dynamicOversetFvMesh with displacementLaplacian/velocityLaplacian does not preserve zeroGradient on overset patches

Summary

Steps to reproduce

The bug can be observed by making a minor modification to $FOAM_TUTORIALS/incompressible/overPimpleDyMFoam/cylinder:

• Change the boundary condition for the walls patch in cylinderAndBackground/0/pointDisplacement to

  • walls
    {
        type            oscillatingDisplacement;
        amplitude       (1 0 1);
        omega           3.14;
        value           uniform (0 0 0);
    }

I have attached this case (bug.cylinder.displacementLaplacian.tgz) below.

Incorrect behaviour can also be produced using the velocityLaplacian motion solver on this case instead of the displacementLaplacian solver; to reproduce this behaviour, make the following modifications to the same cylinder case:

• rename cylinderAndBackground/0/pointDisplacement to cylinderAndBackground/0/pointMotionU
• In the header of cylinderAndBackground/0/pointMotionU, update the object to cellMotionU
• In cylinderAndBackground/0/pointMotionU, change the walls boundary condition to:

  • walls
    {
        type            fixedValue;
        value           uniform (1 0 1);
    }

• In cylinderAndBackground/constant/dynamicMeshDict, replace displacementLaplacian with velocityLaplacian
• In cylinderAndBackground/system/fvSolution, replace cellDisplacement with cellMotionU

I have also attached this case (bug.cylinder.velocityLaplacian.tgz) below.

Example case

As noted above, here are the two cases:

• bug.cylinder.displacementLaplacian.tgz
• bug.cylinder.velocityLaplacian.tgz

What is the current bug behaviour?

As the mesh boundary condition for the overset patch is zeroGradient (overset { patchType overset; type zeroGradient; }), I expect the overset mesh region around the cylinder to rigidly translate over the background mesh. However, in the two cases above, you will see that the overset mesh patch is not acting as zeroGradient. For the displacementLaplacian case, it seems to act as zero fixedValue, whereas, for the velocityLaplacian case, it seems to act as a normal overset patch (i.e. coupled to the background mesh).

As a result, in both cases, the simulation crashes due to extreme mesh deformations in the overset region.

What is the expected correct behavior?

The expected behaviour in both cases is that the overset mesh region rigidly translates over the background mesh and the background mesh does not move.

Relevant logs and/or images

Here is an image of the mesh just before crashing in the displacementLaplacian case, where the overset patch mesh condition seems to act like fixedValue:

Here is an image of the mesh just before crashing in the velocityLaplacian case, where the overset patch mesh condition seems to act like a normal overset condition:

With the user-fix proposed below, the correct behaviour is observed for both cases:

Environment information

• OpenFOAM version : v2212
• Operating system : I have chedk ubuntu and macOS but it should be the same on all OSes
• Hardware info : N/A
• Compiler : I checked gcc (on Ubuntu) and clang (on macOS) but it should be independent of the compiler

Possible fixes

The problem seems to come from the construction of the boundary conditions for the cellDisplacement field in the displacementLaplacian class and the cellMotionU field in the velocityLaplacian class. The zeroGradient condition on the overset mesh patch does not seem to be correctly copied from the pointDisplacement and pointMotionU fields.

A simple user workaround is to provide the cell mesh motion fields in the case, including the correct boundary conditions on the overset patch. These user-provided fields are used because the cell fields are READ_IF_PRESENT in the displacementLaplacian and velocityLaplacian motion solvers.

For example, to fix the displacementLaplacian case, add the following cellDisplacement file to cylinderAndBackground/0/:

FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    object      cellDisplacement;
}

dimensions      [0 1 0 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{
    overset
    {
        patchType       overset;
        type            zeroGradient;
    }

    walls
    {
        type            cellMotion;
        value           uniform (0 0 0);
    }

    ".*"
    {
        type            uniformFixedValue;
        uniformValue    (0 0 0);
    }
}

Similarly, to fix the velocityLaplacian case, add the following cellMotionU file to cylinderAndBackground/0/:

FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    object      cellMotionU;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //                                                                                   

dimensions      [0 1 0 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{
    overset
    {
        patchType       overset;
        type            zeroGradient;
    }

    walls
    {
        type            cellMotion;
        value           uniform (0 0 0);
    }

    ".*"
    {
        type            uniformFixedValue;
        uniformValue    (0 0 0);
    }
}

Here are the two modified cases:

• fix.cylinder.displacementLaplacian.tgz
• fix.cylinder.velocityLaplacian.tgz

This user fix is a simple workaround; however, it would be better if this was fixed within the code, such that zeroGradient conditions on overset patches are correctly mapped from point fields to cell fields.