21 research outputs found
Discharge of elongated grains in silos under rotational shear
The discharge of elongated particles from a silo with rotating bottom is investigated numerically. The introduction of a slight transverse shear reduces the flow rate Q by up to 70% compared with stationary bottom, but the flow rate shows a modest increase by further increasing the external shear. Focusing on the dependency of flow rate Q on orifice diameter D, the spheres and rods show two distinct trends. For rods, in the small-aperture limit Q seems to follow an exponential trend, deviating from the classical power-law dependence. These macroscopic observations are in good agreement with our earlier experimental findings [Phys. Rev. E 103, 062905 (2021)]. With the help of the coarse-graining methodology we obtain the spatial distribution of the macroscopic density, velocity, kinetic pressure, and orientation fields. This allows us detecting
a transition from funnel to mass flow pattern caused by the external shear. Additionally, averaging these fields in the region of the orifice reveals that the strong initial decrease in Q is mostly attributed to changes in the flow velocity, while the weakly increasing trend at higher rotation rates is related to increasing packing fraction. Similar analysis of the grain orientation at the orifice suggests a correlation of the flow rate magnitude with the
vertical orientation and the packing fraction at the orifice with the order of the grains. Lastly, the vertical profile of mean acceleration at the center of the silo denotes that the region where the acceleration is not negligible shrinks significantly due to the strong perturbation induced by the moving wall
Particle flow rate in silos under rotational shear
Very recently, To et al. have experimentally explored granular flow in a cylindrical silo, with a bottom wall
that rotates horizontally with respect to the lateral wall [Phys. Rev. E 100, 012906 (2019)]. Here we numerically
reproduce their experimental findings, in particular, the peculiar behavior of the mass flow rate Q as a function
of the frequency of rotation f . Namely, we find that for small outlet diameters D the flow rate increased with
f , while for larger D a nonmonotonic behavior is confirmed. Furthermore, using a coarse-graining technique,
we compute the macroscopic density, momentum, and the stress tensor fields. These results show conclusively that changes in the discharge process are directly related to changes in the flow pattern from funnel flow to mass flow. Moreover, by decomposing the mass flux (linear momentum field) at the orifice into two main factors, macroscopic velocity and density fields, we obtain that the nonmonotonic behavior of the linear momentum is caused by density changes rather than by changes in the macroscopic velocity. In addition, by analyzing the spatial distribution of the kinetic stress, we find that for small orifices increasing rotational shear enhances the mean kinetic pressure (pk) and the system dilatancy. This reduces the stability of the arches, and, consequently, the volumetric flow rate increases monotonically. For large orifices, however, we detected that (pk) changes nonmonotonically, which might explain the nonmonotonic behavior of Q when varying the rotational shear
Silo outflow of soft frictionless spheres
Outflow of granular materials from silos is a remarkably complex physical
phenomenon that has been extensively studied with simple objects like
monodisperse hard disks in two dimensions (2D) and hard spheres in 2D and 3D.
For those materials, empirical equations were found that describe the discharge
characteristics. Softness adds qualitatively new features to the dynamics and
to the character of the flow. We report a study of the outflow of soft,
practically frictionless hydrogel spheres from a quasi-2D bin. Prominent
features are intermittent clogs, peculiar flow fields in the container and a
pronounced dependence of the flow rate and clogging statistics on the container
fill height. The latter is a consequence of the ineffectiveness of Janssen's
law: the pressure at the bottom of a bin containing hydrogel spheres grows
linearly with the fill height
Two scenarios for avalanche dynamics in inclined granular layers
We report experimental measurements of avalanche behavior of thin granular
layers on an inclined plane for low volume flow rate. The dynamical properties
of avalanches were quantitatively and qualitatively different for smooth glass
beads compared to irregular granular materials such as sand. Two scenarios for
granular avalanches on an incline are identified and a theoretical explanation
for these different scenarios is developed based on a depth-averaged approach
that takes into account the differing rheologies of the granular materials.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
Discharge of elongated grains from silo with rotating bottom
We study the flow of elongated grains (wooden pegs of length L = 20 mm with circular cross section of diameter d(c) = 6 and 8 mm) from a silo with a rotating bottom and a circular orifice of diameter D. In the small orifice range (D/d approximate to 15 degrees to the motion of the bottom
The role of the particle aspect ratio in the discharge of a narrow silo
The time evolution of silo discharge is investigated for different granular materials made of spherical or elongated grains in laboratory experiments and with discrete element model (DEM) calculations. For spherical grains, we confirm the widely known typical behavior with constant discharge rate (except for initial and final transients). For elongated particles with aspect ratios between 2 < L/d < 6.1, we find a peculiar flow rate increase for larger orifices before the end of the discharge process. While the flow field is practically homogeneous for spherical grains, it has strong gradients for elongated particles with a fast-flowing region in the middle of the silo surrounded by a stagnant zone. For large enough orifice sizes, the flow rate increase is connected with a suppression of the stagnant zone, resulting in an increase in both the packing fraction and flow velocity near the silo outlet within a certain parameter range
Rheological response of nonspherical granular flows down an incline
We present an extensive numerical and experimental study, investigating a three-dimensional (3D) granular flow of elongated particles down an inclined plane. Similarly to sheared systems, the average particle orientation is found to enclose a small angle with the flow direction. In the bulk, this behavior is independent of the shear rate. At the surface, however, the particles move in more dilute conditions, and the average orientation strongly depends on the shear rate. A systematic numerical study varying the particle aspect ratio and the plane inclination reveals that the particle size perpendicular to the flow direction, deff, is an appropriate length scale to define an effective inertial number Ieff, which fully captures the impact of the particle shape on the system's rheology. Like in the case of spheres, density and friction result in well-defined functions of the effective inertial number Ieff, Thus, we quantify and explain the dependence of the rheological parameters on the aspect ratio, based on the micromechanical details
The role of the particle aspect ratio in the discharge of a narrow silo
The time evolution of silo discharge is investigated for different granular materials made of spherical or elongated grains in laboratory experiments and with discrete element model (DEM) calculations. For spherical grains, we confirm the widely known typical behavior with constant discharge rate (except for initial and final transients). For elongated particles with aspect ratios between 2 < L/d < 6.1, we find a peculiar flow rate increase for larger orifices before the end of the discharge process. While the flow field is practically homogeneous for spherical grains, it has strong gradients for elongated particles with a fast-flowing region in the middle of the silo surrounded by a stagnant zone. For large enough orifice sizes, the flow rate increase is connected with a suppression of the stagnant zone, resulting in an increase in both the packing fraction and flow velocity near the silo outlet within a certain parameter range
Nucleation and Bulk Crystallization in Binary Phase Field Theory
We present a phase field theory for binary crystal nucleation. In the
one-component limit, quantitative agreement is achieved with computer
simulations (Lennard-Jones system) and experiments (ice-water system) using
model parameters evaluated from the free energy and thickness of the interface.
The critical undercoolings predicted for Cu-Ni alloys accord with the
measurements, and indicate homogeneous nucleation. The Kolmogorov exponents
deduced for dendritic solidification and for "soft-impingement" of particles
via diffusion fields are consistent with experiment.Comment: 4 pages, 4 figures, accepted to PR