Particle-like and fluid-like settling of a stratified suspension

Abstract

Abstract. The gravitational settling of inhomogeneously suspended particles in fluid has been investigated. Of particular interest is whether collective or individual motion of particles is dominant during their settlings, i.e., whether the particles settle as a continuous suspension or they settle individually relative to surrounding fluid. We observed the settling of a stratified suspension which has the lower and upper concentration interfaces in quasi-two dimensional vessel. In some cases, the suspension behaves perfectly as a continuous fluid and the motion of the constituent particle is subject to bulk flow caused by the interfacial instability. In other cases, the particle behaves individually relative to surrounding fluid. The existence of concentration interface plays a significant role in these extreme behaviors of suspension. The transition from the collective to individual behaviors can be predicted quantitatively by a parameter which expresses the border resolution of concentration interface.

Collective settling of fine particles in a narrow channel with arbitrary cross-section

“…As can be seen, finger-like instability develops at concentration interface and the settling of suspended particle is subject to macroscopic motion of the interface instabilities. As described in our previous article (Harada et al, 2012), such fingering instability is fundamentally the same as Rayleigh-Taylor instability and it behaves as immiscible fluids if the particle size is small and the concentration is high,…”

“…Before that, we reconsider the experimental results of the gravitational settling of various-sized particles described in our previous article (Harada et al, 2012). Fig.9 is the settling velocity of particles in quasi-two dimensional closed channels.…”

“…Völtz et al The dispersion relation for two immiscible fluids with no interfacial tension in a quasi-two dimensional channel have found in our previous article (Harada et al, 2012) and is as follows.…”

“…Such instability of suspension-fluid interface greatly influences the settling motion of constituent particles. Fig.1 is the settling behavior of particulate suspension arranged in laminae for various particle and fluid conditions in quasi two-dimensional closed channel (Harada et al, 2012). If the particle size is small and the concentration is large, the suspension behaves as an immiscible fluid even though there is no definite interface and the suspended particles settle with the growth of fingering instability at lower suspension-fluid interface.…”

Numerical simulation of concentration interface in stratified suspension: Continuum–particle transition

“…For the second case in which the concentration interface points down, Harada et al (2012) have experimentally examined the collectivity of settling particles caused by the Rayleigh-Taylor instability. They found that the transition from individual to collective settling of suspended particles is determined by a dimensionless parameter given by the ratio of the interparticle separation to the dominant wavelength of the instability; this ratio is related to the number density of the suspended particles.…”