Flow Induced by Collective Vertical Migration: Impact of Swimmer Distribution, Buoyancy, and Wake Interactions

Citation

Mohebbi, Nina (2025) Flow Induced by Collective Vertical Migration: Impact of Swimmer Distribution, Buoyancy, and Wake Interactions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hz5j-g795. https://resolver.caltech.edu/CaltechTHESIS:05082025-223940273

Abstract

Various animal species exhibit collective motion, characterized by coordinated movement within groups of organisms. A prominent oceanic example is diel vertical migration (DVM), wherein zooplankton migrate vertically from deeper waters during the day to shallower regions at night, often covering distances of approximately 1 kilometer. Despite numerous field measurements, laboratory observations, and theoretical studies of biogenic mixing resulting from collective swimming, the scale of fluid mixing induced by DVM remains unresolved. A key challenge is linking the behavior and flows created by large numbers of individual organisms to collective-scale fluid dynamics. Since most swimmers involved in DVM operate at intermediate Reynolds numbers, the dynamics of these systems are nonlinear and span a wide range of spatial and temporal scales.

This thesis investigates flow scaling generated by vertical migration of brine shrimp (Artemia salina) aggregates, using laboratory measurements complemented by semi-analytical modeling. A volumetric laser scanning system first measured swimmer behaviors and flow interactions during laboratory-induced vertical migrations. Swimmers consistently maintained vertical swimming velocities under varying environmental conditions, showed a Gaussian horizontal distribution within the tank cross-section, and exhibited a pronounced tendency toward the tank center, where illumination was brightest. A scaling relationship between swimmer buoyancy, ascent speeds, and resulting flow velocities was developed to contextualize these results.

A semi-analytical model was then developed to estimate the flow generated by wakes of multiple swimmers in proximity. Individual swimmer behaviors were informed by empirical observations and combined through an iterative approach that conserves mass and momentum, providing an aggregation-scale flow solution. Numerical results indicated that induced upstream flows within the aggregation were relatively insensitive to downstream swimmer presence, that average flow speeds approached a plateau beyond a critical aggregation length, and that closer swimmer spacing significantly enhanced induced flow velocities.

Thesis Files