Setting Coupled Implicit Solver Properties

To set the Courant number for the Coupled Implicit solver, select the node for that solver in the Solvers node as shown below. The Courant Number property can be entered in the Properties window.

The default value of Courant number for the implicit scheme is 5. While this value may be fine for many simulations, it may not be optimal in all cases.

Generally, there will be some optimal value of Courant number that balances the total number of iterations against the time per iteration. This optimum is a function of many factors such as whether or not the flow is compressible, how close the solution is to convergence, mesh quality and the magnitude of the body forces.

If the solution diverges for more than a few iterations after initialization, or if the solver reports that corrections are being limited, it is necessary to reduce the Courant number.

If the Courant number is reduced automatically by the solver for more than 1 or 2 iterations, it should be manually reduced.

If the Courant number is too high, especially during initial iterations, the convergence might be slowed due to the multigrid solver working too hard. A suggested procedure for monitoring this is as follows:

Select the Solvers Coupled Implicit AMG Linear Solver node.

Set the Verbosity property to low. This will cause the number of inner multigrid iterations to be displayed.

There should be about 3-4 inner iterations per global iteration. If there are many more, then it is necessary to reduce the Courant number.

If the Courant number is set very high and the residuals do not decline at some point after many iterations, it is appropriate to try reducing the Courant number.