Vertical Entrainment of Mean Kinetic Energy in Offshore Wind Farms*

Fixed bottom and floating wind farms are two prevailing design methodologies for offshore wind, depending on the depth of the ocean floor. Fixed bottom farms are coupled to wave-induced wake modulation while floating wind farms include rotor misalignment due the dynamic response to turbulent aerodynamic and hydrodynamic forcing. The present work performs scaled wind tunnel experiments exploring the effects of different waves on various performance metrics for both fixed-bottom and floating wind farms. Experiments are conducted in the Portland State University wind and wave tunnel with a four-by-three wind farm spaced at 5D with 15 cm diameter turbines. Two wave steepness are used at four wavelengths, which are set as multiples of the farm spacing to elucidate the presence of harmonics. Coupled motion-capture and power measurements are made of the floating wind farm. Stereoscopic particle image velocimetry is used to capture multiple 2D-3C, streamwise-vertical planes. Time series of power-motion and row-to-row motion correlations are presented. Vertical momentum entrainment is calculated and compared across various wave cases, isolating the effects of waves on wake recovery. Wavelength is shown to cause phase-misalignment with turbine dynamics, which affects, and under certain conditions can enhance, wake recovery. This work provides deeper insight towards offshore farm optimization for both existing fixed-bottom turbines and future floating farms.