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Using a simple model, we study the transport dynamics of active, swimming particles advected in a two-dimensional chaotic flow field. We work with self-propelled, point-like particles that are either spherical or ellipsoidal. Swimming is modeled as a combination of a fixed intrinsic speed and stochastic terms in both the translational and rotational equations of motion. We show that the addition of motility to the particles causes them to feel the dynamical structure of the flow field in a different way from fluid particles, with macroscopic effects on the particle transport. At low swimming speeds, transport is suppressed due to trapping on transport barriers in the flow; we show that this effect is enhanced when stochastic terms are added to the swimming model or when the particles are elongated. At higher speeds, we find that elongated swimmers tend be attracted to the stable manifolds of hyperbolic fixed points, leading to increased transport relative to swimming spheres. Our results may have significant implications for models of real swimming organisms in finite-Reynolds-number flows.

We report numerical simulations of a simple model of flocking particles in the presence of an uncertain background environment. We consider two types of environmental perturbations: random noise applied separately to each particle, and spatiotemporally correlated 'noise' provided by a turbulentlike flow field. The effects of these two types of noise are very different; surprisingly, the applied flow field tends to destroy the global order of the flocking model even for vanishingly small flow amplitudes. Local order, however, is preserved in smaller sub-flocks, although their composition changes dynamically. Our results suggest that realistic perturbations must be considered in assessing the stability of models of collective animal behavior, and that random noise is not a sufficient proxy.

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