Riccardo Artoni

Effective boundary conditions for dense granular flows

We derive an effective boundary condition for dense granular flow taking into account the effect of the heterogeneity of the force network on sliding friction dynamics. This yields an intermediate boundary condition which lies in the limit between no slip and Coulomb friction; two simple functions relating wall stress, velocity, and velocity variance are found from numerical simulations. Moreover, we show that this effective boundary condition corresponds to Navier slip condition when the model of G. D. R. Midi [Eur. Phys. J. E 14, 341 (2004)] is assumed to be valid, and that the slip length depends on the length scale that characterizes the system, viz. the particle diameter.

Scaling laws for the slip velocity in dense granular flows.

In this Letter, the two-dimensional dense flow of polygonal particles on an incline with a flat frictional inferior boundary is analyzed by means of contact dynamics discrete element simulations, in order to develop boundary conditions for continuum models of dense granular flows. We show the evidence that the global slip phenomenon deviates significantly from simple sliding: a finite slip velocity is generally found for shear forces lower than the sliding threshold for particle-wall contacts. We determined simple scaling laws for the dependence of the slip velocity on shear rate, normal and shear stresses, and material parameters. The importance of a correct determination of the slip at the base of the incline, which is crucial for the calculation of flow rates, is discussed in relation to natural flows.

Effective Wall Friction in Wall-Bounded 3D Dense Granular Flows

We report numerical simulations on granular shear flows confined between two flat but frictional sidewalls. Novel regimes differing by their strain localization features are observed. They originate from the competition between dissipation at the sidewalls and dissipation in the bulk of the flow. The effective friction at sidewalls is characterized (effective friction coefficient and orientation of the friction force) for each regime, and its interdependence with slip and force fluctuations is pointed out. We propose a simple scaling law linking the slip velocity to the granular temperature in the main flow direction which leads naturally to another scaling law for the effective friction.

Collapse of quasi-two-dimensional wet granular columns

This paper deals with the experimental characterization of the collapse of wet granular columns in the pendular state, with the purpose of collecting data on triggering and jamming phenomena in wet granular media. The final deposit shape and the runout dynamics were studied for samples of glass beads, varying particle diameter, liquid surface tension, and liquid amount. We show how the runout distance decreases with increasing water amount (reaching a plateau for

Discrete particle simulations and experiments on the collapse of wet granular columns

w>1 %

Shear bands in granular flow through a mixing-length model

) and increases with increasing Bond number, while the top and toe angles and the final deposit height increase with increasing water amount and decrease with decreasing Bond number. Dimensional analysis allowed to discuss possible scalings for the runout length and the top and toe angles: a satisfying scaling was found, based on the combination of Bond number and liquid amount.

A Dissipative Coulomb Model for Dense Granular Flows

Small quantities of liquid in a granular material control the flow dynamics as well as the triggering and jamming phases. In order to study this problem, some experimental collapse tests conducted in a rectangular box were reproduced with a 1:1 scale numerical model using the Discrete Element Method. In simulations the effect of the capillary bridges has been investigated implementing a mid-range attractive force between particles based on the minimum energy approach. Also a bonding-debonding mechanism was incorporated in the algorithm and the volume of each sessile drop on the particle surface was considered during its motion. The influence of some variables was investigated with respect to the final slope profiles and the runout lengths: the initial liquid content, the particle size, the solid density, the liquid surface tension, and the liquid-solid contact angle. Also the crucial effect of the confinement walls on the collapse phenomenon was investigated: wet particles adhere to the lateral walls prov...

Electronic Nose Aided Verification of an Odour Dispersion Model for Composting Plants’ Applications

We discuss the advantages and results of using a mixing-length, compressible model to account for the shear banding behaviour in granular flow. We formulate a general approach based on two functions of the solid fraction to be determined. Studying the vertical chute flow, we show that the shear band thickness is always independent of flow rate in the quasistatic limit, for Coulomb wall boundary conditions. The effect of bin width is addressed using the functions developed by Pouliquen and coworkers, predicting a linear dependence of shear band thickness on channel width, while the literature reports contrasting data. We also discuss the influence of wall roughness on shear bands. Through a Coulomb wall friction criterion we show that our model correctly predicts the effect of increasing wall roughness on the thickness of shear bands. Then a simple mixing-length approach to steady granular flows can be useful and representative of a number of original features of granular flow.

A Fluctuating Energy Model for Dense Granular Flows

We address the slow, dense flow of granular materials as a continuum with the incompressible Navier‐Stokes equations plus the fluctuating energy balance including granular temperature. The pseudo‐fluid is given an apparent viscosity which depends on an order parameter which we choose to be granular temperature. We derive the fluctuating energy balance following Babic [1]; this balance includes a ‘mobility enhancing’ term due to ‘viscous heating’ effects and a dissipative term which we assume, as a constitutive hypothesis, dependent on the isotropic part of the stress tensor and on shear rate, based on local friction considerations. For its particular structure the model is called Dissipative Coulomb Model. Solving the equation system we get an appreciable agreement with experiments in complex geometries (silos, hoppers). The model predicts also the typical behaviour of stresses in these configurations, thus predicting phenomena like the hourglass effect.

Simulation of dense granular flows: Dynamics of wall stress in silos

The dispersion of odour from a composting plant was calculated with the CALPUFF modeling system, where site specific meteorology and geophysical informations were taken into account. The odour emissions, both from forced and free‐convection sources, were measured by means of dynamic olfactometry and implemented in the model. The results obtained from the model were verified with a MOS sensor based Electronic Nose equipped with a preconcentrator, placed in two target sites 50 m and 250 m far from the plant. Odour episodes, detected by electronic nose, were compared with model’s forecast; a procedure for tuning the model parameters is needed, in order to reproduce Electronic Nose measurements.