On the Interplay Between Flow and Pressure Fields Around an Airfoil Undergoing Dynamic Stall

Dynamic stall is an unsteady flow phenomenon that occurs as angle of attack of an airfoil is increased rapidly beyond the static stall angle, resulting in a phase lag between the flow structures and the motion of the foil. Owing to its ubiquitous occurrence around maneuvering aircraft, helicopter blades, wind turbines, insect wings etc., the characterization of flow features and loading due to dynamic stall is an important field of research in unsteady aerodynamics. In this study, time resolved stereo Particle Image Velocimetry experiments are performed on a pitching NACA 0015 airfoil in the refractive index-matched water tunnel, enabling simultaneous measurements of velocity on both sides of the foil. The experiments are performed for a wide range of Reynolds numbers Re and reduced frequencies k. The instantaneous pressure fields are computed from the velocity distribution using Omni-directional integration of material acceleration. To estimate the pressure, the flow is assumed to be 2D and the effects of viscous terms are neglected. The instantaneous aerodynamic loads and moments acting on the foil are calculated by surface integration of pressure fields around the airfoil. The velocity and pressure fields show the existence of a phase lag in the development of suction side flow structures during upstroke and downstroke, as seen from the shift in the peak lift angle. The formation, detachment and migration of the dynamic stall vortex (DSV) varies with k and Re. The delay in stall onset increases with increasing k and Re. Migration of the DSV is determined to be a prominent contributor to the angle- and reduced frequency-dependent location and magnitude of the pressure minimum. Other pressure minima develop near the leading edge either when the flow remains attached or at the onset of separation. The massive DSV is convected downstream and fragmented, creating a diffused pressure region towards the trailing edge. The DSV also entrains the pressure side vorticity creating a low pressure region near the trailing edge, which causes a large increase in the leading edge pitching moment. The pressure gradients are used to estimate the vorticity flux and determine the location of flow reversal. A DSV based time scale is used to collapse the lift curves for different Re and k. Modal analysis using Proper Orthogonal Decomposition (POD) of velocity and pressure fields help in understanding the dominant modes associated with flow. Most of the pressure fields and aerodynamic loading can be reproduced using limited number of modes, with 5 velocity POD modes corresponding to about 85% of the energy. The effect of heave due to structural vibration are observed in a few cases and found to be an caused by added mass effect