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Boundary conditions for flow in nozzle

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Old   July 26, 2015, 12:46
Default Boundary conditions for flow in nozzle
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Hello all!

I need to model the flow of air (considered as a compressible ideal gas) passing through a nozzle. The simulation is in 2D.
I use a density based solver, steady state flow, k-omega SST model, implicit method, start with first order upwind scheme.

I know my total pressure inlet as well as the mass flow rate.
Now, my question is: what should I set for the boundary conditions? Does it depend on the flow regime? It can be either subsonic, transonic or supersonic depending on my total pressure inlet.

-> If all the flow is subsonic, I impose inlet and outlet pressure (outlet is atmospheric pressure).
-> If the flow is transonic, again inlet and outlet pressure (atmospheric because flow is subsonic) should give a correct result. But a first question arises here: when is the flow transonic and when is is supersonic? I can calculate it using isentropic flow equations but this is an approximation. Is it the way to do it?
-> If the flow is completely supersonic, what should I do? Can I use inlet pressure and outlet pressure with target mass flow rate? In the user-guide, it is written that "The pressure outlet will not achieve the target mass flow rate if the flow becomes choked". Which is the case isn't it?

Can I use the same boundary conditions regardless of the flow regime? Pressure inlet and pressure outlet with target mass flow rate?

Thanks in advance for your help
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boundaries condition, nozzle, supersonic, target-mass-flow-rate

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