Wake and power fluctuations of a model wind turbine subjected to pitch and roll oscillations. (1st November 2019) - Fu, Shifeng
Highlights: We isolated factors affecting turbine power and wake dynamics due to turbine motions. Pitch and roll induced distinct effects on the structure of the wake and power output. Mean power increased with moderate tower oscillation regardless of the type of motion. Oscillation-driven power fluctuations may be due to changes in the blade aerodynamics. Abstract: Wind-tunnel experiments were performed to inspect the impact of a variety of pitch and roll oscillations of a model wind turbine on the instantaneous power output and wake. Particle image velocimetry and hotwire anemometry were used to characterize the flow in the wake; instantaneous power output was also obtained in each of the configurations. For comparison, measurements were also performed in a fixed wind turbine. Results show that the wake at the turbine symmetry plane is significantly altered by the imposed motions, where rolling induced the lowest momentum deficit. The mean power output of the turbine increased with moderate tower oscillations, namely ≲ 10 °, independent of the type of motion. We argue that this is due to, at least, two distinctive processes. Namely, a relative gain due to the cube of the relative incoming velocity impinging the rotor in the pitching, and a momentum replenish in the rolling motion The power fluctuations exhibited a peak on the spectral content of the spectrum Φ P coincident with the frequencies of the pitching and rolling. They also revealed the effects of the oscillation within the low-frequency content of Φ P, which was likely due to the oscillation-driven changes in the aerodynamics of the blades. In particular, the pitch reduced the energy of the power fluctuations within frequencies below that of the pitching frequency, with stronger effect at larger amplitude of oscillations, θ . However, the roll motions reduced the energy of the power fluctuations in a relatively narrow band, and notorious only with θ ≳ 10 ° .