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[abbas]

Hello everyone,

 

I want to simulate the flow inside the nozzle in CFX. Because at the inlet of the nozzle, we consider the velocity is near zero and the only parameter is Pressure. At first, I chose the static pressure=12 Mpa and outlet pressure 4Mpa but when the simulation got converged, I noticed that the flow velocity at the beginning of the nozzle is near 100m/s which is opposite of my first assumption. What should I do, in order to get the velocity near zero at the beginning of the nozzle? I notice that CFX has the parameter which it is Zero gradients. Can I use this parameter to get velocity=0 in the inlet?

 

Best regards

Abbas

[Amine Ben Hadj Ali]

Very unreliable boundary specification. This combination is no recommended as total pressure and system mass flow are both implicit result of the prediction. Actually every momentum flux would satisfy the set. Always use Total pressure instead of static pressure and adjust the input as well as the pressure field to get the desired velocity /mass flow level.

[abbas]

Thanks for your quick reply.

When we say that pressure at the exit of the compressor is 12MPa, it means that we are talking about static pressure. Therefore, how can find the correct total pressure, when I don't have any information about the flow velocity.

 

Best regards

Abbas

[Amine Ben Hadj Ali]

That is your user input. Either you know the stagnation quantities or the mass flow /velocity. Problem is not outlet here the problem is at the inlet. If the velocity is near zero at the inlet that means that total pressure is somewhat equal to static pressure. The solver will try to get the massflow rate based on the input from the interior cells. That is why total pressure/static pressure combination is sensitive to initial pressure field. You can first start trying top get quiescent flow (no velocity): this is always a challenge to get a zero flow and then ramp the total pressure to get the desired massflow.