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-   -   reference pressure and compressible flow (http://www.cfd-online.com/Forums/cfx/123374-reference-pressure-compressible-flow.html)

bingo10 September 11, 2013 07:32

reference pressure and compressible flow
 
Hi,
I know that there are already several threats on the subject of reference pressure, but I'm still not sure which settings I need to apply in my case.

I'm trying to simulate a 25 compression ramp at Mach 8.2 for laminar flow.
The inlet conditions should be velocity u =1746 m/s, static pressure = 524 Pa and static temperature = 113 K to equal the conditions in an experiment.

I already did some calculations with the reference pressure set to 0 Pa, because I was told that it would not really matter. But I was never able to get a gonverged solution for this case, because I get some disturbances inside the seperation bubble, which leeds to a solver breakdown in the end. Or the seperation bubble just keeps on growing untill it breaks.

Since I was never able to solve this problem I thought that maybe my reference pressure is the cause of this problem. Because I red that it was rather inportant if you did a compressible calculation. Maybe someone can tell me if the reference pressure is capable of causing such problems in any way?

I then tried to check if the reference pressure made any diffrence. I changed my reference pressure to 10000 Pa because the pressure variable ranged from 400 Pa to 15000 Pa . I set the ref static pressure in the supersonic inlet tab to -9476 Pa. Would that be correct to get the same static pressur for the inlet flow?

I checked the results some time in the calculation and the values for absolut presures were 400 -15000 Pa. The values for the pressure variable at the inlet was -9476 for the inlet. I just do not really know how to interprete this findings. Also the Machnumber was rather lower than in the results before, which I think should not be the case.

I also do not know how the reference pressure is to be used in connection to the reference pressure set in the air ideal gas tap which i'm using. I always thought they were unconnected but some threats here hint otherwise. Maybe the connection between the two creats the diffrence in my Machnumber?

It would be very nice if anyone could tell me which values I should apply for both the reference pressures and the ref. static pressure at the inlet, so I can check if that causes any of my problems in the calculation since i'm quite sure that cfx should be able to give a reasonable calculation for this case.

Thanks in advance


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