Starting September, 2013
Granular materials are ubiquitous in a large variety of natural as well as industrial systems. One of their most fascinating properties is their ability to either sustain elastic stresses as solids, or to flow as fluids, depending on the applied solicitation. The present project will tackle the fundamental mechanisms arising when both a stagnant granular zone (quasi-static) and a dense-liquid granular zone (inertial regime) coexist with mainly the help of well-documented and well-calibrated DEM and FEMLIP numerical simulations.
Two model granular systems will be addressed: a lid-driven cavity and an inclined free-surface granular flow with a dead zone trapped by a wall.
The expected breakthrough concerns the identification of the role of fluctuations under well-controlled boundary conditions. This project is of utmost importance for a better understanding of
granular flows around structures and forces exerted on those structures.
Modelling of an unstable granular bank flow over a wall (click on the image to run the
Kneib, F.; Faug, T.; Dufour, F. & Naaim, M. Numerical investigations of the force experienced by a wall subject to granular lid-driven flows: regimes and scaling of the mean force. Computational Particle Mechanics, 2016, 3, 293-302
Kneib, F.; Faug, T.; Nicolet, G.; Eckert, N.; Dufour, F. & Naaim, M. Force fluctuations on a wall in interaction with a granular lid-driven cavity flow. Phys. Rev. E 96, 042906 – Published 16 October 2017
Kneib, F.; Faug, T.; Dufour, F. & Naaim, M. Forces experienced by the walls of a granular lid-driven cavity. 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23). Byron Bay, Australia, 9-12 December 2014, S.T. Smith (Ed.)
Kneib, F.; Faug, T.; Dufour, F. & Naaim, M. Force fluctuations experienced by a boundary wall subjected to a granular flow in two distinct systems. Powders and Grains 2017 –
8th International Conference on Micromechanics on Granular Media. Montpellier, France, July 3-7, 2017, F. Radjai, S. Nezamabadi, S. Luding and J.Y. Delenne (Eds.)
Congrès Français De La Mécanique, Lyon - 2015. Forces experienced by the walls of a granular lid-driven cavity.
IV International Conference on Particle-Based Methods, Barcelone - 2015. Forces experienced by the walls of a granular lid-driven cavity.
PhD defense: 2nd of June 2017
The existing studies dealing with the design of civil-engineering structures against snow avalanches are generally based on force times series that are smoothed over time. However the strong heterogeneity of snow leads to systematic observations of a high level of force fluctuations. This PhD thesis aims at characterizing the force fluctuations exerted on an obstacle that is overflowed by a granular flow. Numerical simulations based on the discrete elements method are implemented to model the interaction between the snow, represented by an assembly of spherical particles, and a rigid motionless wall-like obstacle. A key feature of this work is the broad range of flow regimes investigated, from quasistatic to collisionnal. Two model systems are studied in order to focus on a zone restricted to the upstream of the obstacle, and to allow a full control of the macroscopic flow variables (shearing velocity, confinement pressure, system sizes). The first one confines the grains between four walls from which the top one imposes a constant shearing velocity while the force signals are measured on the wall facing the corresponding displacement. The second system confines the grains between a static bottom wall, a shearing top wall, and a periodic boundary condition in the shear direction, while the wall-like obstacle is fully immersed in the grains. Each system is studied through a time-averaged analysis then the fluctuations are characterized from the instantaneous force time series.The macroscopic inertial number built from the shear velocity and the confinement pressure imposed to the system turns out to be the main control variable of both the mean dynamics and the fluctuations in the systems. An empirical law has been established to predict the mean force transmission on the obstacle as a function of the macroscopic inertial number for each of the two systems, and the measurement of local strain and stress tensors revealed that the granular flow µ(I)-rheological law is respected nearly everywhere in the samples. The autocorrelations of force signals on the obstacle at the mesoscopic scale revealed the presence of a memory effect of both systems at low inertial numbers which vanishes with the transition from the quasistatic to the dense inertial flow regimes. The force distributions at three different spatial scales are also controlled by the macroscopic inertial number: for slow regimes the distributions are tightened and resemble Gaussian shapes, for fast regimes the distributions are rather exponential. Truncated log-normal probability density functions (with three parameters) have been established in order to predict empirically the force distributions on the obstacle.This work contributes to advance the knowledge on both the time-averaged and the fluctuating components of the force exerted on a wall subjected to a granular flow. The results enable to look forward with the modeling of gravity-driven systems approaching real flow conditions, thus allowing a comparison with laboratory experiments and full-scale measurements, with the aim of better designing of civil engineering structures impacted by avalanches.
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