# 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 New Member   Sam Join Date: Aug 2016 Posts: 3 Rep Power: 9 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 over-exaggerate 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 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,731 Rep Power: 143 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.cfd-online.com/Wiki/Ansys..._inaccurate.3F

 August 3, 2016, 08:14 #3 New Member   Sam Join Date: Aug 2016 Posts: 3 Rep Power: 9 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 New Member   Sam Join Date: Aug 2016 Posts: 3 Rep Power: 9 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.

 Tags cfd post, wall shear stress