# Dynamic pressure higher than total pressure?

Hi,

Any help would be hugely appreciated on this confusing issue.

I'm modelling an external supersonic flow of M=2 around an aircraft. The boundary conditions are a pressure far field.

Using the isentropic flow relations, for my altitude (18,300m) the static pressure is 7,158 Pa, which using the isentropic flow relations gives a total pressure as 56,008 pa. I input this as the gauge pressure in the far field boundary condition.

Operating pressure is set to zero.

I set the simulation to provide me with dynamic pressure as I need to calculate a drag coefficient, however upon viewing it in Post, the dynamic pressure in the far field is ~3x higher than the total pressure I input??

Is there something I'm missing here? I've gone over my setup over and over and it seems fine.

Thanks very much in advance.

Ed.

## Comments

Hi Amine,

Thanks very much for posting an answer. from the ansyshelp.ansys.com website, which I accessed by clicking the help button in fluent, it says the following for high Mach numbers:

So I was under the impression this boundary condition was correct?

Please let me know any further thoughts on this.

Thanks again.

Ed

I do not understand your post sorry. Your input for Gauge Pressure is static pressure which you get from the isentropic relationship. For Ma=2, you need to get the stagnation pressure at that altitude (7158) and then you get the input for boundary (914 Pa round about)