Motion of anisotropic particles in: sedimentation, uni-directional shear flows and turbulence

Abstract

This thesis broadly deals with inertial effects on the motion of passive anisotropic particles and can be divided into four parts. Orientation dynamics of spheroids settling in turbulence The first part of the thesis is concerned with analyzing the motion of ice crystals in Cirrus clouds (chapter 1). This problem is motivated by the studies of Liou [1986]; Baran & Francis [2004]; Pandit et al. [2015] which comment on the crucial contribution of Cirrus clouds to the planetary greenhouse effect. Cirrus clouds are composed of ice crystals settling in an ambient turbulent flow and the orientation distribution of these ice crystals affects the sunlight scattered by the clouds which, in part, determines the earth-atmosphere radiation budget. The ice crystals are highly anisotropic, and occur in a variety of shapes ranging from needles to thin plates [Pruppacher & Klett, 2012]. The small-scale scenario in a Cirrus cloud is mimicked by simulating the motion of ice crystals, modeled as spheroids of an arbitrary aspect ratio κ, settling under gravity through a homogeneous isotropic turbulent field. The ice crystals in Cirrus clouds are comparable to or smaller than the Kolmogorov scale for atmospheric turbulence, which may therefore be treated as a fluctuating linear flow, and one may write down the turbulent-shear-induced torque as the Stokesian torque acting on a spheroid suspended in general linear linear flow