[DJF]

I want to set a velocity profile at the inlet boundary as a function of the vertical distance to the non-planar ground surface. I'm using OpenFOAM v2106. So far I've obtained a volScalarField of wallDistance using the checkMesh utility and setting in system/fvSchemes:

wallDist
{
    method      directionalMeshWave;
    normal      (0 0 1);
}

The volSacalarField is in 0/wallDistance and looks like this:

FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    location    "0";
    object      wallDistance;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 1 0 0 0 0 0];


internalField   nonuniform List<scalar> 
2610223
(
34.4376
36.2435
36.5364
...

In the 0/U file I want to set for the inlet boundary the x-component of U as a function of the wallDistance using an expression. (It works using the z-coordinate pos().z() but I need instead the wallDistance since the ground surface ist not planar):

FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    object      U;
}
// ============================================== //

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

internalField   uniform (7.9 0 0);

boundaryField
{
    ground
    {
        type            noSlip;
    }
    
    "(sides|top)"
    {
        type            symmetry;
        value          $internalField;
    }
   
    inlet
    {
        type    uniformFixedValue;
        uniformValue
        {
            type    expression;

            variables
            (
                "z0 = 0.08" //[m] roughness length
                "zRef = 4.5" //[m] reference height
                "Uref = 7.9" //[m/s] wind speed @zRef
            );

            expression
            #{
                vector(Uref/log(zRef/z0)*log(wallDistance/z0), 0, 0) //[m/s] wind profile log-law
            #};
        }
    }
  
    outlet
    {
        type            pressureInletOutletVelocity; 
        value          $internalField;
    }
}

However the Object wallDistance doesn't exist for the solver when I run the simulation. How can I use the volScalarField wallDistance in an expression for the inlet boundary? Should I read it before?
I've tried the solutions in wallDist already without success