Aerodynamic interaction of bristled wing pairs in fling

Tiny flying insects of body lengths under 2 mm use the "clap-and-fling" mechanism with bristled wings for lift augmentation and drag reduction at a chord-based Reynolds number (Re) on O ( 10 ) . We examine the wing-wing interaction of bristled wings in fling at Re = 10 as a function of initial inter-wing spacing (δ) and degree of overlap between rotation and linear translation. A dynamically scaled robotic platform was used to drive physical models of bristled wing pairs with the following kinematics (all angles relative to vertical): (1) rotation about the trailing edge to angle θ_r, (2) linear translation at a fixed angle (θ_t), and (3) combined rotation and linear translation. The results show that (1) the cycle-averaged drag coefficient decreased with increasing θ_r and θ_t and (2) decreasing δ increased the lift coefficient owing to increased asymmetry in the circulation of leading and trailing edge vortices. A new dimensionless index, reverse flow capacity (RFC), was used to quantify the maximum possible ability of a bristled wing to leak the fluid through the bristles. The drag coefficients were larger for smaller δ and θ_r despite larger RFC, likely due to the blockage of inter-bristle flow by shear layers around the bristles. Smaller δ during early rotation resulted in the formation of strong positive pressure distribution between the wings, resulting in an increased drag force. The positive pressure region weakened with increasing θ_r, which in turn reduced the drag force. Tiny insects have been previously reported to use large rotational angles in fling, and our findings suggest that a plausible reason is to reduce drag forces.