# sr #109148: CFD exact cell hight (wall distance) and growth rate(s)?

 Submitter: Klaus Burkart Submitted: Tue 20 Sep 2016 04:29:35 PM UTC Category: Open discussion Priority: 5 - Normal Severity: 3 - Normal Privacy: Public Assigned to: None Open/Closed: Open Operating System: GNU/Linux
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## Discussion

Tue 20 Sep 2016 07:48:07 PM UTC, comment #2:

I quickly hacked a function to create the offset curve for the boundary layer. See cfd_2d_2.py and resulting mesh in naca_0015_5.png.
For clarity of the image, I only used 5 elements over the thickness. The cell thickness can be varied, by specifying a list of relative node positions (see example in the script).

Benedict Verhegghe <bverheg>
Tue 20 Sep 2016 06:50:37 PM UTC, comment #1:

The idea is to define an extra curve between the airfoil and the outer boundary. The easiest way to create that curve would be to scale it from its center. But since you probably want to have a good control over the boundary layer thickness, a better approach would be to create a fixed offset from the boundary curve.

pyFormex currently does not have such a function for curves, but there is a similar one for triangulated surfaces (in pyformex/trisurface.py). Here is its implementation:

def offset(self,distance=1.):
"""Offset a surface with a certain distance.

All the nodes of the surface are translated over a specified distance
along their normal vector.
"""
n = self.avgVertexNormals()
coordsNew = self.coords + n*distance
return TriSurface(coordsNew, self.getElems(), prop=self.prop)

A similar method could be created for curves.

Benedict Verhegghe <bverheg>
Tue 20 Sep 2016 04:29:35 PM UTC, original submission:

Some types of airfoil simulations require a very fine mesh near the airfoil surface to resolve the physics of the boundary layer.

The boundary layer mesh for such cases is defined by the number of cells around the airfoil profile (cross section) and the cell thickness of the near surface cell layers. The latter is defined by the cell thickness (wall distance) of the first layer (which can be calculated from physics/boundary layer laws) and a layer thickness growth rate applicable to the subsequent cell layers.

The optimum would be to be able to define two growth rates for the cell layer thickness:

A) For the actual boundary layer mesh (usually 20-30 cell layers)

B) For the rest of the flow domain

What would be an approach to define the wall distance = first cell layer thickness in combination with growth rate(s)?

What appears in the attached picture to be a thick white contour is actually the fine boundary layer mesh which is continued with larger cells in the flow domain.

Klaus Burkart <klausb>

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file #38575:  cfd_2d_2.py added by bverheg (2KiB - text/x-python)
file #38576:  naca_0015_5.png added by bverheg (24KiB - image/png)

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