KNOWN ISSUE: Transient MFX cases with multiple stagger iterations per timestep may give incorrect results (Class 3 Error)

An MFX (2-way FSI) case with multiple stagger iterations per timestep may give incorrect results. The results are expected to be only slightly wrong in many cases.

If an MFX case has multiple stagger iterations per timestep, some variable updates within the CFX-Solver which should be performed only every timestep are being performed every stagger iteration instead. The affected quantities include the following.
a) Rhie-Chow discretization. A value which is used for Rhie-Chow discretization is being updated when it should not be. This results in different mass flows being predicted. So long as the Rhie-Chow correction remains small, the effect of this error should also be small.
b) Transient Statistics. Transient Statistics have contributions from every stagger iteration rather than every timestep.
c) Courant number. This may affect any CEL which depends on Courant number, timestep initialisation (if that is set to use the Automatic initialisation which is based on Courant number), and the SAS and DES turbulence models (which use the Courant number to apply a limiter).

No workaround is currently available.

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

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