WIND User's Guide - Keyword Reference, DQ

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DQ - Delta Q limiter

DQ [LIMITER] [OFF | ON [DRMAX drmax] [DTMAX dtmax]]

This keyword may be used to limit the change in density and/or temperature over a single iteration. This may be helpful, especially in the initial stages of a calculation, in maintaining stability, and allow the use of larger CFL numbers than might otherwise be possible.

DRMAX drmax drmax specifies the fractional change allowed in the density over an iteration. I.e., specifying "DQ LIMITER ON DRMAX 0.2" means the density will not be permitted to change by more than ±20% from one iteration to the next.
DTMAX dtmax dtmax specifies the fractional change allowed in the temperature over an iteration.

The default is to not limit the change in density or temperature. When limiting is turned on, the defaults for drmax and dtmax are both 0.5. Thus, if just "DQ LIMITER ON" is specified, both density and temperature will be allowed to change by no more than ±50% over an iteration.

The use of the DQ LIMITER keyword becomes somewhat problematic when multiple sub-iterations are used for the Navier-Stokes equations (using the NAVIER-STOKES ITERATIONS keyword) and/or the turbulence model equations (using the ITERATIONS option with the TURBULENCE keyword). By default, the number of sub-iterations for both the Navier-Stokes and turbulence model equations is one. In this case, each iteration for the Navier-Stokes equations is followed by an iteration for the turbulence model equations, and if the limiter is applied to the Navier-Stokes solution, it is also applied to the turbulence model solution.

If the number of Navier-Stokes sub-iterations is one, and if multiple sub-iterations are used for the turbulence model equations, then if the limiter is applied to the Navier-Stokes solution, it is applied to each turbulence model sub-iteration.

Also, if multiple Navier-Stokes sub-iterations are used, it's possible that the computed flow field is such that the limiter is applied during some sub-iterations, but not others. In this case, if the limiter is applied during the final Navier-Stokes sub-iteration, it is again applied to each turbulence model sub-iteration. If the limiter is not applied during the final Navier-Stokes sub-iteration, it will not be applied at all during the turbulence model iteration.

For these reasons, it is recommended that the number of sub-iterations for the Navier-Stokes and turbulence model equations be left at their default values of one when the DQ LIMITER keyword is used.

	`DRMAX` drmax		drmax specifies the fractional change allowed in the density over an iteration. I.e., specifying "`DQ LIMITER ON DRMAX 0.2`" means the density will not be permitted to change by more than ±20% from one iteration to the next.
	`DTMAX` dtmax		dtmax specifies the fractional change allowed in the temperature over an iteration.