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-   -   definition of totalPressure (http://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.
I'll be glad if you let me know your progress.

doubtsincfd March 9, 2013 22:13

I was using simpleFOAM (steady state) and so was not concerned with the transient solution

the relaxation factors prevented solution from blowing up but the problem for first few iterations still persists

immortality March 15, 2013 05:38

did anyone find a way or have an idea to limiting velocity at initial iterations when using total pressure at inlet?


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