Stephen L. Conway

A Taylor vortex analogy in granular flows

Fluids sheared between concentric rotating cylinders undergo a series of three-dimensional instabilities. Since Taylors archetypal 1923 study, these have proved pivotal to understanding how fluid flows become unstable and eventually undergo transitions to chaotic or turbulent states. In contrast, predicting the dynamics of granular systems—from nano-sized particles to debris flows—is far less reliable. Under shear these materials resemble fluids, but solid-like responses, non-equilibrium structures and segregation patterns develop unexpectedly. As a result, the analysis of geophysical events and the performance of largely empirical particle technologies might suffer. Here, using gas fluidization to overcome jamming, we show experimentally that granular materials develop vortices consistent with the primary Taylor instability in fluids. However, the vortices observed in our fluidized granular bed are unlike those in fluids in that they are accompanied by novel mixing–segregation transitions. The vortices seem to alleviate increased strain by spawning new vortices, directly modifying the scale of kinetic interactions. Our observations provide insights into the mechanisms of shear transmission by particles and their consequent convective mixing.

Density waves and coherent structures in granular Couette flows

Density inhomogeneities in granular flows can dramatically influence microscopic and macroscopic properties. Here, we numerically examine dilute rapid granular flows in the Couette geometry via large-scale particle-dynamic simulations, and characterize development of nonuniform particle distributions. For monodisperse grains we observe density waves in two- and three-dimensional computational domains of varying aspect ratios. Both fully developed and transient states are quantified using Fourier methods. For inelastic, planar (two-dimensional) flows exceeding a minimum solids fraction, one-dimensional, high-density clusters—well-known features of inelastic materials—align parallel to the walls. Above a critical streamwise length, these are destabilized by two-dimensional antisymmetric modes with wavelength ∼100 particle diameters. We relate oscillatory behavior to an underlying physical mechanism of the slow drift of clusters towards walls and their subsequent bursting. Further streamwise or spanwise expa...

Density waves in gravity-driven granular flow through a channel

A molecular-dynamic computer simulation is used to examine rapid granular flow in a vertical channel. A two-dimensional event driven algorithm is used with periodic boundary conditions in the flow direction and solid walls in the lateral direction. Flow in the channel leads to an inhomogeneous distribution of the particles and two distinct types of density waves are identified: An S-shaped wave and a clump. The density waves are further characterized by quantifying their temporal evolution using Fourier methods and examining local and global flow properties of the system, including velocities, mass fluxes, granular temperatures, and stresses. A parametric study is used to characterize the effect of the system parameters on the density waves. In particular we are able to show that the dynamics of large systems are often qualitatively and quantitatively different from those of small systems. Finally, the types of density waves and dominant Fourier modes observed in our work are compared to those that are predicted using a linear stability analysis of equations of motion for rapid granular flow.