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.
The default value of -1 leads to a second-order upwind scheme, which is the most stable. A value of 0.33 leads to a blended upwind/central scheme, which is recommended for low subsonic flows to reduce dissipation.
orderOfAccuracy determines whether the solver will use 1st or 2nd order spatial discretization. The 1st order solver is faster, cheaper and most importantly, it is more dissipative, making it less likely to diverge. However, such numerical dissipation may also significantly impact the accuracy of the solution.
When initializing the flow field for unsteady cases with rotating components, such as simulating a rotor enclosed in a sliding interface, the user typically needs to run the 1st-order solver for around 1 or 2 revolutions. Once the flow field has been initialized, the user can fork the first-order case and switch
orderOfAccuracy from 1 to 2 for the child cases.
While adjusting the
orderOfAccuracy for the
navierStokesSolver, the turbulenceModelSolver should also be adjusted.
timeStepping is slightly different for the 1st and 2nd order cases. For more details, see Rotational Angle per Step, maxPseudoSteps and CFL
If the case is transonic or supersonic, the user should set
TRUE in the Navier Stokes solver parameters section of their input file.
linearIterations controls the number of linear iterations in each pseudo-step. Typically,
linearIterations is set to 25~35 for the NS solver. The user might need to increase it to 50-55 for challenging cases to improve convergence.
The default value for
updateJacobianFrequency is 4, which means that the Jacobian for evaluating the NS equation is updated every 4 pseudo-steps. For some challenging cases, reducing
updateJacobianFrequency from 4 to 1 may help, however, this may slow the NS solver by up to approximately 30%.