Granular-front formation in free-surface flow of concentrated suspensions

Abstract

A granular front emerges whenever the free-surface flow of a concentrated suspension spontaneously alters its internal structure, exhibiting a higher concentration of particles close to its front. This is a common and yet unexplained phenomenon, which is usually believed to be the result of fluid convection in combination with particle size segregation. However, suspensions composed of uniformly sized particles also develop a granular front. Within a large rotating drum, a stationary recirculating avalanche is generated. The flowing material is a mixture of a viscoplastic fluid obtained from a kaolin-water dispersion with spherical ceramic particles denser than the fluid. The goal is to mimic the composition of many common granular-fluid materials, such as fresh concrete or debris flow. In these materials, granular and fluid phases have the natural tendency to separate due to particle settling. However, through the shearing caused by the rotation of the drum, a reorganization of the phases is induced, leading to the formation of a granular front. By tuning the particle concentration and the drum velocity, it is possible to control this phenomenon. The setting is reproduced in a numerical environment, where the fluid is solved by a lattice-Boltzmann method, and the particles are explicitly represented using the discrete element method. The simulations confirm the findings of the experiments, and provide insight into the internal mechanisms. Comparing the time scale of particle settling with the one of particle recirculation, a nondimensional number is defined, and is found to be effective in predicting the formation of a granular front.

Modeling Two‐Phase Debris Flows With Grain‐Fluid Separation Over Rugged Topography: Application to the 2009 Hsiaolin Event, Taiwan

“…If we infer the coarser grains to a lower solid basal friction, the high solid concentration snout meets the fact of a coarse‐grained front observed in laboratory investigation as well as reported by event witness or video record in natural conditions (e.g., Iverson, ; Iverson & Denlinger, ; Takahashi, ). However, with increased initial volume fraction of the solid phase, the formation of a granular front, observed in the rotating drum experiments by Leonardi et al (), was not obtained in our numerical tests. This is suspected to be caused by the assumption of a nearly uniform velocity distribution along the flow thickness in our depth‐averaged model, where the shearing is neglected and the corresponding reorganization of phases cannot be induced consequently.…”

“…The chosen values presented above do not perfectly match reality, but we suppose they would be sufficient to highlight the impacts of these parameters on the flow behavior. However, for a more realistic condition, the density ratio α ρ = 1,420/2,600 = 0.5462, as given in Leonardi et al (), is additionally taken into consideration in this parameter study. With m and ρ f = 1,420 kg/m 3 , N R = 268 corresponds to a viscosity of 0.5245 Pa·s.…”

Effects of coarse grain size distribution and fine particle content on pore fluid pressure and shear behavior in experimental debris flows

“…[] and Leonardi et al . []. This latter effect was more pronounced for the mixtures that contained few or no fine particles and for the mixtures where the GSD of the coarse fraction was narrow.…”

Visualization of dominant stress-transfer mechanisms in experimental debris flows of different particle-size distribution

“…The mechanics of debris flows is made complex by the large range of particle sizes within the flowing mass, from boulders to clay, which segregate during motion. Larger particles tend to focus toward the flow front while fluid concentrates in the tail, altering the behaviour in time and space (Pierson 1986;McArdell et al 2007;Leonardi et al 2015). At the microscale, the dynamics of such particle-fluid systems involves momentum exchange processes caused by inertial granular collisions, friction between grains, viscous shear, and solid-fluid interactions (Iverson 1997).…”