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 chord-based Reynolds number (\(Re\)) on \(\mathcal{O}\)(10). We examine wing-wing interaction of bristled wings in fling at \(Re\)=10, as a function of initial inter-wing spacing (\(\delta\)) 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 \(\theta_\text{r}\); 2) linear translation at a fixed angle (\(\theta_\text{t}\)); and 3) combined rotation and linear translation. The results show that: 1) cycle-averaged drag coefficient decreased with increasing \(\theta_\text{r}\) and \(\theta_\text{t}\); and 2) decreasing \(\delta\) increased the lift coefficient owing to increased asymmetry in 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 fluid through the bristles. Drag coefficients were larger for smaller \(\delta\) and \(\theta_\text{r}\) despite larger RFC, likely due to blockage of inter-bristle flow by shear layers around the bristles. Smaller \(\delta\) during early rotation resulted in formation of strong positive pressure distribution between the wings, resulting in increased drag force. The positive pressure region weakened with increasing \(\theta_\text{r}\), which in turn reduced drag force. Tiny insects have been reported to use large rotational angles in fling, and our findings suggest that a plausible reason is to reduce drag forces.