[Show abstract][Hide abstract] ABSTRACT: In this experimental and numerical thesis we have studied the behavior of a granular assembly in two different situations. In the first part, a sand track is subjected to repeated passages of vehicles, under certain conditions a regular pattern of ripples appears spontaneously: this is known as the washboard road instability. This phenomenon, very common on dirt roads, is of course annoying for drivers but is also very dangerous because of the lack of adhesion it causes. We reproduced this instability with a laboratory scale set-up and also thanks to soft spheres molecular dynamics simulations. With these experimental and numerical tools we have highlighted some properties of instability. In particular, we have measured the dispersion relation of the pattern and shown that this phenomenon follows pitchfork instability. By measuring the stress acting on the vehicle we were then able to build a linear stability analysis that predicts quantitatively the threshold of the instability and the wavelength of the pattern. Finally, we have studied more complex cases where several vehicles were simultaneously on the track. We have also considered the case where the track was wet and therefore cohesive. In a second part, we have studied the behavior of a silo filled with grains subjected to temperature variations. A slow creep motion of the grains in the column is observed. Two flow regimes are observed according to the amplitude of temperature cycles. We obtained an expression of the critical temperature between the two regimes as a function of the microscopic parameters of the grains (elasticity and surface roughness). We have also studied the motion of each individual grain in order to interpret the macroscopic dynamics of the pile.
[Show abstract][Hide abstract] ABSTRACT: In this letter, we report results on the effect of temperature variations on
a granular assembly through Molecular Dynamic simulations of a 2D granular
column. Periodic dilation of the grains are shown to perfectly mimic such
thermal cycling, and allows to rationalize the link between the compaction
process, the local grains dynamics and finite size effects. Here we show that
the individual grain properties, namely their roughness and elastic modulus
define a minimal cycling amplitude of temperature \Delta Tc below which the
dynamics is intermittent and spatially heterogeneous while confined into
localized regions recently coined "hot spot" [Amon et al., Phys. Rev. Lett.
108, 135502 (2012)]. Above \Delta Tc, the whole column flows while the grains
dynamics ranges continuously from cage-like at the bottom of the column to
purely diffusive at the top. Our results provide a solid framework for the
futur use of thermal cycling as an alternate driving method for soft glassy
[Show abstract][Hide abstract] ABSTRACT: When submitted to the repeated passages of vehicles unpaved roads made of sand or gravel can develop a ripply pattern known as washboard or corrugated road. We propose a stability analysis based on experimental measurements of the force acting on a blade (or plow) dragged on a circular sand track and show that a linear model is sufficient to describe the instability near onset. The relation between the trajectory of the plow and the profile of the sand bed left after its passage is studied experimentally. The various terms in the expression of the lift force created by the flow of granular material on the plow are determined up to first order by imposing a sinusoidal trajectory to the blade on an initially flat sand bed, as well as by imposing a horizontal trajectory on an initially rippled sand bed. Our model recovers all the previously observed features of washboard road and accurately predicts the most unstable wavelength near onset as well as the critical velocity for the instability.
Physical Review E 01/2013; 87(1-1):012203. DOI:10.1103/PhysRevE.87.012203 · 2.29 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We studied the drag and lift forces acting on an inclined plate while it is dragged on the surface of a granular media, both in experiment and in numerical simulation. In particular, we investigated the influence of the horizontal velocity of the plate and its angle of attack. We show that a steady wedge of grains is moved in front of the plow and that the lift and drag forces are proportional to the weight of this wedge. These constants of proportionality vary with the angle of attack but not (or only weakly) on the velocity. We found a universal effective friction law that accounts for the dependence on all the above-mentioned parameters. The stress and velocity fields are calculated from the numerical simulations and show the existence of a shear band under the wedge and that the pressure is nonhydrostatic. The strongest gradients in stress and shear occur at the base of the plow where the dissipation rate is therefore highest.
[Show abstract][Hide abstract] ABSTRACT: When a wheel of plow is dragged at a constant velocity on a granular bed, a ripple pattern known as washboard road forms if the velocity is above a critical value. Although much work has been recently devoted to this topic the underlying mechanisms remain unclear. We have studied the phenomenon using both an experimental setup consisting of a circular track on which a wheel or plow is dragged and 2D DEM simulations. Here we focus on the lift and drag forces exerted by the sand onto the wheel or plow. We found that these forces do not seem to depend on the velocity. We also found a linear relation between the lift and drag forces. These results are typical of static friction which is somewhat surprising considering the complexity of the granular flow advected by the wheel of plow. These results are a first step to the development of a stability analysis of washboard roads.
[Show abstract][Hide abstract] ABSTRACT: The orientation fluctuations of the director of a liquid crystal are measured after a quench near the Fréedericksz transition, which is a second order transition driven by an electric field. We report experimental evidence that, because of the critical slowing down, the liquid crystal presents several properties of an aging system after the quench, such as power law scaling in times of correlation and response functions. During this slow relaxation, a well defined effective temperature, much larger than the heat bath temperature, can be measured using the fluctuation dissipation relation.