Flow structure and performance of a flexible plunging airfoil

Abstract

An investigation was performed with the intent of characterizing the effect of flexibility on the flow structure and aerodynamic performance of a plunging airfoil, over a parameter space applicable to birds and flapping MAVs. Both the material properties of the airfoil and the kinematics of its motion were characterized optically. The vortex dynamics associated with the plunging motion were mapped out using particle image velocimetry (PIV), and categorized according to the behavior of the leading edge vortex (LEV) and its interaction with the trailing edge vortex (TEV). The development and shedding process of the LEVs was also studied, along with their flow trajectories. Results of the flexible airfoils were compared to similar cases performed with a rigid airfoil, so as to determine the effects caused by flexibility. Aerodynamic loads of the airfoils were also measured using a force sensor, and the recorded thrust, lift and power coefficients were analyzed for dependencies, as was the overall propulsive efficiency. Thrust and power coefficients were found to scale with the Strouhal number defined by the trailing edge amplitude, causing the data of the flexible airfoils to collapse onto a single curve. The lift coefficient was likewise found to scale with trailing edge Strouhal number. On the other hand, the wake classification and the propulsive efficiency were more successfully scaled by the reduced frequency of the motion. The circulation of the LEV was found for each case and the resulting data was scaled using a parameter developed for this study, which provided significant collapse of the data throughout the entire parameter space. iii

Flow structures and aerodynamic loads of a rolling wing in a free stream

“…This hypothesis was briefly tested by Akkala et. al [1] for a purely plunging wing however no conclusions were drawn. For future work, perturbing the flow with forced methods can help support or weaken the argument that secondary vorticity is the mechanism controls LEV attachment.…”

Understanding and controlling vorticity transport in unsteady, separated flows

“…(c) Dye visualization of the leading and trailing edge vortex structures formed by a plunging, flexible wing. Unpublished picture from previous work by the author [3]. through cross-cancellation [118,117].…”

“…The small size of the SLV generally causes it to be neglected, yet Akkala [3] showed that its presence within the flow can help to prevent the OSV from circumnavigating the leading edge. The results obtained in this experiment also illustrate this phenomenon.…”