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-   -   BC's for compressor cascade flow (http://www.cfd-online.com/Forums/fluent/125352-bcs-compressor-cascade-flow.html)

MichiMuffin October 23, 2013 18:18

BC's for compressor cascade flow
 
Hi Folks,

i try to analize the flow over an compressor cascade, using fluent 14.5 . As usual, i want to have a look at the behaviour of the deflection of the cascade in dependence on different inflow angles, at prescribed flow velocity.

i set up the case with the following BC's:

incomressible
operating pressure = 103520 PA
inlet: velocity inlet, with the given flow velocity and an angle of attack
outlet: pressure outlet , gauge pressure = 0
periodic conditions on the two remaining channel sides.

Well, that worked quit well. But i realized that due to the high inlet velocity, the local flow velocity above the cascade became really high in several local spots, so the Ma is >0,2 (max. Ma = 0,54) .
Now i want to investigate a compressible case, due to high Ma numbers. In the manual of fluent, its said that velocity inlet conditions shouldnt be used for compressible flow.
i changed the inlet BC to pressure inlet, but heres the problem. How can i setup the BC's that the inlet velocity will reach the desired, prescribed value?

i tried it like this:

the incompressible case for a certain inflow angle delivered a dynamic pressure at the outlet. i tried to use this value for the compressible calculation.

operating pressure = 0
inlet: pressure inlet,
with desired direction, Total Gauge pressure = 103520 Pa
outlet: pressure outlet, Gauge Pressure = 103520 - dynamic pressure
from compressible calculation


Well, the calculated inlet velocity by fluent is as half as big as in the incompressible case. And Ma is <0,2 in the whole flow area, what makes an compressible calculation unnecessary.

Is there a possibilty, to set up the BC's in a way, that a desired inlet velocity can be reached?

any advice?

thanks in advance
greeting
Muffin


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