[scipy]

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]

Please give some briefs in new thread

[alenglaro]

http://www.cfd-online.com/Forums/flu...tml#post363878

[Far]

few updates are available here

http://www.cfd-online.com/Forums/ans...tml#post364079

[nick.17]

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.

[Anna Tian]

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.

[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...

[Anna Tian]

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?

[Far]

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.

[mahantesh]

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?

[orangesky]

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.

[Far]

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).

I've managed to get 1 % agreement with experimental results with a hybrid prism/tetra mesh for the same domain, so I wonder, is the necessity for a larger domain purely because of the hexa elements or? Since I thought they were higher quality elements and as such should experience even less problems than tetras?

In any case, I know for a fact that people from the car industry recommend upstream of about 7-10L max (or at least 100 cells in the direction of the flow before the stagnation point) and downstream of 12-15L, so, why the need for 30L?

It is not 2L (upstream), it is rather 2.38 L. And in experiments it is 2L

[Rozna]

hai i have a problem with my icem mesh, while loading it to fluent i get the following error:
WARNING: The mesh contains high aspect ratio quadrilateral, hexahedral, or polyhedral cells. The default algorithm used to compute the wall distance required by the turbulence models might produce wrong results in these cells. Please inspect the wall distance by displaying the contours of the 'Cell Wall Distance' at the boundaries. If you observe any irregularities we recommend the use of an alternative algorithm to correct the wall distance. Please select /solve/initialize/repair-wall-distance using the text user interface to switch to the alternative algorithm.
pls can u help me

[Rozna]

Originally Posted by Far

It is not 2L (upstream), it is rather 2.38 L. And in experiments it is 2L

hai i have a problem with my icem mesh, while loading it to fluent i get the following error:
WARNING: The mesh contains high aspect ratio quadrilateral, hexahedral, or polyhedral cells. The default algorithm used to compute the wall distance required by the turbulence models might produce wrong results in these cells. Please inspect the wall distance by displaying the contours of the 'Cell Wall Distance' at the boundaries. If you observe any irregularities we recommend the use of an alternative algorithm to correct the wall distance. Please select /solve/initialize/repair-wall-distance using the text user interface to switch to the alternative algorithm.
pls can u help me

[Gert-Jan]

Can be anything. Check & smooth your mesh in ICEM. Then smooth the mesh in fluent.
If it does not help then remeseh it in ICEM with better settings.

[Far]

Originally Posted by Rozna

hai i have a problem with my icem mesh, while loading it to fluent i get the following error:
WARNING: The mesh contains high aspect ratio quadrilateral, hexahedral, or polyhedral cells. The default algorithm used to compute the wall distance required by the turbulence models might produce wrong results in these cells. Please inspect the wall distance by displaying the contours of the 'Cell Wall Distance' at the boundaries. If you observe any irregularities we recommend the use of an alternative algorithm to correct the wall distance. Please select /solve/initialize/repair-wall-distance using the text user interface to switch to the alternative algorithm.
pls can u help me

.

Try to run this case in Fluent and see what happens. Use double precision solver

[Rozna]

Originally Posted by Far

.

Try to run this case in Fluent and see what happens. Use double precision solver

i got this warning while running in fluent.
i also got an error: divergence detected in ang solver: pressure coupled