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Cannot achieve negative wall shear stress values in CFD Post (ANSYS) 

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August 2, 2016, 11:52 
Cannot achieve negative wall shear stress values in CFD Post (ANSYS)

#1 
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Sam
Join Date: Aug 2016
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Hello all,
For reference, I'm using ANSYS 17.1 CFX I am working with a simulation that models 1/6 the geometry of a circular disk (so it basically looks like a slice of pizza). I input a rotational sine wave on the fluid above the disk, which should cause a shear stress equivalent sine wave with an amplitude of +/ 2 Pa. (Labeled Target on graph) In CFD Post, I am extracting the radial component of the shear stress with the transform Radial Wall Shear Stress = (0.5 Wall Shear Stress X + 0.866 Wall Shear Stress Y) (from a trig transform). When the sine wave should be changing direction and going negative down to 2 Pa, instead, the waveform achieves a minimum value of ~ 0.4 Pa (Waveform Labeled "0" on graph). I have tried to overexaggerate the input angular velocity of the negative portion of the sine wave, and when I do this, I get a curve full of positive "humps" (think the first half period of a sine wave, repeating over and over at the applied amplitude). (Waveform Labeled "1" on graph) Do you think this could be a problem because I'm modeling 1/6 of the geometry as a symmetry plane? Or is there some problem with my post processing? TL;DR = Cannot achieve negative wall shear stress values in simulation with a sine wave input (and I'm not exporting the absolute value or magnitudes of shear stress, either). Any help is much appreciated! Last edited by SAtkins; August 2, 2016 at 12:06. Reason: Image Attachment 

August 2, 2016, 20:39 

#2 
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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The variable "Wall Shear Stress" is the magnitude of the vector so is always positive. But "Wall Shear Stress X/Y/Z" are the components and should be able to go negative when they need to. So I do not know why you are seeing what you are seeing.
Your result also looks wobbly so I am suspicious there is general accuracy problems as well. Have a look at the FAQ: http://www.cfdonline.com/Wiki/Ansys..._inaccurate.3F 

August 3, 2016, 08:14 

#3 
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Sam
Join Date: Aug 2016
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Right, I am extracting just the Wall Shear X and Wall Shear Y components now  my Wall Shear X goes negative in the appropriate places while my Wall Shear Y is very positive (2 times the magnitude of X). My equation for radial shear stress should take care of the positive values, though, as it adds 0.866 Wall Shear Y.
I've looked at that link  my solutions are definitely converging. I've ran a mesh analysis and don't see any differences in output when changing the grid size. At this point I'm pretty stuck 

August 3, 2016, 14:27 

#4 
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Sam
Join Date: Aug 2016
Posts: 3
Rep Power: 8 
Update:
I have figured out the problem  and it's my bad trig. Instead of using the formulas I did to pull out what I called radial, which is really circumferential, shear stress  I just needed to use simple trigonometry. Circ = cos(30)WSSx  sin(30)WSSy Circ 1 = cos(3)WSSx + sin(30)WSSy Circ produced a waveform that was inverted from my target. Circ 1 seemed to produce the output I was looking for (minus, minor inertial effects not taken into the WSS to AV transform I used  which is expected). Anyways, I wanted to update with the hope that anyone having this problem in the future might get some help on transforming x and y WSS components into a circumferential shear stress component tangential to the direction of a rotational flow. 

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cfd post, wall shear stress 
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