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How to set fully developed flow aty inlet?

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Old   September 25, 2015, 15:59
Default How to set fully developed flow aty inlet?
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Fernando Soares Alves
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Hi there Foamers,

I was wondering how to set properly a fully developed velocity profile WITH the pressure gradient required to push the fluid (newtonian). As far as I have checked mostly what has been adopted regards only mapping the velocity profile at the geometry inlet however with zeroGradient for the pressure field.

Is it just me or does these b.c. sound quiet off? Suppose I do know the pressure gradient in order to assure the development of the flow, which b.c. should I adopt at the outlet? I mean for a simple geometry, e.g. a backward facing step for instance.

I was trying something like

U:
Code:
    inlet
    {
        type            fixedValue;
        value           nonuniform List<vector>
     (developed velocity profile here ...)
     }
    outlet
    {
        type            inletOutlet;
        inletValue      uniform (0 0 0);
        value           uniform (0 0 0);
     }
p:
Code:
    inlet
    {
        type            fixedGradient;
        gradient        uniform (~known Pressure Gradient);
    }
    outlet
    {
        type            outletInlet;
        outletValue     uniform 0;
        value           uniform 0;
    }
and I get no good convergence whatsover. (FYI I start the run with a mapped solution obtained from a previous result for a smaller geomtery). A plot of the residuals I am getting for simpleFoam with standard kEpsilon model is attached to this thread.

How is one suppose to set such a boundary condition in order to assure the velocity profile is in fact developed (i.e. with pressure gradient at the inlet)? I've already used zeroGradient for pressure, but it seems to me such condition doesn't agree with definition of developed flow.

Best regards to all and many thanks in advance for any advise.

F. Soares

PS: I found this thread on the subject emphasizing what I mean.
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Old   September 28, 2015, 02:57
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Nima Samkhaniani
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instead of using zeroGradient, employing fixedFluxPressure at inlet for pressure will consider your concerns. because fixedFluxPressure sets the pressure gradient based on velocity field
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Old   September 30, 2015, 13:44
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Fernando Soares Alves
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Sorry for the late reply, but thanks for your advise. I am trying to use the b.c. you mentioned but I am getting the following error message

Code:
--> FOAM FATAL ERROR: 
updateCoeffs(const scalarField& snGradp) MUST be called before updateCoeffs() or evaluate() to set the boundary gradient.

    From function fixedFluxPressureFvPatchScalarField::updateCoeffs()
    in file fields/fvPatchFields/derived/fixedFluxPressure/fixedFluxPressureFvPatchScalarField.C at line 151.

FOAM exiting
I must say I am not sure how to set it in my 0/p file. According to the damBreak tutorial

Code:
    leftWall
    {
        type            fixedFluxPressure;
        value           uniform 0;
    }

    rightWall
    {
        type            fixedFluxPressure;
        value           uniform 0;
    }

    lowerWall
    {
        type            fixedFluxPressure;
        value           uniform 0;
    }
although these are used at walls not at an inlet.

Are you familiar with the proper setting for fixedFluxPressure? What am I missing here?

Once again thank you for your help.

F. Soares
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