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Old   January 19, 2007, 05:11
Default vitual _ real
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I'm a new user of Gambit_Fuent and I have a question, maybe a bit stupid but... what is the difference between real and virtual when for example, creating a volume? What are the differences in the further treatment of that volume? Is it possible to mesh a virtual volume?

thanks in advance

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Old   January 19, 2007, 09:06
Default Re: vitual _ real
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Real geometry is what you want to work in when you're manipulating the geometry. Real geometry is what is created using the typical geometry creation tools (Like Extrude, Create block, etc) and through most import methods (like importing a Step or Iges file). The ACIS engine (which is the core for Gambit) works on the real geometry, which makes it more stable and more robust (not perfect though, it can still run into errors and crash, especially with complex imported geometry).

Virtual geometry is another "layer" of geometry which Fluent added on top of the ACIS engine to be able to "clean-up" geometry to make it mesh-able. The whole point in virtual geometry is to allow you to simplify the geometry that's going to be meshed (merging faces is the most common operation). It also allows you to create faces on the fly out of existing geometry (like using the split faces tool) which can be helpful in cleaning up geometry or controlling your mesh.

The best way of putting it is that Real geometry allows you to create a control volume, and then once you're happy with the volume you've created, you can go in and clean up the individual faces using virtual geometry to make it easier to mesh. Once you've cleaned up the faces, then you can mesh the geometry.

Virtual geometry has many of the geometry operations that real geometry does, but in reality, these operations typically don't work. I use real geometry, and the only steps left when I'm ready to go virtual are to merge/split faces to make it easier to mesh, and occasionally I may leave a "virtual-connect" if doing a "real-connect" would have been too complicated. In my experience, any geometry operations beyond these are unreliable with virtual geometry.

If you don't have to use virtual geometry, then don't. If you're using virtual geometry, then I recommend saving a backup of the geometry before going virtual. You may realize you forgot to do something to the geometry that can't be done using virtual geometry, so it's good to have the backup (I know, you can just re-run the journal, but un-meshed geometry files are small, and if you're doing a lot of steps, re-running a journal can be a slow process).

Hope this helps, and good luck, Jason
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Old   January 19, 2007, 09:15
Default Re: vitual _ real
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in Gambit when you create a cube or you import a CAD geometry these are real geometries.

If to make a geometry simple you merge two faces with a slightly irregular form (not two squares for example) gambit creates a virtual geometry which cannot be made real. This geometry really exists, but has properties different from the real ones. For example, you can do cuts and Boolean opetations with real geometry, but you have many constraint for doing the same things with virtual geometries (I think that Boolean operation are completely impossible vith virtual geom.)

The usage of virtual geometries helps very much when the volumes (or faces in 2D) of your model are those final before creating the mesh (this means that you don't need any more cuts or geometry modifications before meshing). In fact, using virtual geometries you can simplify the geometry, eliminating little faces or joning volumes to make meshing easier. What I learnd from my experience, is: pass to virtual geometries ONLY in this case, i.e. only when the geometry id the final one. Do that just before meshing, because otherwise, coming back to modify the geometry will be evil.

From waht I said above, you can see that meshing a virtual volume is possible and, done at the right moment, can brings many advantages.


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Old   January 22, 2007, 04:31
Default Re: vitual _ real
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thanks a lot Jason and MCB. I think I see now the difference. I'm working now on the mesh, so If I use Virtual geometry now, it should be ok. Another question, my first attempts to mesh have failed and Gambit told me each time: "....line 6059: found-1 && Found_2 while meshing...". Is there a link where I could find the meaning of this kind of message, how I could interpret them ecc...

Thanks a lot Deneb
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