Reorientation of elongated particles at density interfaces

Density interfaces in the water column are ubiquitously found in oceans and lakes. Interaction of settling particles with pycnoclines plays a pivotal function in nutrient transport between ocean layers and settling rates of marine particles. We perform direct numerical simulations of an elongated particle settling through a density interface and scrutinize the role of stratification on the settling dynamics. It is found that the presence of the density interface tends to turn the long axis of an elongated particle parallel to the settling direction, which is dramatically different from its counterpart in a homogeneous fluid. Although broadside-on settling of the elongated particle is enhanced upon approaching the interface, the long axis rotates toward the settling direction as the particle passes through the interface. We quantify turning couples due to stratification effects, which counteract the pressure-induced torques due to the fluid inertia. A similar behavior is observed for different initial orientations of the particle. It is shown that the reorientation of an elongated particle occurs in both sharp and linear density stratifications.