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August 30, 2009, 20:46 
wall shear stress Gradient

#1 
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Hi All
Could anyone tell me how can i calculate wall shear stress gradient in normal and tangential direction in cfxpost. Thanks 

August 31, 2009, 07:43 

#2 
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Glenn Horrocks
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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?


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 "Flowinduced Wall Shear Stress in Abdominal Aortic Aneurysms: Part I – Steady Flow Hemodynamics Computer Methods in Biomechanics and Biomedical Engineering , 2002 Vol. 5 (4), pp. 309–318" "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 tˆ and nˆ 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, 

August 31, 2009, 18:46 

#4 
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Glenn Horrocks
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The variable tu has not been defined. What is it?
I consider calculating wall shear stress gradient directly in CFDPost. 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. 

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?
thanks 

August 31, 2009, 19:42 

#6 
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Glenn Horrocks
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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 nonphysical to me. 

January 28, 2011, 05:35 

#7 
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Lance
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Sorry for bumping an 1½ 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 

January 28, 2011, 06:23 

#8 
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Glenn Horrocks
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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).


January 28, 2011, 07:07 

#9 
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Lance
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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... 

April 16, 2011, 03:06 

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ali
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Hi guys
Did any body succeed in finding a way to calculate WSSG in cfx post? I really need this 

April 16, 2011, 07:21 

#11 
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Glenn Horrocks
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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.


April 16, 2011, 07:51 

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ali
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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?


April 17, 2011, 07:25 

#13 
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Glenn Horrocks
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? But calculating gradients like this should be easy. You can set up some orthogonal lines in CFDPost 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.


April 18, 2011, 13:07 

#14 
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Lance
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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 121 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. 

April 18, 2011, 18:23 

#15 
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Glenn Horrocks
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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. 

June 5, 2011, 21:42 

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Hi
I was wondering can we use Poisson equationg in cfxpre for additional variables for calculationg WSS gradient?? 

June 6, 2011, 07:41 

#17 
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Glenn Horrocks
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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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