Experimental study of non-stationary aerodynamic effects on the wind turbine nacelles under extreme wind events

The wind pressure coefficients on the nacelle model of a typical wind turbine are experimentally examined in nonstationary wind flows adopting an active-controlled multi-blade system installed in an atmospheric boundary layer wind tunnel. Various nonstationary winds, characterized by flow parameters of acceleration and initial velocity, are grouped as ramp-up and ramp-down flows. Particular attention is devoted to the influence of nonstationary attributes of incoming flow on the wind pressures acting on the nacelle envelope. The experimental results indicate that the wind pressures on the nacelle surfaces are significantly modified by the incoming nonstationary wind conditions. The overshoot ratios of wind pressure on the nacelle's top surface is increased with the increasing acceleration in the ramp-up flows, while the maximum overshoot ratio occurs at a middle-level acceleration magnitude. Moreover, the overshoot ratios increase with the reduction in the initial velocity of nonstationary winds. Additionally, the extreme pressure coefficients of the nacelle in the ramp-down flow are much increased compared with those in ramp-up winds with equivalent acceleration magnitude. The findings from this study are expected to be instrumental in improving the understanding of the unique characteristics of nonstationary wind pressures and their effect on the nacelle envelope.