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 M32: Granular Flows IV |
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Chair: Elisabeth Guazzelli, Aix Marseille University Room: 33C |
Tuesday, November 20, 2012 8:00AM - 8:13AM |
M32.00001: What is the granular response to a high-speed impact? Abe Clark, Lou Kondic, R.P. Behringer Although many studies of impact on a granular material exist, the connections between the local granular response, the microscopic processes which dissipate kinetic energy, and the intruder dynamics are unclear, largely due to experimental difficulties in obtaining very fast data at the grain scale. We use high-speed imaging (40 kHz) of an intruder striking a quasi-2D system of photoelastic disks, yielding both the intruder dynamics and the force response of individual grains. The frame rates are fast enough to resolve rich acoustic activity on the particle scale. For long time scales, the intruder dynamics are consistent with previously used empirical force laws. However, for short time scales, we observe very large fluctuations in the deceleration, which we connect to the intermittent acoustic activity beneath the intruder as it moves. We show that these intense, intermittent acoustic pulses, which travel much faster than the intruder along networks of grains, are the primary microscopic mechanism of energy loss. These pulses carry energy away into the medium, and they decay roughly exponentially with distance. We examine the statistics of these fluctuations in order to better understand their origin and behavior. [Preview Abstract] |
Tuesday, November 20, 2012 8:13AM - 8:26AM |
M32.00002: Particles impacting on a granular bed John Hinch An asymptotic analysis is made to find the penetration depth and the stopping time for a particle impacting a granular bed. Newton's equation is solved with a drag force with two terms, one term proportional to the square of the velocity and one term linear in the depth. The penetration depth is found to increase with the logarithm of the impact velocity, while the stopping time is found to decrease with the inverse of the square root of the logarithm of the impact velocity. [Preview Abstract] |
Tuesday, November 20, 2012 8:26AM - 8:39AM |
M32.00003: Flow-mediated coupling on projectiles falling in a superlight granular medium Gabriel A. Caballero-Robledo, Juan M. Solano-Altamirano, Vincent Kamphorst, Felipe Pacheco-V\'azquez, J.C. Ruiz-Su\'arez Interesting collective motion emerges when several heavy disks fall in a quasi 2D granular bed of extremely light grains [F. Pacheco-V\'azquez and J.C. Ruiz-Su\'arez, \textit{Nat. Comms.} \textbf{1}, 123 (2010)]. In particular, when two disks impact side by side they initially repel, then they attract each other, until they finally stop. We perform experiments and Discrete Element Soft-Particle simulations to determine the range of action and the origin of these attractive and repulsive flow-mediated forces. Our findings suggest that repulsion results from jamming of grains between intruders while attraction would be due to a ``granular pressure'' drop in the region between intruders caused by a high flow velocity of grains: a Bernoulli-like effect. [Preview Abstract] |
Tuesday, November 20, 2012 8:39AM - 8:52AM |
M32.00004: Force measurements after granular impact using instrumented spheres Sylvain Joubaud, Tess Homan, Yoann Gasteuil, Detlef Lohse, Devaraj van der Meer Impacts of solid spheres on soft and dry sand may result in the rapid sinking of the sphere into the sand. This in turn can lead to a jet shooting up from the surface of the sand. The dynamics of the sphere is affected by the impact velocity and the ambient air pressure. In this work, we performed direct measurement of the acceleration using instrumented spheres from {\bf {\it Smart}INST}. These spheres non-invasively and continuously transmit the force acting on themselves as they penetrate into the sand using a radio signal, {\em i.e.}, without disturbing the behavior of the granular medium which any invasive measurement technique would do. The signal reveals very complex dynamics including a signature of the jet formation. These results are compared to various phenomenological models. [Preview Abstract] |
Tuesday, November 20, 2012 8:52AM - 9:05AM |
M32.00005: Fill-level symmetry and minimization of energy states in rotating tumblers with polygonal cross-sections Nicholas A. Pohlman, Daniel F. Paprocki, Jr., Yun Si Typically in rotating tumblers, constant rotation rates and circular cross-sections are used as they jointly produce a steady, uniform flowing layer at the free surface. On the other hand, experiments conducted in polygon-shaped tumblers produce unsteady conditions due to the rapidly changing flowing layer length. Results analyzing free surface properties indicate that the particle dynamics within the flowing layer attempt to minimize energy of the flowing system: The arithmetic difference between the angle of repose and the tumbler orientation has a functional relationship with the instantaneous flowing layer length in the form of a catenary. The peaks of the catenary are affected by the number of sides on the polygon cross-section as well as the symmetry around the critical 50\% fill fraction. Furthermore, oscillation of the flowing layer position appears to affect the free surface curvature. This result is likely due to the rapidly increasing and decreasing length of the free surface and the rotational inertia of particles entering the flowing layer. [Preview Abstract] |
Tuesday, November 20, 2012 9:05AM - 9:18AM |
M32.00006: Slow axial drift in three dimensional tumblers Zafir Zaman, Umberto D'Ortona, Paul Umbanhowar, Julio M. Ottino, Richard M. Lueptow We recently demonstrated the existence of coherent axial drift of monodisperse particles in partially filled spherical tumblers from DEM simulations and experiments. This motion occurs solely in the flowing layer: particles move gradually toward the pole at the top of the flowing layer and toward the equator at the bottom of the flowing layer. The drift is small relative to streamwise displacements, thus particles require many passes through the flowing layer to progress from the equator to the pole and back. Since axial drift is negligible in cylindrical containers except near the endwalls, this suggests that axial variation in tumbler diameter is required for axial drift. To understand how axial variations in flowing layer length, $L$, determines drift, we conducted new experiments and simulations of partially filled double cone tumblers of varying wall slope. Axial drift remains present in the conical geometry, and the drift speed increases with the equator diameter for fixed tumbler length. Results from both the double cone and the spherical tumbler reveal that the axial drift velocity depends on $L$ and the axial position. Funded by NSF Grant CMMI-1000469. [Preview Abstract] |
Tuesday, November 20, 2012 9:18AM - 9:31AM |
M32.00007: Competing segregating effects of gravity and shear rate gradients in dense granular flows in a drum: theory and simulations Kimberly Hill, Danielle Tan A well-known rule of thumb for sheared mixtures of different-sized (same density) particles is that larger particles tend to go up (toward the free surface), and the smaller particles, down, commonly referred to as the ``Brazil-nut problem'' or ``kinetic sieving.'' However, it has been recently shown that in a sheared granular mixture, larger particles may rise or fall relative to the small particles, or even rise only partway to some steady-state height in a sheared mixture. We present a theory that accounts for this complex behavior as a balance between gravity-driven segregation effects and, effectively, granular temperature gradients driven by the shear. Then, we test this theory using discrete element method simulations of different mixtures rotated in a partially-filled drum. Using the theory and simulations, we show that for all mixtures we test, the segregation fluxes are driven by the difference between the partitioning of kinetic and contact stresses among the species in the mixture subjected to a gravity-induced contact stress gradient. Specifically, all particles bear a fraction of the local contact stress equal to their local concentration in the mixture, but the smaller particles bear a higher fraction of the local kinetic stress (akin to granular temperature). This presents a new physical mechanism for kinetic sieving: even where the total granular temperature is small, the higher granular temperature of the smaller particles segregates them downward, in the direction of gravity, towards high stress and low temperature regions. [Preview Abstract] |
Tuesday, November 20, 2012 9:31AM - 9:44AM |
M32.00008: Investigation of the mobile granular layer in bed-load transport Elisabeth Guazzelli, Pascale Aussillous, Julien Chauchat, Mickael Pailha, Marc Medale The mobile layer of a granular bed composed of spherical particles is experimentally investigated in a laminar rectangular-channel flow. Both particle and fluid velocity profiles are obtained using particle image velocimetry for different index-matched combinations of particles and fluid. While the Shields number controls incipient motion, it is not the most appropriate parameter for describing bed-load transport. The experimental observations suggest that the appropriate length-scale is the fluid height and that the proper control parameter is the dimensionless fluid flow-rate. A two-phase continuum model having a frictional rheology to describe particle-particle interactions can capture most of the experimental observations. Rheological constitutive laws having increasing degree of sophistication are discussed. [Preview Abstract] |
Tuesday, November 20, 2012 9:44AM - 9:57AM |
M32.00009: Shear-Induced Diffusion in a Dense Frictional Disk Packing Joshua Dijksman, Jie Ren, Robert Behringer We study shear-induced diffusion in a dense disordered packings of frictional photoelastic disks. We induce diffusion by subjecting the packing to uniform oscillatory shear cycles. We can track both displacement and rotational motion, and measure interparticle forces obtained from the photoelastic response of the disks. The shear deformation is volume conserving, so each experiment corresponds to a well defined density. We then vary the density to probe its impact on diffusion; we also study the influence of the shear amplitude. Surprisingly, we find that both rotational and translational diffusion increases with density for all but the highest densities -- clearly steric hindrance only becomes relevant at the highest packing fractions. At the onset of mechanical stability, as indicated by an increase in the total pressure in the system, we also find profound changes in both the amplitude, time dependence and directionality of the particle diffusion. [Preview Abstract] |
Tuesday, November 20, 2012 9:57AM - 10:10AM |
M32.00010: Compression and shear in ultra low friction soft sphere packings: an experimental exploration Hu Zheng, Joshua Dijksman, Robert Behringer We study the quasi-static deformation of three dimensional sphere packings close to the onset of mechanical rigidity. We perform the experiments on slightly polydisperse, nearly frictionless soft hydrogel spheres in a tri-axial shear apparatus. We can access both system pressure and structural flow information by index matched scanning on the submersed, transparent hydrogel spheres. We address whether sheared frictionless spheres display dilatancy pressure, we measure the non-linear force response of a disordered packing under compression and explore the plastic rearrangements inside cyclically sheared and com pressed packings. [Preview Abstract] |
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