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Error: Negative volume and Creating empty surface

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Old   February 14, 2013, 04:45
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Originally Posted by sadjad.s View Post
hi mate,
actually with 6DOF option in dynamic mesh, fluent automatically considers buoyancy force between solid(ie. box) and fluid(water).
my problem was that i considered rather little mass for box (in the main problem it was sphere) so that it went up!
in this example(2d version), i made a structred grid around box and let it move with the same speed as box.(as you can see in picture).
and used remeshing method for unstructured grid.
i use fluent v14.0 64 bit, if you want, post me your email so i send you case & data and udf.
Attachment 16128
I did the same!
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Old   February 14, 2013, 04:53
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Originally Posted by subha_meter View Post
Hi Sadjad,

Although the dynamic mesh model worked for heavier particle (density > liquid). for lighter particle (particle density < liquid density), it seems there's some problem. The particle bounces off the interface instead of floating on the liquid. Any suggestion?

I was able to simulate a buoyant particle fully immersed in a fluid just setting the acceleration of gravity = 0 and the operating density ( in operating conditions) equals to the fluid density. It works.
For your case...have you seen the ansys tutorial about a 2D buoyant box? This is the video of simulation , but you can find the tutorial on web
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Old   March 1, 2013, 00:59
Default 2D airfoil O Grid- negative volume
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I'm carrying out a 2D pitching airfoil simulation using dynamic meshing with a "O" Grid.It is a structured mesh. The dynamic conditions i have given were
1) airfoil - rigid body with UDF with pitching over X=0.25
2)interior - deforming with the min and max length scale from zone scale info
3) fluid -deforming with the min and max length scale from zone scale info.

The spring constant i gave was 0.001 and
convergence tolerance 0.0001
No of iterations 150.

The time step tat i used is 0.001.

After running the iterations and after about 500 time steps i'm getting the following error

negative volume detected
dynamic mesh update failed.
i have worked around with the spring constant and also with the time step nothing worked.
could someone help me out rectify this issue..
thanks in advance,
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Old   March 1, 2013, 01:39
Default negative volume - dynamic mesh failure
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Subhasish Mitra
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Hi Sam,

In dynamic mesh problems, the mesh around the solid body keeps deforming during each iteration. Now, when the mesh becomes too distorted that the centroid of the cell lies outside the cell boundary, a negative volume is detected triggering the solver failure.

In a structured mesh, cell distortion is always higher than the unstructured mesh. You may tighten the the remeshing criteria i.e. the minimum cell size, skewness ratio etc. however the best way to get rid of the problem is to use "unstructured" (tri/tetrahedral) mesh. This works.
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