Effect of Reduced Frequency and Reynolds Number on Hysteresis Behavior of Flow Past an Oscillating Airfoil

Abstract

Wind tunnel experiments were conducted on NACA-0015 airfoil model to investigate the effect of the varying reduced frequency from 0.001 to 0.5 at constant Reynolds Number varying from 0.2 to 0.7 millions on the aerodynamic characteristics and hysteresis behavior associated with the oscillating motion of the airfoil. Surface pressure measurements were conducted on the mid span of the airfoil for quantitative results. Other oscillating parameters which may affect the hysteresis behavior such as airfoil geometry, amplitude of oscillation, mean angle of incidence are kept unchanged. The initiation of the stall process from the trailing edge with laminar separation bubble is believed to be characteristic of the NACA-0015 airfoil section tested. Conditions for fully developed dynamic stall characteristics are determined. Lower reduced frequencies for which no or weaker primary and secondary LEVs are traced, Re effects are significant, which provides substantial rise in the hysteresis behavior and shift in the lift stall angle. Beyond the quasi-steady conditions, these incremental rates of hysteresis rise and stall angle shift gets lowered with the increase in Re. Under such conditions where no significant change in hysteresis behavior is observed, the Re effect are limited to in weakening the strength of laminar separation bubble, inception of moment stall, secondary LEV (if any). The point where the fully developed separated flow reattaches during the pitch down motion is independent of Re. However, the effect of reduced frequency dominates when compared with the Re effect. As the reduced frequency increases, hysteresis loop gets enlarged, inception of moment stall is strengthened and lift stall angle also increases due to increment in lag associated with the movement of separation point towards leading edge during pitching motion. This time lag also causes delay in the flow reattachment. Significance of normal force defect, pitch damping, and cross correlations of coefficients of normal force and pitching moment are obtained to understand the chronology of dynamic stall events. Effect of Leading edge contamination, independent of reduced frequency and Re, is also depicted to cause incremental shift in the stall angle.