Particle Size Segregation in Granular Mass Flows With Different Ambient Fluids

Abstract

Size segregation, which is a robust feature of sheared granular mixtures and geophysical mass flow deposits, is found to diminish in the presence of a viscous fluid. We study this inhibitive effect through coupled fluid-particle simulations of granular flows fully immersed in different ambient fluids. Granular-fluid mixture flows are modeled according to three distinct flow regimes-free fall, fluid inertial, and viscous-at different angles of inclination. Each flow regime corresponds to distinct flow dynamics and segregation behaviors. We find that segregation is indeed weaker and slower in the presence of an ambient fluid, which is more so as the flow becomes more viscous. The ambient fluid affects segregation in two major ways. First, buoyancy reduces the contact pressure gradients which are needed to drive large particles up, while at the same time reduces the particles' apparent weight. On the other hand, the streamwise drag force substantially changes the flow rheology, specifically the shear rate profile, thereby modifying the segregation behavior in the normal direction. Surprisingly, the fluid drag in the normal direction is negligible regardless of the fluid viscosity and does not affect segregation in a direct manner.

Mechanisms of size segregation in granular flows with different ambient fluids

“…4. Note that these values are significantly higher than those reported in [9] due to different partitioning criteria applied [12].…”

Viscous Effects on the Particle Size Segregation in Geophysical Mass Flows: Insights From Immersed Granular Shear Flow Simulations

“…The pioneering work of Savage and Lun (1988) through buoyancy as a function of  . Our recent work (Zhou et al, 2020) showed that size segregation in immersed granular flows over rough inclines (Figure 10a) is inhibited due to the weakening of contact pressures owing to buoyancy and due to the low shear rates resulting from the development of plug zones induced by an opposing streamwise drag. Three distinct ambient fluids: air, water, and a viscous "slurry" (a fluid 500 times more viscous than water but having the same density), were considered.…”

“…However, Figure 10c shows that scaling by I is inadequate to collapse the data, as fluid-related mechanisms that govern the difference in the segregation behavior are not fully considered. Here, with the scaling established in this paper, it is possible to revisit the data in Zhou et al, (2020) and show whether the proposed scaling works for different flow scenarios.…”

“…In our recent work (Zhou et al., 2020), segregation was studied in immersed gravity‐driven flows in which the viscous drag force was not found to directly hinder the upward rise of large particles but instead influences segregation by modifying the flow profiles. The segregation process is sensitive to the local shear rate of the flow (Itoh & Hatano, 2019; Siman‐tov & Brodsky, 2018; Staron & Phillips, 2015), hence, when streamwise drag forces induce the formation of plug flow regions wherein shear rates are significantly reduced, the rate of size segregation likewise slows down (Zhou et al., 2020). However, since segregation was studied in highly inhomogeneous flows (Zhou et al., 2020), its inherent dependence on fluid properties, such as density and viscosity remains unclear.…”

Acceleration of Debris Flow Due to Granular Effect

“…Grain size segregation in debris flow also affects the movement process (Iverson et al, 2010). Studies have shown that grain size segregation has a profound impact on the mobility of immersed granular flows, and is strongly associated with the pore pressure distribution (Bagnold, 1954;Middleton, 1970;Remaitre et al, 2011;Jing et al, 2017;Cui et al, 2020;Zhou et al, 2020). In the granular flow and geophysical flow communities, the mechanism of the development of pore water pressure and its critical role in determining mobility have been widely concerned, and some quantitative relationships have been obtained (Iverson, 2005;Rondon et al, 2011;Meruane et al, 2012;Yang et al, 2020).…”