Collaborative Research: Swimming and Settling in Stratified Fluids

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1066545/1066566Ardenkani/StockerMany aquatic systems are characterized by regions where water density varies over depth, often due to temperature or salinity gradients. These pycnoclines are associated with intense biological activity and can affect carbon fluxes by slowing the descent of particles. The low to moderate Reynolds number regime is particularly important, because the vast majority of organisms and particles are small (µm-cm) and their motion predominantly viscous. Despite this, the fundamental fluid dynamics of settling and swimming in a stratified fluid have remained largely unexplored. This is partly due to the widespread belief that the relevant length scale of stratification is orders of magnitude larger than organisms. The PIs have recently showed this not to be true, and that typical aquatic stratifications can in fact affect the flow field of particles and organisms as small as O(100 ìm). This opens the door to a broad new set of questions on viscous motion in stratified fluids a novel area of fluid mechanics. The proposed research will take first strides into this new area by determining and rationalizing the effects of stratification on swimming organisms and settling of elongated particles through a combination of experimental, theoretical, and computational research. New tools will be developed to solve for the flow field of swimming organisms in stratified fluids and conduct a broad, in-depth investigation on the effects of buoyancy, viscosity, inertia and diffusion on fundamental hydrodynamic parameters, including swimming speed, velocity decay rates and energy expenditure. The proposed research will address the important component of the geometrical complexity of natural particles and organisms, by focusing on the role of elongation on settling. A novel hypothesis is developed in this proposal and will be tested both theoretically and experimentally: that a buoyancy-induced torque reorients elongated particles and considerably affects their descent.During the last few decades, important correlations have been discovered between regions of fluid stratification and a wide range of environmental processes, including algal blooms, accumulation of marine snow particles, and vertical migration of aquatic organisms. Although this is often the realm of aquatic scientists and oceanographers, what is missing is a fundamental understanding of the fluid mechanics in this new, unexplored regime where both stratification and viscous effects are important. This study will yield the first physical insights on the hydrodynamics of this regime within the broad context of particle settling and organism motility. These new insights, along with the state-of-the-art experimental and numerical techniques to be developed, will (i) provide fertile ground for a broad range of other researchers (mathematicians, engineers, oceanographers, limnologists, ecologists) at the interface between fluid mechanics and the aquatic sciences; and (ii) inform a broad range of processes in aquatic ecosystems, of ecological and societal value, for example by contributing to improved management practices to prevent eutrophication (e.g. algal blooms), providing better estimates of particle fluxes for biogeochemical ocean models and furthering the understanding of the fate of oil droplets dispersed from oil plumes in the marine environment. This grant will provide training for three graduate students. The participation of women and members of underrepresented groups will be strongly encouraged through the Women's Engineering Program at Notre Dame and presentations at an all-women's college (Saint Mary's college). The PIs will ensure the participation of undergraduates, particularly in the experimental aspects of the project, through the Undergraduate Research Opportunities Program at both Notre Dame and MIT.