Propulsion performance of bio-inspired tandem flapping airfoils with unequal heaving amplitudes

In this study, the effect of heaving amplitude (h₀) on the propulsion performance and flow structure of two-dimensional tandem flapping airfoils was investigated numerically. Systematic consideration was given to situations in which the amplitudes of the front airfoil (FA) and rear airfoil (RA) were the same or different. In addition, the effect of airfoil spacing was examined when the FA and RA had different h₀ values. The results show that the force generated by the airfoil transitions from drag to thrust as h₀ increases. When the FA and RA have the same h₀, the mean thrust of the tandem system increases compared to that of a single airfoil. Furthermore, it is also found that when the h₀ of the FA is larger than that of the RA, the thrust increment of the tandem system relative to the two isolated airfoils can be further improved. A detailed analysis of typical flow characteristics highlights that the interaction between the wake vortices of the FA and the leading-edge vortex of the RA is a key factor influencing aerodynamic force generation. Our results also show that significant thrust enhancement of the tandem system occurs within the airfoil spacing range of 1.5c−2.5c. These findings offer valuable insights for the design of new bionic flapping wing vehicles.