Transport and Mixing Induced by Swimming Organisms and Settling/Rising Particles

Ocean contains dissolved inorganic and organic materials, particulate matters, organisms and dissolved atmospheric gases. They are carbon carriers. Their mixing and transport processes significantly affect the oceanic carbon cycle. In order to understand corresponding mixing and transport phenomena, this study employs both analytical and numerical approaches, aided by experiments, to perceive the fundamental physics of transport of marine organisms, marine snow particles, and gas bubbles. To this end, two types of processes are discussed in the dissertation: biological and physical transports.

Biological transport corresponds to motion of organisms, prey-predator interactions and other behavioral traits. For small organisms such as planktonic organisms, their propulsion is derived from contractile elements (cilia and flagella) or appendages. For swimming organisms, the unsteadiness arises from unsteady propulsion mechanisms (e.g. unsteady beating of flagella or cilia), turbulent velocity fluctuations of the ambient fluid and velocity disturbances generated by a prey or predator. The fundamental equation of motion is derived for unsteady swimming of small organisms in a non-uniform background flow. Unsteady forces such as history and added mass forces are important as the organism's Stokes number rises above unity. Large organisms with sizes of $O(1 mm-20 mm)$ swim in a range of parameters where the inertial effects significantly affect organisms' locomotion and induced mixing in an aquatic environment. The associated inertial forces influence swimming speed, energy expenditure, and flow signature. The sea surface microlayer contains a large population of microorganisms. The reduced swimming velocity near a free surface is quantified to explain the observed aggregation of microorganisms near a liquid interface. Below the surface mixed layer, there exists abundant zooplankton. Their vertical migration could potentially contribute to the density overturn, enhancing nutrient exchange between nutrient-depleted and nutrient-rich layers.

Rising/settling dynamics of particles (e.g. marine snow sediments, streaming gas bubbles) are also considered. Most marine snow particles are irregular shaped aggregates. For the first time, we investigate the settling dynamics of a circular disk in a linearly stratified fluid. For specific ranges of the Froude and Reynolds numbers, there is a change of stability for the disk orientation, from broadside-on to edgewise settling. For bubble streaming in the ocean, there are frequent collisions between bubbles and air-seawater interfaces. We study the effect of surfactant on a bubble colliding with an air-water interface. The extent of collision is determined by the ratio of adsorption and desorption rate of surfactant at the bubble's surface as well as the air-water interface. From the standpoint of carbon sequestration, rising motion of bubbles containing carbon dioxide or methane gases acts as a carbon source from the ocean to the atmosphere while settling marine snow particles acts as a carbon sink, affecting global carbon cycle.