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-   -   Unphysical behaviour in dynamic mesh/vof model (https://www.cfd-online.com/Forums/fluent/87590-unphysical-behaviour-dynamic-mesh-vof-model.html)

xaero April 24, 2011 07:50

Unphysical behaviour in dynamic mesh/vof model
 
Hello community,

I've been experimenting with dynamic meshes for a while, but, however, was not able to calculate the simple example described below.

I am trying to let a cylinder (diameter 25mm) fall into water (pressure based/vof geo reconstruct/piso)

The whole domain is a 0.3x0.3m face with the cylinder falling from a height of 0.0625m into the water. (approx. 65000 tri elements) There is also a hex boundary layer around the cylinder. The motion is introduced with the 6dof solver and an appropriate udf.

I calculated the mass and moments of inertia of the cylinder assuming a depth of 1m. The density is around 612 kg/m^3.

Problem: The calculation is doing fine until the cylinder dips into the water(with a velocity of about 1 m/s). Then it suddenly accelerates and bounces off the water surface like a rubber ball (velocity ~3m/s), until it crashes at the upper side of the domain.

The UDF i use is pretty the same as from the UDF tutorial:
#include "udf.h"

DEFINE_SDOF_PROPERTIES(stage, prop, dt, time, dtime)
{
prop[SDOF_MASS] = 0.30;
prop[SDOF_IXX] = 0.025;
prop[SDOF_IYY] = 0.025;
prop[SDOF_IZZ] = 0.000234;

printf ("\nstage: updated 6DOF properties");
}

I have uploaded a video to http://www.it-satisfies.com/bouncing.mpeg

Any help will be highly appreciated.

Greetings


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