General Mechanical

Aqwa – Forces between adjacent structures with underwater sides in contact

• tecnico
Subscriber
Hi, we have two floating bodies (for simplicity two boxes) which share a side and are rigidly connected, and we are interested in obtaining the forces across the rigid joint.
We tried the following attempts:
1)     Modelling the two bodies as single part with two point masses (each representing one body). “Splitting forces” can be obtained as a result of the “hydrodynamic diffraction” problem, however no similar results can be obtained (or at least we do not seem able to obtain) for the “hydrodynamic response” problem
2)     Modelling the two bodies as two separate parts. We made each part composed by 4 no. bodies (submerged, emerged, submerged face in contact with the other part, emerged face in contact with the other part). We inserted a rigid joint between the two parts.
Then:
a)       we set the submerged faces in contact with the other part as “non-diffracting” (as per 5.5.1 of the User Manual: “surface bodies … can be manually defined as non-diffracting elements …. where contact occurs underwater between adjacent parts”). A series of errors protesting that “water line element XX on struct YY is beneath mean water surface but not a diff T/QPPL” cause the solve aborting (hydrodynamic diffraction problem).
b)     we suppressed the submerged faces in contact. After a number of warnings informing that X/Y  hydrostatic forces for both parts are imbalanced (no surprise considering that the parts are open, there is one face missing when we suppress it) and warnings about the fact that certain sides of certain elements are not connected to another element (again no surprise considering that the parts are open), calculation is completed successfully (hydrodynamic diffraction problem). Similarly hydrodynamic response problem is solved and forces in the joint can be plot.
Questions are:
1)     is 2b) the correct way to model this configuration?
2)     2a would appear being in accordance with the instructions of the User Manual. If 2a is correct, why are we getting these errors that cause the solve to abort?
3)     how should we model a structure made of two rigidly connected halves to get the forces in the joint between the two halves?
Thanks.
• Mike Pettit
Ansys Employee
Hi @AndreaMatteo, Sorry you haven't received a reply to this sooner - the Forum has been undergoing some updates recently. To answer your questions: in this case the method in 2b) is the best option for estimating the joint forces/moments between the two halves of the structure, and your modelling approach is correct. The errors that you see in 2a) are related to the calculation of the waterline integral, which is required for drift forces/QTFs. I hope this helps. Mike