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[ICEM] meshing airfoil in ground effect

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Old   February 24, 2017, 12:28
Default meshing airfoil in ground effect
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Hi everyone,

Im a beginning cfd user with some experience from numerical courses etc.

Before this I have meshed an airfoil without ground effect and this went well. I have used gambit for meshing in this case but now wanted to use something more advanced.

I am currently trying to mesh an airfoil in ground effect and am having some trouble. I am following the tutorial - ICEM CFD hexa 2d airfoil meshing from Simon Pereira (i believe active on this forum).

I have done most thing similar to the tutorial. The C block around the airfoil is however oriented with the flat side to the ground instead of the downstream side.

My first problem is that the mesh is not following the curves of my airfoil (see picture). I have associated the curves and also followed the "edit edge -> split edge -> automatic lineair " as done in the tutorial.

http://i.imgur.com/9mIQf79.png

I have been trying several times without succes. I think there might be a problem that both my fluid part and my airfoil part have a set of overlaying curves. attempts to associate to either of these have not solved the problem though.

thanks in advance for any advice ( also any tips on how to create a better mesh for a wing in ground effect are highly appreciated)


http://i.imgur.com/ZHw8PWL.png

Last edited by cfd_peter; March 1, 2017 at 07:32.
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Old   February 28, 2017, 12:08
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Dear Peter! It seems that you select wrong edge while creating C-block. You should have choose the one on the downstream side. So try this: Create simple H-grid around your airfoil, then go to Split Block and select the block contains inside your airfoil. After that click Select Edge(s) and select the edge placed downstream on the outer boundary. Unfortunately, I cannot take screenshots right now to be more descriptive. But you can easily find on YouTube some examples of creating C-grid on airfoil. Actually in terms of grid structure it does not matter whether you examine airfoil near the ground or far from it. All the difference mainly contains in boundary conditions.
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Old   March 1, 2017, 07:31
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Dear Kirill Makarov,

thank you very much for your reply. I had made the C curve like this on purpose as i thought it would provide a better meshing result. When i try as you suggest i get something like this:

http://i.imgur.com/RgLkg3E.png

I am not sure how to make this into a nice mesh, if you have any tips please suggest me.

One question i still have remaining is why when i associate edge to curve for the airfoil sometimes the mesh does not follow the airfoil curve (and sometimes is does, i cannot figure it out (15+ attempts)).

I have been adjusting the geometry to get a better mesh, my last attempt was a half circle which unfortunately ICEM didnt let me specify input and output on the first/second half of the circle (it kept overwriting one of the BC on the full circle nomatter what).

Last edited by cfd_peter; March 1, 2017 at 10:39.
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Old   March 1, 2017, 09:34
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Yes, of course, C-grid improves mesh quality. I find an airfoil and make a couple screenshots to illustrate the process. At first, I create simple H-grid. I make edges black just for better visualization, you don't need to do so. After that I go to Ogrid Block section at Split Block menu and choose central block, then I choose corresponding edge on the back boundary. I highlighted it red on screenshot. This is how it looks like in my case. Notice that I click on Around Blocks option. As a result, nice O-grid has been created.

If it still do not work for you, try to create O-grid manually, by using Merge Vertices menu. To do so, you should create a little bit more complexed H-grid like this. Choose option Multiple and set as a base the vertex marked B. Vertices to be merged marked red. Next step is to collapse vertices on the outer boundary. Finally, repeat it with this vertex. And you should obtain nice O-grid..
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Old   March 1, 2017, 10:56
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Thank you very much for your help Kirill it is very clear and the blocking works great:

http://i.imgur.com/Ao5mvaz.png

now im still stuck with having the mesh follow the airfoil curve, is there anything else i can do besides associate edge to curve? :

http://i.imgur.com/adnCtN9.png

Im really confused since some times it does follow the airfoil curve and sometimes it doesnt.
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Old   March 1, 2017, 11:52
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Great! I am more than happy to help. To fix your second issue try to update associations. There is this button in Blocking Association menu. Also try to click Snap Project Vertices button. Don't worry if your grid collapses into weird ravel. It means you just do not complete with making associations. Just Undo last action and carefully check that:
1) all vertices are associated to points or curves
2) all edges are associated to curves
3) all faces are associated to surfaces (impornant in case of 3D simulations)

Just curious which method did you use to make C-grid?
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Old   March 2, 2017, 09:24
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Dear Kirill,

Unfortunately following your steps I still am unable to get the meshing to follow the airfoil curve.

The associations seem to be correct, color change of the edges show the edge to curves are correctly associated. And the vertexes on the airfoil curve are constrained to move over the curve itself.

I am importing my geometry from the designmodeler and am now wondering if it is possible the issue is originating there? My design modeler is resulting in different parts for fluid, airfoil (substracted from fluid), and all 4 in/outlets. :

http://i.imgur.com/CIu7tgQ.png
http://i.imgur.com/tI3KdEr.png


For the C- grid I just followed step by step your explanation,

H-grid -> O-grid @ central block + downstream edge.
The reason the downstream side blocking has a bit of a triangle shape is because i want to have some denser meshing to capture wake flow behind airfoil (which I'm assuming to be roughly in that region).
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Old   March 2, 2017, 10:06
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OK, in this case try to visualize association and actual edge shape. Right click on Edges -> Show Association or Projected Edge/ Mesh Shape. This instruments can help you to figure out what exactly is doing wrong with your mesh.

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Old   March 2, 2017, 11:58
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With both projected view and show associations the program is basically telling everything is correct:

http://i.imgur.com/ZfuPH8W.png (projected is exactly on the airfoil)

is there any options that might interfere with the mesh following following the curve ? something like a maximum curvature option?


Apart from the airfoil the mesh is looking great, and following the airfoil curve at the "beginning of the mesh edges towards the airfoil. just not on the ending of these same edges..

http://i.imgur.com/spTWvcI.png
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Old   March 6, 2017, 03:25
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Ok, that is becoming a little confusing. Did you delete the central block, the one that contained inside the airfoil? Try to do this if you did not. Try also to move nodes over the nose of airfoil, this may help to catch the shape of the geometry. If not - try to split edges reflecting upper and lower part of the airfoil so that they be placed closer to curves they should follow. Finally, try to use function Split Edge in Edit Edge menu. It cuts edges without cuting blocks. Hope some of this helps you.
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Old   March 14, 2017, 09:35
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After several frustrating weeks i have found a solution. It is called pointwise.
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Old   March 14, 2017, 14:06
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Quote:
Originally Posted by cfd_peter View Post
After several frustrating weeks i have found a solution. It is called pointwise.
Good news! What exactly did you do to fix your problem?

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Old   March 14, 2017, 18:30
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Unfortunately i didn't manage to fix it with ICEM. I have instead used the program Pointwise.

In my now short experience this program is far superior in the user-friendly category (at the very least).
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