High-lift devices are widely used on multi-element wings of large civil aircrafts. At large angles of attack, flow separation may occur over upper surfaces of leading-edge slats of multi-element airfoils. The flow separation would increase the thickness of the induced wake flow rapidly and enhance the interaction between the wake and the downstream boundary. This will reduce the circulation and lift and finally result in the stall. This paper puts forward a method to improve the aerodynamic performance of high-lift devices by slotting on leading-edge slats. Flow fields and parametric effect of the slotting are investigated based on numerical simulations by solving Reynolds-Averaged Navier-Stokes equations. Results show that the slots on leading-edge slats can effectively delay the flow separation by inhibiting the development of the wake flow and its interaction with downstream boundary layers. The adverse impact induced by the stall can be alleviated significantly as a result. However, the control effect is closely related to the slot position and the direction of the exiting jet. Therefore, to obtain a favorable improvement of aerodynamic performance, the slot should be reasonably designed according to the shape of the leading-edge slat and also to the surrounding flow field.

In this paper, trailing vortices generated by three wingtip configurations, namely the M6 wing and the M6 wing with a blended or split winglet, are experimentally investigated using the Stereo Particle Image Velocimetry (SPIV) technology. Then, linear stability analysis is performed to investigate instability characteristics. Three corresponding trailing vortex patterns, including the isolated trailing vortex without wake (pattern v) and with wake (pattern v-w), co-rotating vortex pair (pattern v-v), are observed in experiments. The strength of trailing vortices, characterized by circulation, is reduced after installing winglets as expected, and the strength of pattern v-v can be further suppressed compared with pattern v-w. Moreover, instability characteristics, such as the eigenvalue spectrum and perturbation mode, are distinctive among these three vortex patterns. The distribution of eigenvalue spectrums indicates that pattern v and pattern v-w are temporally ‘‘marginally stable”, while pattern v-v is temporally ‘‘unstable.” The primary perturbation mode of pattern v and pattern v-w is the m = –1 helical mode, while |m|>1 for the case of pattern v-v. The effect of vortex dynamics and instability characteristics can be concluded in two aspects. Firstly, the value of induced drag is polluted by about 3% from vortex wandering since vortex wandering affects the tangential velocity and streamwise vorticity of trailing vortices. Secondly, the growth rate and penetration depth perturbation mode affect trailing vortex evolution and further affect induced drag. Specifically, the larger the growth rate and penetration depth are, the more turbulence injects inside the vortex core, thus leading to a quicker and more intense attenuation of trailing vortex, as well as a smaller induced drag. This finding can guide us to manipulate the induced drag in flow control.