Luca Sarno

Pressure Coefficient in Dam-Break Flows of Dry Granular Matter

AbstractThe propagation of dry granular flows, such as rock and snow avalanches, can be described by depth-averaged models. Different from classical shallow-water equations, these models take into account the anisotropy of normal stresses inside the flowing pile through using an earth pressure coefficient in the pressure term. A new regularization function for calculating the pressure coefficient in the Savage-Hutter-type models at the early stages of dam-break flows and collapses is proposed. In such circumstances the flow lines are significantly curved with respect to the basal surface and a special treatment of the earth-pressure coefficient is required for obtaining a satisfactory agreement with experimental data. The comparison between numerical simulations and laboratory experimental data shows an apparent improvement in describing the early stages of dam-break waves over rough beds. The comparison with experiments over smooth bed surface exhibits minor evidence of improvement. Nonetheless, in this ...

A two-layer depth-averaged approach to describe the regime stratification in collapses of dry granular columns

The dynamics of dry granular flows is still insufficiently understood. Several depth-averaged approaches, where the flow motion is described through hydrodynamic-like models with suitable resistance laws, have been proposed in the last decades to describe the propagation of avalanches and debris flows. Yet, some important features of the granular flow dynamics cannot be well delivered. For example, it is very challenging to capture the progressive deposition process, observed in collapses and dam-break flows over rough beds, where an upper surface flow is found to coexist with a lower creeping flow. The experimental observations of such flows suggest the existence of a flow regime stratification caused by different momentum transfer mechanisms. In this work, we propose a two-layer depth-averaged model, aiming at describing such a stratification regime inside the flowing granular mass. The model equations are derived for both two-dimensional plane and axi-symmetric flows. Mass and momentum balances of each...

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

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.

DAM-BREAK FLOWS OF DRY GRANULAR MATERIALON GENTLE SLOPES

INTRODUCTION Debris flows as well as snow and rock avalanches are fast-moving flows that occur in many areas of the world. They are particularly dangerous to life and property because they move with high velocities, destroy infrastructures in their paths, and often strike without warning. In some real world cases, geophysical flows can be triggered by phenomena that are very similar to a dam break. Water flows generated by a dam break have been widely studied and mathematical models for water dam-break waves are available in many textbooks and research papers. Compared to water dambreak waves, debris flow waves display a wider variability and, for their mathematical description, require models with a much greater complexity. As in the case of clear water, particular attention must be given to their numerical integration because of the frequent developing of steep gradients and shock waves. Owing to this complexity, a number of simplifications are put in place and tested under laboratory conditions: the results of the tests are then used to improve the rheological models that underlie the numerical simulations. In the specific case of debris flows, flows arising from a dam-break-like event of dry granular material can help in the process of model validation. Moreover, a good understanding of the mechanics of dry granular flows is also essential in order to set up twophase debris-flow models because two separate modABSTRACT This work examines dam-break flows of dry granular material and investigates the suitability of the depth-averaged models with particular attention being given to the description of the shear stresses and pressure terms. The experimental results of dam-break flows down a gently sloped channel have been reported. Tests were carried out on both a smooth Plexiglas bed as well as a rough one. Measurements of the flow depth profiles and the front wave position were obtained using two digital cameras. In order to compare the prediction of the depth-averaged approach with granular avalanche tests, a specific mathematical and numerical model was implemented. The momentum equation was modified in order to take into account the resistances due to the side walls. The numerical integration of the shallow water equations was carried out through a TVD finite volume method. In order to address the importance of a good estimate of the stress distribution inside the pile, several numerical simulations were performed, calculating with different formulas the pressure coefficient that relates longitudinal and vertical normal stresses in the momentum equation. The simulations present, in general, agree with experimental data. The differences have been outlined between the smooth and rough bed cases.