Wing performance and 3-D vortical structure formation in flapping flight

Numerical simulations of the three-dimensional flow around a modelled insect wing were performed to investigate the performance in flapping flight and to provide insight into the vortex dynamics and associated force generation. Different parameters relevant for three-dimensional flapping wing aerodynamics have been studied, notably the angle of attack in mid-stroke, the Rossby number, the Reynolds number and the stroke kinematic pattern. A parametric study has been made for these parameters, notably for the hovering flight regime. The leading-edge vortex is confirmed to be important for the gain in lift, it being larger and more stable at angles of attack larger than about 30 ° . At smaller angles of attack, the leading-edge vortex development is insufficient to increase the lift, instead the lift decreases. It is observed that the trend of the force development over the cycle and the effect of the angle of attack is similar for revolving and translating wings. However, a flapping wing motion with a revolving character has an important lift-enhancing effect, at a small penalty of drag. Although the variations in lift and drag with Reynolds number are found to be larger at lower Rossby numbers, the lift-enhancing effect of the revolving wing appears not strongly dependent on Reynolds number. Application of a ‘trapezoidal angle of attack’ pattern with increased angular rotation at stroke reversal showed a significant performance increase. It was further shown how the variation in lift and drag can be significantly influenced by introducing deviation in the stroke pattern. A comparison between the three-dimensional simulations and two-dimensional simulations (for forward flight conditions) displayed similar trends with respect to the influence of the angle of attack. However, the latter do not account for finite wing and tip vortex effects which were found to have an important impact on the LEV development.