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FLUENT 6DOF dynamic mesh error pressure

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Old   September 20, 2017, 08:16
Default FLUENT 6DOF dynamic mesh error pressure
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I would like to use Fluent to simulate the movement of a floating body. I am currently experimenting with dynamic mesh and a "simple" model (2D): I use a rigid rectangular body of 1.54*0.08m in a box of 7.7*0.34m. For the boundary conditions I use the same parameters as Fluent tutorial "Solving a 2D Box Falling into Water". The only difference are the dimensions and the fact that my body is already immerse (by 0.04m) in the water (water height is 0.18m). I meshed the whole domain with trias and used the following UDF to parameter the 6dof model:

#include "udf.h"

DEFINE_SDOF_PROPERTIES(sdof_properties, prop, dt, time, dtime)
prop[SDOF_MASS] = 61.49;

prop[SDOF_IXX] = 129.6296;
prop[SDOF_IYY] = 111.1111;
prop[SDOF_IZZ] = 129.6296;


printf ("\n updated 6DOF properties");

Before using dynamic mesh I started the calculation with the body fixed in order to get hydrostatic pressure. Then I allowed the body to move but after roughly 5 time steps, I get the error:

"Warning: non-positive volumes exist.

Error: Update-Dynamic-Mesh failed. Negative cell volume detected.
Error Object: #f"

And when i display the mesh, I can see that the body has moved out of the box... If I stop the calculation after 2 or 3 time steps and display the pressure, I can see that I get huge values. If I calculate the lift force on the body, the force is also bigger than what it should be. If i look at the lift force history, when the body is fixed, I get 610.13984 N but after I allow the body to move it is oscillating with amplitude such as -2468606 N, 1.2394356e+08 N, ... until I get the error message.

From what could this instability come from? I really don't understand why it does something like that... Could someone help me solve this problem?

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