Effects of the wing spacing, phase difference, and downstroke ratio on flapping tandem wings

In this paper, a numerical study on the effects of the asymmetry in flapping duration, wing spacing, and phase difference on the aerodynamic performance of flapping tandem wings is carried out. Three downstroke ratios to flapping period (ξ) are chosen, ξ = 0.35, ξ = 0.5, and ξ = 0.65, where ξ = 0.5 represents a symmetrical stroke. Four values of wing spacing L are selected: 1.25c, 1.5c, 1.75c, and 2c (c is the wing chord length), while the phase difference between the fore and hind wings is ranged from 0° to 270° with an interval of 90°. The effects of these parameters on the lift and thrust generation are investigated. A study of the energy consumption and the flight efficiency has also been taken into consideration. The flow variables in the computational domain around the flapping tandem wings are solved using the commercial software Fluent. The results show that the effects of the downstroke ratio on the aerodynamic performance are significantly higher than the effects of the wing spacing and the phase difference. The thrust generation and efficiency are maximized in the case with faster upstroke (ξ =0.65). However, the maximum lift generation and efficiency are located in the case with faster downstroke (ξ =0.35) instead. The optimal configuration is obtained using a minimum of wing spacing (1.25c) and phase angle (0°) with a downstroke ratio of 0.436.