arXiv subjects

Yifei Duan

Publications and source records attributed to Yifei Duan.

23 records · Page 2Linked to original sources

arXiv2021

Designing non-segregating granular mixtures

In bidisperse particle mixtures varying in size or density alone, large particles rise (driven by percolation) and heavy particles sink (driven by buoyancy). When the two particle species differ from each other in both size and density, the two segregation mechanisms either enhance (large/light and small/heavy) or oppose (large/heavy and small/light) each other. In the latter case, an equilibrium condition exists in which the two segregation mechanisms balance and the particles no longer segregate. This leads to a methodology to design non-segregating particle mixtures by specifying particle size ratio, density ratio, and mixture concentration to achieve the equilibrium condition. Using DEM simulations of quasi-2D bounded heap flow, we show that segregation is significantly reduced for particle mixtures near the equilibrium condition. In addition, the rise-sink transition for a range of particle size and density ratios matches the combined size and density segregation model predictions.

cond-mat.soft↗

arXiv2020

Modelling segregation of flowing bidisperse granular mixtures varying simultaneously in size and density

Flowing granular materials segregate due to differences in particle size (driven by percolation) and density (driven by buoyancy). Modelling the segregation of mixtures of large/heavy particles and small/light particles is challenging due to the opposing effects of the two segregation mechanisms. Using discrete element method (DEM) simulations of combined size and density segregation we show that the segregation velocity is well described by a model that depends linearly on the local shear rate and quadratically on the species concentration. Concentration profiles predicted by incorporating this segregation velocity model into a continuum advection-diffusion-segregation transport model match DEM simulation results well for a wide range of particle size and density ratios. Most surprisingly, the DEM simulations and the segregation velocity model both show that the segregation direction for a range of size and density ratios depends on the local species concentration. This leads to a methodology to determine the combination of particle size ratio, density ratio, and particle concentration for which a bidisperse mixture will not segregate.

cond-mat.soft↗

arXiv2020

Segregation models for density-bidisperse granular flows

Individual constituent balance equations are often used to derive expressions for species-specific segregation velocities in flows of dense granular mixtures. We propose a semiempirical expression for the interspecies momentum exchange in density-bidisperse granular flows as an extension of ideas from kinetic theory and compare it to a previous viscous drag approach that is analogous to particles settling in a fluid. The proposed model expands the range of the granular kinetic theory from short-duration binary collisions to the multiple enduring contacts characteristic of dense shear flows and incorporates the effects of particle friction, concentration ratio, and local flow conditions. The segregation velocities derived from the momentum balance equation using both interspecies drag models match the downward and upward segregation velocities of heavy and light particles obtained from DEM simulations through the flowing layer depth for different density ratios and constituent concentrations in confined shear flows. Predictions of the kinetic theory inspired approach are additionally compared to results from free surface heap flow simulations, and, again, a close match is observed.

cond-mat.soft↗

arXiv2018

A new kinetic theory model of granular flows that incorporates particle stiffness

Granular materials are characterized by large collections of discrete particles of sizes larger than one micron, where the particle-particle interactions are significantly more important than the particle-fluid interactions. These flows can be successfully modeled by the existing Kinetic Theory (KT) models when they are in the dilute regime with low particle-particle collision frequencies, yielding results that agree well with the simulation results of the event-driven hard sphere model or the more sophisticated soft-sphere Discrete Element Method (DEM). However, these KT models become less accurate for granular flows with soft particles (low particle stiffness) at high particle-particle collision frequencies when the predicted collision interval (the time of free flight for a particle prior to the next collision) is comparable to the collision duration; there is a large discrepancy between the results of these KT models and those from the DEM models. In this work we develop a new KT model that could be used to model granular systems of high collision frequencies with a finite particle stiffness. This is done by modifying the fluctuation energy dissipation rate to incorporate the ratio of collision duration to collision interval, a parameter that is determined by both the collision frequency and particle stiffness. We use a linear-spring-dashpot collision scheme to model the elastic potential energy in the system and to uncover the relationship between the constitutive relations of KT and the ratio of collision duration to collision interval.

cond-mat.soft↗

arXiv2017

Incorporation of Velocity-dependent Restitution Coefficient and Particle Surface Friction into Kinetic Theory for Modeling Granular Flow Cooling

Kinetic theory (KT) has been successfully used to model rapid granular flows in which particle interactions are frictionless and near elastic. However, it fails when particle interactions become frictional and inelastic. For example, the KT is not able to accurately predict the free cooling process of a vibrated granular medium that consists of inelastic frictional particles under microgravity. The main reason that the classical KT fails to model these flows is due to its inability to account for the particle surface friction and its inelastic behavior, which are the two most important factors that need be considered in modeling collisional granular flows. In this study, we have modified the KT model that is able to incorporate these two factors. The inelasticity of a particle is considered by establishing a velocity-dependent expression for the restitution coefficient based on many experimental studies found in the literature, and the particle friction effect is included by using a tangential restitution coefficient that is related to the particle friction coefficient. Theoretical predictions of the free cooling process by the classical KT and the improved KT are compared with the experimental results from a study conducted on an airplane undergoing parabolic flights without the influence of gravity [Y. Grasselli, G. Bossis, and G. Goutallier, EPL (Europhysics Letters) 86, 60007 (2009)]. Our results show that both the velocity- dependent restitution coefficient and the particle surface friction are important in predicting the free cooling process of granular flows; the modified KT model that integrates these two factors is able to improve the simulation results and led to a better agreement with the experimental results.

physics.flu-dyn↗