grid normal to wall
Hi all!
Maybe a very stupid question to you but I only just started studying CFD and I was wondering... Why exactly is it necessary to make the angle between the wall and the grid as close to 90° as possible? It is always done like that but what is the mathematical/physical reason behind it? 
It is related to computation of data interpolation. If cells have 90 angle you will have less error.

When you have boundary conditions that include terms like the gradient of the pressure normal to the wall is zero at the wall it's easier to enforce of the grid is also orthogonal. Also, some turbulence models benefits from having grid lines normal to the wall due to terms involving distance to the wall.
However, I'm certain someone can cite a very good reason why the numerics benefits from orthgonality. 
Wall normal grid in a pipe
Dears
I have question regarding the wall normal grid style: In the Wu and Moin paper (A direct numerical simulation study on the mean velocity characteristics in turbulent pipe flow) the mesh that they used is a bit confusing for me. In the centerline (r=0) the mesh size is 1.845e3 and at the wall (r=1) is 9.203e4. But at the distance a bit far from centerline (r=0.409) the mesh size is 9.083e3 which says 5 time bigger than cenerline. Could please someone help me and explain me the reasean. 
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