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Old   January 22, 2002, 04:35
Default big negative absolut pressure
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Dear sir/Madam I am a Star-cd user and presently work on a volute pump. I have difficulty to explain the numerical results obtained using the code and hope to receive some help form you. Steady simulation was set up at pump design condition. At inlet, axial velocity was specified as a constant and relative pressure was set 0; k and epsilon was also sepcified. Reference point was set at the inlet plane. The problem is that in some region of impeller the calculated value of absolute pressure are negative, and this obviously does not make sense. I look forward to getting your help. Many thanks.

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Old   January 27, 2002, 10:50
Default Re: big negative absolut pressure
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Hi Lixhon:

I assume you use the Multiple Reference Frames to solve the problem. Did you do " reframe,static" after you load the .pst file ?


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Old   January 29, 2002, 07:38
Default Re: big negative absolut pressure
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What did you set as your reference pressure?

If you set it at 1 bar, and there is a pressure drop of more than 1 bar then obviously the absolute pressure will go negative.

If this is an incompressible simulation you needn't worry. The values of absolute pressure in your solution are arbitrary (based on your arbitrary choice of reference pressure). If you choose 1 bar as your reference pressure than the results should be identical to a solution in which you choose 5 bar as your reference pressure, in everything apart from the absolute pressure field in which the pressure in each cell will be 4 bar different.

If you need exact values for absolute pressure then you need to be very careful how you choose the value of reference pressure.

If you are certain that the reference pressure is correct and this is a liquid pump, then it could be that the negative pressure suggest the onset of cavitation.


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Old   January 30, 2002, 10:00
Default Re: big negative absolut pressure
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thank you very much.i think it is because of cavitation,i will try it again with transient cavitation,
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