Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session FM: Granular Flows III |
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Chair: Jennifer Galanis, National Institutes of Health Room: Salt Palace Convention Center 251 A |
Monday, November 19, 2007 8:00AM - 8:13AM |
FM.00001: Amplitude equation for under water sand-ripples in one dimension. Teis Schnipper, Keith Mertens, Clive Ellegaard, Tomas Bohr Sand-ripples under oscillatory water flow form periodic patterns with wave lengths primarily controlled by the amplitude $d$ of the water motion. We present an amplitude equation for sand-ripples in one spatial dimension which captures the formation of the ripples as well as secondary bifurcations observed when the amplitude $d$ is suddenly varied. The equation has the form \[ h_t=- \epsilon(h-\bar{h})+\big((h_x)^2-1\big)h_{xx}- h_{xxxx}+ \delta((h_x)^2)_{xx} \] which, due to the first term, is neither completely local (it has long-range coupling through the average height $\bar{h}$) nor has local sand conservation. We discuss why this is reasonable and how this term (with $\epsilon \sim d^{-2}$) stops the coarsening process at a finite wavelength proportional to $d$. We compare our numerical results with experimental observations in a narrow channel. [Preview Abstract] |
Monday, November 19, 2007 8:13AM - 8:26AM |
FM.00002: Secondary bifurcations of under water sand-ripples under oscillatory flow in narrow channels. Tomas Bohr, Teis Schnipper, Keith Mertens, Clive Ellegaard Sand-ripples under oscillatory water flow form periodic patterns with wave lengths primarily controlled by the amplitude $d$ of the water motion. When $d$ is suddenly varied the sand-ripples undergo characteristic secondary bifurcations, which we study experimentally and compare to our proposed amplitude equation (previous lecture). In particular we focus on the so-called doubling transition where, initially, a new ripple is formed in each trough, and show that this transition is well reproduced theoretically for sufficiently large $\delta$ (asymmetry between trough and crest). We finally present experimental results showing that long range coupling is seen to a surprising degree in the initial details of the doubling transition: initially {\em two} new ripples form in every trough, but quickly either the left or the right one wins. And this choice is made collectively for the whole system. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:39AM |
FM.00003: Dynamics of underwater sand ripples in pulsed flow Joachim Kruithof, Jose Eduardo Wesfreid Using a high-definition experimental apparatus, we have obtained results about sand ripples formed by a pulsed flow. We have found a decompaction process linked with the formation of vortex ripples. We have also studied the dynamical properties of the system, such as front propagation speed and global drift speed of the vortex ripples. Finally, we have tested the validity of the Sleath criterion {\$}h/$\backslash $lambda = 0.1{\$} in order to discriminate rolling grain ripples from vortex ripples. [Preview Abstract] |
Monday, November 19, 2007 8:39AM - 8:52AM |
FM.00004: Power Consumption And Granular Flow In A Vibro-Fluidized Stirred Granular Bed James Gilchrist, Kenneth Ford, Hugo Caram This work examines the behavior of a deep granular bed subject to simultaneous vertical vibration and stirring. The power necessary to stir a 5.6 cm diameter and 20 cm deep granular bed of 150 micron glass beads using a 4-blade vane impeller was measured over a wide range of forcing conditions. Impeller rotation rates from 0 - 1000 rpms and vibration accelerations in the range 0 $\le \quad \Gamma =\omega ^{2}$a/g $\le $ 4.0 were explored. Both the power required for stirring and vibration were recorded. Sharp changes in vane power draw indicate flow transitions from dense granular flow to vibro-fluidized flow at a critical acceleration ($\Gamma _{c})$. The total power profile (vane plus vibration power) demonstrates a minimum just above the critical acceleration. Progressive increases or decreases in both vane speed and $\Gamma $ independently show hysteresis as the flow bifurcates between two primary states of dense granular flow and loose-packed, vibrofluidized behavior. Finally, preliminary results of direct density measurements using a localized capacity probe will be discussed. These observations are compared to those found in fluidized systems and flows generated in high-shear granulators. [Preview Abstract] |
Monday, November 19, 2007 8:52AM - 9:05AM |
FM.00005: Fluidization in a centrifugal field generated by the tangential injection of the fluidization gas in a static fluidization chamber Juray De Wilde, Guy B. Marin, Axel de Broqueville The new concept of fluidization in a centrifugal field generated by the tangential injection of the fluidization gas in a static fluidization chamber is experimentally investigated. The tangential injection of the fluidization gas via multiple gas inlet slots in the outer cylindrical wall of the fluidization chamber fluidizes the solids tangentially and induces a rotating motion of the particle bed, the solids experiencing a radially outwards centrifugal force. A radially inwards gas-solid drag force is introduced by positioning a chimney centrally inside the fluidization chamber. As such, the gas is forced to move radially inwards, fluidizing the solids radially. Both the centrifugal force and the radial gas-solid drag force being influenced by the fluidization gas flow rate, the latter is observed to have only a limited effect on the radial bed expansion which is mainly determined by the fluidization chamber design and the type of particles being fluidized. This offers important flexibility with respect to cooling or heating. The centrifugal force can be a multiple of gravity, allowing dense operation at very high gas velocities and, as a result, radial gas-solid slip velocities at least one order of magnitude larger than in conventional fluidized beds. The latter drastically improves the gas-particle heat and mass transfer. [Preview Abstract] |
Monday, November 19, 2007 9:05AM - 9:18AM |
FM.00006: Confinement and symmetry breaking in vibrofluidized granular rods Jennifer Galanis, Wolfgang Losert We study the behavior of vertically vibrated rod-shaped granular materials with an aspect ratio (length-to-diameter ratio) of 40 to 80 confined to a quasi-two-dimensional horizontal plane. The containers are elliptical, with a minor-to-major axis ratio of 0.7 to 1.0. Similar to previously published results, we find that an isotropic to nematic-like transition occurs that is dependent on rod length and density. The shape of the container also influences this phase transition. The critical point of the phase transition decreases as eccentricity increases. Also, the time required to transition from an isotropic to nematic-like state shortens, and the nematic-like state orients along the major axis. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:31AM |
FM.00007: A Granular Smoluchowski-Feynman Ratchet Devaraj van der Meer, Peter Eshuis, Ko van der Weele, Detlef Lohse We experimentally study the Brownian motion of a mill with four vanes that is allowed to rotate freely in a vibrofluidized granular gas. At mild driving, when the granular temperature of the gas is low, the mill is found to move around a distinct equilibrium position. If we increase the driving strength, the mill starts to move between equivalent equilibrium states and explores more and more of its phase space by revolving faster. At very high driving, the mill interacts back with the granular heat bath in which it is submersed. By breaking the spatial symmetry, the mill is turned into a ratchet of the Smoluchowski-Feynman type and starts moving with a finite, non-zero average rotation speed. All these phenomena are analyzed in terms of a diffusion coefficient and the complete probability distribution functions of both position and speed. [Preview Abstract] |
Monday, November 19, 2007 9:31AM - 9:44AM |
FM.00008: Second-order transport due to fluctuations in clustering particle systems Madhusudan Pai, Shankar Subramaniam Particle systems that exhibit clustering are characterized by fluctuations in particle number. We perform molecular dynamics simulations of a freely-cooling granular gas using a hard- sphere event-driven algorithm in order to investigate clustering. We show that the mean number density in the freely- cooling gas remains uniform deep into the clustering regime, although the system exhibits significant clustering in each realization. We further show that fluctuations in particle number are correctly characterized by the second-order density. An important quantity that governs the evolution of the second-order density is the relative velocity between particle pairs. A simple scaling behavior of the relative velocity with restitution coefficient is shown to hold. This second-order analysis can be extended to study clustering in other particle systems, such as nanoparticle aggregation. [Preview Abstract] |
Monday, November 19, 2007 9:44AM - 9:57AM |
FM.00009: Base unidirectional steady flows in Navier-Stokes granulodynamics Francisco Vega Reyes, Jeffrey S. Urbach We present the set of steady unidirectional flows that appear, at low density and constant pressure, in a granular gas at Navier-Stokes order. Two basic energy inputs from the boundaries are considered: temperature and shear sources. We find new steady states not previously reported. Explicit expressions of the hydrodynamic profiles for all the existing steady unidirectional flows are for the first time provided. We determine the natural reduced (microscopic over macroscopic) length scales of the problem. This allows to quantitatively determine the range of validity of the Navier-Stokes hydrodynamics. We describe the properties of the base unidirectional flows, focusing in the not previously reported states. [Preview Abstract] |
Monday, November 19, 2007 9:57AM - 10:10AM |
FM.00010: ABSTRACT WITHDRAWN |
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