Stable schooling arrangements of two and three swimmers*

We present data supporting the existence of two-swimmer schooling arrangements that are stable and arise spontaneously from hydrodynamic forces alone. Water channel experiments were performed with two NACA 0012, AR=3 hydrofoils where both swimmers are free to move in the horizontal plane. Stable arrangements deviate only slightly from a canonical, side-by-side formation for phase synchronies ranging from 160 to 200 degrees. The collective swimming speed is 15% higher than that of an isolated swimmer. These findings are also supported by 2D free-swimming simulations, PIV data and performance measurements. In a second experiment we then take these stable arrangements of two swimmers as a starting point for studies of larger schools. The two swimmers are positioned in a stable arrangement and fixed (translation not allowed in the plane). A third, fully unconstrained swimmer is then added downstream of the two fixed foils. The third swimmer is used to investigate larger schooling formations of fish-like swimmers.