Compressible flow in CFX, valve simulation
 

I am currently performing some analysis in CFX regarding the flow through a high pressure air valve. This valve is designed to be used to prevent air leaks in the event that the airline on the valve outlet fails. I am performing this analysis with an inlet pressure of 207Bar and then I am varying the outlet pressure from 25 Bar up to 175Bar in steps of 25Bar. I am performing this analysis to make a design decision for the valve opening. So far my velocities have been sonic (>340.29m/s) at their peak and this does not feel right. However, I do not know how the density of the air is changing and would be interested in measuring this.

Ummmm - 207 bar in air? This is definitely going to choke and go sonic. Why do you think otherwise?

I have since found out that the system does not drop approximately 1Bar after each seismic gun fires. Instead, the system pressure at the manifold drops to approximately 75% of the line pressure after discussing this with a field service engineer. And this was shown to me using a simulated system.

I have no idea what you are talking about. Please post an image of what you are doing. You are going to have to explain what you are doing if you want us to help you.