Problem of negative volume.......Please help

nims · October 25, 2010, 01:52

Hello all,

I am trying to simulate the oscillatory motion of a vertical cylinder partially submerged in a tank ( can be rectangular or circular).I am using the following features of fluent.
1. VOF model
2. Define_CG_motion udf to give the oscillatory motion
3. K-epsilon model for Turbulence modelling.
Now i couldnt find a proper mesh for the problem.
I tried with a rectangular domain splitting it to 4 quaters. But when the mesh size is too small its causing global courant number value very high. When i solve this problem sometimes negative volume problem coming.
Can anybody help me find a proper meshing....?

-mAx- · October 25, 2010, 03:02

negative volumes are linked with your MDM parameters

nims · October 31, 2010, 01:57

Hi max,

I am using spring smoothing method for mesh update.The below are the parameters which i am using.

Spring Constant Factor 1
Boundary Node relaxation 1
Convergence tolerance 0.001
Number of iterations 20

I tried using the layering method also

But for both the cases the non positive volumes exists after 1 oscillation of the cylinder. can u help me out further..

-mAx- · November 2, 2010, 03:07

Are you able to see where are those neagtive cells?
Check also if your displacement isn't too high in comparion with your cell height (if yes then reduce your timestep)

October 25, 2010, 01:52	Problem of negative volume.......Please help	#1
nims New Member Nimmy Thankom Philip Join Date: Aug 2010 Posts: 6 Rep Power: 16	Hello all, I am trying to simulate the oscillatory motion of a vertical cylinder partially submerged in a tank ( can be rectangular or circular).I am using the following features of fluent. 1. VOF model 2. Define_CG_motion udf to give the oscillatory motion 3. K-epsilon model for Turbulence modelling. Now i couldnt find a proper mesh for the problem. I tried with a rectangular domain splitting it to 4 quaters. But when the mesh size is too small its causing global courant number value very high. When i solve this problem sometimes negative volume problem coming. Can anybody help me find a proper meshing....?

October 25, 2010, 03:02		#2
-mAx- Super Moderator Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,297 Rep Power: 41	negative volumes are linked with your MDM parameters __________________ In memory of my friend Hervé: CFD engineer & freerider

November 2, 2010, 03:07		#4
-mAx- Super Moderator Maxime Perelli Join Date: Mar 2009 Location: Switzerland Posts: 3,297 Rep Power: 41	Are you able to see where are those neagtive cells? Check also if your displacement isn't too high in comparion with your cell height (if yes then reduce your timestep) __________________ In memory of my friend Hervé: CFD engineer & freerider

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October 31, 2010, 01:57		#3
nims New Member Nimmy Thankom Philip Join Date: Aug 2010 Posts: 6 Rep Power: 16	Hi max, I am using spring smoothing method for mesh update.The below are the parameters which i am using. Spring Constant Factor 1 Boundary Node relaxation 1 Convergence tolerance 0.001 Number of iterations 20 I tried using the layering method also But for both the cases the non positive volumes exists after 1 oscillation of the cylinder. can u help me out further..