Flow-mediated formations of a robotic school

  Motivated by schools of fish and flocks of birds, we perform experiments on a “robotic school” consisting of many flapping hydrofoils swimming in tandem.  These foils have a prescribed heaving motion but are free in the swimming direction; the fluid forces on each foil determine its swimming speed and spacing relative to its neighbors.  Previous results have shown that the reverse von Kármán wake generated by a heaving hydrofoil can provide stable positions to a second hydrofoil heaving in the wake (Ramananarivo et al. at NYU).  New results for groups of more than two hydrofoils show that all trailing foils experience stable positions in the wake of their upstream neighbors.  Force measurements confirm that the strength of these interactions weakens only slightly for each successive foil, thus the dominant interaction is that between each foil and the wake generated by its nearest upstream neighbor.  We also find that allowing periodic interactions between the wake of the last foil and the body of the first foil can cause boosts in speed that are passed between successive foils.  We reproduce these results using a simple wake-follower interaction model that can be extrapolated to predict the dynamics of larger groups.