Transition in motility and collective behavior of a simple, self-propelled swimmer at intermediate Reynolds numbers

We propose a simple, self-propelled model swimmer that uses steady streaming flows in a novel way, i.e. for propulsion, and computationally study its motility for a single swimmer and collective behavior for multiple swimmers. Our model swimmer is composed of two unequal spheres that oscillate with respect to each other. For all Re>0, our reciprocal swimmer swims, and interestingly, switches its swimming direction from a small-sphere-leading to a large-sphere-leading regime. Varying a broad range of parameters (viscosity, amplitude, distance between the spheres, sphere radii and sphere-radii ratio), we can collapse the transition point data to a critical value when the appropriate Reynolds number is used. Analyzing the flow fields, we show that propulsion occurs as a result of the interfering steady streaming flows generated by the two spheres forced to oscillate close to one another; the small sphere acts like a flagellum and generates flows that are blocked on one side by the large sphere which causes an asymmetry and net momentum flux. We continue by investigating the interactions between multiple swimmers in both swimming regimes.