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: CahnHilliard Regularization of the "mu(I)" Rheology Joe Goddard, Jaesung Lee Recently Barker et al. [J. Fluid Mech. 779 (2015) 794818] have shown that the popular $\mu(I)$ model for the viscoplasticity of granular media is illposed, exhibiting short wavelength instabilities of the Hadamard variety. As one possible regularization of the model, we employ the dissipative analog of the classical CahnHilliard (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 linearstability 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 highergradient terms in the CH model lead to a large wave number cutoff 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: stresscoordinated cellular constriction chains Michael Holcomb, GuoJie 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 chainlike structures. In contrast, constricted particles tend to form compact clusters when $P$ favors compression. A remarkable similarity of constrictedparticle chains and CCCs observed \textit{in vivo} provides indirect evidence that tensilestress feedback coordinates the apical constriction activity. [Preview Abstract] 
Sunday, November 20, 2016 3:23PM  3:36PM 
D30.00003: Theory for Indirect Conduction in Dense, GasSolid Systems Aaron Lattanzi, Christine Hrenya Heat transfer in dense gassolid systems is dominated by conduction, and critical to the operation of rotarykilns, 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 gassolid systems. Current stateoftheart models for indirect conduction assume that particles are surrounded by a static ``fluid lens'' and that onedimensional 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, singleparticle cases. By contrast, here we quantify these effects for dynamic, multiparticle systems. This analysis is compared to outputs from computational fluid dynamics and discrete element method (CFDDEM) 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, singleparticle 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 crosssectionally 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 nonzero bulk viscosity as well as shearrate 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 wellknown test cases, such as planeshear and pressuredriven 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 radialflow movingbed reactor. Guillaume Vinay, Felaurys Vasquez, Florence Richard In the petroleum and chemical industries, radialflow movingbed reactors are used to carry out chemical reactions such as catalytic reforming. Radialflow 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 inhouse 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] 

D30.00007: ABSTRACT WITHDRAWN 
Sunday, November 20, 2016 4:28PM  4:41PM 
D30.00008: A thermodynamically consistent model for granularfluid mixtures considering pore pressure evolution and hypoplastic behavior Julian Hess, Yongqi Wang A new mixture model for granularfluid 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 granularfluid flows, using a closing assumption in conjunction with the dynamic fluid pressure to describe the pressurelike 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 depthintegrated model. Results presented give insight in the ability of the deduced model to capture the key characteristics of granularfluid flows. [Preview Abstract] 
Sunday, November 20, 2016 4:41PM  4:54PM 
D30.00009: A nonlocal plasticity theory for slow granular flows Prabhu R Nott Recent studies on dense granular materials have shown evidence of nonlocality 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. Nonlocal 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 nonlocal 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 shearinduced dilatancy. Finally, our nonlocal theory plasticity theory is Hadamard wellposed, a significant improvement over the local theories. [Preview Abstract] 

D30.00010: ABSTRACT WITHDRAWN 
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