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Old   July 6, 2006, 22:32
Default Making modifications in GAMBIT
  #1
Vidya Raja
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Hi,

Is there any way modifications to a geometry can be made in Gambit? I have a model that's imported into Gambit from Pro/E? This solid model was then inserted into a cylinder which was created in Gambit. I then subtracted the volume of the model from the cylinder volume to get my flow domain. I then meshed half the geometry and the other half in Tgrid.

I now want to change the length of the cylinder to make it shorter and thus reduce the number of nodes. Is there any way I can do this without disturbing any other part of the geometry or the cylinder?

Thanks, VR
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Old   July 7, 2006, 07:58
Default Re: Making modifications in GAMBIT
  #2
Jason
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Depending on your geometry it may be easier to go back to Pro/E.

You could cut a section of the geometry out (create a block that represents the geometry you want to remove from the flow domain, then subtract it from the flow domain). If the piece you're removing is on the end of the geometry, then you have to re-mesh and you're done, but if you have to cut a chunk out of the middle of the geometry, now you have to move the two pieces back together, and connect the face (just being next to each other doesn't make a continuous volume... you MUST connect the faces or use non-conformal interfaces in Fluent). If you've decomposed your geometry, the only mesh that's affected when you do this operation is the part that the geometry's being subtracted from. If you only have one single volume, then you're going to lose your whole volume mesh. On the plus side, any face that wasn't modified in the subtract and connect operations will maintain their surface mesh. Another point is that this won't work if you've gone virtual. If you're virtual, you can rerun your journal and stop it sometime after you've imported your geometry, but it may be just as easy to go back to Pro/E at that point.

Hope this helps, and good luck, Jason
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Old   July 7, 2006, 11:18
Default Re: Making modifications in GAMBIT
  #3
Vidya Raja
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Hi Jason,

That's not what I meant. My Pro/E model is to be retained as it is. After exporting it to Gambit, I want it to be inserted into a cylinder. The total length of the cylinder should be such that it is one diameter (diameter of the Pro/E geometry) long at the upstream end and about 2-3 diameters at the downstream end.

Right now, it's longer at the upstream end. This has also increased the number of nodes tremendously.

Is there any way these modifications can be made now? Or can any modification be dine in Tgrid?

Thanks, VR
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Old   July 10, 2006, 07:55
Default Re: Making modifications in GAMBIT
  #4
Jason
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Then if your geoemtry is still real, cut off the extra the way I described. If it's virtual, then you have to go back to before it was virtual (if you've saved a copy of the geometry then you can open that, or re-run the journal).

Hope this helps, and good luck, Jason
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Old   July 10, 2006, 09:13
Default Re: Making modifications in GAMBIT
  #5
Vidya Raja
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Hi,

I wanted to do the subtraction in order to get the flow domain. I want to analyse the flow patterns with the device inserted into the cylinder. Then how does cutting off the chunk help?

Thanks.
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Old   July 11, 2006, 07:34
Default Re: Making modifications in GAMBIT
  #6
Jason
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You said that you wanted to reduce the length of the domain. Cutting of a chunk of the domain is the same thing.

Jason
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Old   July 11, 2006, 15:33
Default Re: Making modifications in GAMBIT
  #7
Vidya
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Hi Jason,

I tried cutting off chunks of the parts which were to be subtracted. I assumed that this was to simplify the geometry. Also, to reduce the length of the geometry, I tried to cut off a chunk using the SPLIT VOLUME option.

However, neither worked. Do you have any comments?

Thanks, Vidya
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Old   July 11, 2006, 22:58
Default Re: Making modifications in GAMBIT
  #8
Vidya Raja
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Hi Jason,

I also tried to make a cylinder exactly of the same diameter and height as that of my model, then inserted my model into it, subtracted the model volume from the cylinder volume. This resultant volume was then inderted into the main cylinder and the volume subtracted from the new (bigger) cylinder's volume to get the final flow domain. When I did this subtraction, it cut off parts of my model geometry, thus distorting it. So I'm guessing that this approach doesn't work (or, perhaps I'm messing up somewhere). Is my approach like what you suggested?

My ultimate goal is to mesh my complicated geometry 9part od which is already done, but there are still many parts which are to be meshed and this is the complicated part with which I'm still stuck. Then I want to reduce the length of the main big cylinder that I talked about above to reduce the length of the flow domain.

Thanks, Vidya
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Old   July 12, 2006, 05:38
Default Re: Making modifications in GAMBIT
  #9
Nanda
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hi,excuse da interruption but if i understand correctly what u r saying, u subtracted your ProE model from a cylinder, hence the cylinder now consists a void in the shape of your ProE model and by this i believe u r trying to simulate flow similar to that of a body in a wind tunnel.By subtracting the model again with another cylinder, wouldn't it result in having 2 volumes,and pardon me if i'm wrong,create model which does not actualy represent your problem?

my suggestion is, why not create both volumes,your initial ProE model and the cylinder that u want in accordance to your specs in a CAD software,ProE i guess since u r familiar with it, and import it into gambit in a single session seperately.Now you would have 2 volumes according to how u want both the model to be exactly. then you can proceed to clean it up and subtracting the model.

hope it was helpful

Nanda
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Old   July 12, 2006, 08:00
Default Re: Making modifications in GAMBIT
  #10
Jason
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If I remember correctly, you've been modeling some type of valve system in a pipe... so the way I was reading your request is that you have the inlet, the valve, then the outlet, but the outlet section of your geometry is longer than you wanted, so what I had recommended was creating a volume that engulfed the EXTRA portion of the length and then using the subtract function to remove this portion of length from the rest of the geometry. If you're geometry's virtual, then this will not work. If you're using virtual geometry, then there's no reliable way of modifying the geometry. You can try the virtual splits, but those are hit or miss... they either work or they don't. If you're able to split the geometry, then you can delete the portion you don't need.

If this isn't what you're trying to do, then let me know.

Hope this helps, and good luck, Jason
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Old   July 12, 2006, 08:18
Default Re: Making modifications in GAMBIT
  #11
Vidya
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Hi Jason,

You are right- I'm trying to simulate a valve in a human vein. So the vein is simulated by the cylinder. The valve (modeled in Pro/E) is to be placed in the cylinder (vein) in such a way that the distance from the inlet to the base of the valve is about 1 diameter (of the valve) and the distance from the tip of the valve to the outlet should be about 2-3 diameters. Then the flow domain is obtained by subtracting the volume of the valve from that of the cylinder volume.

Right now, The inlet is much longer than what I want and hence the number of nodes is also very large. I want to reduce the inlet length because of this. Also, the inlet and outlet lengths of the vein (cylinder) is important because it would help to determine the recirculation zones, etc.

After I did the subtraction, the valve geometry has gone virtual and so nothing could be done.

Do you have any suggestions?

Thanks, Vidya
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Old   July 12, 2006, 08:49
Default Re: Making modifications in GAMBIT
  #12
Jason
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You can try the virtual splits, but as I said, they're hit-or-miss. Once you split off the part you don't want, just delete it. But there's no guaruntee that'll work. Virtual geometry in Gambit is sketchy at best. Another option would be to go into your journal, modify the lines that define the length and location of the cylinder, then re-run the journal. It's worth a try at least, and it might solve your problem for you.

Hope this helps, and good luck, Jason
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Old   July 12, 2006, 10:49
Default Re: Making modifications in GAMBIT
  #13
Vidya
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No, the virtual split option doesn't work. The entire valve model is virtual, but the cylinders are still real. Now, is there any way to shorten the inlet cylinder without harming the other parts of the geometry? The valve model is meshed, but the cylinders are still unmeshed.

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Old   July 12, 2006, 11:26
Default Re: Making modifications in GAMBIT
  #14
Jason
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The cylinder's are real... but they're connected to the virtual geometry (i.e. they share a common face with the virtual geometry), correct? If that's the case, then you can't perform "real" operations on the cylinder. And if the virtual splits aren't working, then you have to either modify your journal to change the lengths of the cylinders and re-run the whole thing, or you need to delete the cylinders and re-make them (then re-connect the shared face).

Those are really you're only options at this point. Gambit is not a parametric modeling program like Pro/E, so you can't go in and just modify a property of the geometry like that (they claim that you can do parametric modeling, but what you're really doing is modifying the journal with different values and then re-running the journal with the new values).

Hope this helps, and good luck, Jason
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Old   July 12, 2006, 14:15
Default Re: Making modifications in GAMBIT
  #15
Vidya
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Yeah, what you said is absolutely correct. The cylinders share common faces with the geometry.

I do not have an actual journal file because only one half of the geometry was meshed in Gambit, while the other half was done in TGrid using the method of reflection to get the other half. So I do not have a journal file, as such.

But anyway, if I have to redo the whole thing, right from start, and save a journal of the part done in gambit, how do I rerun a journal, if the need arises?

As far as I know, there's no good journal file that TGrid maintains? Or is there some other option?

Thanks a lot.
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Old   July 12, 2006, 14:58
Default Re: Making modifications in GAMBIT
  #16
Jason
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I don't know of any journal maintained by TGrid, but there could be. Or it could be an option in TGrid to output a journal. You'll have to look into the manuals to find that one out.

Whenever you make geometry in Gambit you get a .jou file. You can modify this file (it's just an ASCII text file) and rerun the geometry. I don't have Gambit running in front of me, but it's under the File menu... You're looking for Edit/Run Journal. Then you can just read the journal and tell it to run.

I would modify the journal that has your half of the geometry. Then you can run it and pull it back into TGrid and work from there. It might save some time, and it'll be good practice for you to modify/run a journal.

The thing to watch out for in a journal is the file save commands... it could overwrite the geometry you already have, which you probably don't want to do. So you might back-up the case you're modifying to a safe directory.

Hope this helps, and good luck, Jason
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