Flow mechanisms and aerodynamic performance of perforated two-dimensional flapping wings

The presence of gaps between feathers is known to enhance the aerodynamic performance of birds during flapping flight. To investigate the underlying flow mechanisms of this phenomenon, we numerically investigate a two-dimensional zero-thickness flat-plate airfoil with a hinged perforation. The immersed boundary-lattice Boltzmann method is employed to simulate the flow over a zero-thickness flat-plate airfoil, where the hinge-representing a feather gap-is prescribed to open during the upstroke and close during the downstroke. The effects of the gap position, size, and maximum opening angle on aerodynamic performance are systematically analyzed. The results demonstrated that, within the studied parameter ranges, upward perforations increased the pressure differential near the leading edge, while downward perforations reduced it. However, hinge-related vortices generated downstream of the perforations partially offset these pressure differential effects. For downward perforations, optimal positioning, larger sizes, and greater maximum opening angles significantly improved the lift and lift efficiency. Upward perforations enhanced the aerodynamic performance only under restricted perforated conditions: positioning at a distance of 0.25 times the chord length from the leading edge, sizes of less than 0.2 times the chord length, and maximum opening angles of less than 30°. Downward perforations generally outperformed upward configurations in terms of lift and lift efficiency, making them preferable for engineering applications, though self-propulsion effects and three-dimensional flow interactions require further investigation. These findings provide insights for optimizing the perforation designs in micro-flapping-wing vehicles.