Unsteady aerodynamic characteristics of a bionic flapping wing in three-dimensional composite motion

Abstract

Owing to continual advances in computational fluid dynamics, simulations of fluids have emerged as an important means of analyzing the aerodynamic characteristics of flapping-wing flight. This study establishes an aerodynamic model of bionic flapping-wing flight and uses dynamic hybrid grid technology to divide its flow field and other parts. We formulate equations to control the discretized flow field and combine the Navier–Stokes equation (N–S equation) with the theory of dynamic vortices to solve the flow field of the motion of flapping wings. We analyzed the lift and drag generated by the flapping wing at different wind speeds, amplitudes, frequencies, and chordal torsion angles. The results show the following: (1) wind speed had a significant influence on the lift resistance of the flapping wing. When the wind speed was 1 m/s, the lift force was 1.544 N and increased 4.3 times to 6.636 N when the wind speed was 5 m/s. The resistance increased from 0.39 to 0.88 N. (2) Changes in the amplitude of flutter had little effect on the average lift resistance. When the amplitude was increased from 15° to 45°, the lift force increased to only 0.757 N and drag changed by little. (3) The increase in the flapping frequency improved flight lift. When the frequency was increased from 1 to 5 Hz, the lift increased by 2.9 N (1.78 times) and resistance increased by only 0.08 N. (4) Increasing the chord torsion angle increased flight lift. The lifts at β = 5° and 15° were 6.636 and 6.654 N, respectively, 0.85 and 0.87 N greater than those at β = 0°. As the torsion angle continued to increase, the lift decreased, while the resistance increased more quickly. When β = 15°, the resistance increased by 1.53 N, 9.6 times larger than that at 0°. Increasing the flight speed and flapping frequency can increase flight lift. A small increase in the chord torsion angle increased flight lift, but an excessively large angle led to a substantial increase in drag. Increasing the amplitude of flapping can increase the instantaneous lift generated but has a smaller effect on the average lift. By revealing the influence of different parameters on the lifting resistance of a flapping wing during flight, this paper provides a theoretical foundation for the design and control of bionic flapping-wing aircraft.