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choked mass flow value

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Old   October 1, 2019, 05:02
Post choked mass flow value
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Hello everybody,

I have a question about a statement in the Ansys CFX Reference Guide 18.1. In chapter 11.1.2, which is about convergence tips, the following is written: "For high speed compressors and turbines, where the machine mass flow is choked, it is generally not possible to specify the specified mass flow at the outlet to match the numerical choked mass flow value. For choked flow conditions, you can either specify static pressure or exit corrected mass flow at the outlet. Both these conditions allow the mass flow through the machine to adjust to the numerical choked mass flow value."

Regarding this statement I have several questions:
  1. In the first sentence, what do they mean by the numerical choked mass flow value and why is it not possible to match it? Assuming I have experimental data of the mass flow, shouldn't I use this one? or what is the problem?
  2. In the second sentence: I donīt know why specifying the corrected mass flow solves the problem? Why is this one capable of adjusting to the numerical choked mass flow? Because it also uses total pressure and temperature?

Thanks a lot for your answers and have a nice day!
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Old   October 1, 2019, 09:33
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Opaque will become famous soon enough
Let me start with a very simple illustration,

Say you have circular pipe in your CAD model, and you decide to mesh it. An incompressible material is flowing through at 1 m/s. Your mass flow will be then

mass flow = Density * Exact area of the circle * 1 m/s

Now you create a coarse mesh, and impose the mass flow above, the software cannot in any way or form produce the expected velocity, Why?

Computed velocity = Specified Mass Flow / (Density * Area_meshed circle)

Area_meshed circle ! = Exact area of the circle

and it will NEVER be exact using a discrete mesh. That means the numerical velocity is not the expected exact velocity.

You can extrapolate from here, and try making sense of the documentation.
Hope it helps,
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Old   October 2, 2019, 04:02
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Never looked at it this way, but it makes sense!

Thank you and have a nice day!
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