Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q08: Granular Flows II |
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Chair: Richard Lueptow, Northwestern University Room: 212 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q08.00001: Granular flow model for stochastic additive manufacturing feedstock distributions William Rosenthal, Amanda Howard, Francesca Grogan Selective laser sintering (SLS) additive manufacturing processes distribute layers of irregular particles which are then fused by exposure to a laser. Physical models to predict microstructure variability, especially pore characteristics, are restricted by unrealistic assumptions of powder regularity and the deposition process. 2D and 3D models for SLS feedstock particle distributions are presented which take into account particle characterization, the mechanical deposition process, and inter-particle cohesive forces. These three factors are shown to have significant effects on sub-optimal packing fractions of feedstock material, which suggest assumptions of sphericity and optimal packing may lead to inaccurate results if used in sintering models. The impact of these assumptions is illustrated by using the granular feedstock model to initialize a phase-field SLS model. Sensitivity analysis of pore structure features shows the assumptions about the granular flow characteristics play a central role in the variability of the sintered microstructure. [Preview Abstract] |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q08.00002: Rayleigh-Taylor and Rayleigh-Benard like instabilities in two-size two-densities granular flow Umberto D'Ortona, Nathalie Thomas Rayleigh-Taylor and Rayleigh-Benard instabilities are among the most classical hydrodynamical instabilities. In both cases, the buoyancy is responsible for the instability. When put into motion, dry granular materials behave as liquids. The granular segregation induces a migration of large particules to the surface, while dense particles move to the bottom. Dense and large particles migrate to the top or the bottom depending on size and density ratios. Here, large and dense particles are chosen such that they migrate to the surface. We show experimentally and numerically that a layer of dense particles put above a layer of less dense particles develops a Rayleigh-Taylor instability while flowing. If dense particles are also larger and subject to upward segregation, the two-layer system destabilizes as well. Furthermore, if the system is initially made of one homogeneous layer, the segregation first induces the formation of a dense layer at the surface which destabilises later. This self-driven Rayleigh-Taylor instability occurs for various density and size ratios. While time evolves, a pattern of parallel stripes forms with convection cells analogous to Rayleigh-Benard cells. The motor of the instability is the granular segregation. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q08.00003: Interplay between hysteresis and nonlocality in granular flows Saviz Mowlavi, Ken Kamrin The jamming transition in granular materials is well-known to exhibit hysteresis, wherein the level of shear stress required to trigger flow is larger than that below which flow stops. From a rheological standpoint, such behavior is typically modeled as a nonmonotonic flow rule. However, the rheology of granular materials is also nonlocal due to cooperativity at the grain scale, leading to increased strengthening of the flow threshold as system size is reduced. We investigate how these two effects -- hysteresis and nonlocality -- couple with each other by incorporating nonmonotonicity of the flow rule into the nonlocal granular fluidity (NGF) model, a nonlocal continuum model for granular flows. Comparing predictions of the model with discrete element simulations in the case of planar shear flow with gravity, we show that the inclusion of nonlocal effects is key to explaining certain features of the hysteretic solid-liquid transition as the applied stress is ramped up and down. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q08.00004: Transitional Granular Packing: Rate-dependent Brittleness Cheng-En Tsai, JC Tsai We discover a route of transition over driving rate that bridges two classic regimes of granular dynamics: fluid-lubricated suspension on the fast end, against the largely plastic regime at the slow limit. Here, densely packed centimeter-sized PDMS particles submerged in fluid are sheared at variable but strictly constant rates. Fluctuations on multiple components of boundary force reveal a transitional regime exhibiting brittle failure of the packing at the intermediate driving rates, accompanied by evidence from simultaneous internal imaging. Rate-dependent statistical distribution of avalanches reveals the development of ductility toward the slow limit. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q08.00005: An investigation of mixing ratio effects on a Couette cell granular flow using magnetic particle tracking method Xingtian Tao, Huixuan Wu Optical based methods are abundant in flow measurements; however, they can hardly be used in opaque environment. Therefore, a non-optical based particle tracking method --- Magnetic Particle Tracking (MPT) ---is investigated and three algorithms of data processing are analyzed. These algorithms are sequential quadratic program, extended Kalman filter (EKF), and particle filter. The reconstructed position and orientation of the magnetic tracer is compared with high-speed camera image result, and the accuracy of MPT with EKF algorithm is in the order of 0.6{\%} in position and 1.5 degree in orientation. This technique is applied to study a sheared dense granular mixture in a Couette cell. The mixture comprises of spheres and cylinders (aspect ratio equals to 1). The trajectory and angle alignment of the tracer particle is reconstructed, and its distribution in the Couette cell is depicted. The effect of the mixing ratio on this Couette cell system is characterized by using finite time Lyapunov exponent between neighboring trajectories. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q08.00006: Segregation force in granular flows: From single intruders to bidisperse mixtures Richard M. Lueptow, Yifei Duan, Lu Jing, Julio M. Ottino, Paul B. Umbanhowar Recent studies have focused on the size segregation force on a single large intruder particle in granular flows. However, a generalized scaling of the force is still lacking for combined size and density segregation as well as for mixtures (rather than a single intruder). Here we first measure the segregation force on a single intruder in DEM simulations using a spring-based force meter and provide a universal scaling law of the segregation force that predicts whether the intruder will rise or sink depending only on the size and density ratios. Interestingly, the scaled force does not increase monotonically but decreases at large size ratios, explaining experimental observations that very large intruders sink. Then we extend the measurement to bidisperse particle mixtures of varying concentration. The resulting scaling law enables prediction of the segregation direction (rise or sink) of each particle species for varying size ratio, density ratio, and species concentration. Surprisingly, the segregation may invert as the species concentration changes because the segregation force depends on the species concentration. The predictions are validated with DEM simulations and experiments in various flow configurations. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q08.00007: Modeling segregation pattern formation in biaxial spherical tumbler flow Mengqi Yu, Paul Umbanhowar, Julio Ottino, Richard Lueptow Flow of size bidisperse granular particle mixtures in a half-full spherical tumbler rotating about two perpendicular axes exhibits segregation patterns that can be observed through the transparent tumbler wall. The patterns resemble predictions based on dynamical systems analysis including non-mixing structures and unstable manifolds, but also depend on the underlying flow field, relative strength of segregation, and collisional diffusion. Discrete element method (DEM) simulations enable precise characterization of the three-dimensional structures of the segregation pattern and statistical analysis of particle movement in the flow. Axial drift of large particles during rotation about a single axis results in migration toward double bands near the rotation poles. At the same time, chaotic advection redistributes large particles in regions outside of non-mixing structures. As a result of both mechanisms, large particles accumulate in regions where axial bands coincide with non-mixing structures. Comparison of particle trajectories in size bidisperse and monodisperse mixtures provides further insight into the interaction between segregation, diffusion, and the underlying flowing field that results in pattern formation. [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q08.00008: Binary Solid-Liquid Fluidized Beds in Very Narrow Tubes Erick Franklin, Fernando Cunez Solid-liquid fluidized beds (SLFB) are commonly found in industry, where usually different grains coexist. In polydisperse cases, segregation occurs and the layer inversion phenomenon, defined as the inversion of already segregated regions, may happen under certain conditions. In addition, if the bed is narrow (bed thickness to grain diameter of the order of 10), segregation and layer inversion are highly affected by wall effects and plug formation. In this study, we investigate the segregation of grains and mimick the layer inversion phenomenon in binary SLFB in very narrow tubes. In our setup, the tube to grain diameters were between 4 and 6, and we placed the lighter grains under the heavier ones in order to force the layer inversion. We found that the characteristic time for inversion is $t_c$/20, where $t_c$ is a proposed time scale. We found also that the average distance traveled by individual grains from the beginning to the end of inversion is within 5 to 8 $h_{mf}$, where $h_{mf}$ is the initial height of the bed. [Preview Abstract] |
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