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Old   August 30, 2009, 20:46
Default wall shear stress Gradient
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Hi All
Could anyone tell me how can i calculate wall shear stress gradient in normal and tangential direction in cfx-post.

Thanks
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Old   August 31, 2009, 07:43
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Wall shear stress only exists on the wall (ie a surface) so how can you have a normal gradient of it?
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Old   August 31, 2009, 09:22
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Hi glen,
I 'm not really sure how to answer your question. I found below formula from this paper
"Flow-induced Wall Shear Stress in Abdominal Aortic Aneurysms: Part I Steady Flow Hemodynamics


Computer Methods in Biomechanics and Biomedical Engineering
, 2002 Vol. 5 (4), pp. 309318"

"The calculation of the local Wall Shear Stress Gradient (WSSG) is based on the predictor equation proposed by Lei and Kleinstreuer [19] at the cellular level:
WSSG=sqrt((dtu/dt^)^2+(dtu/dn^)^2)
where t and n are the local tangential and normal directions to the wall,"

I want to look at rupture of aneurysm so in addition to the magnitude , I need to consider the direction of the WSS as well so I tought this eqation might help me.
Do you have any idea that can help me?
By the way I'm living in sydney too
Best Regards,


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Old   August 31, 2009, 18:46
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The variable tu has not been defined. What is it?

I consider calculating wall shear stress gradient directly in CFD-Post. Have a look in the CEL Expression language documentation (in the reference manual) to see the available functions. It includes gradient functions, but when I try to apply it to a boundary only variable it gives an error.
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Old   August 31, 2009, 19:28
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Tu is a wall shear stress. I couldn't find the gradient function in CCL. I just found an example that suggest a fortran dll. and itroducing a gradient function as an additional variable. I don't have Fortran to complie the dll. Do you have another suggestion?

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Old   August 31, 2009, 19:42
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There are a lot of new gradient calculation functions in V12. Are you using V12?

If tu is the wall shear stress then I go back to my original question - wall shear stress is not defined away from the wall so the gradient is not defined. The formula seems non-physical to me.
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Old   January 28, 2011, 05:35
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Sorry for bumping an 1 year old thread, but does anyone have an idea on how the calculate the gradient of the wall shear?
Using the gradient calculator in POST gives me
Internal error detected:
Gradient won't apply to boundary only variable "Wall Shear X.Gradient"

edit: Im using V13
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Old   January 28, 2011, 06:23
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Yes, that might be tricky in CFX. Wall shear only exists on the walls, and the gradient function only applies to the volume. Not sure what you can do to get the gradient on a wall. You might have to export to another post processor like tecplot, ensight or fieldview (and I am not sure they can do it either).
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Old   January 28, 2011, 07:07
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Hi,
I also think that post cant do this. I have exported the wall shear values to Matlab, and Im trying to calculate the gradient in various ways, none that seems to work though...
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Old   April 16, 2011, 03:06
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Hi guys

Did any body succeed in finding a way to calculate WSSG in cfx post? I really need this
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Old   April 16, 2011, 07:21
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As I said at the time, I suggest you export the wall shear stress and XYZ locations from Post and calculate them using another application. That should be pretty easy.
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Old   April 16, 2011, 07:51
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I do not have time to find out about a way to introduce WSSG in another software all by myself. Has anybody succeeded in solving this problem?
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Old   April 17, 2011, 07:25
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? But calculating gradients like this should be easy. You can set up some orthogonal lines in CFD-Post to export the points from so the gradient calculation is very simple. And packages like Tecplot, Fieldview or Ensight probably have in built gizmos to calculate this sort of thing.
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Old   April 18, 2011, 13:07
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This is what I ended up doing:

Export the WSS and coordinates on the entire surface and compute the gradient (in e.g. Matlab) using the method described in "An implicit upwind algorithm for computing turbulent flows on unstructured grids" Kyle Anderson and Daryl L Bonhaus, Computers & Fluids Volume 23, Issue 1, January 1994, Pages 1-21

It is pretty straightforward, all you need to do is a nearest neighbor search for each node and then implement eqn. 20. It works with unstructured/structured grids.
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Old   April 18, 2011, 18:23
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Good idea Lance. But my comment about extracting orthogonal lines on the surface is probably easier as then you can do simple linear gradients along the lines as there is no geometric cross terms. But on the other hand my suggestion will reduce resolution.

So there you go, two ways of doing it.
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Old   June 5, 2011, 21:42
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Hi
I was wondering can we use Poisson equationg in cfx-pre for additional variables for calculationg WSS gradient??
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Old   June 6, 2011, 07:41
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You can, but note that it will not be accurate as curvature of the surface will distort it. If the surfaces are flat (or at least flat enough) then this is a good idea.
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