Dynamics of an anisotropic particle in geophysical flows

by Himanshu Mishra (Indian Institute of Technology Madras)

Monday 21 Apr 2025, 16:00 → 18:00 Europe/Stockholm

Albano 3: 6228 - Mega (22 seats) (Albano Building 3)

We examine the translation and orientation dynamics of an anisotropic particle in various geophysical flows. Anisotropic particulate matter interacts with the background flow field in a complex manner, influencing oceanic mixing, dispersion, and ice crystal growth in clouds. We analyze multiple problems motivated by geophysical flows, exploring various complexities of the background flows - steady laminar flows, a wavy flow field and a homogeneous isotropic turbulent flow.

First, we study the orientation dynamics of a settling spheroid in a simple shear flow. When viscous effects dominate, a spheroid follows the well-known Jeffery orbits, but fluid inertia from particle translation introduces an inertial torque, leading to broadside-on alignment. The competition between background shear and inertial torque determines the preferential alignment based on the alignment of the flow vorticity with respect to gravity.

Next, we investigate the transport dynamics of a heavy spheroid in the presence of surface gravity waves - an oscillatory shear flow. For neutrally buoyant particles, it is known that a wave-induced preferential alignment exists, which is dependent on the degree of shape anisotropy. We show that for heavy spheroids, the equilibrium orientation also depends on the density ratio and the initial orientation, thus playing a crucial role in long-time dispersion behaviour.

Then, we explore the orientation dynamics of a spheroid in homogeneous isotropic turbulence with an external electric field relevant to ice crystals in cirrus and deep convective clouds. Without the external field, an anisotropic particle in a turbulent flow exhibits random tumbling motion with a propensity to align strongly with the vorticity vector. However, the external field would prefer to align the particle along itself, and thus, there would be competition between the turbulent and electric torques. In the strong field limit, we asymptotically calculate the orientational variance transverse to the field direction as a function of turbulence intensity and compare it with full numerical simulations. On further evaluating the higher orientation moments, we show the strong dependence of the non-Gaussian nature of the turbulent velocity gradient on the orientation dynamics.