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how to make the negative pressure as zero in surface intergration

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Old   September 1, 2015, 02:20
Default how to make the negative pressure as zero in surface intergration
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i work on hydrodynamic fluid bearings. here due to cavitation of fluid i observe negative pressure. when i calculate the surface integrals for the load carrying capacity, the negative pressure is also integrated.

can you please suggest me how can i make the negative pressure as zero in the calculation of the surface integrals?? it should also be zero when i calculate the mid plane pressure plot.

i have attached a mid plane pressure plot. you can see that the curves reach negative pressures too. i want that to be neglected, i.e, zero as it is my boundary condition.

please help.
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Old   September 1, 2015, 02:27
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here it is http://www.cfd-online.com/Forums/flu...t-working.html
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Old   September 1, 2015, 09:57
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it looks like some coding work. can i just not do it using the software abd some tools in it??
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Old   September 1, 2015, 16:38
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I'm not familiar with the cavitation model but isn't the absolute static pressure clipped at 0 anyway? You have a negative absolute pressure or gauge pressure? A negative gauge pressure should be acceptable is it not?
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Old   September 2, 2015, 04:11
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I agree with what LuckyTran said, in my experience the absolute pressure is never below zero...

But to literally answer your question, you can always output results to a text file and read that in matlab, octave, excel or your favourite analysis tool, and set the negative values to zero, and do the integral calculation yourself.
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Old   September 2, 2015, 10:17
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yes i have put reference frame as absolute and the gauge pressure is zero.

i have attached an image wer u can see the negative pressure devloped. i need that part to be zero.

its the fluid part.

how can i do that????

how can i integrate values in excel???? please help.
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Old   September 2, 2015, 10:55
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This is pretty easy in CFD-Post. If you make an expression for positive only pressure:
Code:
max(0[Pa],Pressure)
you can use that to calculate force. For example, the adjusted pressure force in the x direction:
Code:
areaInt_x(PosPressure)@surface
where PosPressure is the expression you defined first and "surface" is the name of the boundary you want to sum forces on. You'll probably want to include shear force as well by integrating the appropriate Wall Shear variable.
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