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Old   April 6, 2005, 17:32
Default GRID MESHING AROUND A MISSILE
  #1
Chris
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Hello everyone, please help!

I am modelling the fluid flow over an AIM9X Sidewinder missile at M0.8, M1.2 and M3.0. It is 3m in length an of dia 0.127m. The fins and wings are assumed to run inline with the length of the missile body.

I have a parabolic shaped (or "C shaped") domain that I need to mesh. Its coordinates are (-15,0) for the "nose" and (33,30), (33, -30) for the right hand side. I will gladly email the .dbs file I have made in Gambit if anyone can use his or her expertise to find a way to do this. I am at a whit's end and have tried all kinds of ways to construct the grid from interfaces around the missile to creating faces "around" the missile to using the subtract operaton….. I have proof of many failed meshes! When I do get a successful mesh the elements immediately around the missile's boundary layer are all skewed as disproportioned.

I am a beginner with this software and am simply using it to complete m final ear project at university.

I need advice on how to construct a good mesh so that I can read the .msh file into Fluent for analysis.

I hope someone can help! Chris

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Old   April 6, 2005, 17:42
Default Re: GRID MESHING AROUND A MISSILE
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Jason
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Use an unstructured mesh and read up on sizing functions. They should helps you grow the mesh from your surfaces appropriately.

Hope this helps, and good luck, Jason
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Old   April 7, 2005, 04:50
Default Re: GRID MESHING AROUND A MISSILE
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Jörn Beilke
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http://www.gridpro.com/gridgallery/missle.html
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Old   April 8, 2005, 07:13
Default Re: GRID MESHING AROUND A MISSILE
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Chris
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Thanks for your advice Jason,

Can you recommend places to look for information on how to create an unstructured mesh and sizing functions in Gambit?

Hope so!

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Old   April 8, 2005, 10:23
Default Re: GRID MESHING AROUND A MISSILE
  #5
Jason
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The gambit tutorials are the best source (they come with the Fluent/Gambit installation... you might have to talk to you your administrator to ask them to install them, or you can find them on www.fluentusers.com).

The other option is to just try it. If you do a search through the archives there's some good information on sizing functions and pitfalls to watch out for. When you try to mesh a face with a size function attached, a successful sizing function will give:

% of cells hitting tree depth = 0 and % of cells below specified tolerance = 100.00

in the transcript window. If any cells aren't below the specified tolerance, or of any cells hit the "tree depth", then go to Edit->Defaults->Tools->SFUNCTION and increase the value for the BGRID_MAX_TREE_DEPTH. Then try meshing it again. Keep increasing the default value until you get 0 and 100. Otherwise, start decomposing your volume so that the sizing functions act over a smaller volume.

Just a heads up, the old versions of Gambit ask you define a distance in sizing functions. That's just how far from the source that the sizing function still is accounted for. The new versions dropped that in favor of just a size limit and the sizing function is accounted until the mesh reaches the size limit, and then remains that size. Also, the new version of Gambit has added the "meshed" size function, which is nice because you can mesh the faces however you like, and then have the mesh grow at a certain rate from those faces. The new version of Gambit computes sizing functions much faster, and you rarely have to play around with default values to get it to work. In fact in the new version of Gambit, I've never played with the defaults for the sizing functions.

Like I said, search through the archives, and look for the tutorials. Those are your best options.

Hope this helps, and goodluck, Jason
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Old   April 8, 2005, 13:19
Default Re: GRID MESHING AROUND A MISSILE
  #6
Father Dougal
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What do you need to get from the analysis? Forces, Moments, Pressure Distributions? Answer these and then decide on the mesh quality you need - very often an Euler calculation will suffice.
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Old   April 8, 2005, 14:18
Default Re: Forces, Moments, etc.....
  #7
Chris
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I am looking to calculate the: turbulence intesity pressure distribution total drag force

and then modify the shape of the missile to reduce them.

I am using Fluent for the fluid dynamics simulation once I have created a good mesh.

Any ideas on the meshing? its a 2D parabolic domain.
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