Molecular dynamics methods are used to study two-dimensional gravity-driven granular flow through a horizontal aperture. Two distinct approaches to modeling the granular particles are studied. ͑a͒ Circular particles subject to a strongly repulsive short-range interaction, together with normal and tangential frictional damping forces. ͑b͒ Rigid nonconvex particles, each consisting of disks arranged as an equilateral triangle, suitably spaced to provide a tangible indentation along each edge; the same repulsive interactions between disks in different grains and normal frictional damping forces are incorporated, but transverse damping is omitted, with the model relying on grain shape to resist sliding motion. In order to allow accurate measurements under steady-state conditions, a continuous-feed approach is adopted, in which grains exiting through the hole are returned to the top of the material in the container. For both models the output flow is measured as a function of aperture size, and the observed behavior is compared with previous theoretical and experimental results. Tests of the degree to which the models reproduce the depth independence of the flow are reported, and the influence of the container width and the nature of the walls are studied. The depth dependence of the pressure, the local stress distribution, and the particle flow patterns are also examined.
The formation of toroidal Taylor vortices in a fluid contained within the annular region bounded by two concentric cylinders, the inner one of which rotates, has been observed using molecular dynamics simulation. The quantitative nature of the vortices has been examined over a range of supercritical Taylor numbers. The Fourier amplitudes of the fundamental radial velocity mode and its low-order harmonics have been analyzed; despite the microscopic system size their functional dependence on the Taylor number is in excellent agreement with theory and experiment. [S0031-9007(98)06397-2]
Molecular (or granular) dynamics methods are used to study the gravity-driven flow of granular material through a horizontal aperture in three dimensions. The grains are spherical and modeled using a short-range repulsive interaction, together with normal and tangential frictional damping forces. The material is contained in a rough-walled cylindrical container with a circular hole in its base, and to permit flow measurements under steady-state conditions a continuous feed approach is employed in which exiting grains are replaced at the upper surface of the material. The dependence of flow velocity and discharge rate on aperture diameter is found to agree with experiment; other quantities such as the kinetic energy and pressure distributions are also examined.
Size segregation is a widely observed consequence of sheared flow in granular media, in which the larger grains rise to the upper surface of the bulk, despite the fact that all grains have the same material density. A two-dimensional molecular dynamics study of the phenomenon is described in this paper. The grains are represented by inelastic disks and the flow occurs down an inclined slope with a rough base. The simulations readily reproduce the segregation effect; the results of a series of measurements of various aspects of this so-called ''inverse grading'' are reported.
Molecular dynamics methods have been used in a quantitative study of the growth and decay of Taylor vortices in a fluid confined between concentric cylinders when the rotation of the inner cylinder is instantaneously started or stopped. Analysis of the temporal evolution of the vortex flow fields shows that the behavior of this microscopic system agrees with experiment. In order to make the analysis entirely self-contained, torque measurements have been used to determine the effective viscosity of the fluid.
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