The scaling and structure of aquatic animal wakes

Animal generated water movements are visualized and quantified using two-dimensional particle image velocimetry (PIV). The resulting vector flow fields allow for the study of the distribution of velocity, vorticity and vortices. Structural and temporal aspects of animal-induced flows covering a range of Reynolds (Re) numbers between less than 1 to more than 10(4) are presented.

Maps of flow induced by continuous foraging and intermittent escape responses of tethered nauplius and copepodid stages of the marine copepod Temora longicornis offer insight in viscosity-dominated flow regimes. Fast escape responses of the equally sized largest nauplius stage and the smallest copepodid stage are compared. The nauplius moves by generating a viscous flow pattern with high velocities and vorticity; the copepodid moves by using inertial effects to produce a vortex ring with a rearward jet through the center.

Larvae and small adult fish (zebra danio) use a burst-and-coast-swimming mode at Re numbers up to 6,000, shedding a vortex ring with the associated jet at the tail during the burst phase. Flow patterns during the coasting phase differ between the small larvae and larger adults due to the changes in importance of viscosity.

A 12 cm long mullet swimming in a continuous mode generates a chain of vortex rings with a backward undulating jet through the centers of the rings at Re numbers of 4 x 10(4) in inertia-dominated regimes. Our empirical results provide realistic insight in the scale effects determining the morphology of the interactions between animals and water.