9 research outputs found
Shocks in sand flowing in a silo
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.Comment: 5 pages, 7 figure
Maximum Angle of Stability of a Wet Granular Pile
Anyone who has built a sandcastle recognizes that the addition of liquid to
granular materials increases their stability. However, measurements of this
increased stability often conflict with theory and with each other [1-7]. A
friction-based Mohr-Coulomb model has been developed [3,8]. However, it
distinguishes between granular friction and inter-particle friction, and uses
the former without providing a physical mechanism. Albert, {\em et al.} [2]
analyzed the geometric stability of grains on a pile's surface. The
frictionless model for dry particles is in excellent agreement with experiment.
But, their model for wet grains overestimates stability and predicts no
dependence on system size. Using the frictionless model and performing
stability analysis within the pile, we reproduce the dependence of the
stability angle on system size, particle size, and surface tension observed in
our experiments. Additionally, we account for past discrepancies in
experimental reports by showing that sidewalls can significantly increase the
stability of granular material.Comment: 4 pages, 4 figure
Angle of repose and segregation in cohesive granular matter
We study the effect of fluids on the angle of repose and the segregation of
granular matter poured into a silo. The experiments are conducted in two
regimes where: (i) the volume fraction of the fluid is small and it forms
liquid bridges between particles, and (ii) the particles are completely
immersed in the fluid. The data is obtained by imaging the pile formed inside a
quasi-two dimensional silo through the transparent glass side walls. In the
first series of experiments, the angle of repose is observed to increase
sharply with the volume fraction of the fluid and then saturates at a value
that depends on the size of the particles. We systematically study the effect
of viscosity by using water-glycerol mixtures to vary it over at least three
orders of magnitude while keeping the surface tension almost constant. Besides
surface tension, the viscosity of the fluid is observed to have an effect on
the angle of repose and the extent of segregation. In case of bidisperse
particles, segregation is observed to decrease and finally saturate depending
on the size ratio of the particles and the viscosity of the fluid. The sharp
initial change and the subsequent saturation in the extent of segregation and
angle of repose occurs over similar volume fraction of the fluid. In the second
series of experiments, particles are poured into a container filled with a
fluid. Although the angle of repose is observed to be unchanged, segregation is
observed to decrease with an increase in the viscosity of the fluid.Comment: 9 pages, 12 figure
Size Segregation of Granular Matter in Silo Discharges
We present an experimental study of segregation of granular matter in a
quasi-two dimensional silo emptying out of an orifice. Size separation is
observed when multi-sized particles are used with the larger particles found in
the center of the silo in the region of fastest flow. We use imaging to study
the flow inside the silo and quantitatively measure the concentration profiles
of bi-disperse beads as a function of position and time. The angle of the
surface is given by the angle of repose of the particles, and the flow occurs
in a few layers only near the top of this inclined surface. The flowing region
becomes deeper near the center of the silo and is confined to a parabolic
region centered at the orifice which is approximately described by the
kinematic model. The experimental evidence suggests that the segregation occurs
on the surface and not in the flow deep inside the silo where velocity
gradients also are present. We report the time development of the
concentrations of the bi-disperse particles as a function of size ratios, flow
rate, and the ratio of initial mixture. The qualitative aspects of the observed
phenomena may be explained by a void filling model of segregation.Comment: 6 pages, 10 figures (gif format), postscript version at
http://physics.clarku.edu/~akudrolli/nls.htm
Swarming ring patterns in bacterial colonies exposed to ultraviolet radiation
Consiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7 Rome / CNR - Consiglio Nazionale delle RichercheSIGLEITItal