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Old   December 21, 2016, 02:05
Default Problems with Prism Layers
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Mathias
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Hello guys, i am Mathias from Germany, studying mechanical Engineering. I am doing a Group Project work at the Moment about the aerodynamics of a formula Student racecar. At the Moment we have lot of Problems with the Generation of the Volumemesh. I try to describe the Problem as good as possible:
I generate a prism layer on our car, the rest of the volume is filled with tets. after the generating of the mesh is finished, the surface of the car is not the original surface. The prism layer is always like a solid Body, to Support your Imagination, its looking a bit like a car in winter, when the snow is laying on top of the surface around the car. So the car is just looking "swollen".
I took the Pictures for you. The Pictures where only taken to Show the Problem and test different Settings to get this Problem solved, so dont wonder about the small wind tunnel! Furthermore the prism layer is three layers thick, so its not that "swollen", with 10 prism layers the car was really getting fat!
By the way, i did meshes for the formula Student Team last year, there i didnt had this Problem with the prism layers.
I hope someone knows a possible way to get a solution for this! We are using Fluent 17.2, before we used 16.1 if this makes a difference.

https://www.imagebanana.com/view/rvr...rismlayers.PNG
https://www.imagebanana.com/view/2b3...ismlayers1.PNG
https://www.imagebanana.com/view/kcd...ismlayers2.PNG
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Old   December 22, 2016, 01:05
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Noone is taking a guess? Or you didnt understand the Problem?
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Old   December 22, 2016, 03:53
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So your problem is the skewness of many of the prisms and some of the tets? There isn't really a straightforward way to resolve that. Meshing complicated geometries can be quite complicated. You should locate the bad cells and figure out what causes them to be so skewed. You could play with the height of the prism layers, to see if that reduces their skewness. Also, make use of growth factors to make sure their successive growth is limited.

What mesher are you using actually?
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Old   December 22, 2016, 04:06
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Thanks for replying!

No thats not my problem. For me its quit hard to describe the problem right, i try again:
The surface of your car is a solid Body. When i grow the prism layers on the surface of my car, they are not only growing on the surface, they even get a part of my surface and so its not my original surface of the car. Or is my understanding of using prism layers wrong?
When i was using Fluent 16.? i never had that problem, i was doing my surface mesh on the car, which is representing my geometry of my car. After that i was doing my volume mesh and also Setting prism layers on the surface of the car. Finally i got a volume mesh and the geometry of the car was represented by the surface mesh.
At the Moment the geometry of my car is represented by my surface mesh plus the prism layers, they are fusioned.

Hope i could explain the problem better, if not pls reply!
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Old   December 22, 2016, 06:01
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I'm still a bit confused. Your car is kind of a boundary of your geometry. So when you apply a prism layer on your car geometry, it will grow from your boundary outward. Like in the example you gave: snow on a car, the snow doesn't get "into" the car. So the prism layers shouldn't protrude into your car's geometry. So is your problem that the cells are actually crossing the boundary given by the car's geometry? Or it's not and you think it should?
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Old   December 22, 2016, 07:07
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Here i have two Pictures of the car, after finishing the surface mesh. One time the mesh is visible, the other invisible. That is the clear geometry of our car.

if you take a look in the Pictures in my first post on top of the thread, you see that after generating the volume mesh with prism layers the complete car is thicker. that is my Problem, that my car has a different geometry after doing the Volume mesh.

In the Pictures of the beginning of my post, its not that clear cause its just 3 layers of prisms. When we are doing 10 layers of prisms, the car gets extrem thicker on every part

https://www.imagebanana.com/view/9rj...1372945736.jpg
https://www.imagebanana.com/view/3mv...1934347774.jpg
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Old   December 22, 2016, 08:01
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But the prism layers are part of your volume mesh. You aren't actually meshing the inner volume of the car, right? Only the outside volume, i.e., the windtunnel volume? Because the prism layer is a volumetric boundary layer, that extends from the car's surface outward into the windtunnel. So if you plot the prism layers on top of the car, of course it looks thicker, because it's the snow around the car in which you solve the flow equations too. They are volume cells, part of your computational domain. Are you suggesting that the prism layers are just 2D meshes?
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Old   December 22, 2016, 08:25
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Yes absolutly right, the prism layers grow from the surface of the car into the volume of the wind tunnel!
now we get closer to a result, of course if the prism layers are visible, the car gets thicker.

Quote:
..around the car in which you solve the flow equations too. They are volume cells, part of your computational Domain.
so if i understand it right, pls correct me if not, the surface of my car still stays a "wall", so the fluid is possible to go through the prism layer cells and it just Looks that weird cause the prism layers are plot on the surface of my car?
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Old   December 22, 2016, 08:34
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Quote:
Originally Posted by matzeee View Post
so if i understand it right, pls correct me if not, the surface of my car still stays a "wall", so the fluid is possible to go through the prism layer cells and it just Looks that weird cause the prism layers are plot on the surface of my car?
Exactly! The prism layers are volumetric boundary layers part of your computational domain. They are included to have a transition from your free stream domain where the cells can be quite larger due to small gradients, to the surface of the car where the cells need to be small due the large gradients. In order words, at the car's surface, where you'll have the no-slip condition, the velocity needs to be zero. To accomplish that and to solve it accurately, you generally have smaller cells near boundaries. Far away from boundaries, most fields are rather constant, so you can have large cells.

So your "problem" isn't a problem at all, it's the way it's supposed to be.

That said, when looking at your 2nd image of the OP, you've got many highly skewed cells, which is really bad for convergence, if it convergences or runs at all! That's an actual concern you've got to fix.
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Old   December 22, 2016, 09:10
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Oke perfect, thanks a lot!
nice when a "Problem" isnt a Problem at all
for me personal it was quiet strange with the prisms, because last year when i did my mesh, the prism layers never were plot on the car surface

The Pictures were only mad for showing the Problem, so for the real Simulation i will put a bit more effort in the mesh

Thanks a lot, was a hard conversation but you helped me alot!
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Old   December 22, 2016, 09:42
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No problem! Good luck with the rest of your work!
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