[AnnaF]

Hello everybody

I have two questions regarding the forces functionObject when using it with interFoam. Sometimes I confuse myself so I would like to clarify some things.

1. When using it with interFoam, I am using "forcesCompressible" and the input for "rho" is "rho" (and not "rhoInf"), as interFoam uses the the rho field because of being a multiphase solver - unlike other incompressible solvers, where rho is constant. Am I right with this?

2. As far as I understand, the force functionObject uses the p field, which is the static pressure, and does not include the dynamic pressure 0.5*rho*U^2 ?
So I thought I can get the total pressure, as in total(p)= p + 0.5*rho*U^2, and use it as the input field "totalp" for the forces calculation. But this doesn't work and I get the error "Could not find U, totalp".
Maybe I misunderstand something?

Would be greatful to get some help

With kind regards,
Anna

[AnnaF]

Hi again,

so my forces file looks like:

forcesFront
{
    type            forces;
    libs            ("libforces.so");
    log             yes;
    patches         (plateFrontPatch_half0);

    enabled         true;
    writeControl    timeStep;
    writeInterval   1;

    p           total(p); //p; 
    U           U;

    rho         rhoInf;  
    rhoInf      1000;    // Fluid density -> water 1000 kg/m3

    CofR        (5 0.005 0); //centre of rotation for moment calulation
}

So i set rho as constant rhoInf (regarding question 1.), as it's an incompressible case.

If I set the pressure field to total(p) instead of p, I get this error message:

--> FOAM FATAL ERROR: 
Could not find U, total(p)
    From function void Foam::functionObjects::forces::initialise()
    in file forces/forces.C at line 197.

...still not sure, why it cannot find the calculated total(p) field.

I would appreciate any ideas I keep feeling that these are stupid newbie-questions...

Best wishes,
Anna

[AnnaF]

To give an update on my questions:

1. I am right with this.

2.
a) To get the total(p) field I am using the already available postProcessing function now (I have not seen that before). So I include in the controlDict:

functions
{
#includeFunc totalPressureCompressible
}

and execute it with:

interFoam -postProcess -fields "(p U)" -func totalPressureCompressible

b) The forcesFunction utility still cannot find the total(p) field and I still don't know why.


c) So if I want to calculate the forces with total(p) am using "areaIntegrate" directly in a dictionary (that I include in the controlDict - functions{} again:

#includeFunc totalForce

totalForce
{
    #includeEtc "caseDicts/postProcessing/surfaceFieldValue/surfaceRegion.cfg"
    operation       areaIntegrate;
    fields          (p total(p));
    regionType  patch;
    name         patch1;

    writeControl    timeStep;
    writeInterval   1
}

[hectorgabriel85]

No idea why you need total pressure on a surface when it is the same as static pressure since U_normal is zero on a solid boundary.

[AnnaF]

Hi Hector

Ah I see, good question. Of course, you are right. I do have a porous zone with an internal patch and wanted to check/try something.

Anna