Learning Forum

[arjun.janardhanan]

Dear ANSYS community,

I am working on a hydrodynamic–structural workflow in which AQWA time-domain pressure-mapping APDL files are read into a Transient Structural analysis. The structure is a floating platform, and I am applying AQWA mapped pressures together with additional AQWA 6-DOF loads from wind, current, additional damping and cable forces. The cable forces are included within the AQWA APDL pressure files. 

The AQWA time history has a time step of 0.2 s, which I believe should be sufficient to capture the relevant transient effects. The AQWA PREP7 file is read once at the beginning, and the individual AqwaLoadMap_LC*.dat files are read sequentially at each load step. The AQWA-imported rigid-body acceleration terms are then set to zero using commands such as:

CMACEL,_Aqhs101,0,0,0
CMOMEGA,_Aqhs101,0,0,0,0,0,0
CMDOMEGA,_Aqhs101,0,0,0,0,0,0

I have Standard Earth Gravity also defined.

The issue is that, in Transient Structural with TIMINT,ON, the platform experiences very large rigid-body motion. My interpretation is that even small residual imbalances between the mapped pressures and the external 6-DOF loads generate large rigid-body accelerations. These artificial rigid-body accelerations then dominate the response and introduce large inertial effects/motions.

As a separate check, I applied the total 6-DOF loads from the AQWA time-domain analysis directly at the centre of gravity. In that case, the motion response obtained in Transient Structural was consistent with the AQWA response. This suggests that the issue may be related to residual imbalance when the distributed pressure field and the external load components are applied separately.

Is there a recommended workflow for applying AQWA mapped pressures to a floating structure in a transient structural analysis? In particular, is a fully free transient analysis not the appropriate approach? (I already have results from-static structural)

Any guidance would be greatly appreciated.

Thank you.