Numerical Study of the Aerodynamic Performance of Two Coaxial Flapping Rotary Wings Under Wake Interaction

Flapping rotary wing (FRW) is a composite flapping wing layout proposed in the last decade for micro air vehicle (MAV) design. FRW flaps actively in the vertical direction coupled with passive horizontal rotation, ensuring that the high-lift mechanisms from wing flapping at low Reynolds numbers and the high aerodynamic efficiency of the rotary wing are applied in this wing layout. In actual MAV designs, multiple FRWs have various arrangements, one of which is the arrangement where FRWs locate coaxially and rotate in the same direction. In this type of arrangement, a complex flow interaction exists between wakes from the upper and lower wings, but it has never been focused on in previous research. Thus, this study investigates the effect of wake interaction on the aerodynamics of two coaxial FRWs by using the lattice Boltzmann method and explores the influence of rotation speed, flapping phase difference (Delta theta), and wing vertical distance between two wings on the lift and rotating moment, which determine the rotation speed of actual FRW. With inverse flapping (i.e., Delta theta = pi), the wings interact strongly, and the MAV can reach the maximum rotating moment. An increase in vertical distance between the two wings and rotation speed can weaken the interaction. Our design allows for high rotating speed and low wing loads, thereby reducing the torque requirements on the motor. This study can enhance our understanding of the complex wake interaction produced by multiple FRWs at low Reynolds numbers and provide theoretical guidance for the design of MAVs, especially FRW MAVs.