The unsteady flow field in the wake of an NACA 23012 pitching airfoil was investigated by means of triple hot-wire probe measurements. Wind tunnel tests were carried out both in the light and deep dynamic stall regimes. The analysis of the wake velocity fields was supported by the measurements of unsteady flow fields and airloads. In particular, particle image velocimetry surveys were carried out on the airfoil upper surface, while the lift and pitching moments were evaluated integrating surface pressure measurements. In the light dynamic stall condition, the wake velocity profiles show a similar behaviour in upstroke and in downstroke motions as, in this condition, the flow on the airfoil upper surface is attached for almost the whole pitching cycle and the airloads show a small amount of hysteresis. The deep dynamic stall measurements in downstroke show a large extent of the wake and a high value of the turbulent kinetic energy due to the passage of strong vortical structures, typical of this dynamic stall regime. The comprehensive experimental database can be considered a reference for the development and validation of numerical tools for such peculiar flow conditions. 1. Introduction The aerodynamics of oscillating airfoils is widely investigated as it represents a good model for the study of the dynamic stall of the retreating blade sections [1, 2]. The dynamic stall phenomenon occurs on the retreating side of the helicopter rotor at a high forward flight speed or during maneuvers at high load factors, and it produces several adverse effects on the helicopter performance. The main detrimental effects due to dynamic stall are the limitation of the forward speed and thrust, high control system loads, the introduction of a high level of vibrations affecting the helicopter dynamic performance in terms of maneuver capability and handling qualities, and the occurrence of the aeroelastic problem known as stall flutter [3] causing blade structural damage and excessive cabin vibration. Therefore, in order to overcome the limitations on helicopter performance introduced by this phenomenon and to expand the helicopter flight envelope and vehicle utility, dynamic stall has become in the recent years one of the most challenging research topics in rotorcraft aerodynamics field. In fact, several research activities both in experimental and numerical fields investigated the effectiveness of different dynamic stall control systems integrated into the blade section (e.g., [4–6]). Moreover, the study of the fine details involved in the physics of this phenomenon was
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