Nature has always inspired us and bio-mimetic Micro Air Vehicles (MAVs) are made by mimicking insect flight. Tiny insects such as dragonflies with their two pair of wings survived millions of years without changing their aerodynamic design. They have a variety of capabilities such as hovering, forward and backward flight, take-off, perch and catching the prey in flight, making them the suitable choice for study. Insect wings flap at high frequency with little inflow during hover. The complex vortex shedding and interaction of the leading and trailing edge vortex (LEV/TEV) give key insight into the lift production during hover. This calls for numerical modelling of the unsteady flow to understand the vortex shedding and lift enhancement mechanisms, contributing to the vertical force production.
The inclined stroke plane flapping used by dragonflies produces higher vertical force compared to the horizontal stroke plane hovering used by several other insects, as the drag contributes to the vertical force production along with the lift. In the current study, the class of flows representing the inclined stroke plane flapping wing is investigated for its unique feature of the vortex dipole and its induced jet. The numerical modelling of the flapping wing during hover reveals that the characteristics of the dipole jet are useful in understanding the vertical force trends and explaining the stroke plane angle used by the dragonflies. Further, the magnitude and region of influence of the jet near the wing, call for the study of ground effect and tandem wing configurations.
The coupled effect of the stroke plane and ground effect not only sheds light on the vortex interactions with the ground but also on possible vertical force enhancements due to the dipole jet. Similarly, the inter-plane distances between the tandem flapping wings for different stroke plane angles are studied to reveal the high vertical force-producing configurations with the help of the dipole jet pattern. The vortex visualisations and vortex identification along with POD become the heart of the studies as the vortex dynamics is being explored for the class of flows. Through the studies, important physical insights are obtained in the dominant lift enhancement mechanisms, vortex shedding classification, force regime classification and shear layer interaction. Last but not the least, the average dipole jet patterns are used to understand the vertical force enhancements.