instabilities have resulted in physically impossible results, and replaces the faulty values with averages from nearby points. Using FIXER may help the solution through difficult transients, but it may also simply smear out instabilities over a larger portion of the flowfield. You should maintain a careful watch over the solution.

Structured Grids

For structured grids, values at points where the density or internal energy is negative are replaced with averages from nearby j and k points in the same i-plane. (I.e. the smoothing is two-dimensional; values from adjacent i-planes are not used.) The process is iterative, with multiple smoothing passes. Only "active" points are examined, not wall, coupled boundary, hole, or fringe points.

In a given i-plane, if more than 50% of the points are "bad", or if the smoothing process fails to remove all the negative values, the run is aborted.

For each i-plane containing negative values of density or internal energy, the zone, iteration number, i index, and number of bad points will be written to the list output (.lis) file. If the PRINT option is specified, the listing will also include the j and k indices of each bad point.

Unstructured Grids

Values at points where the internal energy, density, or pressure is outside the allowed limits are replaced with averages from neighboring cells. The limits on density and pressure may be specified using the Q LIMIT keyword, and the limits on internal energy are computed from those. The defaults, normalized by the freestream static values, are 0.01 and 250 for both density and pressure. [For frozen and finite-rate chemistry, only density and pressure are checked.]

If FIRST ORDER is specified, the bad cells will be flagged and a first order scheme will be used for them for the remainder of the computation.

The MAX ... keyword may be added to specify that the maximum number of bad cells allowed is max_pts, but don't start enforcing this until iteration iter. If the limit is exceeded, the run is aborted. If MAX ... is not used, no limit is set on the maximum number of bad cells.

See Also: Q LIMIT