Effect of Aspect Ratio on Wake Patterns and Thrust Characteristics of Pitching Wings

Recently, flapping wing has attracted much attention due to its potential application prospect in the design of bionic Micro Aerial Vehicles(MAVs). As one of simplified propulsors developed to understand the thrust generation mechanisms of flapping wing, the pitching wing did not raised enough concern in previous studies because of its relatively poor propulsion performance. In particular, the aspect ratio effect and its physical mechanism in thrust generation need to be further clarified. In this paper, three-dimensional numerical simulations on a rectangular wing operating in a pure pitching motion are carried out to investigate the effect of aspect ratio on the vortex structures and thrust performance. For the governing parameters considered, the results indicate that both thrust and the critical Strouhal number(St) of drag-to-thrust transition are not significantly affected by the aspect ratio until St is beyond about 0.5, after which a greater thrust can be acquired for a higher aspect ratio. It’s believed that there exists a critical aspect ratio corresponding to the transition from the bifurcation wake to the deflection wake, above which the effect of aspect ratio on vortex structures can be neglected. To reveal the underlying mechanism of aspect ratio effect, a force estimation method based on finite control volume is used to establish a relationship between flow field and thrust. Three flow mechanisms are found that are responsible for the aspect ratio effect on thrust generation: for a higher aspect ratio, a more intense momentum surplus field and a more intense vorticity field are the mechanisms that generate greater thrust while a more pressure reduction field is the mechanism that generates greater drag. The aspect ratio effect on thrust generation actually results from the competition of these flow mechanisms. This work is expected to improve awareness for principles of flapping-wing propulsion.