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. Kepsilon 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....? 
negative volumes are linked with your MDM parameters

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.. 
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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