Hydrodyn damping matrix adjustment

Wind & Water Computer-Aided Engineering Software Tools
1

Dear all,

I have a question regarding the adjustment of the Hydrodyn linear and quadratic damping matrices. I use a Potential Flow model, combined with linear and quadratic damping matrices to account for viscous effects.

I would like to adjust the matrices based on CFD simulations (looking for the Free Decay in OpenFAST to be as close as possible to the Free Decay in CFD). And I want to adjust the matrix for a platform where the CoG does not coincide with the point (0, 0, 0), as shown in the attached image.

I thought that, in this case, the cross damping terms (non-diagonal terms of the matrix) in Pitch-Heave DOFs might be relevant (coefficientes C35 and C53 of the matrices). And I would like to know your opinion on this: whether the non-diagonal terms are important in this case, or whether they can be neglected. If these coefficients are important, I will run CFD with the 2 DOFs (Pitch and Heave) activated. If not, I will adjust damping coefficients of each DOF separately.

Thank you in advance.

Best regards,

Nicolás

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2

Dear Nicolás,

Yes, I would expect the off-diagonal terms to be important any time there is coupled motion of the platform degrees of freedom. Often this is surge-pitch (C15 and C51) and sway-roll (C24 and C42), but in your case, with the platform offset from the tower centerline, and can foresee other couplings as well, e.g., pitch-heave (C35 and C53).

Best regards,