On the dynamics of a model wind turbine under passive tower oscillations

Laboratory experiments were carried out to quantify the effect of small-amplitude, passive oscillations of a model wind turbine in the structure of the unsteady motions, wake statistics, and mean power output and associated fluctuations at various yawing β∈[0o,30o] every Δβ=5o. Planar particle image velocimetry was used to characterize the flow statistics, and power output was measured at high temporal resolution. Scenarios with a fixed turbine were included to aid insight into the coupled and separated effects of passive oscillations and yawing. Unsteady pitch motions, dominant of the system dynamics, were more predominant at small but non-zero yaw of β≈5o. The spectral structure of the oscillations showed that the unsteady roll motions contributed substantially under high yawing. Using basic concepts, we derived a formulation for the turbine oscillation spectrum that shows good agreement with measurements; it serves as a base to include other input modulation types. The power output decreased monotonically with β in the fixed and oscillating turbines, which was more distinct for β>15o. Passive oscillations produced higher power for given yaw. A simple yaw-correction of the structure of the power output also showed reasonable agreement with measurements. Finally, mean velocity in the wake of the fixed and oscillating turbines exhibited minor differences for a given yawing. However, the turbulence levels showed distinct changes of turbine motions in the degree of symmetry and magnitude for given yaw.