After billions of years of natural selection, flying animals with flapping wings have superior flight and mobility capabilities. The aerodynamic characteristics and the propulsion mechanism of bionic wings have attracted a large number of researchers because they will be beneficial to novel bio-inspired micro air or underwater vehicle design. Except the single activities, for fish, birds, and insects, there is a very popular and interesting biological clustering phenomenon known as schooling. Considering the real biological movements in schooling under low Reynolds number, the study of the flow mechanisms and thrust performance of bionic multiflapping wings in different schooling configurations could be applied to the design of future bionic flapping wing aircraft formation. The unsteady flow mechanisms and the thrust performance of three-dimensional multiflapping wings in three different schooling configurations are numerically investigated using the immersed boundary-lattice Boltzmann method with the Chinese TianHe-II supercomputer. The influences of different schooling configurations and individual distances on the thrust performance of multiflapping wings are thoroughly investigated. Numerical results indicate that the individual horizontal distance has great effects on the thrust performance of multiflapping wings in schooling, and the average thrust coefficient of each flapping wing in different schooling configurations at a specific individual horizontal distance is larger than that of the single flapping wing. There is an optimum distance for different schooling configurations, where the individual interaction lead to best propulsion performance. Different from the simple tandem schooling, the closer the individual distance, the better the overall thrust performance obtained for triangle and diamond schooling.

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In nature, creatures such as birds, insects, and fish have excellent flight and mobility capabilities. The prominent flight performance of many creatures employing flapping wings has attracted researchers to study the aerodynamics of bionic flapping wings, which has potential application in designing micro air vehicles and autonomous underwater vehicles. Bionic movements usually have to adapt to the low Reynolds number environment. It is noteworthy that the flow field of a flapping wing at low Reynolds numbers flow state is closely related to the complex non-linear shedding and viscous phenomenon, especially in a three-dimensional (3D) flapping wing. In order to observe the influence of the viscous phenomenon on flapping wing propulsive performance at low Reynolds numbers, the flow field characteristics of the 3D flapping wing under different Reynolds numbers are discussed using the immersed boundary-lattice Boltzmann method with the Chinese supercomputer TianHe-II in this paper. The influence of kinematic parameters on the flow characteristics at low Reynolds number is particularly emphasized, considering that the biological movement involves many kinematic parameters, the unsteady flow field and vortex structure around the flapping wing are analyzed in detail. This study reports that the law of the flapping wing propulsive performance strongly depends on kinematic parameters that affect the vortex changes. The underlying flow mechanism behind flapping wing performance at low Reynolds numbers has been explored, which will make it possible to apply superior kinematic parameters to improve the propulsive performance of a flapping-like new airplane.

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The excellent performance of many creatures using flapping wings has attracted a lot of research on the performance of a single flapping wing. However, many species generally choose highly organized movements rather than alone in the animal world; there is a very popular and interesting biological clustering phenomenon known as schooling. Understanding the flow mechanisms and thrust performance of flapping multiwings in a schooling could be applied to novel bionic flapping wing aircraft formation design. We perform numerical simulations employing the immersed boundary-lattice Boltzmann method for flow over a single flapping wing and the flapping multiwings in a diamond schooling at different St numbers. Meanwhile, the effects of the difference in individual flapping frequency on the overall propulsive performance of the schooling were investigated. We present the spectra of aerodynamic forces for a single flapping wing and each wing in a diamond schooling at different individual flapping frequencies. Numerical results indicate that the flapping frequency has great effects on the thrust performance of a single wing and the multiwings in a schooling. The average thrust coefficient of a single flapping wing grows with the increase in the St. However, there is an optimal St number to obtain the maximum propulsive efficiency. For a schooling that maintains the same flapping frequency, the overall schooling or each wing in a schooling shows the same trend as a single wing. For a schooling with different individual flapping frequencies, the aerodynamic characteristics of the last downstream wing are more affected by the frequency difference.