On the extraction of path-dependent dynamics from recirculating pulsatile flows using optical and ultrasound imaging
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Recirculation is present in nearly every fluid system, and often jeopardizes the system's primary function. This thesis is motivated by recirculation in the cardiovascular system and how it impacts disease. Developing methods of both measuring and analyzing path-dependent dynamics of recirculating flows is critical to influence diagnostics and therapeutics. The studies in this thesis encompass both state-of-the-art measurement science and recirculating fluid dynamics using both steady and pulsatile flows, and are abstracted to apply generally to recirculation in many systems. A Lagrangian measurement technique, capable of extracting path-dependent quantities, is developed and utilized throughout the four major works to plot the trajectories of hundreds of thousands of unique fluid parcels as they flow through an idealized stenosis geometry. As a step towards in vivo investigations, a novel technique called echo-Lagrangian particle tracking is introduced, which couples the Lagrangian tool with ultrasound imaging. Recirculating-fluid entrainment and depletion mechanics are revealed and suggest that mixing is enhanced by periodic vortex formation in pulsatile flows. Finally, depletion efficiency is compared for flow in a pure-liquid, as well as dilute and dense suspensions. Together, these results begin to expose the complex vortex dynamics that contribute to elevated transit times in pulsatile, two-phase flows.