Induced drift by a self-propelled swimmer at intermediate Reynolds numbers

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Swimming organisms have been proposed to contribute to the mixing of stratified water in the ocean, thereby facilitating the vertical transport of nutrients and dissolved gases. In general, mixing results from increasing the interface available for molecular diffusion between neighboring fluid volumes. At high Reynolds numbers (Re), swimmers generate such interfaces through their turbulent wake structures. At lower Re, however, turbulent mixing becomes ineffective as viscous effects dissipate small-scale fluid motions as heat, and diffusion is not significantly enhanced. In this regime, it appears that the dominant mechanism for mixing by a swimmer is induced drift, i.e., the propagation and stretching of a fluid volume by a moving body's pressure field, which increases the diffusion-enabling interface between the drift volume and surrounding fluid. The ratio of drift volume to body volume is called the “added-mass” coefficient and depends on the shape of the body. Importantly, previous computational analysis suggested that the total drift volume increases at low and intermediate Re, 3 implying that in contrast to turbulent mixing, mixing through induced drift becomes more efficient in viscous conditions. As pointed out by others, the limitation of previous numerical simulations, however, is that the simulated objects were towed through viscous fluid, which is dynamically distinct from a self-propelled swimmer. Using qualitative flow visualization, we here demonstrate the presence of induced drift in self-propelled swimmers operating at intermediate Re (1–100). In these experiments, the spatiotemporal pattern of a fluid volume initially surrounding a juvenile Moon jellyfish ( Aurelia aurita) is visualized using Fluorescein dye (see Fig. 1 ). For details on the experimental methods see supplemental material in Ref. 13 .