Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session L32: Granular Flows III |
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Chair: Meheboob Alam, JNCASR Room: 33C |
Monday, November 19, 2012 3:35PM - 3:48PM |
L32.00001: ABSTRACT WITHDRAWN |
Monday, November 19, 2012 3:48PM - 4:01PM |
L32.00002: Rheology and segregation of a heterogeneous cohesive granular material Pierre Jop, Charles Voivret, Emmanuelle Gouillart Heterogeneous and cohesive granular flows are investigated by contact dynamic simulations. A given fraction of the spherical grains is cohesive and experience an attractive force at contact with other grains. We study first the rheology of such mixture in a shear plane geometry: we show that a simple law accounts for the influence of the cohesive fraction on the effective friction coefficient and provide some links to the microscopic structure. Then the segregation occurring when flowing under gravity on an inclined plane is investigated. We show that the segregation rate and its intensity depend on the cohesive force and on the amount of cohesive particles. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L32.00003: Rearrangements and Rheology in Soft Glassy 2D Material Nathan Keim, Paulo Arratia We report on simultaneous measurements of shear rheology and microstructure of athermal disordered monolayers of particles ($\sim$6 $\mu$m) at an oil-water interface, using an oscillatory magnetic-rod interfacial rheometer. Particle tracking is used to examine the population of dissipative plastic rearrangements that govern mechanical response, and that occur even at small strain. We find that the elastic modulus ($G'$) is approximately 10 times larger than the viscous modulus ($G''$) at small strains for these soft glassy materials. We describe the statistics of these rearrangements, the growth of viscous dissipation and irreversibility as strain amplitude is increased, and changes in behavior as the system is sheared repeatedly. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L32.00004: Dynamical Slowing Down for Sheared Granular Materials Somayeh Farhadi, Robert P. Behringer We have performed Couette shear experiments on both circular and elliptical shaped particles below the isotropic jamming point. The dynamics of the system was studied for density regions of $0.85 \leq \phi \leq 0.87$ in ellipses, and $0.80 \leq \phi \leq 0.83$ in disks. In a very small density region, both systems of ellipses and disks evolve slowly in response to continious shear. In particular, we observe that by starting from an essentially unstressed state and applyinmg shear strain, the average displacements of the particles initially grow rapidly, and then slowly decrease for very large strains. In a similar set of experiments performed on disks, slow relaxation was observed as well. However, we observed fundamental differences between the evolution of systems of disks and ellipses. We characterize this slow dynamics by measuring the evolution of velocity profile, density, and orientational order in the course of experiment. Our data suggests that the slow relaxation in ellipses is associated with the small and slow changes in the orientation of particles, which then allow a more efficient packing. [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L32.00005: Gradient and Vorticity Banding Phenomena in a Sheared Granular Fluid Meheboob Alam, Priyanka Shukla In many complex fluids, including granular systems, the homogeneous shear flow breaks into alternate regions of low and high shear rates (i.e., shear localization), respectively, when the applied shear rate exceeds a critical shear rate and this is known as gradient banding. On the other hand, if the applied shear stress exceeds a critical value, the homogeneous flow separates into bands of different shear stresses (having the same shear rate) along the vorticity (spanwise) direction, leading to ``stress localization.'' Here we outline a Landau-type nonlinear order-parameter theory for both gradient and vorticity banding phenomena in a sheared granular fluid. Our analysis holds for any general constitutive model, but the specific results will be presented for a kinetic-theory constitutive model that holds for rapid granular flows. Our theory predicts that while the vorticity banding [1] can occur via supercritical/subcritical pitchfork and subcritical Hopf bifurcations in dilute and dense flows, respectively, the gradient banding [2] occurs only via pitchfork bifurcations, both resulting in inhomogeneous states.\\[4pt] [1] P. Shukla and M. Alam, (2012, submitted).\\[0pt] [2] J.~Fluid Mech. {\bf 666}, 203 (2011); Phys.~Rev.~Lett. {\bf 100}, 068001 (2009). [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L32.00006: Vibrations and stress relaxation in two-dimensional granular solids Thibault Bertrand, Christopher MacMinn, Corey S. O'Hern, John Wettlaufer, Mark D. Shattuck, Eric Dufresne Wave propagation in granular solids remains poorly understood. The highly non-harmonic vibrational behavior of these systems can lead to events such as large scale rearrangements, fracture, and aging. Extensive studies have been performed to understand the role of non-harmonicity in the melting process and strain weakening of crystalline solids; here, we try to extend these concepts to particulate solids undergoing mechanical excitations that can show stress release through avalanches. We use experiments and numerical simulations to study the onset of non-harmonicity in two-dimensional systems composed of soft particles undergoing uniaxial mechanical excitation. This model system allows us to follow particle trajectories and overall compaction of the packing under uniaxial compression and vibration to disentangle the relation between the loss of harmonicity and the weakening of these 2D granular solids. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L32.00007: Reynolds Pressure and Relaxation in a Homogeneous Sheared Granular System Jie Ren, Joshua Dijksman, Robert Behringer We describe experiments on the constitutive behavior of a system composed of a disordered collection of frictional disks. We use a novel shear apparatus that avoids the formation of inhomogeneities known as shear bands. We probe the evolution of shear jammed states, occurring for packing fractions $\phi_S \leq \phi \leq \phi_J$, where above $\phi_J$ there are no stress-free static states, and below $\phi_S$, all static states are stress-free. Our linearly sheared, fixed $\phi$ system exhibits coupling between the shear strain, $\gamma$, and the pressure, $P$, which we characterize by the ``Reynolds pressure,'' and a ``Reynolds coefficient,'' $R(\phi) = (\partial ^2 P/\partial \gamma ^2)/2$. $R$ depends only on $\phi$, and diverges as $R \sim (\phi_c - \phi)^{\alpha}$, where $\phi_c \simeq \phi_J$, and $\alpha \simeq -3.3$. Moreover, by using asymmetric strain cycles, we find that the observed constitutive relations are limit cycles that are approached logarithmically slowly under cyclic shear. We characterize the relaxation in terms of the pressure asymmetry at cycle $n$: $\Delta P \simeq -\beta \ln(n/n_0)$. $\beta$ depends only on the shear cycle amplitude, suggesting an activated process where $\beta$ plays a temperature-like role. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L32.00008: Ratcheting of Granular Polymer with a Spatial Gradient of Excitation Y.-C. Lin, C.-C. Chang, J.-R. Huang, W.-T. Juan, J.-C. Tsai We study the migration of a short granular chain in response to a sinusoidal vibration whose intensity varies linearly with position. The spatial asymmetry induces ratcheting of the chain that can go, counter-intuitively, either in the direction of lowering the vibration or in favour of its increase, depending upon the position. This spatial divide signals a transition of granular dynamics involving finite-amplitude instabilities. We also demonstrate the roles of cooperative movements both by time-resolving the 3D motion of this macroscopic polymer, and by measuring the persistence and magnitude of its migration that go far above simple predictions based on spatially biased random kicks at the theoretical upper limit. [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L32.00009: Jamming of quasi-2D emulsion droplets: Analogies with granular jamming Eric R. Weeks, Kenneth W. Desmond, Pearl J. Young, Dandan Chen We experimentally study the jamming of quasi-two-dimensional emulsions. Our experiments consist of oil-in-water emulsion droplets confined between two parallel plates. These are somewhat analogous to granular photoelastic disks, although they are softer and do not experience static friction. From the droplet outlines, we can determine the forces between every droplet pair to within 8\% over a wide range of area fractions. Using the data, we observe critical scaling behaviors of the contact numbers and pressure as the jamming transition is approached from above. The scaling behavior agrees well with simulations and is similar to what has been seen previously with photoelastic disks. [Preview Abstract] |
Monday, November 19, 2012 5:32PM - 5:45PM |
L32.00010: Patterns, Segregation and Hysteresis in Vertically Vibrated Granular Mixtures Istafaul Ansari, Meheboob Alam Granular materials under vertical shaking exhibit a variety of interesting phenomena: undulations, surface instabilities, oscilons, ripples, Leidenfrost state and convection. We have investigated these phenomena by conducting experiments on two types of equimolar {\it binary} granular mixtures: (i) glass and steel balls both having diameters of $d=1.0\;mm$ and a density ratio of $\rho_s/\rho_g= 3.0$ and (ii) the delrin and steel balls both having diameters of $1.0\;mm$ and a density ratio of $\rho_s/\rho_d= 5.5$. The particles are held in a quasi-two-dimensional Perspex container which is vibrated harmonically in vertical direction using an electromagnetic shaker. All the experiments are done by increasing the shaking intensity (measured in terms of dimensionless shaking acceleration $\Gamma$) while keeping the shaking amplitude $A/d$ fixed. We uncovered many unhitherto reported novel patterns: (i) the Leidenfrost state coexisting with a granular gas, (ii) horizontal segregation within a Leidenfrost state, (iii) granular convection with a floating particle cloud, and (iv) both vertical and/or horizontal segregation with other patterned states. We further show that the transition from the Leidenfrost state to convection in a binary mixture occurs via a hysteretic transition. [Preview Abstract] |
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