Self-diffusion scalings in dense granular flows.

We report on measurements of self-diffusion coefficients in discrete numerical simulations of steady, homogeneous, collisional shearing flows of nearly identical, frictional, inelastic spheres. We focus on a range of relatively high solid volume fractions that are important in those terrestrial gravitational shearing flows that are dominated by collisional interactions. Diffusion over this range of solid fraction has not been well characterized in previous studies. We first compare the measured values with an empirical scaling based on shear rate previously proposed in the literature, and highlight the presence of anisotropy and the solid fraction dependence. We then compare the numerical measurements with those predicted by the kinetic theory for shearing flows of inelastic spheres and offer an explanation for why the measured and predicted values differ.

Dataset of velocities of dry granular flows in a partially obstructed tilted chute.

The dataset provided in this paper refers to an experimental campaign conducted in Laboratory of Fluid Dynamics (LTDF) of the Free University of Bozen-Bolzano at NOI Techpark aiming to understand the movement of granular material in fluids of low viscosity and density exhibited in debris flows. One experimental test was performed consisting of 31 repetitions. In detail, a three-litre volume of granular material (d = 1.8mm) was suddenly released from an upstream reservoir in a 1.5 m long acrylic chute tilted at 19 degrees and stopped in the outlet area by a vertical barrier. This vertical barrier used is adjacent to the side wall of the chute, with two vertical gaps and a width equal to twice the size of the particles used (s = 2d). The instrumentation included two high-speed cameras (300fps) and one spotlight. Camera 1 (C1) was located upstream at the lock gate location and Camera 2 was placed at downstream part of the chute, focusing on the vertical barrier site. A Particle Tracking Velocimetry (PTV) was applied to the set of images captured by the camera placed in the downstream area of the chute in a region of interest (ROI) of 4000 pixel width and 300 pixel height. Firstly, the raw data concerns to the particles coordinates (x,z), their along-chute and wall-normal trajectories and particle tag, detected with the PTV algorithm for the 31 repetitions held. The previous data was submitted to filtering processes where we converted particle trajectories into maps of these mean quantities by binning and constructing a data ensemble. To remove some detected outliers, a refinement of ensemble data was subsequently applied [1]. All of the solutions computed to build the pointed dataset were performed by means of Matlab algorithms. This dataset allows researchers to characterize the behaviour of granular processes that may occur in inclined channels partially or fully obstructed.

Self-diffusion in inhomogeneous granular shearing flows.

In this Letter, we discuss how flow inhomogeneity affects the self-diffusion behavior in granular flows. Whereas self-diffusion scalings have been well characterized in the past for homogeneous shearing, the effect of shear localization and nonlocality of the flow has not been studied. We, therefore, present measurements of self-diffusion coefficients in discrete numerical simulations of steady, inhomogeneous, and collisional shearing flows of nearly identical, frictional, and inelastic spheres. We focus on a wide range of dense solid volume fractions, that correspond to geophysical and industrial shearing flows that are dominated by collisional interactions. We compare the measured values first with a scaling based on shear rate and, then, on a scaling based on the granular temperature. We find that the latter does much better than the former in collapsing the data. The results lay the foundations of diffusion models for inhomogeneous shearing flows, which should be useful in treating problems of mixing and segregation.

Intermittency of rheological regimes in uniform liquid-granular flows.

We present a detailed analysis of a free surface-saturated liquid-granular mixture flowing over a static loose bed of grains, where the coexistence of layers dominated by collisional and frictional interactions among particles was observed. Kinetic theory was applied to the flow described above and it proved suitable for describing a realistic behavior of the collisional layers, although it failed to interpret the regions of the flow domain dominated by the frictional contacts. The paper provides a conceptual scheme with which to overcome this problem by focusing on the mechanisms governing the transition from the frictional to the collisional regime. In particular we observed that in highly concentrated flows the transition layer exhibits a typical intermittency of the dominating rheological regime, switching alternately from the frictional to the collisional one. By filtering the velocity signal, we introduced an intermittency function that made it possible to extend the validity of the equations derived from dense gas analogy, typical of the collisional regimes, also in the intermittent phase of the flow. Owing to the small values of the Stokes number, in the application of the kinetic theory we accounted for the possible variation of the elastic restitution coefficient along the flow depth.

A procedure for human safety assessment during hydropeaking events.

A method for human safety assessment on a hydropeaked river reach is proposed and applied to an Alpine river. The human safety analysis during hydropeaking events is of particular interest because most of the Alpine watercourses are affected by hydropower plant energy production that cause rapid and frequent flow alterations (hydropeaking), but at the same time these watercourses are used by the population for recreational purposes. In literature, many studies have focused on the effect of hydropeaking on the biota but a study of the interaction between a hydropeaking wave and human safety does not yet exist. The proposed procedure is characterized by the combination of hydraulic numerical simulations to study the characteristics of the flow field with a human safety analysis and is applied to a case study in north Italy. Human safety can be assessed in two different ways: one is by studying human stability during hydropeaking events and the other is exploring the possibility of a "target person" leaving the reach during hydropeaking waves, adapting proper escape strategies. For the escape strategy Dijkstra's algorithm is used, where the distance between adjacent nodes is defined as the difficulty (penalty) of moving from one node to the other. For this reason, an original set of penalty functions is proposed that takes into account the steepness (slope between two adjacent computational cells), the roughness, and the product between the water depth and flow velocity. The results show that the difficulty in escaping increases with the flow rate. Moreover, the areas where the human safety is very low are mainly located in the central part of the watercourse. The present work proposes a possible investigational tool to evaluate and parameterize the risk for the population during hydropeaking events through quantitative indices.