Transition of the wake characteristics of a hovering elliptical airfoil from asymmetric to symmetric by modulating the pitching profile

Previous studies have suggested that the lift coefficient (C_L) of an elliptical airfoil undergoing sinusoidal heaving and pitching in a still fluid exhibits asymmetry between the forward and backward strokes, which is further influenced by the pitch or angle of attack profile. This observation prompted us to investigate the impact of various pitching profiles on an elliptical airfoil hovering in a quiescent fluid. Here, both experimental and numerical analyses are conducted to assess the aerodynamic characteristics of a hovering elliptical airfoil with a chord (c) of 0.04 m. Specifically, sinusoidal and trapezoidal pitching profiles are employed while maintaining a sinusoidal heaving kinematics. The heaving and pitching amplitudes are set to 0.75c and 30°, respectively, with a hovering frequency of 0.833 Hz. Our results show that sinusoidal pitching induces asymmetry in C_L due to the formation of a vortex dipole, resulting from uneven vortex shedding during the upstroke and downstroke. In contrast, trapezoidal pitching produces more uniformly spaced vortices during both strokes, eliminating the vortex dipole and yielding a symmetric C_L profile. Further insights into the underlying vortex dynamics and their role in force generation will be discussed in the presentation.