Stratification-induced reorientation of disk settling through ambient density transition

Magdalena M Mrokowska

doi:10.1038/s41598-017-18654-7

Magdalena M Mrokowska

Open Access

https://doi.org/10.1038/s41598-017-18654-7

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Abstract

Settling due to gravity force is a basic transport mechanism of solid particles in fluids in the Earth. A large portion of particles occurring in nature and used in technical applications are non-spherical. Settling of particles is usually studied in homogeneous ambient conditions, however, stratification is inherent of natural fluids. It has been acknowledged that stratification modifies the velocity of settling spheres and amorphous aggregates. However, the effect of particle shape on the dynamics of settling through density-stratified ambient fluid has not been recognized well enough. Here I show experimental evidence that continuous density transition markedly modifies the settling dynamics of a disk in terms of settling velocity and orientation of a particle. Settling dynamics of a disk are more complex than dynamics of spheres and aggregates studied previously. I found that in a two-layer ambient with density transition, a disk settling in a low Reynolds number regime undergoes five phases of settling with the orientation varying from horizontal to vertical, and it may achieve two local minimum settling velocities in the density transition layer. Moreover, I found that the settling dynamics depends on a density difference between upper and lower homogeneous layers, stratification strength and thickness of density transition.

Highlights

The ocean and lakes are inhomogeneous mediums where dissolved substances and temperature act as density-stratifying agents, and nontrivial amounts of microparticles move downwards due to the gravitational force
Salinity of the ocean is reported to be around 3.5%, which results in a typical density of seawater of about 1.02–1.03 g cm−3 5,6
Salinity of natural waters such as hypersaline lakes may be much higher[7], e.g. salinity of the Dead Sea was reported[8] to reach 28% with a mean density of 1.23 g cm−3. As they move through the water column, particles may encounter density gradients forming density transitions or density interfaces[5]

Summary

Introduction

The ocean and lakes are inhomogeneous mediums where dissolved substances and temperature act as density-stratifying agents, and nontrivial amounts of microparticles move downwards due to the gravitational force. I study the behaviour (orientation and settling velocity) of individual thin disks settling through a two-layer water column with density transition due to the vertical variation of salinity.

Results

Conclusion