Possible supersonic flow detected at the connection of Junction X and Pipe Y

Full Message text is:

Possible supersonic flow detected at the connection of Junction X and Pipe Y. Modeling of supersonic conditions is not supported and the solver was halted.

xStream assumes that the flow is one dimensional and does not become supersonic during the transient. In reality there are situations that could cause supersonic flow to develop behind a shockwave. In addition there could be multidimensional shocks in a real system which are not captured in a one-dimensional solution. xStream will halt the transient if the calculated Mach number exceeds one to prevent inaccurate results from being displayed to the user.

If this error message is shown for a system that should not experience supersonic flow it is recommended to verify the model inputs are correct, consider adjusting the junction transient events, and potentially increase the minimum number of sections per pipe. Supersonic flow is more likely to occur when there is a high pressure differential across a junction during the steady state, when a transient event occurs rapidly, and/or when the section length is relatively large in a pipe.

The configuration which most commonly causes this transient solver error is a valve with a large pressure differential across it that opens rapidly in the transient.