Experimental Investigation on the Effects of the Fixed Boundaries in Channelized Dry Granular Flows

Abstract

The dynamics of granular mixtures, involved in several geophysical phenomena like rock avalanches and debris flows, is far from being completely understood. Several features of their motion, such as non-local and boundary effects, still represent open problems. An extensive experimental study on free-surface channelized granular flows is here presented, where the effects of the fixed boundaries are systematically investigated. The entire experimental data set is obtained by using a homogenous acetal-polymeric granular material and three different basal surfaces, allowing different kinematic boundary conditions. Velocity profiles at both the sidewall and the free surface are obtained by using high-speed cameras and the open-source particle image velocimetry code, PIVlab. Significantly, different sidewall velocity profiles are observed by varying the basal roughness and the flow depth. Owing to sidewall friction and non-local effects, such profiles exhibit a clear rheological stratification for high enough flow depths and they can be well described by recurring to composite functions, variously formed of linear, Bagnold and exponential scalings. Moreover, it has been discovered that transitions from one velocity profile to another are also possible on the same basal surface by merely varying the flow depth. This shape transition is due partly to the sidewall resistances, the basal boundary condition and, in particular, the occurrence/inhibition of basal grain rolling. In most of the experiments, the normal-to-bed velocity profiles and the velocity measurements at the free surface strongly suggest the occurrence of a secondary circulating flow, made possible by a chiefly collisional regime beneath the free surface

Application of the 2D Depth-Averaged Model, FLATModel, to Pumiceous Debris Flows in the Amalfi Coast

“…Conversely, the Voellmy resistance law [31,32] was identified as the most suitable approach. This finding strongly indicates that the investigated pumiceous DFs exhibit a granular behaviour, where the main momentum exchange mechanisms are represented by friction and collisions among grains e.g., [10,25]. In this regard, a pumiceous DF generally behaves as a stony debris flow, at least as long as the run-off water depths are small enough so that buoyancy effects remain negligible.…”

“…In the last decades, these phenomena have attracted huge interest from the scientific community, due to the increasing need to correctly describe their initiation and propagation phases, in order to better assess the risk, demarcate the hazardous areas and develop risk mitigation measures. Besides the theoretical and laboratory experimental investigations on the dynamics of granular flows and granular-liquid mixtures [4][5][6][7][8][9][10], considerable efforts have been recently made in the description of these flows at the field scale [11][12][13][14][15][16]. Modelling the propagation stages of these flows involves mathematical complexity and advanced numerical simulation techniques [17][18][19][20][21][22].…”

Granular surface avalanching induced by drainage from a narrow silo

“…In particular, it is restricted to two-dimensional, quasi-steady flows of monodisperse grains over deep erodible beds of the same composition, assumed governed by wall friction and by the linearized µ(I) rheology. Further research will be needed to overcome these limitations and possibly address other applications like flows partly controlled by non-erodible beds (Parez et al 2016;Fernández-Nieto et al 2018;Sarno et al 2018), and unsteady non-uniform flows like granular column collapse (Chou et al 2012;Ionescu et al 2015;Lee et al 2015).…”

“…The model further assumes that the flow is subject to friction along the walls and governed by a linearized form of the viscoplastic µ(I) rheology (Jop et al 2005;da Cruz et al 2005), such that the base of the flow coincides with the yield locus. As a result, it cannot describe flows over non-erodible beds (Parez et al 2016;Sarno et al 2018) or the transitions between flow and rest observed for shallow granular layers in wide channels (Pouliquen & Forterre 2002;Edwards & Gray 2015).…”

Efficiency of a Digital Particle Image Velocimetry (DPIV) Method for Monitoring the Surface Velocity of Hyper-Concentrated Flows

“…The high density of the natural grains allows an easier application of the PIV procedure with natural tracers but, since the granular material is non-transparent, the measurements inside the flow are generally impossible so that the laser sheets are inadequate to illuminate the particles and alternative lighting systems, such as halogen lamps, have to be used [22]. Pudasaini et al [23,24] employed flashes, Sheng et al [25] used halogen lamps powered by a flickering-free ballast, Sarno et al [26][27][28] employed a high-brightness no-flicker LED lamp.…”