[CeesH]

Dear all,

I'm trying to set up a 6DOF simulation to determine the terminal velocity of a metal ball falling through a tube filled with liquid. I've worked my way through the dynamic mesh tutorial on the falling box, which worked fine. However, when I get to setting up my own simulation, all hell breaks loose.

- I've tried remeshing, as in the tutorial, but no remeshing takes place. Result: negative cell volumes and a crash.

- I've tried layering, as in the missile silo tutorial. But, no collapse takes place. The ball-zone ends up overlapping the regular mesh, stretching the cells on top and reversing the cells below, until eventually negative volumes occur at the sides.

Conclusion; I'm stuck. I can't find any tutorial on dynamic meshing with an axisymmetric domain; the pdf of the missile is available but I don't have mesh files or case files to see where things go wrong. Below are my udf and mesh (at least, part near the ball. it extends further down). If anyone can help me out here, it'd be great. If you need more information, please let me know.

Thanks in advance,
Cees

#include "udf.h"

DEFINE_SDOF_PROPERTIES(test_ball, prop, dt, time, dtime)
{
   prop[SDOF_MASS] = 33.5;
   prop[SDOF_IXX]  = 0.0013400000000000003;
   prop[SDOF_IYY]  = 0.0013400000000000003;
   prop[SDOF_IZZ]  = 0.0013400000000000003;
SDOF_ZERO_TRANS_X, /* boolean, suppress translation in x-direction */
 SDOF_ZERO_TRANS_Z, /* boolean, suppress translation in z-direction */
 SDOF_ZERO_ROT_X, /* boolean, suppress rotation around x-axis */
 SDOF_ZERO_ROT_Y, /* boolean, suppress rotation around y-axis */
 SDOF_ZERO_ROT_Z, /* boolean, suppress rotation around z-axis */
SDOF_SYMMETRY_Y, /* normal vector of symmetry plane for half model */

}

[CeesH]

I don't really like bumping up threads, but seeing as it fell of 1st page and the simulation still doesn't work, I'll break my rule of bump.

[CeesH]

ok, one last try, since the problem is still out there...

[ishangaud]

1)The UDF seems wrong to me. Plz check the UDF below

2)also You cannot have axis-symmetric problem in Y-direction. The axis should always be in X-direcion,

3) Mesh type should always be tet-elements for remeshing if you are using that

4) Use smaller time step like 0.001 or 0.0001



This is the UDF for ball falling/floating in water.


#include "udf.h"

DEFINE_SDOF_PROPERTIES(Lightweight_ball, prop, dt, time,dtime)

{

prop[SDOF_ZERO_TRANS_Y]= TRUE;
prop[SDOF_ZERO_TRANS_Z]= TRUE;

prop[SDOF_ZERO_ROT_X]= TRUE;
prop[SDOF_ZERO_ROT_Y]= TRUE;
prop[SDOF_ZERO_ROT_Z]= TRUE;



prop[SDOF_MASS] = 3.34e-4;

prop[SDOF_IXX] = 7.36e-9;
prop[SDOF_IYY] = 7.36e-9;
prop[SDOF_IZZ] = 7.36e-9;

}

[Hossein1]

I'm simulating axisymmetric collision of a particle and a droplet with a mesh very similar to what is shown above. However, as you can see in the attached figure, when particle moves, after some time steps dynamic mesh fails to follow the particle movement (when simulation starts, A=A' and B=B', but after some time steps you can see that how A and B stretched). Does anyone know what should I do?

PS. Here is what I think: since the particle breaks the 'axis' line, I do not have just one line as axis, and I choose A, B and C as my axis. But I think this disconnectivity on my axis causes problems with dynamic mesh.

I don't need necessarily to mesh inside the particle and I've tried that case as well, but the problem still exists.

[samir_cfd]

Hi
I am simulating the same case in Fluent but I have a problem, when the ball is close to the free surface the solution diverege (divergence problem) I don't know where is the problem exactly and how to solve it?
Many thanks!!