[stergiani]

Hei!


I have read a lot of relative posts, but i haven't managed to solve my problem, so I need some help
I'm using k-epsilon turbulence model with SWF in order to model pressure around a building. My mesh is quite fine and I get y+ values lower than 300. Buuut, they are not in the range 30<y+<300, my minimum y+=3
I use SnappyHexMesh, but I haven't managed to increase the y+ values.
I have read that the first cell size is defined by number of Surface layers, final layer thickness and expansion ratio. I have tried to increase the expansion ratio and finalLayerThickness but I failed.
Should I have relative sizes to 'true' and try to increase the minThickness?

// Settings for the layer addition.
addLayersControls
{

    relativeSizes false;

    // Per final patch (so not geometry!) the layer information
    layers
    {
        z_
        {
            nSurfaceLayers 5;

        }

        targetBuilding_buildingObjects
        {
            nSurfaceLayers 5;

        }

/*
        otherBuildings_wall
        {
            nSurfaceLayers 1;

        }
*/
    }

    // Expansion factor for layer mesh
    expansionRatio 1.0;

    //- Wanted thickness of final added cell layer. If multiple layers
    //  is the
    //  thickness of the layer furthest away from the wall.
    //  Relative to undistorted size of cell outside layer.
//    finalLayerRatio 0.7; 

    finalLayerThickness 0.1;

    //- Minimum thickness of cell layer. If for any reason layer
    //  cannot be above minThickness do not add layer.
    //  Relative to undistorted size of cell outside layer.
    minThickness 0.01;

    //- If points get not extruded do nGrow layers of connected faces that are
    //  also not grown. This helps convergence of the layer addition process
    //  close to features.
    nGrow 1;

Thanks
Stergiani

[simrego]

Hi!


Try with firstLayerThickness instead of finalLayerThickness. So you can define the thickness of the 1st layer directly.

[stergiani]

Hei!

Thanks for the reply simrego!
I tried it but i still get min y+ around 2. the max is okay, around 200
I calculated the y+ from the calculator https://www.cfd-online.com/Tools/yplus.php
Would it be better to turn the relativeSizes to 'true'? And the minThickness can be a lower value or should be the same? I'm a bit confused
Thanks anyway

Stergiani

[simrego]

These y+ calculators are good for an initial guess, but since y+ depends on local variables, it won't be correct on the whole domain.

To get the proper y+ (in your case 30<y+<300) everywhere is nearly impossible in most of the cases.
relativeSizes just set the BL thickness relative to the 1st cell near the wall. With relativeSizes it is more difficult to control the BL thickness, but sometimes snappy can create a better quality BL with it.

Check where do you have this really small y+. Then you can create larger layer on these regions, but then you will need a new sHMDict, new mesh, new run, and check again. So it'll be a long loop.

Or you can use a blended wall function (check wall functions) which will switch based on the y+.




Edit:
But if you check where is this small y+ region, and if it is "small" and far from the interesting region, and your results seems to be reliable or you can compare your results with measurements or hand calculations, then you could say that you results are fine with some uncertainty.

[stergiani]

Thanks, I'll check that