Numerical Investigation of the Performance of Pitching Airfoils at High Amplitudes

Recently, a renewed interest toward vertical-axis wind turbines has developed, due to the increasing need of a rational utilization of renewable energy resources. In vertical-axis wind turbines, each blade undergoes a wide variation of angle of attack during one rotation, with important dynamic stall phenomena. Neglecting wake-to-wake interactions and curvature effects, a pitching airfoil is a good representation of the flow experienced by the blades of a vertical-axis wind turbine. In the present work, the flow around an Eppler 387 airfoil in pure pitching is investigated, exploring a parametric space tailored to vertical-axis wind-turbine applications. In such a case, the angle of attack a varies in the range-40 deg < alpha < 40 deg, resulting in large-amplitude pitching oscillations. The Reynolds number, based on the freestream velocity and the chord length, is set to Re-c = 3 x 10(4). In this sense, the present work differs from earlier studies, where typically the combined heaving and pitching or pure pitching for propulsion purposes have been considered, which are characterized by higher frequencies and smaller amplitudes and mainly positive angles of attack. In this paper, a detailed analysis of the force coefficients and how their evolution is affected by the dynamics of the flow structures generated during pitch-up and pitch-down of the airfoil is presented. Furthermore, it is shown that a modified Strouhal number, which takes into account both frequency and amplitude, is an appropriate metric for the performance of an airfoil in these conditions.