Ticking hour glasses: Experimental analysis of intermittent flow (original) (raw)
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Physical Review Letters, 1993
We have found experimentally that the flow of sand in an hour glass is oscillatory in a certain range of parameters. The oscillatory motion results from a coupling between the flow of sand and convection of air through the sand matrix. The period of the oscillation is remarkably insensitive to the grain size, while the mass of the sand contained in one period strongly depends on it.
Structure Formation and Instability in a Tube of Sand
Physical Review Letters, 2001
A new instability in the combined flow of fine grains and gas is investigated by means of experiments, simulations, and analytic techniques. When a bubble of air rises through a granular packing in a tube, a sequence of smaller bubbles spontaneously forms in front of it. The existence of this instability is shown from the experiments, simulations, and theoretical considerations. Moreover, the simulations and experiments agree on the quantitative level. In particular, when the tube is tilted away from the vertical the experiments and the simulations show the same increase in the speed of the rising bubble.
Flow in an hourglass: particle friction and stiffness matter
New Journal of Physics, 2021
Granular flow out of a silo is studied experimentally and numerically. The time evolution of the discharge rate as well as the normal force (apparent weight) at the bottom of the container is monitored. We show that particle stiffness has a strong effect on the qualitative features of silo discharge. For deformable grains with a Young modulus of about Y m ≈ 40 kPa in a silo with basal pressure of the order of 4 kPa, lowering the friction coefficient leads to a gradual change in the discharge curve: the flow rate becomes filling height dependent, it decreases during the discharge process. For hard grains with a Young modulus of about Y m ≈ 500 MPa the flow rate is much less sensitive to the value of the friction coefficient. Using DEM data combined with a coarse-graining methodology allows us to compute all the relevant macroscopic fields, namely, linear momentum, density and stress tensors. The observed difference in the discharge in the low friction limit is connected to a strong d...
EPL (Europhysics Letters), 2002
We present a simple model of a vibrated box of sand, and discuss its dynamics in terms of two parameters reflecting static and dynamic disorder respectively. The fluidised, intermediate and frozen ('glassy') dynamical regimes are extensively probed by analysing the response of the packing fraction to steady, as well as cyclic, shaking, and indicators of the onset of glassy behaviour are analysed. In the 'glassy' regime, our model is exactly solvable, and allows for the qualitative description of ageing phenomena in terms of two characteristic lengths; predictions are also made about the influence of grain shape anisotropy on ageing behaviour.
The heterogeneous dynamics of sand
JPS Conf. Proc. , 011004 (2014)
"We provide an overview of some recent results on heterogeneities in granular media. In the context of spatial heterogeneity, we discuss the phenomenon of bridges, which are one of its most ubiqui- tous and widely studied instances. Next, we review a model of spatiotemporal inhomogeneities in a granular column near jamming, where temporal heterogeneities occur at distinct spatial points and in particular examine its relevance to recent experiments."
Experimental study of a granular flow in a vertical pipe: A spatiotemporal analysis
Physical Review E, 1999
The spatiotemporal characteristics of the flow of 125 m glass beads have been studied experimentally in a 2.9 mm diameter vertical glass pipe by means of a digital linear charge coupled device camera. The dynamics and propagation of granular density fluctuations have been analyzed in various flow regimes for different values of the granular mass-flow rate and of the degree of humidity H. At low flow rates, the granular flow has a high compactness and periodic intermittency effects occur at high H values: a pulsating bubble appears at the top of the flow channel and its time dependence is studied. At higher granular flow rates, a wave regime is observed with alternate low and high compactness regions: at low H values, the wave velocity is constant with time and can be measured precisely from the spatiotemporal diagrams. At higher humidity contents, periodic oscillations of the wave system are observed: at high amplitudes, transient blockages of the flow and a stick-slip displacement mode have been identified. ͓S1063-651X͑99͒06901-9͔
Reports on Progress in Physics, 1994
In recent years, sand has become a paradigm of complexity in physics; despite its everyday familiarity, it has come to typify what is increasingly regarded as a 'new' state of matter, namely matter in the granular state. Granular matter shows behaviour that is intermediate between that of solids and liquids and manifests fascinating properties like dilatancy and hysteresis. The last few years have seen an explosion of theoretical and experimental activity in the study of its dynamics, in particular to d o with the relaxational behaviour of vibrated powders, segregation phenomena and sandpile avalanches. This article sets out to review the progress that has been made, and to present our current understanding of the dynamics of sand.
The Mechanism of Sand Production Caused by Pore Pressure Fluctuations
Oil & Gas Science and Technology, 2001
Le mécanisme de production de sable provoqué par des fluctuations de la pression de pore -Des échantillons sableux, issus d'une couche de surface d'un réservoir saoudien, ont été testés sous des conditions similaires à celles existant dans le réservoir afin d'étudier l'accroissement de la production de sable avec la fluctuation de pression. L'état de contrainte en fond de puits a été simulé en laboratoire à l'aide d'une presse équipée d'une cellule haute pression Hoek, avec contrôle de la pression de confinement, de la pression de pore et du système d'injection. On a pu ainsi mesurer l'écoulement du fluide dans les conditions in situ. Dans cette étude, une eau salée à 3 % et du pétrole léger de faible viscosité (1,5 cP) ont été utilisés comme fluides de remplissage et fluides de déplacement. Le fluide déplacé et le sable ont été collectés à la sortie de la cellule de Hoek (diamètre = 4,25 mm). On obtient que la production de sable augmente lorsque le processus de production est stoppé pendant 24 h puis redémarré ; elle augmente avec la répétition de l'ensemble de ce processus. Par ailleurs, la quantité de sable produite diminue lorsque la pression de fluide se rapproche de la pression initiale : ce déclin est attribué à la hausse de la pression effective de confinement, qui a tendance à contenir les grains sableux et à contrebalancer ainsi les dommages causés au ciment. Les essais de compression indiquent que la résistance du grès est réduite de 8 à 15 % par la succession de trois cycles de production utilisant eau salée et huile légère. Cette réduction de la résistance de la roche est due aux fluctuations de la pression de pore pendant le processus de production de sable.
Flow-rate fluctuations in the outpouring of grains from a two-dimensional silo
Physical Review E, 2009
We present experimental results obtained with a two-dimensional silo discharging under gravity through an orifice at the flat bottom. High-speed measurements provide enough time resolution to detect every single bead that goes out and this allows the measurement of the flow rate in short-time windows. Two different regimes are clearly distinguished: one for large orifices, which can be described by Gaussian fluctuations, and another for small orifices, in which extreme events appear. The frontier between those two regimes coincides with the outlet size below which jamming events are frequent. Moreover, it is shown that the power spectrum of the flow-rate oscillations is not dominated by any particular frequency.
Shocks in sand flowing in a silo
Journal of Fluid Mechanics, 2002
We study the formation of shocks on the surface of a granular material draining through an orifice at the bottom of a quasi-two dimensional silo. At high flow rates, the surface is observed to deviate strongly from a smooth linear inclined profile giving way to a sharp discontinuity in the height of the surface near the bottom of the incline, the typical response of a choking flow such as encountered in a hydraulic jump in a Newtonian fluid like water. We present experimental results that characterize the conditions for the existence of such a jump, describe its structure and give an explanation for its occurrence.