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-   -   Unexpected behavior for interFoam with fixedGradient pressure BC (https://www.cfd-online.com/Forums/openfoam-solving/112577-unexpected-behavior-interfoam-fixedgradient-pressure-bc.html)

styleworker January 31, 2013 04:47
Unexpected behavior for interFoam with fixedGradient pressure BC
 
2 Attachment(s)
Hello,

for validation of my actual case I'm simulating the interphase flow of the capillary rise tutorial with modified pressure boundary.

In my case, the inlet pressure BC should have a neg pressure gradient. The pressure gradient is higher than the pressure gradient produced by the curvature of the free surface, so that the fluid moves downwards and out of the inlet patch. Basically it does move downwards, but the flow flux is way too low (see case 4 in the appended picture).

I also tested a case with a positive pressure gradient (case 3). As expected the fluid moves upwards, but this case is way slower than the standard capillary rise tutorial (case 2). Actually it should move faster, because there is a additional pressure gradient beside the pressure gradient produced by the interphase.

The applied BC's for pressure are:

Code:
    inlet
    {
        type            fixedGradient;
        gradient        uniform -100;
    }

    atmosphere
    {
        type            fixedValue;
        value          uniform 0;
    }

    walls
    {
        type            fixedFluxPressure;
        adjoint        no;
    }
velocity BC's:
Code:
inlet
    {
        type            pressureInletOutletVelocity;
        value          uniform (0 0 0);
    }

    atmosphere
    {
        type            pressureInletOutletVelocity;
        value          uniform (0 0 0);
    }

    walls
    {
        type            fixedValue;
        value          uniform (0 0 0);
    }
alpha BC's:
Code:
    inlet
    {
        type            inletOutlet;
        value          uniform 1;
        inletValue      uniform 1;
    }

    atmosphere
    {
        type            zeroGradient;
    }

    walls
    {
        type          constantAlphaContactAngle;
        theta0        45;
        limit          gradient;
        value          uniform 0;
    }

styleworker January 31, 2013 10:34
The solution is actually clear. I assumed before, that the reference pressure is the pressure of the ambient air, which does not make sense :-)

Nikhilcfd May 20, 2013 22:56
Hello,

I faced a similar issue with capillary rise in interFOAM. In case 3, (where you applied positive pressure gradient) the fluid should move faster as you explained? Could you please explain how I can specify reference pressure and on what basis?

Whenever I applied positive or negative (or counter) pressure gradient (across the capillary tube/plate), I did not get required rise. When I set interface compression term (Calpha) to zero, the height rise was good in comparison with the analytical solution. For cases where I don't have pressure gradient across the capillary tube (i.e p=0 at both inlet and outlet), I got good results for both Calpha = 1 and 0 (interface was diffuse for c=0 but height rise was good). It appears that the Calpha term is imposing additional pressure on interface for cases with positive/negative pressure gradients across the capillary tubes. Please let me know your suggestions.

Thanks,

NP


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