Hints & Tips: Reducing Pressure Iterations

Sometimes I find it requires an excessive number of pressure iterations to achieve convergence. Is there some way to improve convergence with a smaller number of iterations?

Pressure iterations are analogous to a time-marching solution in which the iterations correspond to time steps. Convergence of the iterative process is equivalent to reaching a time asymptotic solution. Most iterative methods can only propagate pressure changes a distance of one grid cell per iteration because the formula for a pressure change is usually based on interactions between neighboring grid elements. In many flow problems this is sufficient since pressure adjustments are often restricted to local neighborhoods.

When pressure adjustments in an incompressible flow problem must be made over large distances, the iteration count can become large. In these situations there is an option, referred to as automatic limited compressibility, that users can use to improve convergence. The idea is based on the observation that a fluid can generally be treated as "incompressible" when its density changes by less than 1%. This condition corresponds to fluid speeds remaining less than about 0.1 of the speed of sound in the fluid.

Using this observation we have constructed an algorithm that automatically introduces some compressibility to a fluid. The algorithm keeps the number of iterations required for convergence down while at the same time insuring that density variations remain small enough for the fluid to be considered essentially incompressible. The algorithm has also been supplemented with additional logic to allow for the limited compressibility to slowly float up and/or done in response to how hard/easy the solver is working to obtain a solution. In general, the level of compressibility is always reduced to the smallest possible value without increasing the pressure iteration count.

The new algorithm for automatic limited compressibility can be activated in one of two ways. The first is to set imp=2 in Namelist XPUT, which instructs the program to introduce the automatic feature. A second way is to use the ihelp=2 option, which tells the MENTOR system in the program to turn on automatic compressibility if it detects difficulties with pressure convergence.

How can I control file sizes as I set up and run FLOW-3D simulations?

The release of Version 8.0 has made it easier for users to arrange output file size. The estimated size of the results file is displayed on the Output tab of the Model Building panel after the pre-processor runs. This gives you fair warning if you-re about to create a very large file. You can immediately affect your file sizes by changing the frequency of data output. Most importantly, you can choose Selected Data (formerly called Animation Data) directly from the Output tab. You can really reduce file size by setting the time interval for restart data to the finish time (thus only one data jump) and using Selected Date for creating your graphics.