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How to move background mesh with body in overset mesh method?

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Old   July 5, 2020, 07:17
Default How to move background mesh with body in overset mesh method?
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Antoni Alexander
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Hello everyone! I am trying to simulate a boat accelerating case with v1806 which contains overset mesh method. In the case, the accelerating phase is most concerned. The acceleration can be realized by rotating the propeller. However, because the boat moving distance is very long, to save grid I want to simulate the motion by making background mesh follow the boat in x-direction. So how to do that?
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Old   July 8, 2020, 21:15
Smile rigidBodyDynamics seems to be the entry.
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Antoni Alexander
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As OpenFOAM uses the rigidBodyDynamics lib to handle multi-body motions within overset mesh, it seems that the rigidBodyDynamics needs to be modified.

I am currently working on it.
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Old   July 12, 2020, 23:12
Default Background mesh movement is done!
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Antoni Alexander
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I have successfully made the background mesh moving by adding a restraint in rigidBodyDynamics. From the flowing picture, we can see the background moving to the left.

An other question is rising, that is the inlet boundary condition. I used the condition same as the DTCHull tutorial. But the water is flowing away during the simulation. However, if i set the value from to 0 to 1 in the alpha.water file, there is a thin layer of water covering the whole inlet boundary cell, which is still not what I want.

So, how to set he water level at inlet boundary when moving the background mesh?

The following picture is the result of my 2D floating case.
Attached Images
File Type: jpg before1.jpg (58.1 KB, 10 views)
File Type: jpg after1.jpg (59.8 KB, 6 views)
File Type: jpg before2.jpg (58.1 KB, 6 views)
File Type: jpg after2.jpg (60.3 KB, 8 views)
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Old   July 13, 2020, 11:24
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Felix Weiler
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Hi Antoni,


did you use the "setFields" command and "setFieldsDict", respectively? That should set separate the inlet faces between air (0) and water (1) in alpha.water.



Best regards
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Old   July 13, 2020, 21:46
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Antoni Alexander
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Hi Felix, thanks for your reply.

Yes, I used the setFields command in the preprocess, and the setFieldsDict is as follow:
Code:
defaultFieldValues
(
    volScalarFieldValue alpha.water 0
    volScalarFieldValue zoneID 123
);

regions
(
    boxToCell
    {
        box ( -100 -100 -100 ) ( 100 0.1 100 );
        fieldValues ( volScalarFieldValue alpha.water 1 );
    }

    cellToCell
    {
        set c0;

        fieldValues
        (
            volScalarFieldValue zoneID 0
        );
    }
    cellToCell
    {
        set c1;

        fieldValues
        (
            volScalarFieldValue zoneID 1
        );
    }

);
When I changed the fixedValue to the variableHeightFlowRate at inlet for alpha.water, the case run.
alpha.water:
Code:
dimensions      [0 0 0 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    #includeEtc "caseDicts/setConstraintTypes"

    inlet
    {
        //type            fixedValue;
        //value           uniform 0;
        type            variableHeightFlowRate;
        lowerBound      0;
        upperBound      1;
        value           $internalField;

    }

    outlet
    {
        type            variableHeightFlowRate;
        lowerBound      0;
        upperBound      1;
        value           $internalField;
    }


    stationaryWalls
    {
        type            zeroGradient;
    }
    atmosphere
    {
        type            inletOutlet;
        inletValue      uniform 0;
        value           uniform 0;
    }
    floatingObject
    {
        type            zeroGradient;
    }
    bgFrontAndBack
    {
        type empty;
    }

    frontAndBack
    {
        type empty;
    }
}
And the p_rgh and U is as follow:
Code:
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p_rgh;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [1 -1 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{

    inlet
    {
        type            fixedFluxPressure;
        value           $internalField;
       //type            zeroGradient;
    }

    outlet
    {
        type            zeroGradient;
    }
    oversetPatch
    {
        type            overset;
    }
    stationaryWalls
    {
        type            fixedFluxPressure;
    }
    atmosphere
    {
        type            totalPressure;
        p0              uniform 0;
        U               U;
        phi             phi;
        rho             rho;
        psi             none;
        gamma           1;
        value           uniform 0;
    }
    floatingObject
    {
        type            fixedFluxPressure;
        value           $internalField;
    }
    overset
    {
        patchType       overset;
        type            fixedFluxPressure;
    }
    bgFrontAndBack
    {
        type empty;
    }

    frontAndBack
    {
        type empty;
    }
}
Code:
FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Umean 0;
mUmean 0;
dimensions      [0 1 -1 0 0 0 0];

internalField   uniform ($mUmean 0 0);

boundaryField
{
    #includeEtc "caseDicts/setConstraintTypes"



    inlet
    {
        //type            pressureDirectedInletOutletVelocity;//freestreamVelocity;//fixedValue;
        //value           $internalField;
        //freestreamValue    (0 0 0);
        //inletDirection (1 0 0);
        //value           uniform  (0 0 0);

        type            fixedValue;
        value           $internalField;
        
        
    }

    outlet
    {
        //type            outletPhaseMeanVelocity;
        //alpha           alpha.water;
        //Umean           $Umean;
       // value           $internalField;
        type            pressureDirectedInletOutletVelocity;
        inletDirection (1 0 0);
        value           $internalField;// uniform  (0 0 0);


    }
    stationaryWalls
    {
        type           movingWallVelocity;// fixedValue;
        value           uniform (0 0 0);
    }
    atmosphere
    {
        type            pressureInletOutletVelocity;
        tangentialVelocity $internalField;
        value           uniform (0 0 0);
    }
    floatingObject
    {
        type            movingWallVelocity;
        value           uniform (0 0 0);
    }
    bgFrontAndBack
    {
        type empty;
    }

    frontAndBack
    {
        type empty;
    }
}
However, With more complicated meshes the simulation seemed to blow up. I suspect the combination of alpha.water, p_rgh and U is not stable.
Any ideas which combinations for p_rgh and U could work when fixing the inlet alpha.water level?
Hints are highly welcome!
Antoni
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Old   July 15, 2020, 05:59
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Felix Weiler
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If a more complex mesh blows up the simulation, I'd assume that the mesh is responsible, not the boundary conditions.



Is the quality of the mesh acceptable? In particular with respect to overset, a suitable interpolation area must be given.
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