An experimental loading study of a pitching wind turbine airfoil in near- and post-stall regions

Abstract

A series of experimental tests was conducted to present an insight into lift characteristics of an oscillating wind turbine airfoil at nearand post-stall regions. Due to the unsteady nature of the flow around a wind turbine, the blades are subjected to oscillating motions and consequently, unsteady phenomena occur, dynamic stall in particular. Therefore, the unsteady lift of wind turbine blades is of considerable importance in performance and estimation of a wind turbine lifespan. In the current study, at Re = 420000, a detailed survey of parameters affecting lift characteristics in the hysteresis loops are carried out for the critical section of a 660 kW wind turbine blade. To investigate the effects of reduced frequency, mean angle of attack (AOA) and amplitude of oscillation, the characteristics of lift hysteresis loops including maximum lift, width of the loop, crossover point (if exists) and the normal force defect (NFD) are compared qualitatively and quantitatively. The results indicate that increase of reduced frequency leads to decrease in lift curve slope and delay in maximum lift occurrence. Furthermore, the lift curves are the evidence of strong dependence of lift characteristics on mean AOA and amplitude of pitching motion. Increase of amplitude makes the airfoil enter the post-stall region and hence, it delays the maximum lift occurrence. Entering the post-stall region, the airfoil encounters deeper dynamic stall. Also, approaching to post-stall region makes the clockwise part of the loops more dominant.