KNOWN ISSUE: Compressible transient flows can get incorrect answers (Class 3 Error)
If a compressible transient flow is undertaken with only one iteration per time step, then the solution can be incorrect if the Heat Transfer option is not set to Total Energy, or if heat transfer is not included in the simulation.
This is due to the CFX-Solver not extrapolating the pressure at the start of the time step in these circumstances. This means that density is not extrapolated, and so the code cannot calculate an accurate value for drho/dt on the first iteration
Either run with at least two iterations per time step, or use the Total Energy option for Heat Transfer.
No fix is currently available.