QTF input format for OpenFAST

[Ramya.Niranjan]

Dear All,

I am interested in obtaining the second-order forces for a semisubmersible floating platform using OpenFAST v2.4.0. I used ANSYS AQWA to obtain the full QTF sum/difference matrices and converted them to WAMIT (.12d/s) format using a python script. I understand that the WAMIT output file (.12d/s) contains only upper half diagonal matrix. Can I use the full matrix or is it mandatory to use only half matrix with OpenFAST?

Regards
Ramya

[Jason.Jonkman]

Dear Ramya,

Section 6.8.4 of the draft HydroDyn User’s Guide and Theory Manual (drive.google.com/file/d/11WY1LR … sp=sharing) provides the answer:

Second-order hydrodynamic theory dictates that difference-frequency QTFs are conjugate symmetric between frequency pairs and sum-frequency QTFs are symmetric between frequency pairs. Due to this symmetry, the QTFs (the 10d, .11d, or .12d, .10s, .11s, and .12s files) may be upper triangular, lower triangular, a mix of upper and lower triangular terms, or full; however, after applying the symmetry, the matrix of frequency pairs must be fully populated (not sparse). When an element of the QTF is supplied together with its symmetric pairing, HydroDyn will warn the user if the QTF is not properly symmetric.

Best regards,