ANSYS AQWA Spar buoy

I am working with ansys aqwa to study the behavior of a Spar Buoy and I am sending this message hopping that you can help me with some troubles that I am having in my simulation.
So on the bottom of my spar buoy I have the geometry with tube elements and a disc in the bottom to create the viscous drag effect. After I run my simulation, in Workbench 16.0, the tube elements appear all deformed in the pressure and motion analysis.

Also, does the software ansys aqwa simulate the displacement of the water inside of the spar buoy considering a moonpool effect? I am trying to simulate the PTO to include wave energy converter in the buoy and I was thinking on inserting a piston with the density of the water inside the spar buoy, but I don't know how to make an interaction between the two bodies. Can you help me with some alternatives for this kind of simulation?

Thank you, looking forward for you feedback.

 

Hi,

I am not very clear what is the exact situation, but I hope that I can help you.

Are you referring to the diffraction analysis post-processing in Workbench when you say that the morison elements are deforming or you are referring to the time-domain analysis ?

Regarding the displacement of the water inside the buoy. I will strongly advise you to create a separate abstract geometry inside the buoy (just a flat plate) and use it as a external lid with a reasonable damping coefficient (0.1 - 0.3) in order to get some more realistic results. This is due to the fact that AQWA is a potential flow solver which doesn't account for fluid viscosity and you will get really high (unrealistic) free surface displacements inside your buoy.

Further on your next question. In the diffraction analysis you simply define which are the interacting bodies under Structure Selection (both of them must be diffracting bodies as well, this doesn't work between diffracting and Morsion bodies). In this way you will get the hydrodynamic results considering the effect between the bodies. Next in your time domain analysis you need to give constraints between the bodies otherwise they will float into each other ... You can make this by adding fenders between the bodies which will keep the relative position between the both. This point is very important because if in your time domain simulation the interacting bodies move too much relatively to each other the simulation will end with an error.

 

Hi, thank you for your response and for the suggestions.

Yes, I am referring to the diffraction analysis in Pressure and Motion results, if you want I can send you some pictures for you to understand what I mean.

Relative to the time domain analysis, I have already tried fenders (since the movement is linear I put a fender outside of both bodies and a then select their connection points), but the geometry inside of the water column is still moving outside of the buoy, can you please tell me what are the best connections to use for linear movement? I not sure that I can restrict the movement of the plate relative to the buoy.

Thank you once again for your help!

 

I think that when you post-process your diffraction results in Workbench this "issue" with the Morison elements happens. If these elements are all collected in one body in AQWA there is nothing to worry about.

Regarding the fenders. I would suggest to place a floating fenders (probably 4 will be enough in a polar array @ 90deg) inside of the spar. They will keep your inner body from going outside.

Regarding the plate (which I suppose is the external lid) it's important that it is together with another structure in one body. In your case you need to make it one part with the buoy (don't worry that the meshes are not connected - it doesn't matter in this case). Another point is that this plate has to be exactly on the water surface (z=0) if you want it to work.

 

Hi,
The moon pool structure is also included in the structure I studied. I would like to know how to correctly add the mass matrix to the created abstract geometry, and whether it should be set to the same density as water.
Thank you. I look forward to your guidance.