Effect of granular shape on radial segregation in a two-dimensional drum

Granular segregation is widely observed in nature and industry.Most research has focused on segregation caused by differences in the size and density of spherical grains.However,due to the fact that grains typically have different shapes,the focus is shifting towards shape segregation.In this study,experiments are conducted by mixing cubic and spherical grains.The results indicate that spherical grains gather at the center and cubic grains are distributed around them,and the degree of segregation is low.Through experiments,a structured analysis of local regions is conducted to explain the inability to form stable segregation patterns with obviously different geometric shapes.Further,through simulations,the reasons for the central and peripheral distributions are explained by comparing velocities and the number of collisions of the grains in the flow layer.

Entropy productions in granular materials

Granular materials display more abundant dissipation phenomena than ordinary materials. In this paper, a brief energy flow path with irreversible processes is illustrated, where the concept of granular temperature Tg, initially proposed for dilute systems, is extended to dense systems in order to quantify disordered force chain configurations. Additionally, we develop the concept of conjugate granular entropy sg and its production equation. Our analyses find out that the granular entropy significantly undermined the elastic contact between particles, seriously affecting the transport coefficients in granular materials and creating new transport processes.

Prediction of segregation characterization based on granular velocity and concentration in rotating drum

In a binary granular system composed of two types of particles with different granule sizes and the same density,particle sorting occurs easily during the flow process.The segregation pattern structure is mainly affected by the granular velocity and granular concentration in the flow layer.This paper reports on the experimental velocity and concentration measurement results for spherical particles in a quasi-two-dimensional rotating drum.The relationship between the granular velocity along the depth direction of the flow layer and granular concentration was established to characterize structures with different degrees of segregation.The corresponding relationships between the granular velocity and concentration and the segregation pattern were further analyzed to improve the theoretical models of segregation(convection-diffusion model and continuous flow model)and provide a reference for granular segregation control in the production process.

2T1C DRAM based on semiconducting MoS_(2) and semimetallic graphene for in-memory computing

In-memory computing is an alternative method to effectively accelerate the massive data-computing tasks of artificial intelligence(AI)and break the memory wall.In this work,we propose a 2T1C DRAM structure for in-memory computing.It integrates a monolayer graphene transistor,a monolayer MoS_(2)transistor,and a capacitor in a two-transistor-onecapacitor(2T1C)configuration.In this structure,the storage node is in a similar position to that of one-transistor-one-capacitor(1T1C)dynamic random-access memory(DRAM),while an additional graphene transistor is used to accomplish the nondestructive readout of the stored information.Furthermore,the ultralow leakage current of the MoS_(2)transistor enables the storage of multi-level voltages on the capacitor with a long retention time.The stored charges can effectually tune the channel conductance of the graphene transistor due to its excellent linearity so that linear analog multiplication can be realized.Because of the almost unlimited cycling endurance of DRAM,our 2T1C DRAM has great potential for in situ training and recognition,which can significantly improve the recognition accuracy of neural networks.

Numerical simulation of ae-olian sediment transport

A discrete element model (DEM) is developed for aeolian sediment transport in the scope of investigation of the dynamic behavior of grains, in which the motion of individual grain is considered while velocity distribution of wind is given according to bed forms. The above model has been used for simulating the phenomenon of 4000 grains blown by wind, resulting in ripples and saltating paths, which shows good agreements with experimental observations. The result is helpful for revealing the intrinsic mechanism of sand flow.

Kinetic granular temperature and its measurement using speckle visibility spectroscopy

The occurrence of a mesoscopic scale in granular materials leads to a sharp increase in the number of interaction processes at both intra-and inter-scale levels.The mesoscopic scale is the main source of the complex macroscopic properties of granular materials.The quantification of mesoscopic disordered movements is often referred to as granular temperature.In this paper,we first introduce the physical meaning of the original granular temperature of a kinetic nature,Tk,and then briefly summarize the advances made over the past few decades.Our research group has focused on Tk measurement using speckle visibility spectroscopy.The principle of this technique and the instruments developed in our research group are briefly introduced.Our work shows great promise in the measurement of kinetic granular temperature.Finally,a summary of granular temperature and some recent developments in speckle visibility spectroscopy measurements are presented.

STUDIES ON STRUCTURAL AND MECHANICAL PROPERTIES UNDER ISOSTATIC COMPRESSION WITH LARGE-SCALE DISCRETE ELEMENT SIMULATIONS

Granular systems undergo a jamming transition at point J simply by increasing the packing fraction. A large-scale parallel discrete element code （THDEM： TsingHua Discrete Element Method） was used to obtain a satisfying statistical description of the structural and me- chanical properties near point J. The isostatic compressions of 100,000 polydispersed frictionless particles were simulated on high performance computers to clearly observe the sophisticated con- figurations of force chains. The first peak of the pair correlation function, coordination number, spatial distribution of the packing fraction, and stress were calculated to analyze their variations with increasing packing fraction. The critical packing fraction at point J is determined to be 0.62. The incremental stress and coordination number from point J scale well with the power law, and coincide with previous theoretical predications. The distribution of the packing frac- tion is a normal distribution around the average value. The standard deviation decreases with increasing packing fraction, indicating the system is more uniform with a denser packing.

ANALYSIS OF PARAMETERS IN GRANULAR SOLID HYDRODYNAMICS FOR TRIAXIAL COMPRESSION TESTS

Granular materials are omnipresent in industries and in nature. For small strains, elastic-plastic and hypoplastic constitutive relations are widely used in engineering practice, but they are not a significant reflection of the underlying physics. Under a unified thermodynamics framework explaining the physics of materials, granular solid hydrodynamics （GSH） was an ex- tension towards describing granular materials, not only solid-like, but also fluid-like behaviors. In this paper, the fundamentals of GSH are briefly treated and then simplified to analyze quasi- static deformations in triaxial compressions. The calculated stress-strain relations and volumetric strain are compared with experimental results. The influences of the major parameters in GSH, especially their cross coupling influences, are analyzed and their physical meanings are further clarified. After parameters were calibrated, the calculated stress values in the characteristic stress state are found to be within 22% of tested values. Meanwhile, the energy dissipation during triaxial compression is analyzed. The above results support and partially quantify GSH.

Application of granular solid hydrodynamics to a well-graded unbound granular material undergoing triaxial tests

Unbound granular materials(UGMs)are widely used as a base or a subbase in pavement construction.They are generally well graded and exhibit a higher peak strength than that of conventional cohesionless granular materials.By using a simplified version of granular solid hydrodynamics(GSH),a set of GSH material constants is determined for a UGM material.The deviatoric stress and volumetric strain caused by triaxial compression are calculated and then compared with experimental data.The results indicate that the GSH theory is able to describe such a special type of granular materials.

Visualization of force networks in 2D dense granular materials

Dense granular matter is a conglomeration of discrete solid and closely packed particles.As subjected to external loadings,the stress is largely transmitted by heavily stressed chains of particles forming a sparse network of larger contact forces.To understand the structure and evolution of force chains,a photoelastic technique was improved for determining stresses and strains in the assemblies of photoelastic granular disks in this paper.A two-dimensional vertical slab was designed.It contains 7200 polydispersed photoelastic disks and is subjected to a localized probe penetrating at the top of the slab to mimic the cone penetration test.The interparticle contact force distribution was found a peak around the mean value,a roughly exponential tail for greater force and a dip toward zero for smaller force.The force chain network around the probe tip was depicted,and the contact angle distribution of particles in force chains was found to be well aligned in the directions of major principal stress.

Simulations of granular flow along an inclined plane using the Savage-Hutter model

In this work a velocity-dependent friction is introduced into a depth-averaged 5avage-Hutter dynamical model for shallow granular flows. The process of granular material flowing along an inclined plane and then depositing on a horizontal plane is simulated. The surface profiles and evolution of various types of energy are investigated and compared when using the standard Coulomb-type friction versus velocity- dependent friction. Interestingly, there is a small difference between the two different types of friction.

Temporal correlation of force and position in granular materials

We investigate the quasi-static mechanical response of dense granular materials under biaxial compressions by using discrete element simulation. The internal force network and its evolution are observed for different strains. Our results show that correlation of force and position appropriately characterize the bulk response and volumetric strain.

Mesoscopic properties of dense granular materials:An overview

A granular material is a conglomeration of discrete solid particles.It is intrinsically athermal because its dynamics always occur far from equilibrium.In highly excited gaseous states,it can safely be assumed that only binary interactions occur and a number of kinetic theories have been successfully applied.However,for granular flows and solidlike states,the theory is still poorly understood because of the internally correlated structures,such as particle clusters and force networks.The current theory is that the mesoscale characteristics define the key differences between granular materials and homogeneous solid materials.Widespread interest in granular materials has arisen among physicists,and significant progress has been made,especially in understanding the jamming phase diagram and the characteristics of the jammed phase.In this paper,the underlying physics of the mesoscale structure is discussed in detail.A multiscale framework is then proposed for dense granular materials.

Force fluctuations in sheared granular disks

The internal structure established within granular materials,often observed as force chains,is dominant in controlling bulk mechanical properties.We designed a two-dimensional Hele-Shaw cell to contain photoelastic disks,and two servos were used on the top and right boundaries individually.We experimentally monitored the fluctuations in force on the top plate while slowing the shearing of the well-confined disks and keeping the right boundary at a contactconfined force of 0.2 kN.The particle rearrangements were found to correspond to bulk force drops and were observed in a localized zone with a length of approximately 5 particle diameters.These results help reveal the structure and mechanics of granular materials,and further investigations are ongoing.

Velocity profiles and energy fluctuations in simple shear granular flows

Rheology analysis of granular flows is important for predicting geophysical hazards and designing industrial processes. Using a discrete element method, we simulate simple shear flows in 3D under a constant confining pressure of 10 kPa. The inertial number proposed by the GDR MiDi group in France is adopted to distinguish rheology regimes, Both translational and angular velocity profiles are investigated, and both fluid-like and solid-like behavior modes are observed in the flows. The maximum angular velocity occurs near the localized deformation area. We also investigate the energy characteristics of the flows and find that at very small shearing speed, the mean kinetic energy density ek is close to zero, while the mean elastic energy density ec is much greater. At large shearing speed, ek increases. The fluctuating parts of the two types of energy increase with increasing shear speed. Thus, the mean energy density ratio ek/ec can be used in addition to the inertial number to distinguish flow regimes. These results provide insights from energetics into the rheological properties of granular flows.