[Gmsh] Trouble meshing 3d shape in "wind tunnel"

Bob Basham bob at whitecloudaviation.com
Wed Nov 3 02:00:34 CET 2010

Thank you for your information.  I was under this impression, but can't 
seem to get gmsh to actually mesh the volume inside the "tunnel" and 
outside the shape.  Rhino can only model surfaces I believe, not 
solids.  Would this be the problem?  I am sending a step file into gmsh 
that contains the surface of the tunnel and the surface of the shape 
inside, both show up as volumes when I check the statistics.  I am 
wondering about the "merge" command, could this help?  I am not sure 
exactly what it is doing, but could I extrude a solid inside gmsh and 
merge with my non-solid surface?  Is there something in Rhino I can do 
to help?  Thankyou - Bob

César A. Vecchio Toloy wrote:
> Bob, I think you're making a common mistake among CFD newcomers: you're modelling the bodies but not the air. I mean, the solid of your STEP must be the air and not the bodies surrounding it or the bodies in it. I give you the following example:
> Let's suppose you want to model the flow around a sphere. First we make a solid box which delimits the space we will model (even if you are modeling something in free space, you must limit it in the model). Now, to represent the sphere you must "take it out" from the solid box, for example by boolean substracting a solid sphere from the solid box, so you will end up with a solid box having an spherical cavity. That is the correct way of modeling in CFD with these kind of general purpose meshers.
> If you already knew this, I apologize. Check the message console in Gmsh (under "tools" menu) to see if there was some error. Perhaps you could be having extra surfaces or lines, that happened to me with some Rhino versions. If the STEP file is not very big, you can try attaching it.
> César
> On Mon, 01 Nov 2010 15:13:55 -0600
> Bob Basham <bob at whitecloudaviation.com> wrote:
>> Hi All,
>>      I am pretty new to CFD in general.  Just starting to figure out 
>> gmsh in order to run some simulations using ELMER.  I need to put a 3d 
>> shape inside of  a wind tunnel.  I have created the shape in Rhino 3d 
>> and finally have success with importing to gmsh as a STEP file (IGES 
>> gave all sorts of little holes in surface).  I have tried and tried to 
>> get a 3d mesh of the volume inside the "tunnel" and outside the shape, 
>> no luck.  I am sure that I am missing something basic.  Can someone 
>> point me in the right direction.  I have lastly tried merging a STP of 
>> the "tunnel" with a STP of the surface, but can't get a 3d mesh of the 
>> inside.  I get a self intersecting error.  I have successfully used a 
>> half model (like symmetry), the surface and tunnel are joined at the 
>> middle.  I would like to have my surface suspended in the tunnel 
>> though.  Thank you very much!
>> -- 
>> Bob Basham
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>> gmsh at geuz.org
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Bob Basham
White Cloud Aviation LLC