The Effect of Hindwing Trajectories on Wake-Wing Interactions in the Configuration of Two Flapping Wings in Tandem

The present investigations on tandem wing configurations primarily revolve around the effects of the spacing L and the phase difference phi between the forewing and the hindwing on aerodynamic performance. However, in nature, organisms employing biplane flight, such as dragonflies, demonstrate the ability to achieve superior aerodynamic performance by flexibly adjusting their flapping trajectories. Therefore, this study focuses on the effects of phi, as well as the trajectory of the hindwing, on aerodynamic performance. By summarizing four patterns of wake-wing interaction processes, it is indicated that phi=-90 degrees and 0 degrees enhances the thrust of the hindwing, while phi=90 degrees and 180 degrees result in reductions. Furthermore, the wake-wing interactions and shedding modes are summarized corresponding to three kinds of trajectories, including elliptical trajectories, figure-eight trajectories, and double figure-eight trajectories. The results show that the aerodynamic performance of the elliptical trajectory is similar to that of the straight trajectory, while the figure-eight trajectory with positive surging motion significantly enhances the aerodynamic performance of the hindwing. Conversely, the double-figure-eight trajectory degrades the aerodynamic performance of the hindwing.