Learning Forum

[mmd_az]

Hi, I'm trying to simulate the motion of a sphere when we have gravity and buoyancy force by 6DOF and Dynamic Mesh in fluent.

My Fluent settings are

Density-Based

Sst k-omega model

Transient

I have a 3d domain that contains water (density is 998.2 kg/m3) and a sphere (radius is 1m and density is 500 kg/m3). I set gravity -9.81 m/s2 in y-direction. In this case, buoyancy force is greater than gravity force so the sphere ought to go up (fluent force plot shown positive value) but the sphere drops. In every time step convergence is ok(under 50 iterations). Why this happens?

when I set the density of sphere 100 kg/m3 forces to go to infinity and it's given me floating point exception error.

For both cases force plot in y-direction is attached.

Any help regarding this problem is highly appreciated.

Regards

Fy density of sphere = 500.jpg

[Rob]

How big a time step are you using?

[mmd_az]

I used 10^-5 s and bigger than that
I think fluent neglected buoyant force when I explicitly apply the buoyant force in the 6DOF properties UDF e.g prop[SDOF_LOAD_F_Y] = 100 N It seems ok

[Rob]

If the step is too small the solver may fail to update the position as the delta is too low to register. Too big and the object passes through more than one cell in a time step and almost certainly turns the mesh inside out. As I can't open the images (we're not allowed to) it's hard to judge what's happening. For the 6DOF I think you need to set a mass & centre of gravity. knows alot more about this area.