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nims October 25, 2010 00:52

Problem of negative volume.......Please help
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 02:02

negative volumes are linked with your MDM parameters

nims October 31, 2010 00:57

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)

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