Experimental assessment of the effective friction at the base of granular chute flows on a smooth incline (original) (raw)
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Physics of Fluids, 2006
In this study, positions, velocities, and rotations of monodisperse disks confined two-dimensionally in a glass-walled chute are measured using a high-speed camera. Steady, fully developed granular flows ͑SFD͒ down bumpy inclines are systematically investigated in the frictional-collisional ͑dense, rapid͒ regime. Three bottoms with different effective roughness heights and roughness distributions are studied to evaluate the influence of the bottom condition. The granular flows are shallow, having a typical depth of ten disk diameters. In the range of flow rates and inclination angles where SFD flows occur, the mean discharge velocity is approximately proportional to the flow depth. The surface solid fractions slightly decrease from the bottom to the free surface. The streamwise velocity profiles are close to the linear profile at small inclination angles, whereas at large inclination angles, they are best approximated by the Bagnold profile. The mean angular velocity is equal to the half shear rate everywhere in the flow except near the free surface and the bottom. At large inclination angles, relatively deep SFD flows exhibit an S-shaped granular temperature profile, but in the core, the temperature is far from scaling linearly with the square shear rate. The streamwise and crosswise translational temperatures are slightly different from each other, whereas the rotational temperature is only half of the crosswise translational temperature. The rough bottoms have complex influences on the granular flows as revealed by the velocity and temperature profiles.
Bulletin of Volcanology, 2016
Laboratory experiments on granular flows using natural material were carried out in order to investigate the behaviour of granular flows passing over a break in slope. Sensors in the depositional area recorded the flow kinematics, while video footage permitted reconstruction of the deposit formation, which allowed investigation of the deposit shape as a function of the change in slope. We defined the slope-angle ratio as the proportion between slope angle in the depositional area and that of the channel. When the granular flow encounters the break in slope part of the flow front forms a bouncing clast zone due to elastic impact with the expansion box floor. During this process, part of the kinetic energy of the dense granular flow is transferred to elutriating fine ash, which subsequently forms turbulent ash cloud accompanying the granular flow until it comes to rest. Morphometric analysis of the deposits shows that they are all elliptical, with an almost constant minor axis and a variable major axis. The almost constant value of the minor axis relates to the spreading angle of flow at the end of the channel, which resembles the basal friction angle of the material. The variation of the major axis is interpreted to relate to the effect of competing inertial and frictional forces. This effect also reflects the partitioning of centripetal and tangential velocities, which changes as the flow passes over the break in slope. After normalization, morphometric data provided empirical relationships that highlight the dependence of runout from the product of slope-angle ratio and the difference in height between granular material release and deposit. The empirical relationships were tested against the runouts of hot avalanches formed during the 1944 AD eruption at Vesuvius, with differences among actual and calculated values are between 1.7 and 15 %. Velocity measurements of laboratory granular flows record deceleration paths at different breaks in slope. When normalized, the velocity data show third-order polynomial fit, highlighting a complex behaviour involving interplay between inertial and frictional forces. The theoretical velocity decays were tested against the data published for volcaniclastic debris flows of the 5-6 May 1998 event in the Sarno area. The comparison is very good for nonchannelized debris flows, with significant differences between actual and calculated velocities for the channelized debris flows.
Experiments on Chute Flows of Granular Materials
Experiments on continuous, steady flows of granular materials down an inclined channel or chute were made with the object of acquiring information on the rheological properties of the granular material flow and the nature of the boundary condition on the base of the channel. Specifically measurements were made of the mean material velocities and velocity profiles on all boundaries of the flow using cross-correlation of two neighboring fibre-optic displacement probes. The output from these probes was used to obtain (1) the unsteady or random component of the particle velocity in the longitudinal direction and (2) a measure of the volume fraction of the flow in contact with the base by counting the frequency of passage of the particles. Measurement was also made of the depth of the flow, the mass flow rate and the shear stress on the base. The latter employed a strain-gauged shear force plate built into the base.
A Model of the Flow of Granular Materials Down Chutes
2005
This paper develops models and constitutive equations that are intended for the calculation of granular flows down inclined channels. The flows are assumed to involve fairly large particles at high concentrations. Furthermore, it is assumed that the flows are rapid, and that the particle velocity fluctuations are vigorous. We make use of results derived from the kinetic theories of granular flows to develop simplified expressions that can be used to determine stresses, and velocity and density profiles in channelized flows. The paper focuses on the particle collisional stress contributions, but the effects of the 'frictional' stresses that come from the enduring interparticle contacts are also considered. A solution for steady uniform flow is also presented. The results are verified through comparisons to a few available experimental measurements. The comparisons indicate that predicted velocities and flow rates are close to the experimental measurements.
Granular flows down inclined and vibrated planes: influence of basal friction
EPJ Web of Conferences
We present an experimental study about granular avalanches when external mechanical vibrations are applied. The results of the flow properties highlight the existence of two distinct regimes: (i) a gravity-driven regime at large angles where scaling laws are in agreement with those reported in the literature for non-vibrating granular flows and (ii) a vibration-driven regime at small angles where no flow occurs without applied vibrations. The flow in this regime is well described by a vibrationinduced activated process. We also propose an empirical law to capture the evolution of the thickness of the deposits as a function of the vibration intensity and the inclination angle.
High-speed confined granular flows down smooth inclines: scaling and wall friction laws
Granular Matter, 2020
Recent numerical work has shown that highspeed confined granular flows down smooth inclines exhibit a rich variety of flow patterns, including dense unidirectional flows, flows with longitudinal vortices and supported flows characterized by a dense core surrounded by a dilute hot granular gas [1]. Here, we further analyzed the results obtained in [1]. More precisely, we characterize carefully the transition between the different flow regimes, including unidirectional, roll and supported flow regimes and propose for each transition an appropriate order parameter. Importantly, we also uncover that the effective friction at the basal and side walls can be described as a unique function of a dimensionless number which is the analog of a Froude number: F r = V / √ gH cos θ where V is the particle velocity at the walls, θ is the inclination angle and H the particle holdup (defined as the depth-integrated particle volume fraction). This universal function provides a boundary condition for granular flows running on smooth boundaries. Additionally, we show that there exists a similar universal law relating the local friction to a local Froude number F r loc = V loc / P loc /ρ (where V loc and P loc are the local velocity and pressure at the boundary, respectively, and ρ the particle density) and that the latter holds for unsteady flows.
Gravity granular flows of slightly frictional particles down an inclined bumpy chute
Journal of Fluid Mechanics, 1996
Gravity-driven granular flow of slightly frictional particles down an inclined, bumpy chute is studied. A modified kinetic model which includes the frictional energy loss effects is used, and the boundary conditions for a bumpy wall with small friction are derived by ensuring the balance of momentum and energy. At the free surface, the condition of vanishing of the solid volume fraction is used. The mean velocity, the fluctuation kinetic energy and the solid volume fraction profiles are evaluated. It is shown that steady granular gravity flow down a bumpy frictional chute could be achieved at arbitrary inclination angles. The computational results also show that the slip velocity may vary considerably depending on the granular layer height, the surface boundary roughness, the friction coefficient and the inclination angles. The model predictions are compared with the existing experimental and simulation data, and good agreement is observed. In particular, the model can well predicate the features of the variation of solid volume fraction and fluctuation energy profiles for different particle-wall friction coefficients and wall roughnesses.
Monodisperse dry granular flows on inclined planes: Role of roughness
The European Physical Journal E - Soft Matter, 2003
Recent studies have pointed out the importance of the basal friction on the dynamics of granular flows. We present experimental results on the influence of the roughness of the inclined plane on the dynamics of a monodisperse dry granular flow. We found experimentally that it exists a maximum of the friction for a given relative roughness. This maximum is shown to be independent of the angle of the slope. This behavior is observed for four planes with different bump sizes (given by the size of the beads glued on the plane) from 200 µm to 2 mm. The relative roughness corresponding to the maximum of the friction can be predicted with a geometrical model of stability of one single bead on the plane. The main parameters are the size of the bumps and the size of the flowing beads. In order to obtain a higher precision, the model also takes into account of the spacing between the bumps of the rough plane. Experimental results and model are in good agreement for all the planes we studied. Other parameters, like the sphericity of the beads, or irregularities in the thickness of the layer of glued particles, are shown to be of influence on the friction.
Experimental investigation of high speed granular flows down inclines
EPJ Web of Conferences
We report on laterally confined granular flow experiments on steep slopes. We provide evidences for the existence of different flow regimes with secondary flows. At moderate mass flow, we observe a first flow regime with a pair of longitudinal vortices which are localized close to the lateral walls and span progressively over the whole flow width with increasing flow rate. They are counter-rotative and induce a vertical upward motion of the grains at the wall. Upon a further increase of the mass flow rate, a transition is evidenced by a reversal of the rotation direction of the vortices which trigger in contrast a downward motion of the grains close to the lateral walls and an upward motion at the center of the channel. We argue that these flows bear some resemblance with the flow regimes reported recently in discrete element simulations.
Transition to movement in granular chute flows
Chemical Engineering Science, 2001
This experimental investigation deals with the observation of the behaviour that dense granular materials present when they ow in steady regime on a rough chute, focusing the attention on the transition to movement of the bed and on quantities involved as the internal friction angle. An important aspect of the study is the identiÿcation of parameters that distinguish granular from uid ows, aiming to verify the possibility to describe a granular bed as it was a pseudo-uid having a particular rheological behaviour. In the experiments we have not used idealised particles (spheres, rods or disks) but sieved powders of ethylenediaminetetraacetic acid (EDTA), constituted of non-spherical particles with polydisperse size distribution and surface roughness. A static and a owing (dynamic) layer are clearly identiÿed. The thickness of the observed layers (static and dynamic) along the chute has been measured for di erent inclination, ÿnding out that they collapse into a single curve when considered in non-dimensional scale. On the ground of the experimental data we propose a direct way of measuring the dynamic friction angle from chute observations and a simple constitutive law for granular materials in the frictional regime of motion. The law has been tested using velocity proÿles obtained by ÿlming the owing granular bed. ?