Extensible collective behavior of three self-propelled beams in tandem configuration

Although the spontaneous collective behavior of swimmers has attracted considerable attention, there has been little physical discussion of the extensibility of the collective system. This paper numerically studies the extensible collective behavior of a self-propelled beam integrated in a stable tandem two-beam system. Like the beams in the two-beam system, the inserting beam is driven by the heaving motions of its leading edge. Different cases are simulated according to different inserting positions and self-propelled motion with different phases. Two types of extensible collective configurations, the regular configuration and the compact configuration, are observed. The swimming speed, input power and propulsion efficiency of the beams are compared. It is found that the extensible collective behavior is conducive to the improvement of the propulsion performance of the whole system. Analyses of the flow-mediated interaction between the beams indicate that the vortex-locking mechanism can be used to predict the formation process and the extensible collective structure of the three-beam system. The vortex-induced hydrodynamic forces experienced by the inserting beam at different inserting positions determine the hydrodynamic behavior of the extensible collective system. It is recommended that the additional individual is inserted from the near front of the two-beam system. •The extensible collective behavior of three self-propelled beams is physically studied.•Both the regular and compact configurations of three-beam system are observed.•Performances of three tandem beams are promoted in the compact configuration.•Formation mechanism of the extensible collective behavior is revealed.