The sequence of events constituting the formation of a leading-edge vortex (LEV) has been investigated for a periodically plunging nominally two-dimensional flat plate and a similarly articulated plate of aspect ratio two. Particle image velocimetry applied in multiple parallel planes and unsteady surface pressure measurements were used to quantify the sources and sinks of vorticity governing the growth of circulation in a control region moving with the plate in each case. In the two-dimensional case, the initial accumulation of (negative) vorticity in the nascent LEV produces a strong surface diffusive flux of vorticity that erodes the connection between the LEV and downstream boundary layer through cross-cancelation, initiating the ‘roll up’ of the LEV. Despite the significant diffusive flux earlier in the vortex development, there is no significant accumulation of secondary vorticity until after the severing occurs. The growth of the secondary vortex reduces the suction near the leading edge, such as to result in a self-limiting mechanism on the diffusive flux. In the finite-aspect-ratio case, a similar development is observed, except that the formation process is regulated or reversed by the spanwise convection of vorticity, which opposes the action of the surface diffusive flux. The physical mechanisms of vortex formation or reversal identified here can provide a basis for the design of passive or active flow control strategies to regulate vortex development.
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
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.