Period-Doubling Phenomenon Observed in the Dynamic Stall Vortex Patterns

Dynamic stall is a nonlinear unsteady aerodynamic phenomenon resulting in stall delay during the time dependent movement of an airfoil at angles of attack higher than its static stall angle. This is observed during helicopter forward flights, rapid pitching of fighter aircraft, or in turbine rotor blades, and so on. The most important features of the flowfield are the leading and trailing edge vortex structures. Their growth, evolution, subsequent shedding into the outer field, and their mutual interactions influence aerodynamic loads significantly during the dynamic stall process. Various past and recent studies have been dedicated to visualizing the vorticity flowfield as system parameters are varied, experimentally and numerically. One of the earlier studies on dynamic stall was reported by McCroskey et al. [1]. The influence of various airfoil profiles and leading edge geometries was investigated. Experimental investigations on rapidly pitching airfoils was reported by Walker et al. [2]. Numerical simulations for a rapidly pitching airfoil with a compressible flow model were presented by Visbal and Shang [3] and Visbal [4]. Numerical simulations for a sinusoidally pitching airfoil were taken up by Tuncer et al. [5]. The effect of reduced frequency, albeit in a low range of variation, was highlighted. Ohmi et al. [6] presented experimental as well as numerical simulation results for a sinusoidally oscillating airfoil. Among other parameters like the Reynolds number, mean angle of attack, and elastic axis, reduced frequency was found to be the most significant one. Akbari and Price [7] have also investigated a similar parametric variation in a sinusoidally oscillating airfoil, though at lower range of reduced frequencies