Abstract
Settling of solid particles in a stratified ambient fluid is a process widely encountered in geophysical flows. A set of experiments demonstrating the settling behaviour (the pattern of trajectory, variation of particle orientation, and settling velocity with depth) of thin disks descending through a nonlinear density transition was performed. The results showed complex hydrodynamic interactions between a particle and a liquid causing settling orientation instabilities and unsteady particle descent in low to moderate Reynolds number regime. Five phases of settling were observed: two phases with stable horizontal, one with stable vertical disk position, and two reorientation phases; moreover, two local minima of settling velocity were identified. It was demonstrated that thresholds for local minima and the first reorientation depend on the settling dynamics in an upper layer, stratification conditions, and disk geometry. The comparison of settling behaviour of thin disks varying in diameter revealed that settling dynamics is sensitive to particle geometry mainly in the upper part of density transition with a non-obvious result that the first minimum velocity is smaller for a disk with a larger diameter than for a disk with a smaller diameter. The analysis of settling trajectory showed that two reorientations are accompanied with a horizontal drift, which may be important in the context of interactions between particles settling in a group.
Highlights
Density stratification occurs in various fluid components of natural environment and affects to a large extent the vertical transport of particles
Disks were translating broadside on, which is in line with phase diagrams describing dynamics of disks in terms of the relation between Re and dimensionless moment of inertia, I* (Field et al 1997; Willmarth et al 1964) with the range of parameters observed in the study, Re between 5.0 and 11.1, and I* of the order 1 × 10–3, corresponding to the steady falling mode
Settling dynamics of thin disks descending through a nonlinear density transition were studied experimentally
Summary
Density stratification occurs in various fluid components of natural environment (ocean, atmosphere, and the Earth’s interior) and affects to a large extent the vertical transport of particles. Sharp density gradients known as pycnoclines (haloclines or thermoclines, with salinity or temperature acting as a stratifying agent, respectively) form in favourable conditions Atmosphere stratification affects the transport dynamics of various particles including dust, aerosol, pollens, volcanic ashes, and pollutants. The stratification of atmosphere affects the fate of eruption columns and the transport of volcanic particles including settling of ashes (Woods 1995). In the context of Earth’s interior, it has been demonstrated that the presence of sinking crystals may considerably accelerate mixing between rhyolitic and basaltic magmas, suggesting that particle settling should be considered in the magma mixing process (Renggli et al 2016)
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