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November 2, 2006, 08:31 
Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#1 
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Hi everybody,
I'm trying to work with dynamic meshes with a DEFINE_GRID_MOTION UDF. The UDF does work, so the boundary for which it is defined does move, but the fluidmesh stays the same so that I get after some iterations problems with negative cells. So does someone know this kind of problem and can tell me what I forgot to enable so that the interiornodes will also change their places so that I will not get negative cells Thanks 

November 2, 2006, 11:04 
Re: Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#2 
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maybe your time step is too large. in one step the boundary moved so big distance that u got the negative cells


November 3, 2006, 02:45 
Re: Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#3 
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I don't think that is the problem, because I can run 6 timesteps till the distance the boundary moves is so big that I get negative Volume cells. The problem is the surrounding cells do not react on the slowly motion of the boundary....


November 3, 2006, 03:57 
Re: Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#4 
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1) pay attention to collaps factor,... ur mesh is not going to collaps,.. chang it,..
2) decrease ur time step,.. it could work, if ur collapse factor is little bit wrong,... if its not work,.. inform me ur grid size,.. and time step,.. 

November 3, 2006, 05:02 
Re: Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#5 
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20 minutes ago I got it. Now it works. The mistake was the I took no trinangular cells. I took rectangular cells. But its for me a little bit strange that it doesn't work because of rectangular cells.... does somebody know how I can get it work with rectangular cells?
Thanks for the response. What is a collapse factor and where can I check and change it? Maybe this problem will appear someday and it would be good to know the answer... Thanks a lot! 

November 3, 2006, 05:47 
Re: Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#6 
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It should work with both mesh,..
if u like, u could send me case file and udf at cfdxue@yahoo.com,.. best luck 

November 3, 2006, 12:59 
Re: Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#7 
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Deformation of hexmeshes is initially swithced off in Fluent. You can turn this feature on through the TUI command 'def mod dmc sp soas', alternative set the scheme variable dynamesh/spring/allelementtype? to #t
Regards, Eirik 

November 3, 2006, 14:22 
Re: Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#8 
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That would explain why it doesn't work with rectangular mesh. Unfortunatly I don't know how to use both of them... can you tell me how and where I have to make the changes, please?
Thanks a lot! 

November 3, 2006, 15:25 
Re: Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#9 
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In Fluent's Text User Interface (TUI  perhaps main window is a better phrase), just type the command I gave you. Typing return after each word will bring you to the next menulevel, like it's written brings you all the way down in one step. You'll then be prompted whether or not to turn the feature on, just answer yes.
 Eirik 

November 3, 2006, 16:34 
Re: Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#10 
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Tanks a lot! I will try it on Monday morning first! (Here it is 22:40 at the moment Friday evening....) have a nice weekend!


November 6, 2006, 03:28 
Re: Problems with Dynamic Mesh: DEFINE_GRID_MOTION

#11 
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Thanks a lot!
It does work! Claudia 

September 25, 2013, 14:23 

#12 
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Alexandre Felipe Medina Correa
Join Date: Aug 2013
Posts: 3
Rep Power: 4 
Dear Eirik,
I am having the same problem as Claudia. I tried to set the command you gave on TUI, but it didn't recognize the comand dmc... I tried part by part, first with def, then mod, works fine. But the 'dmc so soas' does not work that well. Could it be a version problem? I am using 14.5 here in university. Thanks, Alex 

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