Surface Wind Shear due to Large Ocean Waves

(Moving this post from General to this more apt category)

Dear Dr. Jonkman,

Does OpenFAST yet model surface wind turbulence caused by ocean swells? This would entail a special inflow model. The effects could have significance to fatigue life of turbine rotors, and vary greatly according to location.

kPower followed the Makani GoogleX AWE R&D project from 2007 on. I got to meet and know the founders at their Alameda Island retired Navy base, which was near KiteShip where I trained and worked, based at North Sails’ old loft. kPower did constant Makani failure mode analysis, by which any complex system can be judged. When Makani pivoted offshore, due to problems like high noise and risk to populations, there was a basket of new challenges, like seabird poop fouling hot-running motor-gens, fouling laminar-flow wing surfaces, and even clogging critical pitot tubes.

A special area of concern stood out; launching and landing from a “perch” (spar buoy). Makani’s over-scaled M600 was already marginal with respect to reserve hover lift, and at risk of settling under power, where a ring vortex sets up around a rotor rather than a downward jet. kPower determined the risk was especially great during high ocean swells. These entrain air and create strong wind shear, the very same shear that enables albatrosses to Dynamic Soar (DS) the ocean. Thus, the Makani platform could find itself not just trying to dock on a heaving swaying perch, but perhaps literally have the air sucked out from under it.

Modeling a HAWT in such inflow wind shear seems like a reasonable goal, if its not being done already. The CFD is hyper-complex, as sea state, bottom interaction, and wind vary so many ways. Its a very interesting problem.

Dear Dave,

In OpenFAST, the wind and wave conditions are specified distinctly, with no direct air-sea interaction modeled. So, it is up to the user to specify wind conditions that are rightly associated with the wave conditions specified. Typically this is done statistically for design based on a joint probability of wind and wave conditions. But if more detailed data is available (such as direct measurements with wave buoys and met towers or LiDARs), this could be done with time-synchronized wind and wave events.

NREL is also know working to improve high-fidelity modeling capability within ExaWind, including coupled air-sea two-phase flow.

Best regards,

Thank You,

Preprocessed coupled wind/sea-wave models would fill the gap.

The ultimate limit case is a “rogue wave hole” event. Its practical to set these up rather than wait for an Exaflop Sim to randomly create them. Many places experience significant events almost yearly, but almost no one is there to log them. Perhaps Sat data would be a good start.

Observation of rogue wave holes in a water wave tank
A. Chabchoub,N. P. Hoffmann,N. Akhmediev,

Seeing that the spar-buoy concept just presented in Index-General would plunge (heave down) badly in a hole. A fix could be fabric damper panels at the spar-bottom mooring point. Other mooring concepts confined to the upper wave interaction layer could just fall into a rogue hole. These scenarios can be missed by high-fidelity sims that only model a few seconds of most-probable turbulence.