KNOWN ISSUE: RMS residuals for scalar equations in multi-domain cases may be incorrect (Class 3 Error)

RMS residuals for scalar equations in multi-domain cases may be incorrect.

For multi-domain cases, the CFX-Solver combines the RMS residuals for each equation in each domain to give a combined RMS residual for that equation across all relevant domains. This combined residual is plotted in the CFX-Solver Manager residual plots by default, is printed in the table of residuals in the CFX Output File and is used to determine whether the run has converged (if the criteria for convergence is set to use RMS residuals).

However, an error in the CFX-Solver means that when the residuals for each domain are combined, the weighting is not always done correctly for scalar equations. As a result the reported combined RMS residual may be incorrect. Scalar equations include energy, mass fractions, turbulence, etc., but not momentum and mass. The same incorrect weighting factors are used throughout a run. The errors will be worst when the domains contain significantly different numbers of mesh nodes.

This problem will not affect properly converged results but it may affect whether a run is judged to be converged or not. Some cases may appear to reach the specified convergence criteria after fewer iterations than they would if the correct combined residual was used, and hence may be stopped before they have truly reached the specified convergence criteria.

No workaround is currently available. Correct residuals can be visualised in the CFX-Solver Manager by selecting to plot residuals by domain, which can be done by using the Monitors menu from the main menu of the CFX-Solver Manager.

ANSYS CFX Release 13.0. A custom executable for ANSYS CFX Release 12.1 is available on request.

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