Stratification-induced reorientation of disk settling through ambient density transition

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.

Viscoelastic and shear-thinning effects of aqueous exopolymer solution on disk and sphere settling

“…Experiments were carried out in a tank with interior dimensions 0.060 m × 0.060 m and a height of 0.500 m. The experimental set-up was similar to one used in previous settling experiments performed in the laboratory 60 and is presented in Supplementary Fig. S11.…”

“…Instantaneous particle settling velocity, u y , was evaluated from the sequence of recorded images using Particle Tracking Velocimetry method. First, the position and orientation of the particle in each image was assessed by image analysis methods described in previous study 60 . Then, knowing a time step between consecutive frames, the particle time-resolved position was evaluated.…”

Particles, Drops, and Bubbles Moving Across Sharp Interfaces and Stratified Layers

“…The settling of anisotropic bodies, especially ellipsoids and disks, has been considered at moderate Reynolds number (Re 100), under conditions where their motion in a homogeneous fluid reduces to a steady broadside-on fall. Although the background stratification is stabilizing for bodies released broadside-on, density disturbances acting on a slightly tilted oblate or prolate body settling in a stratified fluid may destabilize its initial orientation and make it eventually settle edge-on , Mrokowska 2018. The instability is rooted in the offset between the body axis and the location at which the upward jet emerges at the back of the body (Figure 5a).…”

Terminal fall velocity: the legacy of Stokes from the perspective of fluvial hydraulics

“…Response of the terminal fall velocity to key factors (a) Effects of particle shape Natural particles are hardly spherical. A large variety of non-spherical natural and artificial particles is used in engineering applications; for instance, disc [54], oblate spheroid [55], ice crystals [56], snowflakes [57,58], mineral dust [59], volcanic ash [60] and shell fragments as littoral sediments [15,17]. As a result, the particle shape is worth considering while estimating the drag coefficient and, in turn, the terminal fall velocity [17,61,62].…”

“…In addition, the surface irregularity might induce instability to the particle, yielding rotation and vibration of the particle, which eventually reduces the terminal fall velocity. It is worth highlighting that Mrokowska [54] studied specifically the effects of particle shape on a particle settling through a stratified fluid. In a two-layer fluid with a density transition, it was found that a disc exhibits five phases of settling.…”