Dear @Jason.Jonkman,
I am a PhD student working with a semi-submersible FOWT floating platform for which the first order hydrodynamic coefficients, force transfer functions, and the second order QTFs have been calculated using a different potential flow (PF) solver than WAMIT. This PF solver uses its local system of axes (LSA) at the body’s center of gravity (CoG). The outputs including hydrodynamic coefficients, force transfer functions, and QTFs are given in this LSA system and in SI units.
Could you please advise on the necessary modifications needed for these coefficients, force transfer functions, and QTFs and in the hydrodyn.dat file, elastodyn.dat file, or elsewhere, so that I can correctly convert them to the WAMIT format and proceed with my research?
If you need any specific details, please let me know.
Dear @Touhidul.Islam,
Assuming you are using a recent version of OpenFAST, the reference position of the potential-flow solution is user-specified via PtfmRefxt/PtfmRefyt/PtfmRefzt in the HydroDyn input file. So, you should be able to set this point to the body CoG and use your data directly (as long as it is in WAMIT data format).
Nevertheless, if you want to shift your potential-flow data from the CoG to a different point, such as SWL, see a related forum post: Time domain analysis by AQWA.
Best regards,
Thank you very much @Jason.Jonkman.
I’ve read the post you mentiond about the shifting of potential-flow data from the CoG to a different point here.
Can I use the TransMat^T to convert IMASS, IDAMP and ISTIF data in IEA-15-240-RWT_2nd.frc to transfer them to CG using the following relation?
F_cg = TransMat^T * F_swl
Is this TransMat is defined considering the origin at swl with z-axis upward?
Regards,
Dear @Touhidul.Islam,
I agree with your equation for F_cg (copied from my forum post), but to transfer the matrices you must pre- and post-multiply by TransMat, e.g.,
IMASS_cg = TransMat^T * IMASS_swl * TransMat
IDAMP_cg = TransMat^T * IDAMP_swl * TransMat
ISTIF_cg = TransMat^T * ISTIF_swl * TransMat
TransMat is defined with (x_cg, y_cg, z_cg) in body-fixed coordinates relative to swl, with the z-axis pointed upwards.
Best regards,
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