CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

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

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 2, 2016, 11:52
Default Cannot achieve negative wall shear stress values in CFD Post (ANSYS)
  #1
New Member
 
Sam
Join Date: Aug 2016
Posts: 3
Rep Power: 8
SAtkins is on a distinguished road
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
SAtkins is offline   Reply With Quote

Old   August 2, 2016, 20:39
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,139
Rep Power: 134
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
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
ghorrocks is offline   Reply With Quote

Old   August 3, 2016, 08:14
Default
  #3
New Member
 
Sam
Join Date: Aug 2016
Posts: 3
Rep Power: 8
SAtkins is on a distinguished road
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
SAtkins is offline   Reply With Quote

Old   August 3, 2016, 14:27
Default
  #4
New Member
 
Sam
Join Date: Aug 2016
Posts: 3
Rep Power: 8
SAtkins is on a distinguished road
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.

SAtkins is offline   Reply With Quote

Reply

Tags
cfd post, wall shear stress

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Natural convection in a closed domain STILL NEEDING help! Yr0gErG FLUENT 4 December 2, 2019 00:04
Calculation of Wall Shear Stress Dave442 CFX 36 July 12, 2016 04:12
Wall shear stress in channel flow sreeyuth OpenFOAM Running, Solving & CFD 2 September 5, 2014 10:02
How to get a value of wall shear stress for UDF shileijerry Fluent UDF and Scheme Programming 0 June 18, 2013 10:50
WALL SHEAR STRESS Min-Hua Wang CFX 0 April 3, 2002 17:40


All times are GMT -4. The time now is 16:28.