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BC for pressure-pressure run in simpleFoam

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Old   March 25, 2020, 01:23
Default BC for pressure-pressure run in simpleFoam
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Hello all,

I am trying to run a simpleFoam case with pressure inlet and pressure outlet. I know this is not be best way to do (but it runs in the commercial solvers I know). However, in this case I only have pressure BCs available. I tried following BCs:

Pressure: "totalPressure" for inlet patch, "fixedValue" for outlet pach -> looks reasonable

Velocity: "pressureNormalInletOutletVelocity" for both inlet and outlet patches. I want the solver to calculate velocity based on fluxes and pressure and make the direction normal to the patch faces. However, this gives me a velocity field of more or less zero everywhere in the domain and therefore zero mass flow as well. -> not reasonable

What are the correct/best BCs for the velocity field for this kind of BCs? Thanks.
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Old   March 25, 2020, 04:35
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This is what I've used succesfully.

0/U
Code:
    inlet
    }
        type            pressureInletOutletVelocity;
        value           uniform (0 0 0);

    }

    outlet
    {
        type            inletOutlet;
        value           uniform (0 0 0);
        inletValue      uniform (0 0 0);
    }
0/p
Code:
    inlet
    {
        type            totalPressure;
        p0              uniform 25.5426;
        value           uniform 25.5426;
    }

    outlet
    {
        type            fixedValue;
        value           uniform 0;
    }
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Old   March 25, 2020, 07:29
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Thanks. This was one of my previous trys. Good news: Results where he same as for my last try. Bad news: Velocity results are not physical.
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Old   March 25, 2020, 07:31
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Remember to use normalized pressure.

OF uses p/rho in in-compressible cases.
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Old   March 25, 2020, 08:36
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I know, I did use normalized pressure. Volume flow is far from expected (e-10 instead of e-4). Indeed I did run the same case with velocity-inlet before and I know what pressure drop should give what volume-flow.
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Old   March 26, 2020, 03:50
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Ok, I quick-checked with the "squaredBend" tutorial with OpenFOAM-v1906. The BCs given from linnemann above works as expected for this geometry.
I am currently investigating why it does not work as expected for my model.
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Old   March 27, 2020, 03:51
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bastil,

I agree with the bc combination suggest by linnemann. For your particular case, is the dynamic pressure a significant fraction of the total pressure? If so, you may simply be setting the inflow total pressure too low.
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Old   March 27, 2020, 04:57
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Quote:
Originally Posted by tas38 View Post
For your particular case, is the dynamic pressure a significant fraction of the total pressure? If so, you may simply be setting the inflow total pressure too low.
No it's not. However, my geometry is widely spreading over serveral arms and just merging together those arms at in- and outlet. Seems there is kind of numerical flutuation between the arms that make a solution impossilbe so far.
Suggestions on how to solve this are welcome.
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