Abstract
Settling of non-spherical particles in a stratified fluid exhibits complex dynamics in a low-to-moderate inertia regime. Although this process is involved in a wide variety of phenomena in natural fluid systems, its fundamental mechanisms are still unexplored. Understanding of particle settling in microscale is particularly important to explain challenging problems associated with ecological and biogeochemical processes in the ocean due to the delayed settling of particulate matter at pycnoclines. Here, I explore interactions between disk-shaped particles and a stratified fluid with a density transition. By laboratory experiments, I demonstrate that the settling dynamics of the disk crossing a density transition are tightly coupled with the wake structure evolution, and I observe for the first time in a two-layer ambient configuration a bell-shaped structure that forms on a jet after the wake has detached from the particle. Furthermore, I identify hydrodynamic conditions for the variations of settling velocity and particle orientation instabilities. These findings shed light on particle settling mechanisms necessary to explain dynamics of marine particles such as plankton, faecal pellets, and microplastics and may improve the estimation methods of sedimentation processes in various areas of earth sciences and engineering.
Highlights
Settling of non-spherical particles in a stratified fluid exhibits complex dynamics in a low-to-moderate inertia regime
Phenomena related to particle settling are significant in natural waters where the vertical variation of temperature and/or salinity induces density gradients in the ocean, estuaries, coastal regions, lakes, and deep hypersaline anoxic basins affecting the dynamics of particles settling in a water column[2,5]
I have shown that the settling dynamics of disks descending through a two-layered liquid column with a density transition in a low-to-moderate Reynolds numbers (Re) number regime follows complex patterns accompanied by the evolution of wake structure
Summary
Settling of non-spherical particles in a stratified fluid exhibits complex dynamics in a low-to-moderate inertia regime. A large group of experimental studies on particle settling in stratified ambient fluid has been motivated by a potential application to plankton and marine snow dynamics in pycnoclines present in the ocean. These studies have used experimental configuration with homogeneous upper and lower layers with density transition between them referring to a pycnocline in natural w aters[7,13,20,23,24,25]. The increase of salinity by several dozen (psu) over one meter depth has been reported in the Orca B asin[5] and a sharp halocline has been reported in Ursu Lake (Romania) with salinity difference between homogeneous upper and lower layers about 300 (psu) over 1 m29
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