Energetics in the Wake of Self-Propelling Pitching Airfoil

The present study aims to understand the role of flexibility in self-propelling bodies, mainly how it affects the speed and efficiency of propulsion. We experimentally investigate the energetics of the wake of a rigid airfoil (NACA0015 section) and a composite airfoil (NACA0015 section with a flexible appendage) pitching about 1/3rd chord-length of rigid section. We employ Particle Image Velocimetry (PIV) to obtain the instantaneous velocity field in the wake of the self-propelling foil for different pitching amplitude and frequencies. Complex wake patterns are observed in both cases. Since the net force on a self-propelling body is zero, energetic efficiency is defined by a Power Loss Coefficient (CPL) which is essentially the non-dimensional kinetic energy of the wake. While there are specific interesting comparisons between the rigid and the composite airfoil for the same set of parameters, the rigid airfoil has a wider jet-like wake. In contrast, the wake of the composite airfoil is narrower in comparison making it more efficient. The presence of a flexible appendage results in a pattern of vortex shedding that is nearly aligned to the center-line. With these insights, we try to obtain valuable pointers to design efficient underwater or micro-aerial vehicles.