CFD Online Discussion Forums - [swak4Foam] Dynamically morphing airfoil using groovyBC

Hello

I am trying to simulate 2D flow around a dynamically morphing airfoil using OpenFOAM-v2106. The airfoil surface near the trailing edge is supposed to morphed downwards with time. I believe this can be achieved using groovyBC. I set everything up and the mesh seems to move, but I am facing a weird issue.

The original C-type mesh is created in ICEM CFD, and exported to OpenFOAM. I want to keep the first cell height near the wall as constant throughout the simulation in order to keep a constant y+. But as you can see from the screenshots, in the very first iteration something weird happens and the mesh moves in an abnormal way. In the first time step, the airfoil wall didn't move, but the mesh around it moved. This results in a deformed mesh where the wall distance become reduced on the top side but keeps as expected in the bottom side. Later from the next time-step on-wards, the mesh deforms in the same manner I expected, and from thereon the wall distance no longer changes.

I cannot post the case file here, because the mesh size is large. A minimalistic cellDisplacementDict to reproduce this is given below.

Code:

FoamFile

{

    version     2.0;

    format      ascii;

    class       volVectorField;

    object      cellDisplacement;

}

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //



dimensions      [0 1 0 0 0 0 0];



internalField   uniform (0 0 0);



boundaryField

{

    TOP

    {

           type          groovyBC;

         value        uniform (0 0 0);

          valueExpression    "vector(0,df,0)"; 

    

           variables

           (

        "df=-0.3*time();" 

           );

    }



    BOTTOM

    {

           type          groovyBC;

         value        uniform (0 0 0);

          valueExpression    "vector(0,df,0)"; 

    

           variables

           (

        "df=-0.3*time();" 

           );

    }

    

    FIXEDWALLS

    {

        type            fixedValue;

        value           uniform (0 0 0);

    }

    INLET

    {

        type            fixedValue;

        value           uniform (0 0 0);

    }



    OUTLET

    {

        type            fixedValue;

        value           uniform (0 0 0);

    }



    frontAndBack

    {

        type            empty;

    }

}

This is dynamicMeshDict

Code:

FoamFile

{

    format      ascii;

    class       dictionary;

    location    "system";

    object      dynamicMeshDict;

}



// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //



 dynamicFvMesh   dynamicMotionSolverFvMesh;



 motionSolverLibs ("libfvMotionSolvers.so");



 motionSolver    displacementLaplacian;



 diffusivity  directional (20 0.1 0);

What am I doing wrong? Any suggestions would be greatly appreciated.