Effect of stroke plane inclination on the hovering aerodynamic performance of tandem flapping foils

Abstract

The four-winged form of dragonfly and damselfly allows them to fly with great agility and endurance, which is accomplished by independently controlling the kinematics of each wing. In this study, we have performed numerical simulations of two tandem airfoils oscillating along an inclined stroke plane at Re=157. We investigated the effect of stroke plane angle (β) of forefoil and hindfoil on the aerodynamic performance of dragonflies (or damselflies) hovering flight. The simulations are carried out for parallel and non-parallel stroke planes of forefoil and hindfoil oscillating with three phase differences i.e., ϕ=0^o, 90^o and 180^o. For parallel stroke planes, the results show that total lift increases with β whereas total thrust decreases. Also, the total lift as well as thrust reduces with an increase in ϕ. The forefoil performance is affected by the flow induced by hindfoil leading-edge vortices (LEVs), and the hindfoil interaction with forefoil wake vortices significantly affects the hindfoil performance. The results show that non-parallel stroke planes have detrimental effects on the total lift for ϕ=0^o and 90^o. However, for ϕ=180^o, lift augmentation of 46% is obtained in the case where the forefoil stroke plane angle is smaller than the hindfoil. The results obtained during this investigation can help in optimizing the wing kinematics during the Micro-Air-Vehicles (MAVs) development.