# [ICEM] Mesh Quality

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 May 30, 2012, 12:17 #41 Senior Member     Alex Pasic Join Date: Aug 2011 Location: Croatia Posts: 184 Rep Power: 8 Did you read any of the Ahmed body papers? Usual turbulence intensity of the free stream airflow in a windtunnel is around 0.25 %, so me using 1 % is already a bit higher and more "real world"-like, but 2.5 is unnecessary. You (and most others) seem to confuse a no slip wall and a no shear wall. No slip wall is what I've used for the ground and the ahmed body (since that's what they are, stationary walls with shear stress or 0 slip). However, for the side and top wall - if you are going after ultimate accuracy and recreating exact wind tunnel conditions, then those walls should also have a boundary layer (same as the ground and the Ahmed body itself), but since most of the time they're far enough away to only affect the solution a little bit (if left as no slip), they can be given no shear stress boundary conditions so their viscosity effects will be disregarded. Since this is mathematically the same as a symmetry BC for Fluent, I used that for simplicity. /edit, seems you have read about turbulence intensity Far likes this.

 May 30, 2012, 12:17 #42 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,285 Blog Entries: 6 Rep Power: 43 Please give some briefs in new thread

 May 30, 2012, 12:21 #43 New Member   alessandro Join Date: Oct 2010 Posts: 28 Rep Power: 7 Far likes this.

 May 31, 2012, 10:49 #44 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,285 Blog Entries: 6 Rep Power: 43 few updates are available here NEW : Tutorial on road vehicle

 September 11, 2012, 10:19 #45 New Member   nicolas Join Date: Sep 2012 Location: grenoble Posts: 4 Rep Power: 6 Hi, I'm meshing Ahmed Body with Pointwise V17. I'd like to know wich is the right mesh dimension on the body, because I see in the pics here posted that are used cells far bigger than mines. I'm using the following average dimensions: in the length, a point every 2 mm; in the height, a point every 1,6 mm; so the average dimension of the cell is 2x1,6 mm. thanks a lot.

February 1, 2014, 20:23
#46
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Meimei Wang
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Quote:
 Originally Posted by scipy I think he's actually trying to replicate the results of the LSTM wind tunnel testing on the Ahmed body done by Lienhart and that wind tunnel is 1.4 m high, 1.87 m wide (so half of that since symmetry is used), the downstream length is 5L (5*1044) and upstream of 1.3L (he chose 2L).
Why symmetry BC can be used in this case? Is this BC usage verified by testing or it is widely recommended especially for this kind of case? Theoretically the physics won't be correct if symmetry BC is used especially when we run transient simulation.
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 February 2, 2014, 00:05 #47 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,285 Blog Entries: 6 Rep Power: 43 symmetry boundary condition is used becuase we are not giving any angle of attack or side slip angle and flow will be same for both sides. Therefore to reduce the computational cost we must use symmetry BC...

February 2, 2014, 06:51
#48
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Meimei Wang
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Quote:
 Originally Posted by Far symmetry boundary condition is used becuase we are not giving any angle of attack or side slip angle and flow will be same for both sides. Therefore to reduce the computational cost we must use symmetry BC...
Even though the geometry is completely symmetric, the wake flow could still be not symmetric especially if we run the transient simulation. The vortex shedding could be a very significant effect on the prediction results. If the symmetric BC is used, the vortex at the rear part will be symmetric so that the vortex shedding won't be predicted. So people usually do a test to know whether vortex shedding is important or not. Then if it is not important, symmetric BC can be used.

So I ask, has someone done test or not? Or the usage of symmetry BC is from any experience? Or this is just a widely known setting?
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 February 2, 2014, 08:45 #49 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,285 Blog Entries: 6 Rep Power: 43 You can run both cases and see what is happening. Generally speaking flow structure will not be symmetric at the particular instant of time, but on average flow variables won't be affected by symmetry condition.

 March 6, 2015, 04:35 unstructured meshing of flying wing #50 New Member   mahantesh Join Date: Oct 2014 Posts: 4 Rep Power: 3 hello.. i am doing unstructured meshing (tetra) with patch indepedent and robost octree method on flying wing and after volume meshing , i gone for prism mesh but prism layers are not created. And one more thing is quality of the mesh is very poor before creating prism layers and after it is still worst in patch dependent scheme ...its about 8*e-6(quality) and orthogonal quality is 0.0003.....if any suggestions related to above topic?

 July 20, 2015, 17:21 #51 New Member   Join Date: Mar 2014 Posts: 7 Rep Power: 4 I found this topic looking to solve a similar issue I had with ICEM mesh not translating properly into FLUENT. In my case, ICEM was in mm while FLUENT was in m, so there was a discrepancy of 10^3 when importing into FLUENT, causing my high quality mesh to become extremely low quality and cause divergence. I would get an error of 'invalid/low volume faces' during mesh check. This was fixed by going into mesh and scaling the model. Simple error but just in case anyone else has a similar issue, might be that.

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