Decoupling wind-wave-wake interactions in a fixed-bottom offshore wind turbine

The interest and benefits of offshore wind energy have also brought along legitimate challenges. In the golden age of renewables, offshore wind-energy holds the most potential for growth, but the burgeoning benefits of offshore energy are also entangled in dynamics not fully understood, such as the dynamic coupling of the atmospheric boundary layer, the wind-turbine generated wake, and the surface waves. This study establishes the first experimental turbulent-interaction between the traditionally distinct fields of airflow dynamics above the air-sea interface and the characterization of wind-turbine wakes. The study details a non-trivial experimental setup combining a wave tank, wind tunnel, and scaled fixed-bottom wind turbine. Particle image velocimetry (PIV) was performed on three successive image planes to visualize wind-wake, wind-wave, and wave-wake interaction far downstream of the turbine. The wave phase-dependent dynamics of the turbine wake on the passing ocean-wave profile and location are outlined. The velocity and stress profiles showed a horizontal wake-pumping motion along with vertical wake-lifting of the offshore turbulent wake. The data were decomposed into incremental wave-phases revealing localized and predictable velocity maxima, stress maxima, and wake modulation that is normally obscured by a time-averaged mean. The results quantified wake pumping and meandering, as well as the Reynolds stress and wave-induced phase-averaged fluctuations. There is a phase-dependent oscillation in both the horizontal (streamwise) direction, as well as a vertical displacement of the wake. The shear stress and advection terms show this imbalance along the vertical direction of the turbine. The results illustrate a more complete picture of offshore wind-energy dynamics, which have implications for insidious mechanical issues, design optimization and/or control strategies.