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Old   May 23, 2000, 06:07
Default Static vs Total Pressure
Alfonso Ferrandez
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Dear all,

I've found out something that has been driving me mad lately, and I wanted to share it with you, as well as to try and get a solution to the problem.

My UDF in Fluent 5.2 requires to know the value of the TOTAL pressure in certain threads in the domain. So far I've been using an average pressure calculated with the F_P() macro.

Only today I realised that this macro returns in fact the STATIC pressure and NOT the TOTAL pressure. Obviously with flow acceleration I get drops in STATIC pressure in favour of DYNAMIC pressure.

So here's the conundrum for all Fluent Guru's out there:

Is there any way at all I can extract the value of the TOTAL pressure instead of the STATIC pressure out of the domain?

Thanks in advance to you all! AL

P.S. If you want me to post the STATIC pressure calculation routine, let me know. That would be another one for the repository
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Old   May 23, 2000, 08:48
Default Re: Static vs Total Pressure
Sung-Eun Kim
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In FLUENT 5, total pressure is a "derived" quantity (static pressure is a "solution variable). And FLUENT 5 does not store total pressure in the memory. So whenever you need it, you need to compute it. One thing you should keep in mind is that static pressure in FLUENT 5 is a gague pressure.

The UDF should look like then:

... real op_press = RP_Get_Real("operating-pressure"); real p_tot;

begin_c_loop(c,t) {

real Vmag = cellVelocityMagnitude(c,t);

p_tot = C_P(c,t)+0.5*C_R(c,t)*Vmag*Vmag+op_press;

... }
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Old   May 23, 2000, 08:53
Default Re: Static vs Total Pressure
Alfonso Ferrandez
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Thanks Kim for such a prompt and complete response! I shall implement your solution straight away.

Cheers! AL
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