[Astrodan]

Hello there,

I have a (sloped) channel, for which I have an inlet patch defined. Now I don't want the whole inlet to act as such, but only a small, 1cm high part of it (see attached image).
My approach was to select all faces of the original inlet patch, remove the 1cm part (which starts 2cm above the bottom) and then declare the selected faces as wall patch.

This process generally works, but I can't select the correct cells dynamically. My topoSetDict looks as follows:

actions
(
    {
        name        inletWallSet;
        type        faceSet;
        action      new;
        source      patchToFace;
        sourceInfo
        {
            name    inlet;
        }
    }
    
    {
        name        inletWallSet;
        type        faceSet;
        action      delete;
        source      expressionToFace;

        sourceInfo
        {
            variables
            (
                "inletStart=0.02;"
                "inletHeight=0.01;"
                "zMin=min(pos().z);"
                "start=zMin+inletStart;"
                "end=zMin+inletStart+inletHeight;"
            );
            debugCommonDriver 1;
            expression  "fpos().z>=interpolate(start)&&fpos().z<=interpolate(end)";
        }
    }
);

The idea was that I get the minimum z value on the patch in zMin, and use this to set the start of my actual inlet. However, min(pos().z) returns the minimum z-position over the complete domain, and since my channel is sloped this returns the wrong result.

I have tried working with onPatch(inlet), (e.g. zMin="(onPatch(inlet) ? fpos().z : surf(999);"), but this either fails or always pretends that the minimum value is 0 (which is right, incidently, but when i translate the mesh in z direction it still returns 0, even if this is outside of the bounding box).

Is there anything I'm missing? Does anyone have an alternative idea?

Thanks for any replies,
Timm

[Astrodan]

Ok, due to the mass of replies I choose another way and used some basic algebra which I had completely forgotten since leaving school.

However, it now seems to be running, so I present the new topoSet source "patchPartToFace". An entry for this looks lie this:

    {   
        name        selectFaceSet;
        type        faceSet;
        action      new;
        source      patchPartToFace;
        sourceInfo
        {
            name        inlet;
            radius      0.03;
            direction   (1 0 0); 
            inside      true;
        }
    }

How does it work?
Based on the name, it selects a patch and tries to figure out what is the least distance of the vector given by dir and the coordinates of all patch faces is. This basically moves the given direction vector onto the plane that is the patch.
When this is done, all cells that are within radius around this vector will be selected (inside = true) or not selected (inside = false).

To use it, extract the attached .zip file, and run "wmake lib" in the main directory "topoSet". The library will be install in $FOAM_USER_LIBBIN as libtopoSet. If you want to use it, you will also need to add it in the controlDict under libs();

Example: To achieve the goal I posted in the image of the first post, I used the following commands:

    {   
        name        inletWallSet;
        type        faceSet;
        action      delete;
        source      patchPartToFace;
        sourceInfo
        {
            name        inlet;
            radius      0.03;
            direction   (1 0 0); 
            inside      true;
        }
    }   

    {   
        name        inletWallSet;
        type        faceSet;
        action      add;
        source      patchPartToFace;
        sourceInfo
        {
            name        inlet;
            radius      0.02;
            direction   (1 0 0);
            inside      true;
        }
    }

Note: Sadly, the calculations seems to be highly inaccurate when using a direction that is not aligned to the coordinate system close to (0 0 0), where "close" is approximately 0.1 m. I suppose it's some numerical reason that could be fixed, but right now I just don't care enough. A workaround would be to first use transformPoints -translate '(1 1 1)', then use topoSet and finally transform the points back.