Session G28: Geophysical Fluid Dynamics: Transport and Mixing

A Boundary Integral Approach to Breaking Up Marine Aggregates

Near the surface of the ocean, particulates and other microorganisms tend to cluster and form fractal aggregates as they meet. The mechanisms that lead to their formation have been studied extensively. It is also well known, but not well understood, that marine aggregates often break up as they settle under gravity, or because of stresses induced by some background flow. Here we propose a Boundary-Integral formulation of the Stokes Equations that allows us to compute the internal stresses felt by fractal aggregates subjected to different background flows. When the maximum internal stress computed exceeds a pre-defined threshold, aggregates are broken up in two. For aggregates with various fractal dimensions, we study the relationship between the stresses and the size of the original objects and quantify the fractal dimensions of the newly formed objects. This study provides a more realistic description of aggregates observed in experiments, and gives insight on how to build more accurate disaggregation models.   

Environmental monitoring at coastal scale using Lagrangian Coherent Structures

In this talk, I will describe how Lagrangian Coherent Structures can be used to study environmental problems at the sub-mesoscale. A specific coastal application related to fuel spill management will be presented [1]. Additionally, links between Lagrangian Coherent Structures and Uncertainty Quantification are discussed [2].   

Mass Transport By Oceanic Mesoscale Eddies

Mass transport in the ocean plays a crucial role in regulating Earth's cli- mate and natural marine resources. The oceanic circulation spans a vast range of scales, spanning from O(1) mm to tens of thousands of kilometers. Mesoscale eddies, which range in size from tens of kilometers to a few hundred kilometers, account for a significant fraction of the ocean's total kinetic energy. Previous works analyzed the transport using eddy detection techniques, which consider eddies as closed contours dominated by vorticity. In this study we employ coarse-graining and Lagrangian tracking techniques to analyse mass transport induced by the mesoscales. We rely on data from satellite altimetry, which provides ocean surface currents globally.   

The role of ocean turbulence on the vertical motion of biofouled microplastic particles in a marine environment

This research investigates the influence of turbulence on the sinking of biofouled microplastic particles in the ocean. Marine plastic pollution has been identified as a global environmental crisis with over 8 million metric tons of plastic entering the ocean every year causing extensive damage to aquatic fauna and flora. Plastic particles smaller than 5 mm in size are categorized as microplastics. Buoyant microplastic particles travel deeper into the ocean due to the increase in density caused by the growth of a biofilm. Biofouling has been recently implemented in oceanographic computational models that attempt to track the spatial and temporal evolution of microplastic particles. Turbulence plays a crucial role in such models and the present study highlights the strong interactions between turbulence and the biofouling process. Our results show that polyethylene plastic particles with sizes in the micrometer range act similarly to Lagrangian tracers following the flow trajectories. On the opposite side, particles with sizes in the millimeter range oscillate in the upper ocean water column. Our ongoing research shows that the final particle tracking model and results would provide crucial insight into the fate of plastic particles in the ocean.   

Convection and thermal mixing in super-confined systems

The destabilizing geothermal gradient across the Earth's lithosphere can drive convection in subsurface water reservoirs, especially in regions considered hot spots, where geothermal energy peaks. Such regions include mid-ocean ridges and tectonically active areas on Earth, where deep hot waters meet cold surface water that penetrates from the surface through open faults. Investigating the thermo-fluid dynamics of these super-confined systems is not only stimulating but also relevant for better understanding (1) thermal ecosystems that depend on minerals transport and mixing; and (2) geothermal reservoirs. Adopting the canonical features of Rayleigh-Bénard convection to establish a destabilizing thermal gradient through a fluid, we utilize Hele-Shaw cells as an analogue to investigate convection and thermal mixing in fault systems. Combining experimental and numerical experiments, we examine the relationship between thermal forcing, energy transport's spatial distribution, and global mixing quantified by the scalar fluctuations destruction rate. We aim to give insights into the fluid dynamics that can be expected in hard-to-reach hydrothermal systems.