Added mass in Hydrodyn

Wind & Water Computer-Aided Engineering Software Tools
1

Dear NERL members,

As for offshore floating wind turbine, I am interested in the issue about added mass of floater. I have one question, I will appreciate for your kind help.
1.In free decay test.
It is indicated we should always use the added mass at infinite wave frequency from WAMIT results, is it also same for free decay test?
Thanks a lot.

 Best regards,
 Shining. Zhang
2

Dear Shining,

The short answer is, “yes.”

The frequency-dependence of hydrodynamic added mass and damping is captured in the time domain by applying a constant added mass and damping expressed via a convolution integral. The approach taken in HydroDyn is to take the infinite-frequency limit as the constant added mass and the Kernel of the convolution integral based on the cosine-transform of the frequency-dependent damping matrix. That is, the use of the infinite-frequency limit of added mass is compatible with the approach used to calculate the Kernel of the convolution integral. See section 2.4.2.1 of my PhD thesis-turned NREL report for more information: nrel.gov/docs/fy08osti/41958.pdf.

Best regards,

3

Dear sir,
The FOWT equation of motion in time domain can be expressed as follows:

image
image739×80 2.33 KB

where M is the FOWT mass matrix; A_∞ is the added mass at infinite frequency, respectively, I want to know what does added mass mean at infinite frequency. I have read your doctoral thesis, but I don’t understand it very well. Please tell me more details, thank you!

4

Dear @Hairui.Jiang,

The hydrodynamic wave-body potential-flow solution solved by WAMIT or an equivalent pre-processor is frequency dependent. By “infinite frequency”, it is meant that the frequency-dependent solution is found at the asymptotic limit as frequency increases.

Best regards,