[arun7328]

Hello All,

I have worked on this problem for a long time and would be happy if someone could guide me regarding my doubts.

I have been working on swaying and heaving of cylinder on free surface to find out the added mass and damping. The motion of the cylinder in both cases is sinusoidal with an amplitude of 0.03 and frequency ranging from 1 to 6 rad/sec. This was chosen to compare with the experimental values. Rather than straight away calculating the added mass and mass and non-dimensionalizing it, I have compared the forces obtained from the CFD simulation to the experimental values of forces.

After a lot of trail and error and playing the right mesh, I managed to get very good results in swaying when compared with experiments. In this case I created a force report in the x-direction and compared with the horizontal force from the experiments.I have used morphing for both cases.(x is longitudinal, y is vertical and z is a small 3D width(3m)).

I have now tried to compare results for heaving. The problem I am facing is to extract the hydrodynamic component. I have defined a number of reports and wanted to see the results from each.

1. I defined a static pressure field function. My cylinder center lies at 0,0,0. So a simple field function was defined as
($$Centroid[1] < 0) ? -$Densitywater*(Centroid[1])*9.807 : 0.

This would give me the staic pressure about the mean free surface line. I created a surface integral of this pressure field function to get the force component.

2. I defined a surface integral report of the total pressure field function(default). According to the definition in STAR CCM+, total pressure is the pressure when the flow is isoentropically brought to rest? Then it has both static and velocity components.

3. I defined a surface integral of the static pressure field function(default).

4. I also defined a force report in the y direction excluding shear(only pressure).

When i compared all values I get no relevance . The static and total seems to be very close with the difference only about 50 N which is wrong as per the experiments.

The static pressure defined by me shows values greater than the total pressure.

When i check the vertical force it oscillates about a mean which is not zero. actually ,none oscillate with 0 mean, which is also understandable. So when i check the vertical force and initialize it to 0 mean and compare, it is the same as the hydrodynamic heaving force as per the experiments. I am unable to explain the results and the values of pressure and forces that STAR CCM+ gives me.

Please any one there could explain. The force report as per STAR is the surface force when the body is not defined as rigid? When using rigid body simulation it includes body forces.

Why am I not getting the dynamic component when i subtract the total and static? Why am I getting the dynamic component in the vertical force report?


Please any reply would help me.

The problem is inviscid. I have not yet applied a turbulent model. The domain is large enough not to have any wave reflection and I have also given wave damping. The cylinder oscillates on still water and waves are created only due to the oscillations.

I can share the model if you need.

Thanks
Regards
Arun

[arun7328]

Can any one please give me some reply.?
regards
Arun

[arun7328]

I did not find any thread also which discusses about hydrodynamic force. So please advice if you know the answer. I have completed the simulation for all 6 frequencies and even the vertical force is not what i want.


Thanks
Regards
Arun