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min y+ that can be gain with snappyHexMesh around airfoil

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Old   April 9, 2013, 01:54
Unhappy min y+ that can be gain with snappyHexMesh around airfoil
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s.m
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saeideh mohamadi
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Hi,
i have a one element and a three element airfoil, and i wand use low Re turbulence models so i need yPlus < 50 for my mesh. i use snappyHexMesh.
my problem is to create a boundary layer with snappyHexMesh. How to control the number and thickness of the layers?
with what numbers in add layerControls we can reach to y+ < 50 ?
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Old   April 23, 2013, 10:57
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HECKMANN Frédéric
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First you have to "guess" the thickness of your first cell: http://www.cfd-online.com/Tools/yplus.php
You cannot calculate exactly what is the thickness needed because y+ depends of your solution. The link I gave you can however give you a good estimation.

I don't use snappyHexMesh but I can tell you that your first cell height must be of a close value from the estimation given by the link and the growth rate must be lower than 1.2.

Hope somebody can help you with Snappy
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Old   April 23, 2013, 18:00
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Phillip
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Quote:
Originally Posted by s.m View Post
Hi,
i have a one element and a three element airfoil, and i wand use low Re turbulence models so i need yPlus < 50 for my mesh. i use snappyHexMesh.
my problem is to create a boundary layer with snappyHexMesh. How to control the number and thickness of the layers?
with what numbers in add layerControls we can reach to y+ < 50 ?
Hey s.m,

when you use a so-called "low-Re" turbulence model the yPlus values are usually much lower in this assumption (y+ ~ 1) or (y+ < 5, with attention).

In snappyHexMesh you can control the number and thickness of the layers with:
Code:
addLayersControls
{
    relativeSizes true; // when set to false --> change thickness to absolute values 
    layers
    {
        myWall
        {
            nSurfaceLayers 5;
        }
    }

    expansionRatio 1.0;
    finalLayerThickness 1.0;
    minThickness 0.1;
    nGrow 0;
    featureAngle 30; 
    slipFeatureAngle 30;
    nRelaxIter 5; 
    nSmoothSurfaceNormals 1;
    nSmoothNormals 3;
    nSmoothThickness 10;
    maxFaceThicknessRatio 0.5; 
    maxThicknessToMedialRatio 0.3;
    minMedianAxisAngle 80; 
    nBufferCellsNoExtrude 0;
    nLayerIter 50;
}
For more details look at: http://openfoamwiki.net/images/f/f0/Final-AndrewJacksonSlidesOFW7.pdf

For a good mesh you have to change the upper parameters, because this is only a example of usage...
hope i can help you? For more details post your case and entitle your problem with some code-examples and pictures...
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Old   April 24, 2013, 04:58
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Andreas Wendy
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hi,

like Frédéric said you can guess use the tool to calculate the thickness of the first wall layer. the final layer thickness in snappy hexmesh is the layer thickness wich is tha last layer (not the one at the wall) so you have to calculate final oundary layer depending on the expension ratio, number of layers and desired first layer thickness. yfinal = y*ratio^(n-1) (I think).
be sure to set relativeSize to false!

if you want to resolve your y+ should be smaller than 1 but at least below 5!

best wishes

Andreas
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