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ch July 5, 2005 12:24

importing mesh into fluent

I'm a new fluent user and I can some problems on creating mesh.

Let say I've a cylinder and at the bottom of the cylinder, I have some holes, i.e. I just draw some circles on the base surface of the cylinder. I also define these circles as outlet while the rest of the surface area is defined as wall.

When I read this mesh in fluent, somehow fluent is not able to read the holes. As such, I get a wall boundary condition at the base of the cylinder. Did I miss out any step during creating the mesh?

many thanks

dirk July 5, 2005 12:47

Re: importing mesh into fluent
Maybe you forgot to subtract the circles from the cylinder base.

ch July 5, 2005 16:57

Re: importing mesh into fluent
thanks. but why should the circles be subtracted from the cylinder base? If you look from the bottom, fluid should be coming out from the cylinder through tbe holes. So, if I subtract the circles from the base, fluid will be flowing out via the space between the circles. Correct?

I've tried using the merging function in Gambit, i.e. merging the circles and the base. Perhaps, my understanding of the merging function is insufficient. It doesn't seem to work.

Jason July 6, 2005 09:13

Re: importing mesh into fluent
The merge function is for virtual geometry, and it is the equivalent of the unite command (I'm assuming you're familiar with the boolean operations unite, subtract, and intersect...). What you're looking for is an "impression" of the smaller circles on the larger circle. You can accomplish this with the split command. Now you will have a large round face with "holes" in it, and then smaller round faces that fill those "holes". When you define your BCs, then the larger face is a wall, and the smaller faces are your inlets.

Good luck, Jason

dirk July 6, 2005 09:24

Re: importing mesh into fluent
This is what I wanted to express.

ch July 6, 2005 12:04

Re: importing mesh into fluent
many thanks to both of you!

Just one more question. What's the use of the virtual geometry. In what way can the virtual geometry help to create meshes? The manual doesn't seem to explain this.

Jason July 6, 2005 12:27

Re: importing mesh into fluent
Going virtual is the last thing you want to do before you mesh, and if you don't have to do it, DON'T!!!

Its used to simplify the geometry to make meshing easier. Typically, you only have to do it when you're importing geometry. When you import geometry you can end up with small faces (compared with the size of the local mesh desired), or edges that come together at a very acute angle. These can cause highly skewed elements. To fix this problem, you use virtual geometry. You can merge faces and edges to simplify the geometry and this will make meshing easier. Also, I've run into problems where the imported geometry has a weird feature in the middle of a face (like a bump or a divit that really shouldn't be there). If this happens, I'll usually split the face through this feature. That way I can better control the mesh in the problem area. Also, you can have fillets or chamfers that are too small to really mesh, so you collapse them with the surrounding faces.

Basically you use virtual geometry to get rid of features that are too small to mesh.

Hope this helps, Jason

ch July 6, 2005 17:24

Re: importing mesh into fluent
thanks Jason. Your advice certainly comes handy. I'm just curious about your point on "going virtual is the last thing you want to do".

What is so bad about going virtual?

Jason July 7, 2005 08:39

Re: importing mesh into fluent
It's not bad if you're happy with the way all of your geometry is set up. Once you go virtual, all you can really do is split and merge faces and edges. You lose the ability to work with volumes (sometimes you can still do some work on a volume, but it's rare... the commands are there, they just typically come back with "ACIS Error XXXX"). A common problem is people import a geometry and try to clean it up by going virtual thinking the geometry is simple now, but then they can't subtract it from a control volume or anything. Usually what I do is before I go virtual I do a "save as" and add "-virtual.dbs" to the end of the filename. This way if it turns out I need to modify the geometry, I can go back (and its quicker than going back through the journal).

Like I said though, it's not a bad thing, it just should be the last thing you do before you mesh. It's a very useful tool, and I couldn't have gotten any of my models running without it (I work with step files coming out of Pro/E), but understand that once you go virtual, you are limited in what you can do. Even if you have multiple volumes that are connected... if one goes virtual you lose some functionality in all of them.

Hope this helps clarify what I meant, Jason

ch July 7, 2005 12:20

Re: importing mesh into fluent
Indeed, your point on "people importing a geometry, try to clean it up by going virtual and think that the geometry is simpler now" is very true. This is the impression I got after go through the manual! Many thanks for the piece of advice.

I need to work with imported geometry as well. So, what I'm doing now is to delete the volume, do the necessary cleaning up in the "real mode" and stitch the pieces back to form my geometry again. I'm not sure if this is the way experienced CFD-users would go about doing this. Do you have some quick some advice for beginners like me?

Once again, thank you for all your tips!

Jason July 7, 2005 14:29

Re: importing mesh into fluent
I would be careful... Gambit gets fussy when you're trying to stitch faces together to create a volume. Try to do as much work as you can directly on the imported volume. Also, try to clean up the geometry as much as possible in the original CAD model (a common example is filling in bolt holes and such, or merging an assembly into a single piece) before pulling it into Gambit. You can save yourself days (or weeks) of heartache.

Basically, you import your model, create your control volume (CV), and subtract your model from the CV. From there, you can decompose the CV into smaller, easier to mesh volumes (keep the "connected" option when you split the volume and don't apply any BCs to these faces so that when you export the mesh gambit will treat it as one continuous volume). If you have to, then you can do any work you need to on your geometry in "real mode" but try to work on the volume level... using unite, subtract, and split. You can split a face in real mode with another face, even if it's connected to a volume. I think this is the best method for creating your inlet holes. Once the geometry is how you like it, then go virtual, apply sizing functions (if you're using them), and mesh.

Gambit is adaquate at working with geometry. It's not great, but it works. Try to keep your geometry work in Gambit simple, and do anything complicated you have to in the original CAD model.

Hope this helps, and good luck, Jason

ch July 7, 2005 16:42

Re: importing mesh into fluent
Thanks for all those great advice! They are definitely better than going through the manual for beginners like me. Deeply appreciated!

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