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-   -   definition of totalPressure (https://www.cfd-online.com/Forums/openfoam-solving/114111-definition-totalpressure.html)

 immortality March 5, 2013 11:18

definition of totalPressure

in the totalPressure BC at this part:
Code:

```if (psiName_ == "none" && rhoName_ == "none")     {         operator==(p0p - 0.5*(1.0 - pos(phip))*magSqr(Up));     }     else if (rhoName_ == "none")     {         const fvPatchField<scalar>& psip =             patch().lookupPatchField<volScalarField, scalar>(psiName_);         if (gamma_ > 1.0)         {             scalar gM1ByG = (gamma_ - 1.0)/gamma_;             operator==             (                 p0p               /pow                 (                     (1.0 + 0.5*psip*gM1ByG*(1.0 - pos(phip))*magSqr(Up)),                     1.0/gM1ByG                 )             );         }         else         {             operator==(p0p/(1.0 + 0.5*psip*(1.0 - pos(phip))*magSqr(Up)));         }```
there is this well-known formula.but there is some questions
1) why "(1.0 - pos(phip))" term is put in this formula?what it means?
2)does Up and psip stand for internalField(p) and 1/(R*internalField(T))?
Code:

```p0p               /pow                 (                     (1.0 + 0.5*psip*gM1ByG*(1.0 - pos(phip))*magSqr(Up)),                     1.0/gM1ByG```
3)I have written this formula(isentropic relation) in groovyBC in this manner:
Code:

`p0_2/pow(1+(gamma-1)/2*magSqr(internalField(U))/(gamma*R*internalField(T)),3.5)`
is it equivalent correctly to what there is in OF totalPressure code?

 doubtsincfd March 5, 2013 11:31

We had some discussion on the total pressure bc sometime back.
See if it helps you:

http://www.cfd-online.com/Forums/ope...tml#post401213

 immortality March 5, 2013 13:04

thanks a lot.
then its incorrect to set totalPressure at outlet?
I have 2 totalPressure BC at inlet and outlet.at inlet it behaves somewhat good(except supersonic velocity at 3-4 begining time steps that should be subsonic in real but it modifies at later time steps and becomes subsonic) but when outlet opens pressure goes too little and velocity increases a lot and problem diverges.
could anyone argue about the formula I have written at above post?
thanks.

 immortality March 8, 2013 04:58

any more remarks?

 doubtsincfd March 8, 2013 09:59

Hi Ehsan,

Regarding the bcs, I have replied on http://www.cfd-online.com/Forums/ope...tml#post412540

I also faced the similar problem i.e velocity going supersonic and solution blowing up. I then changed the relaxation parameters suggested by niklas in the post mentioned above.

In my case, I monitored the absolute pressure at inlet and it went negative for first few iterations. Are you experiencing the same problem?

 immortality March 8, 2013 19:45

i set fixedValue at outlet and it fixed the problem!
But in inlet the issue is remain and because my case is transient i think its important and i can't use relaxations.i tried to limit velocity by groovyBC but i didn't have any effect.