Visualization on vortical structures in the wake of a pair of pitching wings with asymmetrical motion

Excerpt

With the rapid development of micro-air vehicles (MAVs), the aerodynamic characteristics of flapping wings have attracted growing interests in the past two decades (Sun 2014). The unsteady aerodynamic mechanisms of pitching wings undergoing simple harmonic motion, together with their induced flow-field structures, have been well studied (Andersen et al. 2017; Yu et al. 2017). However, for many animals, like insects and birds (Betts and Wootton 1988; Tobalske and Dial 1996) and jellyfish (Ichikawa and Mochizuki 2008), their wing/appendage stroke patterns are asymmetric during up- (or forward) and down- (or backward) stroke phase. Experiments (Rival et al. 2009) and numerical simulations (Lu et al. 2013) have already shown that an asymmetric-flapping airfoil can significantly alter the flow-field structures in the wake. As well as, computational analysis of 2D and 3D jellyfish models shows that change in the force duration (the time required for contraction exercise) will influence the vortex structures and improve the propulsive efficiency (Alben et al. 2013; Park et al. 2014; Goon et al. 2015). Therefore, it is interesting to ask whether such a time asymmetry can affect the aerodynamic characteristics and efficiency of jellyfish-like pitching wings, which have been proved to be an ideal configuration for stable hovering (Ristroph and Childress 2014; Fang et al. 2017; Zhang et al. 2018). …