Simulations of settling marine aggregates in a stratified fluid.

Settling marine aggregates play an important role in transporting dissolved carbon dioxide (CO2) from the surface ocean to the deep ocean. While sinking, they accumulate in thin layers where sharp density stratifications are present which become nutrient hotspots for bacteria and animal activity. We present a method to simulate settling aggregates in a stratified fluid. We assemble fractal aggregates as collections of cubic particles to model the marine aggregates. The flow around the aggregates is computed in the limit of zero Reynolds number using a boundary integral method. Due to variable density, a term involving a volume integral is added to the boundary integral formulation. For rapid computation in three dimensions, we use the fast multipole method with a modified Laplace's kernel. We couple the solution with the advection-diffusion equation to track the heat density or salt concentration in time. We use this method to quantify how the presence of stratification affects the settling speed of the aggregates.