Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session A12: Granular Flows I |
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Chair: Tony Rosato, New Jersey Institute of Technology Room: 140 |
Sunday, November 20, 2022 8:00AM - 8:13AM |
A12.00001: Segregation of Free-Sifting Fine Particles Richard M Lueptow, Song Gao, Julio M Ottino, Paul B Umbanhowar Flowing dense mixtures of bidisperse particles tend to segregate by size, and the large to small particle diameter ratio, R, determines whether the segregation is shear-induced (small R) or free-sifting (large R). Free-sifting segregation remains largely unexplored and exhibits qualitatively different segregation dynamics from shear-driven segregation, where segregation occurs only when large and small particles flow together. To consider free-sifting, we use discrete element method simulations of gravity-driven motion of fine particles through static random packings of large particles for 4 ≤ R ≤ 7.5. For R > 6.464 (the geometric trapping threshold), fine particles flow freely through the voids between large particles, and the percolation velocity is constant and decreases only slightly with decreasing R. For R decreasing from 6.464 to 4, fine particles flow a decreasing distance before being trapped in the bed of static large particles, but up to the point where they are trapped their velocity is nearly as large as that for R > 6.464. Preliminary results for dense flows of both particles with R > 4 indicate that the fine particles freely percolate between the flowing large particles leading to the fine particles flowing independently from the large particles. |
Sunday, November 20, 2022 8:13AM - 8:26AM |
A12.00002: Comparison of two size segregation models for binary granular mixtures flowing over a periodic chute Vishnu Kumar K Sahu, Soniya Kumawat, Anurag Tripathi We study the size segregation of binary granular mixtures flowing over an inclined plane under the influence of gravity using Distinct Element Method (DEM) simulations. The DEM results are compared with two different types of size segregation models. The first approach [1] is based on the empirical relation of the species percolation velocity with the shear rate and the other species concentration in a binary mixture of different size particles. The second approach is based on the forces acting on the particles [2]. While both of the approaches have been successful in predicting segregation in different flow configurations, a detailed comparison between the two approaches is not yet reported. In this study, a comparison between the theoretically predicted concentration profiles of two approaches over a wide range of compositions (ranging from a very low to a high concentration of one species) at different inclination angles will be presented. This comparison allows us to understand the strengths and weaknesses of the two segregation models and their limits to predict the segregation in chute flows. |
Sunday, November 20, 2022 8:26AM - 8:39AM |
A12.00003: Time-dependent density segregation of binary granular mixtures flowing over a periodic chute Soniya Kumawat, Vishnu Kumar K Sahu, Anurag Tripathi We study time-dependent density segregation of binary granular mixtures flowing over an inclined plane. Discrete Element Method (DEM) simulations in a periodic box are performed for mixtures having an identical size and different density particles flowing under the influence of gravity. The particle force-based density segregation theory (Tripathi and Khakhar, J. Fluid Mech., 717:643–669, 2013) has been used to predict time-dependent segregation by accounting for the inter-coupling of segregation with rheology. A two-way coupled continuum model is developed to solve the momentum balance equations along with particle force-based segregation transport equation and mixture rheology using Matlab PDEPE solver. Various flow properties of interest such as species concentration, velocity, pressure and shear stress at different times steps are compared with DEM results. The time-dependent evolution of flow properties from the continuum model is found to be in good agreement with the DEM simulations for different density ratios of heavy to light particles over a wide range of compositions at different inclination angles. |
Sunday, November 20, 2022 8:39AM - 8:52AM |
A12.00004: Cooperative motion of intruders amid smaller grains Douglas D Carvalho, Erick Franklin From the locomotion of animals in sand to the anchoring of buildings to the ground, the question of why it is so difficult to move an object within grains is ubiquitous in nature and human activities. Unlike fluids, granular systems transmit forces through interparticle contacts, with preferred directions according to the external applied force. We investigate numerically the forces and structures in a two-dimensional granular system displaced by an intruder. By varying the physical conditions of the problem, we verified that the contact networks percolate forces from the intruder towards the walls, being responsible for jammed regions (which, in some cases, can lead to the complete stop of the intruder's movement) and high values of the intruder’s drag force (Carvalho et al., Phys. Rev. E, 2022). In addition, we investigate how the motion of a set of intruders affects the system dynamics. We observed that the intruders present a cooperative behavior, with a final configuration related to their initial arrangement in space, and that there are optimal separations between intruders for minimum drag. Our results bring new insights into the cooperative dynamics of intruders, paving the way for mitigating the excessive drag suffered by these objects. |
Sunday, November 20, 2022 8:52AM - 9:05AM |
A12.00005: Experiments on Drag Force in Granular Flow Past a Cylinder Aqib Khan, Aadarsh Kumar, Deepika Chimote, Yash Jaiswal, Rakesh Kumar, Sanjay Kumar Granular flows are observed in natural phenomena such as landslides, and snow avalanches and pose a serious threat to infrastructure and the community living in the hilly areas. Granular avalanches often interact with obstacles such as large electric poles, aerial lifts, cable cars, etc. that leads to significant forces on these structures. A good understanding of the interaction of granular flows with solid structures and the resultant drag force is therefore very important. The present experimental study embarks on studying the interaction of shallow granular flows with cylindrical obstacles with a particular focus on estimating the drag force acting on the cylinder as a granular avalanche impacts it. An inclined chute is fabricated to generate slow and rapid granular flow with the facilities for drag calculation and shock wave visualisation at various chute inclinations. Velocity profiles are estimated using the particle image velocimetry method and the drag force acting on the models is calculated using the load cells with a strain gauge force measurement system. Scaling analysis is performed to extract generalised trends in the drag force. |
Sunday, November 20, 2022 9:05AM - 9:18AM |
A12.00006: Vibrofluidic Manipulation of Granular Fluids: Towards “Dry” Lab-on-a-Chip William Ristenpart, Shelly X Zhang, Dhruva Adiga, Timothy Hui, Gregory H Miller Many “wet” techniques have been developed to manipulate liquids at small scales for lab-on-a-chip applications. An important and ubiquitous class of laboratory operations, however, involves dry techniques, where solid powders or other granular materials must be manipulated prior to addition of any liquids. To date, no facile techniques exist to manipulate dry granules in lab-on-a-chip devices. Here, we present a “vibrofluidic” technique to pump, mix, and separate granular fluids simply by vibrating a solid substrate. In contrast to existing vertical vibratory methods for fluidizing granular materials that engender no net motion, here we apply a horizontal, non-antiperiodic vibratory waveform, as is readily applied with a standard subwoofer. The direction and velocity of the flow are tuned by modulating the sign and amplitude, respectively, of the vibratory waveform, with typical pumping velocities on the order of centimeters per second. Different types of granules are mixed by combining them at Y-shaped junctions, and mixtures of granules with similar size but different friction coefficients can be separated by judicious choice of the vibratory waveform. We present asymptotic analyses of the granular fluid motion based on a frictional Froude number, and we discuss the implications for using vibrofluidics in conjunction with other established technologies for lab-on-a-chip and other applications. |
Sunday, November 20, 2022 9:18AM - 9:31AM |
A12.00007: Segregation force in dense bidisperse granular mixtures Yifei Duan, Jing Lu, Paul B Umbanhowar, Julio M Ottino, Richard M Lueptow Granular segregation is typically driven by the combined effects of gravity and shear. Recent studies have characterized the size segregation force on a single intruder with a scaling law that consists of two additive terms: a buoyancy-like gravity-induced pressure gradient term and a shear rate gradient term, both of which depend on the particle size ratio. However, a generalized scaling of the force is still lacking for segregation in finite-concentration mixtures (rather than a single intruder). Here we explore the mixture segregation force, particularly the shear rate gradient term, in DEM simulations of controlled-velocity granular flows where the shear profile is varied. Measuring the mixture segregation force using a modified virtual-spring-based 'force meter' technique reveals that the concentration dependence of the force is well-characterized by a hyperbolic tangent function with no additional free parameters. The extended concentration-dependent scaling law accurately predicts the mixture segregation force in size-bidisperse particle mixtures over a range of pressures, shear rates, and particle size ratios. |
Sunday, November 20, 2022 9:31AM - 9:44AM |
A12.00008: A particle-scale force approach to granular segregation Lu Jing, Julio M Ottino, Paul B Umbanhowar, Richard M Lueptow Particle segregation in granular flows is challenging to model due to its complex, and sometimes contradictory, phenomenology. State-of-the-art segregation theories rely on configuration-specific closure relations at the flow level, but a general constitutive segregation model is missing. Here, an emerging bottom-up approach for segregation modeling is presented, which relies instead on detailed characterization of the forces relevant to segregation at the particle level. Using discrete element method simulations, we first characterize the driving and resisting forces of segregation on single intruder particles exerted by the surrounding particles in the flow, resulting in scaling laws for a buoyancy-like force, a shear-rate-gradient force, and a Stokes-like drag force. Then, we show how these force models lead to accurate prediction of the propensity for and velocity of gravity-induced size and density segregation, which match extensive experimental and numerical results. Finally, we discuss the extension of the force models from the single intruder limit to the finite-concentration mixture regime, which may lead to upscaled closure relations for continuum modeling of granular segregation. |
Sunday, November 20, 2022 9:44AM - 9:57AM |
A12.00009: Rheology of high speed accelerating granular flows Satyabrata Patro, Anurag Tripathi The inertial number-based rheology, popularly known as the JFP model, is well known for describing the rheology of granular materials in the dense flow regime. While most of the recent studies focus on the steady-state rheology of granular materials, the time-dependent rheology of such materials has received less attention. Owing to this fact, we perform three-dimensional DEM simulations of frictional inelastic spheres flowing down an inclined bumpy surface varying over a wide range of inclination angles and restitution coefficients. We show that steady, fully developed flows are possible at inclinations much higher than those predicted from the JFP model rheology. We show that, in addition to a modified effective friction law, the rheological description also needs to account for the stress anisotropy by means of a first and second normal stress difference law. |
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