In-phase and out-of-phase pitch and roll oscillations of model wind turbines within uniform arrays

We experimentally explored the impact of in-phase and complete out-of-phase pitch and roll oscillations on the mean power output and the structure of the power fluctuations of turbines within 3×2 uniform arrays. The flow past selected configurations was also characterized with hotwire anemometry. The parameter space included combinations of three amplitudes and two frequencies of the turbine oscillations, and two turbine layouts; measurements with fixed units are also included for comparison. Results show that the structure of the flow past the wind-turbine arrays was significantly affected by the frequency of the tower oscillations. The turbines under in-phase pitching induced a signature that extended in the intermediate wake, which was measured up to 7 rotor diameters downwind of the last row of turbines. The roll oscillations produced an increase of the combined power output independent of the amplitude and frequency of the oscillations, where the complete out-of-phase motions resulted in larger values with a maximum at the largest amplitude of oscillations (β=20°) of over than 10%. Opposite behavior occurred with the turbines under pitching, which led to negligible changes or decrease of the combined power with a maximum reduction of ~10%. The spacing of the turbines substantially affected the power from the roll motions; however, this was not the case for the pitching. Finally, an inspection of the cross-correlation of the instantaneous power output of the turbines in the first and second rows revealed the strong modulation of the induced motions on the structure of the power fluctuations.