Session R2: Granular Flows IV

Correlating Grain and Slider Dynamics in a Stick-Slip Experiment

We describe an experiment to characterize the stick-slip nature of sliding friction. In the experiment a solid slider is pulled by a spring moving at constant velocity across a 2D granular bed of bidisperse photoelastic discs confined to a vertical channel. A force sensor attached to the spring measures the pulling force on the slider, while two accelerometers on the slider provide angle and acceleration data. Synchronous photoelastic images of the granular bed acquired by a camera moving along with the slider allow us to correlate all features of the slider and granular dynamics. We particularly focus on the correlation of the slider acceleration and changes in the granular force network. This experiment can help us better understand both atomic scale friction experiments, and stick-slip at the geophysical scale.   

Flowing layer thickness in a granular tumbler

The thickness of the flowing layer in a tumbler varies substantially depending on flow conditions, but no predictive approach is available. We have studied monodisperse granular flows in air, water, and glycerine in a quasi-2D tumbler, focusing on the rolling regime to highlight common features and differences between dry and wet granular flows. For dry granular flows, the flowing layer thickness measured in units of particle diameter scales with a dimensionless flow rate based on the tumbler rotational speed and radius as well as the particle diameter. Using appropriate characteristic time scales, the data for the dry and liquid experiments also collapse fairly well. The Stokes number is a key dimensionless parameter to characterize the flow of granular material in liquids.   

Anatomy of flow and jam of a two dimensional granular flow in a hopper

We seek an understanding of the physics of jamming for hopper flow using high speed spatio-temporal video data for photoelastic disks flowing through a two-dimensional hopper. We have found experimental support for the hypothesis that jamming events of granular flow in a hopper occur as a Poisson process. Through measuring the density field and stress distribution, we demonstrate that ``dome-structured'' stress chains affect the density and velocity fields, leading to the mean flow profile in the hopper. We also measure how jamming statistics depend on the orifice sizes and the wall angles of a hopper. By calculating stress fluctuations, we conclude that instead of the density field, the formation of localized force chain arches controls the jamming transition of granular flow in a hopper. These data are part of an IFPRI-NSF Collaboratory for comparing physical data and simulations.   

Anisotropic surface friction

Contrary to the literature, we find that altering the surface roughness has a large effect on intruder drag in the quasi-static regime. Moreover we pattern the surface with a sawtooth texture and observe anisotropic drag: when the texture is comparable in size to the bead diameter the frictional force is 1/3rd greater for the flow directed against the sawtooth versus the opposite flow. We present a systematic study showing the dependence of the anisotropy on the texture's orientation.   

Microstructural theory for colloidal suspensions in active microrheology

The active microrheology problem where a probe particle is pulled through a bath of colloidal particles with a constant external force $F^{ext}$ is studied using a theoretical framework based on the Smoluchowski equation, with emphasis on concentrated colloidal suspensions far from equilibrium. The probability distribution of bath particles with respect to the probe, $g(r)$, is determined from an integro-differential equation solved by iterative numerical methods at different set of $Pe$ (ratio of external to Brownian forces) and volume fractions. The role of inter-particle interactions on microstructure is studied by using different types of pair potentials with the intention to model systems ranging from hard-spheres to soft colloids. The obtained distribution is then used to compute the apparent shear viscosity. The predictions of microstructure and rheology are compared with our Accelerated Stokesian Dynamics simulations and available experimental results.   

Effect of particle devolatilization on bed dynamics during biomass thermochemical conversion

Fluidization is a technique of choice for the thermochemical conversion of biomass. At conversion temperatures however, the amount of gases released by the biomass is large enough to impact the mixing of the reactive particles with the inert sand, and modify the bubbling frequency and intensity. This, in turn, may significantly affect the chemical processes and the final product distribution. In this context, optimizing reactor design and operating conditions requires a better understanding of the actual bed dynamics in the presence of reactive particles. In this work, two- and three-dimensional simulations of biomass conversion in a lab-scale fluidized bed reactor are conducted using a Lagrangian approach to handle the solid phase. The biomass devolatilization chemistry is described using a commonly used global model taking into account each constituent of the biomass. Statistical analysis of the particles and velocity fields is conducted and results are compared to non-reactive cases to quantify the effect of devolatilization on particle mixing, especially segregation, and on the bubbling pattern of the bed.   

Surface Chemistry Effects in Submerged Granular Flows of Hydrophobic Grains

We experimentally investigate submerged granular flows of hydrophobic and hydrophilic grains in a rotating drum. While slurry and suspension flows are common in nature and industry, effects of surface chemistry on flow behavior have received relatively little attention. The experiment consists of a cylindrical drum containing various concentrations of hydrophobic and hydrophilic grains of sand submerged in water rotated at a constant angular velocity. Sequential images of the resulting avalanches are taken and analyzed. While it is known that at slow speeds, submerged avalanches appear qualitatively similar to dry flows, our results suggest that the surface properties of the grains affect underwater flow significantly. High concentrations of hydrophobic grains result in an effectively cohesive interaction between the grains forming aggregates. We present data on the size of the aggregates, slope, and avalanche statistics with changes in concentration. The formation and nature of the aggregates depends significantly on the presence of air in the system. At concentrations larger than 75{\%} hydrophobic sand, the avalanches do not behave in a manner which is typical for sand, but as the concentration decreases, the aggregates are smaller, the angle of repose decreases, and the grains start showing rheological properties similar to those in regular sand.   

Percolation theory applied to the force field in granular material

We analyze the structure of the force field in slowly compressed granular system by the means of discrete element simulations. Using the tools of percolation theory, we compute the quantities describing the force field and discuss their dependence on polydispersity and frictional properties of the granular particles. Then, we correlate the results to the ones obtained using topological approach based on Betti numbers which measure the number of clusters as a function of force thresholds.   

Material point method simulation of dense granular material

Accurate modeling and simulation of granular flow or deformation require a numerical method with Lagrangian description to account for history dependence of the material. However, large deformation or flow of the material requires an Eulerian description. Numerically, different descriptions of the material result in different codes and applications. Unsatisfactory results have been reported by many modelers using both methods. For instance, element deletion scheme is used in the finite element method, a Lagrangian description, to eliminate the highly distorted elements, which results in artificial reduction of inertia from the problem. In codes using the finite volume method, an Eulerian descriptions, how to advect brittle damage of the material is a significant issue. To address these issues we use the material point method, which uses Lagrangian material points and Eulerian mesh simultaneously. Improvements are made to the original material point method for our applications. It is found that the improvements are critically important to granular flows results from brittle damage of the material, while it is marginally important to ductile materials.   

Couette and Fourier planar ows for a low density granular gas

We study in this work steady laminar flows in a low density granular gas. Our system is excited by shear and temperature sources at the boundaries (two infinite parallel walls). We describe previously unreported types of non-Newtonian granular flows. We obtain also their corresponding rheologic and hydrodynamic transport coefficients, following three independent methods: 1) an analytical solution, obtained from Grad's method applied to the inelastic Boltzmann equation, 2) a numerical solution of the inelastic Boltzmann equation, obtained by means of the Direct Simulation Monte Carlo method, and 3) molecular dynamics simulations. We show that the three procedures yield the same general classification of planar Fourier and Couette flows for the granular gas.