Aerodynamic characteristics of unsteady gap flow in a bristled wing

Abstract

A micro-scale flying insect has a unique wing configuration consisting of a central frame and several bristles. For the low-Reynolds-number regime in which the insect lives, the bristled wing utilizes a virtual fluid barrier inside gaps produced by strong viscous diffusion of shear layers to overcome its morphological limitations. Considering the unsteady flapping motion of such a wing, the aerodynamic characteristics of gap flow formation are investigated numerically using a two-dimensional bristled wing model for a wide range of Reynolds numbers. Inside a gap between bristles, the development of a stopping vortex during the deceleration phase and its effect on the extinction of an existing vortex generated at the same edge are dependent on the Reynolds number, which leads to a significant change in vorticity distribution at stroke reversal even with just a small change in the Reynolds number. As the Reynolds number decreases, the gap flow responds more rapidly to wing motion, and its pattern does not deviate significantly from the kinematics of the wing. A noticeable difference is also observed in the behavior of the aerodynamic force acting on each bristle at low and high Reynolds numbers. With regard to aerodynamic force generation by the bristles, each bristle behaves independently and produces similar force because of strong gap flows relative to the wing at high Reynolds numbers. Meanwhile, at low Reynolds numbers, each bristle experiences a different force depending on its relative position, which indicates the existence of collective interaction of bristles through a virtual fluid barrier.