Passive locomotion of a simple articulated fish-like system in the wake of an obstacle

The behaviour of a passive system of two-dimensional linked rigid bodies in the wake of a circular cylinder at Re=100 is studied computationally. The three rigid bodies are connected by two frictionless hinges, and the system (‘fish’) is initially aligned with a streamwise axis three diameters behind the cylinder. Once flow symmetry is broken, the wake rolls up into a Kármán vortex street in which the fish is stably trapped, and the passing large-scale vortices induce an undulatory shape change in the articulated system. It is found that, for certain fish lengths relative to cylinder diameter, the fish is propelled upstream toward the cylinder. Furthermore, the fish is propelled equally effectively when the hinges are locked, confirming that induced body undulation is not necessary for achieving a net thrust. An analysis of the forces on constituent bodies shows that leading-edge suction and negative skin friction on the forward portion of the fish are in competition with positive skin friction on the aft portion; propulsion is achieved when the forebody contributions dominate those on the aftbody. It is shown that the so-called ‘suction zone’ behind the cylinder that enables this passive propulsion is double the length of that without a fish present.