When using the Automatic option for Timestep Initialisation in a transient compressible flow calculation, the convergence can be poor and in extreme cases can cause severe divergence and solver failure.
Automatic Timestep Initialisation is intended to give a better initial guess for the next timestep by extrapolating values from the previous timestep using local flow conditions. The decision to use extrapolation is based on the local flow-velocity Courant Number. However, in compressible flow cases a more appropriate quantity includes the local speed of sound. The exclusion of this quantity makes the decision to extrapolate more aggressive in these cases and can therefore lead to problems with convergence.
Use the Previous Timestep option for Timestep Initialisation.
ANSYS CFX Release 12.0.