A quasi-coupled wind wave experimental framework for testing offshore wind turbine floating systems

Two sets of experiments in the St. Anthony Falls Laboratory (SAFL) wave tank facility and atmospheric wind tunnel are integrated to provide a scaled representation of a floating wind turbine under heave and pitch motions due to ocean waves. The quasi-coupling is established by controlling the turbine rotor speed to generate a thrust force mimicking steady or fluctuating wind gusts in the wave tank, and by using two actuators to oscillate a miniature turbine in the wind tunnel. Measured pitch and heave motions under varying waves are scaled down using rotor geometry and the wake meandering frequency to study the effect of the floating platform kinematics on the evolution and characteristics of the oscillating turbine wake. For a limited case of experimental conditions results provide a phenomenological and quantitative description of the floating-turbine system under variable waves and simulated wind gusts. Specifically, we demonstrate that wind gusts contribute to increase the platform pitch range, and that periodic large scale flow patches of high and low momentum flow are generated by the oscillating rotor in the turbulent boundary layer and are coherently convected through the wake. Both mechanisms could amplify the pitch response of downwind floating turbine units within the offshore power plant, in particular if the wave and/or wind forcing frequencies happen to approach the pitch natural frequency of the floating system.