grid normal to wall

lavdwall · June 13, 2013, 11:38

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?

Far · June 13, 2013, 12:22

It is related to computation of data interpolation. If cells have 90 angle you will have less error.

jchawner · June 16, 2013, 18:06

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.

hnemati · January 7, 2014, 04:19

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 1.845e-3 and at the wall (r=1) is 9.203e-4. But at the distance a bit far from centerline (r=0.409) the mesh size is 9.083e-3 which says 5 time bigger than cenerline. Could please someone help me and explain me the reasean.

June 13, 2013, 11:38	grid normal to wall	#1
lavdwall New Member Laurien Vandewalle Join Date: Jun 2013 Location: Ghent, Belgium Posts: 29 Rep Power: 12	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?

June 16, 2013, 18:06		#3
jchawner Senior Member John Chawner Join Date: Mar 2009 Location: Fort Worth, Texas, USA Posts: 275 Rep Power: 18	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. __________________ John Chawner / jrc@pointwise.com / www.pointwise.com Blog: http://blog.pointwise.com/ on Twitter: @jchawner

January 7, 2014, 04:19	Wall normal grid in a pipe	#4
hnemati Member SAM Join Date: Apr 2012 Posts: 74 Rep Power: 13	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 1.845e-3 and at the wall (r=1) is 9.203e-4. But at the distance a bit far from centerline (r=0.409) the mesh size is 9.083e-3 which says 5 time bigger than cenerline. Could please someone help me and explain me the reasean.

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June 13, 2013, 12:22		#2
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	It is related to computation of data interpolation. If cells have 90 angle you will have less error.