CiteULike: dchen's Kudrolli

Lubrication effects on the flow of wet granular materials

Qing Xu — 2008-01-23T19:06:56-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

We investigate the dynamics of a partially saturated grain-liquid mixture with a rotating drum apparatus. The drum is partially filled with the mixture and then rotated about its horizontal axis. We focus on the continuous avalanching regime and measure the impact of the volume fraction and viscosity of the liquid on the dynamic surface angle. The inclination angle of the surface is observed to increase sharply to a peak and then decrease as a function of liquid volume fraction. The height of the peak is observed to increase with rotation rate. For higher liquid volume fractions, the inclination angle of the surface can decrease with viscosity before increasing. The viscosity where the minimum occurs decreases with the rotation rate of the drum. Limited measurements of the flow depth were made, and these were observed to show only fractional changes with volume fraction and rotation speeds. We show that the qualitative features of our observations can be understood by analyzing the effect of lubrication forces on the time scale over which particles come in contact.

Velocity correlations in dense granular gases

Daniel Blair — 2008-05-02T21:34:09-00:00

Physical Review E, Vol. 64, No. 5. (24 October 2001), 050301.

Diffusion and Mixing in Gravity-Driven Dense Granular Flows

Jaehyuk Choi — 2008-01-23T19:08:39-00:00

Physical Review Letters, Vol. 92, No. 17. (27 April 2004), 174301.

We study the transport properties of particles draining from a silo using imaging and direct particle tracking. The particle displacements show a universal transition from superdiffusion to normal diffusion; as a function of the distance fallen; independent of the flow speed. In the superdiffusive (but sub-ballistic) regime; which occurs before a particle falls through its diameter; the displacements have fat-tailed and anisotropic distributions. In the diffusive regime; we observe very slow cage breaking and Péclet numbers of order 100; contrary to the only previous microscopic model (based on diffusing voids). Overall; our experiments show that diffusion and mixing are dominated by geometry; consistent with long-lasting contacts but not thermal collisions; as in normal fluids.

Velocity profile of granular flows inside silos and hoppers

Jaehyuk Choi — 2008-03-24T22:32:04-00:00

Journal of Physics: Condensed Matter, Vol. 17, No. 24. (2005), pp. S2533-S2548.

We measure the flow of granular materials inside a quasi-two-dimensional silo as it drains and compare the data with some existing models. The particles inside the silo are imaged and tracked with unprecedented resolution in both space and time to obtain their velocity and diffusion properties. The data obtained by varying the orifice width and the hopper angle allow us to thoroughly test models of gravity driven flows inside these geometries. All of our measured velocity profiles are smooth and free of the shock-like discontinuities ('rupture zones') predicted by critical state soil mechanics. On the other hand, we find that the simple kinematic model accurately captures the mean velocity profile near the orifice, although it fails to describe the rapid transition to plug flow far away from the orifice. The measured diffusion length b, the only free parameter in the model, is not constant as usually assumed, but increases with both the height above the orifice and the angle of the hopper. We discuss improvements to the model to account for the differences. From our data, we also directly measure the diffusion of the particles and find it to be significantly less than predicted by the void model, which provides the classical microscopic derivation of the kinematic model in terms of diffusing voids in the packing. However, the experimental data are consistent with the recently proposed spot model, based on a simple mechanism for cooperative diffusion. Finally, we discuss the flow rate as a function of the orifice width and hopper angles. We find that the flow rate scales with the orifice size to the power of 1.5, consistent with dimensional analysis. Interestingly, the flow rate increases when the funnel angle is increased.

Swarming and Swirling in Self-Propelled Polar Granular Rods

Arshad Kudrolli — 2008-02-26T22:07:35-00:00

Physical Review Letters, Vol. 100, No. 5. (2008)

Using experiments with anisotropic vibrated rods and quasi-2D numerical simulations, we show that shape plays an important role in the collective dynamics of self-propelled (SP) particles. We demonstrate that SP rods exhibit local ordering, aggregation at the side walls, and clustering absent in round SP particles. Furthermore, we find that at sufficiently strong excitation SP rods engage in a persistent swirling motion in which the velocity is strongly correlated with particle orientation.