Experimental Investigation of a Passively Deforming Airfoil Under Dynamic Flow Conditions

A rigid and a passively deforming airfoil, designed to alleviate fatigue causing load fluctuations that appear during normal operation of wind turbines, is investigated under unsteady conditions in two dimensional wind tunnel experiments. In a first series of experiments, a vertical gust encounter is generated by means of an active grid. This approximates the wind turbine blade’s passage through the atmospheric boundary layer and corresponds to the theoretical formulation of the Sears problem. In a second experiment the airfoil is oscillated in a steady free stream, which approximates the bending and twisting motion of a wind turbine blade and corresponds to the theoretical formulation of the Theodorsen problem. The frequency dependent dynamic lift response of the rigid airfoil under attached flow conditions is compared to the Theodorsen and Sears function. If the airfoil is oscillated around its zero lift angle, experimental results and theoretical prediction agree. Contrary to theoretic assumptions, a substantial dependence on the mean angle of attack is observed: If the airfoil is oscillated around higher mean angles of attack, the dependence on the reduced frequency is is inverted. The deforming airfoil shows good performance in terms of gust load alleviation over a wide range of operating conditions: At small mean angles of attack, up to 60% of the fluctuating loads are alleviated. Under high mean angles of attack, leading edge vortices are efficiently suppressed, yielding up to 30% less load fluctuations