I also observe in the transient analysis of a 2D CFD that as the Mach number transitions to sonic from 0.8 Mach it starts to create a negative pressure or vacuum at point 1 and as I continue to iterate the timestep with a result of 1.6 Mach the result has no physical meaning already where the surrounding atmosphere after point 3 has a 10KPag pressure while at point 1 has a negative pressure or vacuum. Therefore with this result for this particular case it can be validated the max flow is only sonic Mach = 1 (choked flow).

Thanks Mariog for the statement that the inertial effects of air in a transient event so far can only be represented by pressure. I think the next challenge for simulation is the interaction of still air to the moving fluid due to PSV release. Am I right in saying this? Or this can already done in CFD simulation? Please correct if this statement is wrong.

Any other opinion is greatly appreciated.

Many Thanks,

Cheers!!!