A Question about CFX Supersonic Flow Solving Theory

Zhifeng · December 15, 2020, 09:17

Hi,

I have a question about CFX solving theory. When I choose the supersonic inlet for the CFX, I need to provide three parameters about the flow (e.g. pressure, temperature, and velocity), it is reasonable because the supersonic flow meets the upwind form, and the CFX needs to calculate the inlet flow state only through these parameters, without the information from the flow downstream. But when I use the subsonic inlet for a compressor and the flow is supersonic in the rotating framework, how can the CFX get enough information for it? It is from the reference pressure, or the pressure level information in solving control - advanced control, or the CFX guesses a pressure for it?

It seems that the results in my simulation are correct, but I don't know why.

Thanks!

Zhifeng

Zhifeng · March 27, 2021, 09:23

OK, several months later, let me try to answer this question myself. For turbomachinery, no matter the inflow is supersonic or subsonic, the axial velocity is always subsonic, and the flow in this direction doesn't meet the upwind form, hence, the inlet condition should always defined as subsonic. As for the circumferential velocity, it is calculated by the rotating speed, and directly defined as a value, so it is not relevant to the inlet condition.

December 15, 2020, 09:17	A Question about CFX Supersonic Flow Solving Theory	#1
Zhifeng New Member Zhifeng Yan Join Date: Nov 2020 Posts: 6 Rep Power: 6	Hi, I have a question about CFX solving theory. When I choose the supersonic inlet for the CFX, I need to provide three parameters about the flow (e.g. pressure, temperature, and velocity), it is reasonable because the supersonic flow meets the upwind form, and the CFX needs to calculate the inlet flow state only through these parameters, without the information from the flow downstream. But when I use the subsonic inlet for a compressor and the flow is supersonic in the rotating framework, how can the CFX get enough information for it? It is from the reference pressure, or the pressure level information in solving control - advanced control, or the CFX guesses a pressure for it? It seems that the results in my simulation are correct, but I don't know why. Thanks! Zhifeng

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March 27, 2021, 09:23		#2
Zhifeng New Member Zhifeng Yan Join Date: Nov 2020 Posts: 6 Rep Power: 6	OK, several months later, let me try to answer this question myself. For turbomachinery, no matter the inflow is supersonic or subsonic, the axial velocity is always subsonic, and the flow in this direction doesn't meet the upwind form, hence, the inlet condition should always defined as subsonic. As for the circumferential velocity, it is calculated by the rotating speed, and directly defined as a value, so it is not relevant to the inlet condition.