Wall Shear on concave plate

sherifkadry · April 24, 2013, 13:37

So I'm modeling a concave plate, that mimics a blade pressure surface. I setup a case in CFX, my mesh is basically "2D" that is one element thick, with an inlet patch an outflow, a wall patch and the top bc is just a free slip wall.

I'm using the gamma-theta transition model, actually just the intermittency aspect of it, and I'm specifying a onset Retheta of 300. I'm interested in wall shear and wall heat transfer, mainly Stanton numbers on the wall.
My mesh is pretty simple as I said earlier, and seems to be good, Y+ at the wall 0.3 to 0.8. Aspect ratios reasonable, the case converges well no issues there. Mesh created with ICEM block, and is a hex mesh.

However my wall shear as a function of plate length is always zig-zaggy in nature. Even if I create a contour of the wall shear at the wall patch, very zig-zaggy in nature.

I did a totally flate plate wish similar mesh parameters and solver params, but do not get that same zig-zaggy nature of the wall shear vs. x plot. It is nice and smooth.

Any ideas why I keep getting this zig-zaggy plot? I have also refined the mesh 3 times and still get this zig-zag plot. I've also tried a 3D mesh of the plate but still get the same thing. Any ideas?

ghorrocks · April 25, 2013, 08:19

With mesh refinement are the zig zags unchanged or do they refine with the mesh?

Please post an image of the zig-zags.

CFX does, on rare occasions, have problems with the Rhie-Chow coupling which can lead to defects like this. You might have a rare case where you need to fiddle with the Rhie-Chow settings.

sherifkadry · April 25, 2013, 10:08

sherifkadry · April 25, 2013, 10:11

ghorrocks,
Thanks for the reply, as you can see from the image above that the zig-zag nature of the Stanton number at the wall, gets slightly better, but I had to refine the mesh by 10 times to get that ever so slight improvement.

ghorrocks · April 25, 2013, 19:48

I have seen this before in some work I was doing years ago - no matter how much I refined the mesh the zig zags were always there. A closer inspection of my geometry showed that my geometry was facetted, and the mesh was refining to the facets and not resolving the curvature better. Have you checked that your more refined meshes are resolving the curvature better and not being affected by facetting of the geometry?

sherifkadry · April 25, 2013, 23:02

Glenn, thanks for the reply, by faceted I assume you mean sawtoothed of sorts, and the mesh is snapping to those features? Will check, doubt it but it could be the case I will fiddle around with the tolerance in ICEM. Thanks.

Sherif

ghorrocks · April 26, 2013, 08:25

Yes, that is what I mean. And it produces exactly the results you are seeing so I highly suspicious this is the cause.

sherifkadry · April 26, 2013, 11:07

Glenn,
You were spot on, I went into ICEM, reduced my tolerance substantially, 1e-9. Ran the case again and zig-zag pattern is gone.

Thanks for your help.

April 24, 2013, 13:37	Wall Shear on concave plate	#1
sherifkadry Member Sherif Kadry Join Date: May 2009 Posts: 38 Rep Power: 17	So I'm modeling a concave plate, that mimics a blade pressure surface. I setup a case in CFX, my mesh is basically "2D" that is one element thick, with an inlet patch an outflow, a wall patch and the top bc is just a free slip wall. I'm using the gamma-theta transition model, actually just the intermittency aspect of it, and I'm specifying a onset Retheta of 300. I'm interested in wall shear and wall heat transfer, mainly Stanton numbers on the wall. My mesh is pretty simple as I said earlier, and seems to be good, Y+ at the wall 0.3 to 0.8. Aspect ratios reasonable, the case converges well no issues there. Mesh created with ICEM block, and is a hex mesh. However my wall shear as a function of plate length is always zig-zaggy in nature. Even if I create a contour of the wall shear at the wall patch, very zig-zaggy in nature. I did a totally flate plate wish similar mesh parameters and solver params, but do not get that same zig-zaggy nature of the wall shear vs. x plot. It is nice and smooth. Any ideas why I keep getting this zig-zaggy plot? I have also refined the mesh 3 times and still get this zig-zag plot. I've also tried a 3D mesh of the plate but still get the same thing. Any ideas?

April 25, 2013, 19:48		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	I have seen this before in some work I was doing years ago - no matter how much I refined the mesh the zig zags were always there. A closer inspection of my geometry showed that my geometry was facetted, and the mesh was refining to the facets and not resolving the curvature better. Have you checked that your more refined meshes are resolving the curvature better and not being affected by facetting of the geometry? sherifkadry likes this.

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April 25, 2013, 08:19		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	With mesh refinement are the zig zags unchanged or do they refine with the mesh? Please post an image of the zig-zags. CFX does, on rare occasions, have problems with the Rhie-Chow coupling which can lead to defects like this. You might have a rare case where you need to fiddle with the Rhie-Chow settings.

April 25, 2013, 10:08		#3
sherifkadry Member Sherif Kadry Join Date: May 2009 Posts: 38 Rep Power: 17

April 25, 2013, 10:11		#4
sherifkadry Member Sherif Kadry Join Date: May 2009 Posts: 38 Rep Power: 17	ghorrocks, Thanks for the reply, as you can see from the image above that the zig-zag nature of the Stanton number at the wall, gets slightly better, but I had to refine the mesh by 10 times to get that ever so slight improvement.

April 25, 2013, 23:02		#6
sherifkadry Member Sherif Kadry Join Date: May 2009 Posts: 38 Rep Power: 17	Glenn, thanks for the reply, by faceted I assume you mean sawtoothed of sorts, and the mesh is snapping to those features? Will check, doubt it but it could be the case I will fiddle around with the tolerance in ICEM. Thanks. Sherif

April 26, 2013, 08:25		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Yes, that is what I mean. And it produces exactly the results you are seeing so I highly suspicious this is the cause.

April 26, 2013, 11:07		#8
sherifkadry Member Sherif Kadry Join Date: May 2009 Posts: 38 Rep Power: 17	Glenn, You were spot on, I went into ICEM, reduced my tolerance substantially, 1e-9. Ran the case again and zig-zag pattern is gone. Thanks for your help.