Purely hydrodynamic origin for swarming of swimming particles

Three-dimensional simulations with fully resolved hydrodynamics are performed to study the collective motion of model swimmers in confinement. We show that certain swimming mechanisms can lead to traveling wave-like collective motion even without any direct alignment mechanism. It is also shown that by varying the swimming mechanism, this collective motion can be suppressed, contrary to the perception that hydrodynamic effects are completely screened at high volume fraction. From an analysis of bulk systems, it is shown that this traveling wave-like motion, which can be characterized as a pseudo-acoustic mode, is mainly due to the intrinsic swimming property of the particles.