Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D30: Granular Flows: Computation and Modeling |
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Chair: Joe Goddard, University of California, San Diego Room: F151 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D30.00001: Cahn-Hilliard Regularization of the "mu(I)" Rheology Joe Goddard, Jaesung Lee Recently Barker et al. [J. Fluid Mech. 779 (2015) 794-818] have shown that the popular $\mu(I)$ model for the viscoplasticity of granular media is ill-posed, exhibiting short wave-length instabilities of the Hadamard variety. As one possible regularization of the model, we employ the dissipative analog of the classical Cahn-Hilliard (CH) model, with dissipation potential given by: $\psi(\nabla{\bf v}, \nabla\nabla{\bf v}) = \psi_0({\bf D}) + k||\nabla\nabla{\bf v}||^2$, with ${\bf D} = \rm{Sym}(\nabla{\bf v})$ and $ k>0$, with stress for the standard $\mu(I)$ model given by $\partial\psi_0/\partial{\bf D}$, and with hyperstress given by $\partial\psi/\partial\nabla\nabla{\bf v}$. Following the linear-stability analysis of Barker et al. of the momentum balance and continuity equation, we obtain a modification of their dispersion relation giving growth rate in terms of spatial wave number. It is found that the higher-gradient terms in the CH model lead to a large wave number cut-off of the instability, so that the model provides a possibly useful regularization of the $\mu (I)$ model. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D30.00002: Embryo as an active granular fluid: stress-coordinated cellular constriction chains Michael Holcomb, Guo-Jie Gao, Jeffrey Thomas, Jerzy Blawzdziewicz Mechanical stress plays an intricate role in gene expression in individual cells and sculpting of developing tissues. Motivated by our observation of the cellular constriction chains (CCCs) during the initial phase of ventral furrow formation in the \textit{Drosophila melanogaster} embryo, we propose an active granular fluid (AGF) model that provides valuable insights into cellular coordination in the apical constriction process. In our model, cells are treated as circular particles connected by a predefined force network, and they undergo a random constriction process in which the particle constriction probability $P$ is a function of the stress exerted on the particle by its neighbors. We find that when $P$ favors tensile stress, constricted particles tend to form chain-like structures. In contrast, constricted particles tend to form compact clusters when $P$ favors compression. A remarkable similarity of constricted-particle chains and CCCs observed \textit{in vivo} provides indirect evidence that tensile-stress feedback coordinates the apical constriction activity. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D30.00003: Theory for Indirect Conduction in Dense, Gas-Solid Systems Aaron Lattanzi, Christine Hrenya Heat transfer in dense gas-solid systems is dominated by conduction, and critical to the operation of rotary-kilns, catalytic cracking, and heat exchangers with solid particles as the heat transfer fluid. In particular, the indirect conduction occurring between two bodies separated by a thin layer of fluid can significantly impact the heat transfer within gas-solid systems. Current state-of-the-art models for indirect conduction assume that particles are surrounded by a static ``fluid lens'' and that one-dimensional conduction occurs through the fluid lens when the lens overlaps another body. However, attempts to evaluate the effect of surface roughness and fluid lens thickness (theoretical inputs) on indirect conduction have been restricted to static, single-particle cases. By contrast, here we quantify these effects for dynamic, multi-particle systems. This analysis is compared to outputs from computational fluid dynamics and discrete element method (CFD-DEM) simulations of heat transfer in a packed bed and flow down a heated ramp. Analytical predictions for model sensitivity are found to be in agreement with simulation results and differ greatly from the static, single-particle analysis. Namely, indirect conduction in static systems is found to be most sensitive to surface roughness, while dynamic systems are sensitive to the fluid lens thickness. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D30.00004: Discrete particle modelling of granular roll waves Jonathan Tsang, Stuart Dalziel, Nathalie Vriend A granular current flowing down an inclined chute or plane can undergo an instability that leads to the formation of surface waves, known as roll waves. Examples of roll waves are found in avalanches and debris flows in landslides, and in many industrial processes. Although related to the Kapitza instability of viscous fluid films, granular roll waves are not yet as well understood. Laboratory experiments typically measure the surface height and velocity of a current as functions of position and time, but they do not give insight into the processes below the surface: in particular, the possible formation of a boundary layer at the free surface as well as the base. To overcome this, we are running discrete particle model (DPM) simulations. Simulations are validated against our laboratory experiments, but they also allow us to examine a much larger range of parameters, such as material properties, chute geometry and particle size dispersity, than that which is possible in the lab. We shall present results from simulations in which we vary particle size and dispersity, and examine the implications on roll wave formation and propagation. Future work will include simulations in which the shape of the chute is varied, both cross-sectionally and in the downstream direction. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D30.00005: A nonlinear description of the viscosity and dilatancy of granular suspensions Davide Monsorno, Christos Varsakelis, Miltiadis Papalexandris In the first part of this talk we present a rheology law for granular suspensions based on the representation theorem of isotropic tensors. The proposed law has a number of desirable properties, namely, it is free of singularities, it vanishes at equilibrium, and it predicts non-zero bulk viscosity as well as shear-rate dependent normal viscous stresses. Next, we present an evolution equation for the volume fraction of the granular phase that can describe the dilatancy of granular suspensions in a consistent manner. Its derivation is based upon the introduction of the volume fraction and its gradient as internal degrees of freedom. The resulting model has been applied to a number of well-known test cases, such as plane-shear and pressure-driven flows, and its predictions are presented and compared with experimental data. In particular, we show that this model can successfully predict important features of granular suspensions such as normal stress differences and particle migration. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D30.00006: Numerical study of cavitation and pinning effects due to gas injection through a bed of particles: application to a radial-flow moving-bed reactor. Guillaume Vinay, Felaurys Vasquez, Florence Richard In the petroleum and chemical industries, radial-flow moving-bed reactors are used to carry out chemical reactions such as catalytic reforming. Radial-flow reactors provide high capacity without increased pressure drop or greatly increased vessel dimensions. This is done by holding the catalyst in a basket forming an annular bed, and causing the gas to flow radially between the outer annulus and the central tube. Catalyst enter the top of the reactor, move through the vessel by gravity to the bottom where it is removed and then regenerated. Within the catalytic bed, the combined effects of particles motion and radial injection of the gas may lead to cavitation and pinning phenomenon that may clearly damage the reactor. We study both cavitation and pinning effects using an in-house numerical software, named PeliGRIFF (\underline {www.peligriff.com/}), designed to simulate particulate flows at different scales; from the particle scale, where fluid/particle interactions are directly solved, to the particles suspension scale where the fluid/solid interactions are modeled. In the past, theoretical and experimental studies have already been conducted in order to understand the way cavitation and pinning occur. Here, we performed simulations involving a few thousands of particles aiming at reproducing experimental experiments. We will present comparisons between our numerical results and experimental results in terms of pressure drop, velocity, porosity. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D30.00007: ABSTRACT WITHDRAWN |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D30.00008: A thermodynamically consistent model for granular-fluid mixtures considering pore pressure evolution and hypoplastic behavior Julian Hess, Yongqi Wang A new mixture model for granular-fluid flows, which is thermodynamically consistent with the entropy principle, is presented. The extra pore pressure described by a pressure diffusion equation and the hypoplastic material behavior obeying a transport equation are taken into account. The model is applied to granular-fluid flows, using a closing assumption in conjunction with the dynamic fluid pressure to describe the pressure-like residual unknowns, hereby overcoming previous uncertainties in the modeling process. Besides the thermodynamically consistent modeling, numerical simulations are carried out and demonstrate physically reasonable results, including simple shear flow in order to investigate the vertical distribution of the physical quantities, and a mixture flow down an inclined plane by means of the depth-integrated model. Results presented give insight in the ability of the deduced model to capture the key characteristics of granular-fluid flows. [Preview Abstract] |
Sunday, November 20, 2016 4:41PM - 4:54PM |
D30.00009: A non-local plasticity theory for slow granular flows Prabhu R Nott Recent studies on dense granular materials have shown evidence of non-locality in the mechanical response, wherein the motion of an intruder is aided by shearing the material far from it. This behaviour is not explained by classical plasticity theories, which also have other serious failings. Non-local theories proposed earlier are either of phenomenological origin, or based on the introduction of an additional field variable whose mechanical origin is debatable. Here we present a non-local plasticity theory whose mechanical origin is easy to comprehend, involves no additional field variables, and captures rather simply the physical picture of plastic events in a spatial point influencing its neighbourhood. Most crucially, the theory is able to predict the kinematics of simple shear flows, in particular the exponentially decaying velocity profile in simple shear, and shear-induced dilatancy. Finally, our non-local theory plasticity theory is Hadamard well-posed, a significant improvement over the local theories. [Preview Abstract] |
Sunday, November 20, 2016 4:54PM - 5:07PM |
D30.00010: ABSTRACT WITHDRAWN |
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