Free Floating Vertical Cylinder
 

Hello all,

I am trying to do a 3D free heave decay test on a floating vertical cylinder with the following properties in its un-displaced equilibrium : KG = 0.1431m, KB = 0.1925m. GM = 0.0510m, Ixx=Iyy = 0.2708 kgm^2. The cylinder is having a free board of 0.1m. The initial displacement of 0.055m is given in the heave direction.

The domain is 4*4*3m with a water depth of 2.5m.

The cylinder is loaded as a .stl file. I am getting confused with the X 182 command for the translation especially in the z-direction what value has to be given.

Also do I need to subtract the water depth from the output file of 6DOF-position or any other calculation is needed in my case to find the heave decay amplitude?

And X 11 0 0 1 0 0 0 has to be used or X 11 1 1 1 0 0 0 is the right one for heave deacay?

It will be really helpful if these are cleared.

Regards,
Arya

Hi Arya,
The X 182 option moves the stl object by the dx, dy, dz values that you provide, as you have correctly understood.
To know what values are appropriate, you will need to know the coordinate system in the stl file. You can determine this by simply opening the stl file in Paraview. Depending on the source of the stl, (as in who made it, in which program etc.), the origin of the stl could be at different locations.

Once you know the origin of the stl file, you should be able to decide on the values for X 182. Since you are doing a heave decay test, you will "push" the cylinder 0.055 m in the direction (up or down) from the CG.

Yes, the values in the files will be wrt CG. So if you want the signal to vary around 0, then you will need to subtract the Cg value for the direction.

Yes, X 11 0 0 1 0 0 0 will give you a 1 DOF heave decay solution.
Technically you won't be wrong switching all of them on, but the simulation will be slower.

Thanks a lot.
This was so helpful.

Hi,

I was trying to validate the free pitch decay by inputting the initial pitch angle in X 183. I have enabled only pitch in X11 but in the 6DOF_position.dat file roll DOF is also showing values. I am using the version 21.11. To carry out the validation study, I have taken the experimental data from Palm, J., Eskilsson, C., Paredes, G.M., Bergdahl, L., 2016. Coupled mooring analysis for floating wave energy converters using CFD: Formulation and validation. Int. J. Mar. Energy 16, 83–99. http://dx.doi.org/10.1016/j.ijome.2016.05.003.

Can you please help me out to find the solution for the above problem?

Also can you explain the 6 values to be used in X 183 a bit more?

Thanking you,