Vortex Shedding Modes of the Self-Propelled Airfoil

In the present research, we study the vortex formation and vortex shedding of an oscillating NACA0012 airfoil that is free to "self-propel" in the streamwise direction. Experiments are conducted by imposing heaving and pitching motions of varying amplitude onto the airfoil, resulting in a "Heave-Pitch Diagram". We consider two cases for the self-propelled airfoil. In the first case, the airfoil's oscillatory motion is symmetric, resulting in zero mean angle of attack and no net lift production over a cycle. In the second case, the airfoil's motion becomes asymmetric by introducing an offset angle into the pitching motion trajectory. Consequently, the airfoil experiences a nonzero mean angle of attack and generates a net lift force over a cycle. Through PIV flow visualization throughout our Heave-Pitch diagram, we characterize the wake structure and vortex dynamics for both the airfoil undergoing symmetric oscillations and the airfoil undergoing asymmetric oscillations. While the symmetric configuration yields the inverse von Kármán street, the vortex dynamics of the asymmetric configuration leads to the formation of a series of vortex pairs in the wake. As a consequence of vortex-vortex interactions, these pairs can deflect at angles relative to the flow's center plane.