Jumpy Wall Shear Behavior
 

Hello,

when I took a look at some of my results today I noticed a rather strange wall shear behavior. A screenshot is attached.
Has anyone else encountered such 'spiky' wall shear behavior?
Velocity and other stuff is fine.

Leif

I do not know what you are graphing and I do not know what spike you are concerned about. Please explain what you are doing.

Sorry about that. I am graphing the wall shear in streamwise direction over the span of an airfoil.
The spikes I am talking about are those in the graph - as far as I know it should be a smooth curve.
The case has reached convergence and other stuff, for example the velocity field around the airfoil, look just fine.

Turbulence Model used was SST Gamma Theta for Transition.

I also made some more pictures ... maybe those help understanding the problem ... I am still kinda new to cfd stuff so I might give too little / wrong information - if thats the case please tell me what you need!

Which spike don't you like?

Don't forget that you will have very sharp pressure gradients near the leading and trailing edges. Also at a separation or a laminar to turbulent transition you will also get big pressure gradients. So sharp pressure gradients can be real.

Hey,

thanks for the effort you put in this!
Its not a single spike I dont like - it is the „jumpy behavior“ ... sorry dont know how to put this in better words ... the graphs should be more smooth I think.
I found a graph of the drag coefficient over the span of an airfoil at multiple RE numbers - the behavior of the graphs should be the same as wall shear in x direction.
Apparently the calculations were also made with CFX so it should be possible to get results that are of similar quality.
The magnitude of my results is just fine (in terms of drag and lift coefficient etc) - it just would be nice to have good looking results since I want to show the jump from laminar to turbulent behavior.

This is sounding like a FAQ now: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Pay particular attention to mesh resolution. Jumpy curves like what you see are commonly caused by too coarse meshes.

Unfortunately I do no think this is the case ... I did a mesh study on this considering the parameters

Y+
Streamwise grid refinement
Expansion Rate of Cells

with no change in the behavior.

I also tried doing multiple transition iterations - thats when I ran out of ideas :(

Just looked into it again ... the velocity gradient (perpendicular to airfoil surface) shows the same behaviour ... which makes sense since

http://www.cfd-online.com/W/images/m...d495dddff5.png... cant really figure out why though ... I will try to run the simulation with a different turbulence model and see what happens.

Another issue to look into is how well the curvature is resolved. The solid modelling may have facetted the face.

Sorry dont really understand your advice....
how well curvature is resolved = how many samples cfxpost takes ?

No. The solid modelling package (DesignModeller in particular) approximates curved surfaces with facets. For most applications the approximation is good and no problems occur. But for simulations which rely on very accurate definition of curvature (airfoil modelling is a good example) this does cause problems sometimes.

If you zoom into your geometry on designmodeller you will see it is facetted. There is a setting in DM to set the geometry tolerance to reduce this.