A discussion on vortex growth and saturation in dragonfly-inspired flight

Abstract

This report is devoted to examining vortex formation and interactions as they occur with isolated and tandem pitching and plunging airfoils. Motivation for understanding such flow phenomena is inspired from dragonflies, which exhibit a wide range of acrobatic flight maneuvers, possibly interesting for the realization of flapping drones known as Micro Air Vehicles (MAVs). Experimental investigations are described in which the kinematics of the forefoil and hindfoil could be independently varied, attaining any combination of pitch and plunge movements up to reduced frequencies of approximately k=0.3 in forward flight. Furthermore, the Reynolds number in these studies was varied between Re=3000 (hover) and Re=30000 (forward flight). Particle Image Velocimetry (PIV) was used to extract information on leading-edge vortex (LEV) and trailing-edge vortex (TEV) circulation, which in turn was useful in examining the growth rate and saturation often referred to as formation number. The following results will be presented: the effect of airfoil kinematics on LEV and TEV development; the influence of hindfoil interaction on the vortex formation and lift generated by the forefoil; and a quantification of the vortex interaction found in the hover condition. Finally, some thoughts regarding a new model of vortex growth will be discussed. © 2012 Springer Berlin Heidelberg.