I have a question about the reference frame for the calculation of the forces in HydroDyn. In theory, linear potential flow theory assumes small motions of a body around its equilibrium position. In the case of more important motions (when the body is far from it’s equilibrium position), I wonder how the hydrodynamic forces are calculated in HydoDyn/FAST.
Is the hydrodynamic force applied to the platform (if HasWamit=True) calculated in the global inertial frame or is it considered in the body’s frame ?
I mean, for instance: when the platform is pitched (let’s say 5 degrees) because of the wind, do we consider that the vertical force applied to the platform is vertical in the global inertial frame ? Or is it “vertical” in the instantaneous frame attached to the platform, which is pitched at a 5-degrees angle (directed along its z vector). That would mean that the hydrodynamic force has to be transported to the global inertial frame before the resolution of the motion equation.
I’m not really sure I understand your question. When you refer to the “vertical force applied to the platform”, which force are you referring to (hydrostatic, wave-excitation, etc.)?
Section 6.5 (titled, “Impact of Substructure Motions on Loads”) from the draft HydroDyn User’s Guide and Theory Manual is related to your question–I suggest you review it if you haven’t already: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.
Thank you for your quick answer. I think that part answers to my question, sorry for asking about something that I did not find in the theory manual.
I was not mentionning a particular force (hydrostatics or other), but taking an example. I was asking if hydrodynamic forces were applied to the undisplaced or displaced position of the substructure. Sorry for not being understandable.
I found the answer in the last bullet point of the part 6.5 of the manual :
There is some information that I am still missing. In my previous post, I quoted :
But this does not tell me in what frame the force is defined. I can read the force is applied to the displaced position. But, considering the frames defined in FAST v7 user"s guide (part : Model Description), is the force defined in the Inertial Frame Coordinate System or in the Tower-Base Coordinate System ?
This can alter the results as the floating platform is moving and pitching : z_t is not purely vertical if the turbine is pitched.
Other than the exceptions mention in Section 6.4 of the draft HydroDyn User’s Guide and Theory Manual: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf, the hydrodynamic loads in HydroDyn are calculated at the undisplaced position and orientation of the substructure. These loads are applied at the displaced position of the substructure, considering their original orientation e.g. buoyancy always points vertically upward even if the substructure pitches.