Dear Jason,
Jason.Jonkman:
The axial coefficients at joints apply to ends of members that are exposed to water e.g. the bottom of the OC3-Hywind spar. See slides 11-14 of the presentation found here for more information: wind.nrel.gov/public/jjonkman/Pr … onkman.pdf .
Now I got it. I wasn´t aware that the surface normal to the member surface where the distributed loads are applied is ment here. Slide 13 from your link was very helpfully.
Jason.Jonkman:
You must always specify all coefficients (you can’t leave any blank), but you can set them to zero if appropriate. For the OC3-Hywind model, hydrodynamics are treated as a hybrid potential flow plus strip theory solution, so, only the viscous-drag terms from strip theory are used. Without marine growth and the use of model 1, this means that SimplCd is the only coefficient actually used by HydoDyn. The axial terms in models 1-3 are applied only for tapered members. We don’t have great documentation on this yet, but just be aware that the axial terms in models 1-3 are unused for members that are not tapered i.e. where dR/dz = 0 (where the change in radius along the length of the member is zero).
The Froude-Kriloff term is derived by integrating the dynamic pressure of the undisturbed incident waves around the body. In Morison’s equation, the long-wavelength approximation implies that this term can be calculated using the fluid acceleration at the body centerline with a coefficient of unity. In HydroDyn, we allow the coefficient to be different than unity (e.g. to correct for members that are not exactly circular or for other calibration purposes). See slide 12 of the presentation linked above for more information.
Thank you, this helps alot!
Jason.Jonkman:
The hydrodynamic loads are calculated locally at nodes along a member so that they can be properly transferred to the structural model for analysis. While FAST currently treats a floating platform as a rigid body (so integrating the hydrodynamic load along the member is OK), FAST can model bottom-fixed substructures as flexible (via the SubDyn module), so, it is important to calculate the hydrodynamics locally.
Okay that makes sense to me, that single nodes are necessary for the load mapping procedure in SubDyn.
Jason.Jonkman:
For the OC4-DeepCwind model, hydrodynamics are treated as a hybrid potential flow plus strip theory solution. The terms AddCLin(4,4) and AddCLin(5,5) are nonzero because of the ballasted (fluid-filled) members. See section 6.8.3 of the draft HydroDyn User’s Guide and Theory Manual for more information: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf .
I hope that helps.
I got one question with regard to MoorDyn. I am not sure if I should open a new thread on this. I will try it here anyow.
How do I set up pretension of the mooring lines? I didn´t find any hint on this for MoorDyn while I found out that for FEAMooring it is possible to specify pretension.
Once again thank you and your team for your time and engagement in providing those tools and support.
Best regards,