A sudden alteration in the self-propulsion of tandem flapping foils

The self-propulsion of tandem flapping foils, which can self-propel identically and maintain a fixed gap distance, is numerically studied in this paper. It is found that there is a sudden alteration in the self-propulsion of tandem flapping foils, in which the speed of the tandem-foil system dramatically changes from the highest to the lowest. Moreover, the emergence of the sudden alteration is determined by the gap distance and phase difference between tandem foils. This can be described as the equivalent distance G. The critical points at which the sudden alteration occurs are located at G = (2N − 1)/2, where N = 1, 2, 3…. In the interval between two adjacent critical points, the propulsive speed, power consumption, and propulsive efficiency of the tandem-foil system increase monotonically with the rise of G. Additionally, compared with the isolated flapping foil, the tandem-foil system shows performance enhancement at every interval where G = 0.5. Furthermore, the mechanism behind the sudden alteration is analyzed, and it is found that this sudden alteration is associated with the potential landscape. Finally, the hydrodynamics of each foil are discussed in detail. The results obtained here may shed some light on understanding the performance of dual-flipper/wings in nature.