When running unsteady cases, the
relativeTolerance is typically set to 1e-2 or 1e-3. Once the nonlinear residuals drop by 2 or 3 orders of magnitude, the solver will continue to the next physical step. The
relativeTolerance is ignored for steady cases.
As recommended in the orderOfAccuracy of navierStokesSolver, when solving unsteady cases, it is typically necessary to initialize the flow field with
orderOfAccuracy set to 1. Once the flow field has been initialized, the user can create a child case and switch the
orderOfAccuracy back to 2.
When adjusting the
orderOfAccuracy for the
turbulenceModelSolver, the navierStokesSolver should be adjusted as well.
The turbulence solver is typically easier to converge than the NS solver. Therefore, the value of
linearIterations for the turbulence solver, typically set to ~20, is less than
linearIterations for the NS solver. However, for some challenging cases where the turbulence solver diverges, increasing
linearIterations up to ~50 could be helpful.
Similar to the NS solver, the default value for
updateJacobianFrequency is 4, indicating that the Jacobian for evaluating the turbulence equation is only updated every 4 pseudo-steps. For more challenging cases,
updateJacobianFrequency may need to be reduced from 4 to 1. This will not significantly slow down the solver, since the turbulence equation is not as computationally expensive as the NS equation.
As mentioned above, the turbulence equation is typically easier to converge than the NS equations.
Therefore, by default,
equationEvalFrequency is set to 4, meaning that the turbulence equation is only evaluated every 4 pseudo-steps. For challenging cases,
equationEvalFrequency may need to be reduced from 4 to 1 as well. This change will not significantly impact the solver’s performance.