The 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, the AVERAGE mode will take values at points where the density or internal energy is negative and replace them 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. If the CHECKPOINT option is specified, a checkpoint will be made and the solution saved before terminating.

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.

For structured grids, the default is FIXER OFF. If the FIXER keyword is used without specifying OFF or AVERAGE, then AVERAGE is assumed.

Unstructured Grids

For unstructured grids, the AVERAGE mode will take values at points where the density or pressure is outside the allowed limits and replace them with averages from neighboring cells. The limits on density and pressure may be specified using the Q LIMIT keyword. The default minimum and maximum values, normalized by the freestream static values, are 0.01 and 250 for both density and pressure.

The THRESHOLD mode will merely cap values at the allowed minimum/maximum values rather than averaging.

If PRINT is specified, the list output will contain detailed information of each bad point.

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 the CHECKPOINT option is specified, a checkpoint will be made and the solution saved before terminating. If MAX ... is not used, no limit is set on the maximum number of bad cells.

For unstructured grids, the default is FIXER AVERAGE with iter=1. If the FIXER keyword is used without specifying OFF, AVERAGE, or THRESHOLD, then AVERAGE is assumed.

See Also: Q LIMIT