[k-epsilon]

Hello together,
till present, unfortunately I have still no experience in the area of CFD modeling but I have experience in structural calculations. Using ANSYS is not completely new for me. Hence, I turn to your forum and hope for your advice.
To the problem:
I would like to connect a nozzle to an air pressure pipe. On this occasion the exhaust velocity interests me in and after the nozzle (NO Laval nozzle!).
The boundary conditions known to me are the pressure in the air pressure pipe (p1) and the atmospheric pressure (p2). Further the geometry has to be designed with the help of simulation.
I have build a small practise model which is shown in illustration 1. It is a rotation-symmetrical part. The nozzle is between both chambers (Between Inlet and Outlet).
Now I am not shure about the boundary conditions. Can I use STATIC PRESSURE as Inlet boundary condition and an OPENING Condition with a Pressure at the Outlet?
If I use this boundary condition I get very high velocities (up to supersonic) after the nozzle (red point). Im irritated because of the physics. When you expand compressible gas there is a maximum possible speed which is the sonic velocity, right? So im not shure if my model is correct.
What do you think about the model, could it be correct or is it absolutely wrong?

Thanks,
Robert

Illustration 1:
model.png

[ghorrocks]

On your boundary condition choice (static pressure inlet, opening outlet), the CFX documentation has the following to say:

Very Unreliable: Static pressure at an inlet and static pressure at an outlet. This combination is not recommended, as the inlet total pressure level and the mass flow are both an implicit result of the prediction (the boundary condition combination is a very weak constraint on the system).

The choice of boundary condition should most importantly match whatever data you have above what the flow conditions are.

[k-epsilon]

Hello Mr. Horrocks,
thank you for your fast response and your quote. I haven't seen it in the cfx manual before. I already thought that the boundary conditions are not the best one,...

But is it possible to get reliable results with my known boundary conditions? This Boundary conditions are the only one which i know exactly. I think it is a common technical Problem which i would like to simulate. How would you model this Problem (just a short hint)?

Robert

[ghorrocks]

You probably know the total pressure at the inlet, not the static pressure. Total pressure inlet to a static pressure outlet is a much more reliable choice of boundary condition.