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Exit Boundary Condition for Compressor Stall Inception and Rotating Stall Simulation

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Old   March 29, 2021, 02:00
Default Exit Boundary Condition for Compressor Stall Inception and Rotating Stall Simulation
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Hi Everyone,

I will run a compressor rotating stall simulation in CFX. What I want to calculate is the stall inception mechanisms (modal waves and spikes) and the formation of rotating stall. When compressor undergoes rotating stall the exit pressure will drop and mass flow will change with time. Thus, imposing a static pressure or mass flow at the exit BC will not work. In literature a BC is used which is called throttle model. "This BC allows the variation of exit static pressure to match the compressor exit mass corrected to the exit total condition." The formula is attached. Within the formula ρ is density, Cxe is velocity in the axial direction, U is the middle span blade speed, kb is the reference coefficient, k – throttle coefficient and Pout is the pressure at the exit.

The paper used this BC is: " NUMERICAL STUDY OF STALL INCEPTION IN A TRANSONIC AXIAL COMPRESSOR ROTOR BASED ON THE THROTTLE MODEL"

They studied the stall in CFX and imposed the mentioned BC. My question, is there any option such as this BC in CFX, if not how can I impose this and run stall simulation in CFX ?
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Old   March 29, 2021, 05:57
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This is just a pressure boundary condition with the defined pressure is the function you describe. That function can easily be implemented as a CEL expression.

Whether it is numerically stable and accurate is another thing, but implementing it is simple.
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Old   March 29, 2021, 07:07
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Glenn thank you for your answer.

After I had read a couple of papers more, I realized that this is a simple pressure outlet BC as you said. Throttling will be done via changing the throttling coefficient during run. I will just write some expressions for these.

Regarding the numerical stability and accuracy, this BC was proposed in many studies in literature. The location of the BC might be important since it is a reflecting boundary. For a stall inception simulations, reflecting artificial waves from boundaries may deteriorate the signals.

Another option in the literature, using a choked nozzle at exit and throttling the compressor by reducing the exit area of the nozzle. Since the exit is sonic, no information comes from the downstream into compressor.
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