68 research outputs found
Slow axial drift in three-dimensional granular tumbler flow
Models of monodisperse particle flow in partially filled three-dimensional
tumblers often assume that flow along the axis of rotation is negligible. We
test this assumption, for spherical and double cone tumblers, using experiments
and discrete element method simulations. Cross sections through the particle
bed of a spherical tumbler show that, after a few rotations, a colored band of
particles initially perpendicular to the axis of rotation deforms: particles
near the surface drift toward the pole, while particles deeper in the flowing
layer drift toward the equator. Tracking of mm-sized surface particles in
tumblers with diameters of 8-14 cm shows particle axial displacements of one to
two particle diameters, corresponding to axial drift that is 1-3% of the
tumbler diameter, per pass through the flowing layer. The surface axial drift
in both double cone and spherical tumblers is zero at the equator, increases
moving away from the equator, and then decreases near the poles. Comparing
results for the two tumbler geometries shows that wall slope causes axial
drift, while drift speed increases with equatorial diameter. The dependence of
axial drift on axial position for each tumbler geometry is similar when both
are normalized by their respective maximum values
Designing non-segregating granular mixtures
In bidisperse particle mixtures varying in size or density alone, large
particles rise (driven by percolation) and heavy particles sink (driven by
buoyancy). When the two particle species differ from each other in both size
and density, the two segregation mechanisms either enhance (large/light and
small/heavy) or oppose (large/heavy and small/light) each other. In the latter
case, an equilibrium condition exists in which the two segregation mechanisms
balance and the particles no longer segregate. This leads to a methodology to
design non-segregating particle mixtures by specifying particle size ratio,
density ratio, and mixture concentration to achieve the equilibrium condition.
Using DEM simulations of quasi-2D bounded heap flow, we show that segregation
is significantly reduced for particle mixtures near the equilibrium condition.
In addition, the rise-sink transition for a range of particle size and density
ratios matches the combined size and density segregation model predictions
Wavelength Scaling and Square/Stripe and Grain Mobility Transitions in Vertically Oscillated Granular Layers
Laboratory experiments are conducted to examine granular wave patterns near
onset as a function of the container oscillation frequency f and amplitude A,
layer depth H, and grain diameter D. The primary transition from a flat grain
layer to standing waves occurs when the layer remains dilated after making
contact with the container. With a flat layer and increasing dimensionless peak
container acceleration G = 4 pi^2 f^2 A/g (g is the acceleration due to
gravity), the wave transition occurs for G=2.6, but with decreasing G the waves
persist to G=2.2. For 2.2<G<3.8, patterns are squares for f<f_ss and stripes
for f>f_ss; H determines the square/stripe transition frequency
f_ss=0.33(g/H)^0.5. The dispersion relations for layers with varying H collapse
onto the curve L/H=1.0+1.1[f(H/g)^0.5]^(-1.32 +/- 0.03) (L is the wavelength)
when the peak container velocity v exceeds a critical value v_gm of
approximately 3 (Dg)^0.5. Local collision pressure measurements suggest that
v_gm is associated with a transition in the horizontal grain mobility: for
v>v_gm, there is a hydrodynamic-like horizontal sloshing motion, while for
v<v_gm, the grains are essentially immobile and the stripe pattern apparently
arises from a bending of the granular layer. For f at v_gm less than f_ss and
v<v_gm, patterns are tenuous and disordered.Comment: 21 pages, 15 figures, submitted to Physica
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