Aspect ratio effects on the aerodynamic performance of flapping wings in tandem configuration

Dragonflies are frequently used as bioinspired models for micro air vehicles due to its great manoeuvrability. However, the complex mechanisms underlying the wing-wing interaction are not properly understood yet. In this work, direct numerical simulations have been used to study the aerodynamic performance of a pair of heaving-pitching wings in horizontal tandem configuration in forward flight. The kinematics of the wings are chosen from a 2D configuration optimised to maximize the propulsive efficiency. The Reynolds number based on the free-stream velocity and chord of the wing is $Re = U c / \nu = 1000$. The influence of the aspect ratio (AR) of the hind-wing on the aerodynamic performance of the system is analysed, considering hind wings with $AR = 4, 3$ and $2$, keeping $AR = 4$ for the fore wing. Results show that the propulsive efficiency is similar for all studied cases as the force coefficients are barely affected when the hind-wing aspect ratio is changed. Decreasing the hind-wing aspect ratio helps to avoid the fore-wing tip vortex but also increases the finite wing effects. Funding: Spanish Ministry of Economy and Competitiveness (DPI2016-76151-C2-2-R) and Red Espa\~nola de Supercomputaci\'on (IM-2019-3-0011).