UC Merced

Settling marine aggregates are essential in transporting dissolved carbon dioxide from the surface ocean to the deep sea. While sinking, they may accumulate in thin layers where density stratification is present, becoming nutrient hotspots for bacteria and animal activity. We simulate settling marine aggregates in an ambient homogeneous and a density-stratified fluid to study their dynamics. We model a marine aggregate as a fractal collection of cubes. In a homogeneous fluid, the flow is computed in the Stokes regime using a boundary integral method. We focus on analyzing the forces acting on the aggregate in the presence of various background flows. We have identified an effective hydrodynamic radius that captures the settling dynamics accurately. We couple the velocity 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 and time of the aggregates.We also present the study of complex fluids with higher-order rheology. We introduce rate-dependent flows having non-Newtonian behaviors. We pay attention to a flow of granular material, which is the second most commonly used material in industry after water. We model a second-order stress tensor in strain rate for granular material within an Adaptive Mesh Refinement (AMR) framework, AMReX-incflo, to solve for incompressible flows. We also formulate a regularization of the yield stress terms to account for viscosity divergence at low strain rates. To incorporate a higher-order strain rate tensor with the incompressible solver numerically, we added a two-stage Runge-Kutta scheme in the AMReX-incflo code and the default solver, the explicit Euler method.