The DQ LIMITER keyword may be used to limit the change in the solution 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.

Structured Grids

For structured grid zones, the keyword limits the change in density and/or temperature over a single iteration.

	OFF		Don't limit the change in density or temperature.
	ON		Use the MacCormack ΔQ limiter. [MacCormack, R. W., "Considerations for Fast Navier-Stokes Solvers", presented at the conference on Advances of Flow Simulation Techniques, Davis, CA, May 1997.] With this limiter, the entire solution, including the turbulence variables, is limited throughout the zone, such that the specified fractional changes in density and temperature are not exceeded. (See below for some further considerations when multiple sub-iterations are being used.)
	NXAIR		Use the ΔQ limiter from the NXAIR code. This limiter acts locally; the solution is limited only at those points that exceed the specified fractional changes in density or temperature. Unlike the MacCormack limiter, the NXAIR limiter is not applied to the turbulence model equations (although the models of course use the limited Navier-Stokes variables, so there is an indirect influence). The NXAIR limiter assumes a perfect gas when limiting the temperature. The solution will be aborted if it is specified for flows with multiple species.
	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, when DQ LIMITER is not used, is to not limit the change in density or temperature. If just DQ LIMITER is specified, the MacCormack ΔQ limiter is used. 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 DQ LIMITER ON 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 DQ LIMITER ON is used.

Unstructured Grids

For unstructured grid zones, the keyword limits the change in energy over a single iteration. It also may be used to set the change allowed in the density at inviscid walls.

	OFF		Don't limit the change in energy.
	ON		Apply the ΔQ limiter using the default or user-specified limits.
	DEMIN demin		demin specifies the fractional decrease allowed in the energy over an iteration, and must be a negative value. I.e., specifying "DQ LIMITER ON DEMIN -0.2" means the energy will not be permitted to decrease by more than 20% from one iteration to the next.
	DEMAX demax		demax specifies the fractional increase allowed in the energy over an iteration, and must be a positive value.
	DQRATIO dqratio		dqratio specifies the fractional change allowed in the density over an iteration at inviscid walls.

The entire solution, including the turbulence variables, is limited throughout the zone, such that the specified (or default) fractional increase or decrease in energy is not exceeded. And at inviscid walls, the change in density is not allowed to exceed the specified (or default) limit.

The default values for demin, demax, and dqratio are −0.5, 2.0, and 10.0, respectively.

See Also: Q LIMIT