System Of Spheres Research Articles

Shear of granular materials composed of ellipses

Granular systems of disks (2D) and spheres (3D) under shear have been studied extensively. However, less attention has been paid to systems of non-spherical particles, i.e., ellipses, polygons, etc. Here we studied a quasi 2D granular system composed of ellipses under homogeneous simple shear. By tracking the positions and orientations of the ellipses, we have observed that they gradually rotate to align along a preferred direction as the shear strain increases. This preferred direction asymptotically approach a density-independent value at large strain. While first reported for disk systems, shear jamming is also observed for our ellipse system, as indicated by the increase of system pressure and coordination number with shear strain.

Experimental and numerical study of coarse particle conveying in the small absorber sphere system: Overview and some recent CFD-DEM simulations

Pneumatic conveying of absorber spheres in the pebble-bed high-temperature gas-cooled reactor is a special application of pneumatic conveying technique in the nuclear engineering. It may be regarded as an intermittent circulation of absorber spheres between the side reflector boring and the storage bin in the reactor. The whole process consists of several sub-processes, e.g., the granular discharge from the sphere storage bin into the boring, granular discharge from the side reflector boring into the feeder, particle entrainment in the feeder, sphere conveying in the transport pipe, and gas-solid separation in the storage bin. We give a brief summary of the experimental and discrete element simulation work done recently at the Institute of Nuclear and New Energy Technology of Tsinghua University, mainly from the viewpoint of gas-solid flow and granular flow. CFD-DEM coupling simulation is recently conducted to investigate the coarse particle conveying in the small absorber sphere system. We present some CFD-DEM simulation results of coarse particle conveying in ambient air and high-pressure helium gas. Based on the needs of engineering practice and scientific research, some research requirements are suggested. Further work is still needed, to make a comprehensive understanding of the conveying process, and to optimize the design and operation of the conveying system.

Time-varying Brinkman electrophoresis of a charged cylinder-in-cell model

Abstract The time-evolving electrophoresis of the swarm of parallel charged circular cylinders in the transverse to their axe’s direction through an electrolyte-saturated porous medium when suddenly applying an external electric field is studied semianalytical approach using the unit cell model. The electric double layer in the porous continuum surrounding the circular cylinder in a unit cell is supposed to be thin but finite, and the effect of dynamic electroosmosis within it is included. Explicit formulas of the time-periodic electrophoresis and settling velocities of the circular cylinders in the Laplace transform as functions of the relevant physical parameters of the problem are performed for two different boundary conditions on the outer virtual surface of the cell model. The calculation results indicate that the time scale for the development of electrophoresis and sedimentation is small for a high permeability, a swarm of circular cylinders through a charged porous Brinkman medium with a higher volume fraction of the particle, and a smaller particle to porous fluid density ratio; the electrophoresis mobility increases monotonically with an increase in the electrokinetic radius of the cylinder at any instant. The time-dependent electrophoresis velocity has a monotonic decreasing function with increasing the volume fraction of the particle at a low value of the cylindrical particle-to-medium density ratio, but it may increase as the volume fraction of charged cylindrical particles increases at a high value of this density. The result of particle interaction in a suspension on the start-up of electrophoresis is much weaker than that on the time-varying sedimentation of the circular cylindrical particles. The comparison between calculations of the cell model with associated boundary conditions at the fictitious surface of the cell is derived and depicted graphically. The cell model predicts that, under otherwise specified conditions, the electrophoresis mobility in a Brinkman medium constructed by a system of parallel cylinders in the transverse direction, in general, is weaker than that of a system of spheres.

Chain formation in a two-dimensional system of hard spheres with a short-range anisotropic interaction.

We analyze a generalization of the hard-sphere dipole system in two dimensions in which the range of the interaction can be varied. We focus on the system in the limit in which the interaction becomes increasingly short ranged, while the temperature becomes low. By using a cluster expansion and taking advantage of low temperatures to perform saddle-point approximations, we argue that a well-defined double limit exists in which the only structures which contribute to the free energy are chains. We then argue that the dominance of chain structures is equivalent to the dominance of chain diagrams in a cluster expansion, but only if the expansion is performed around a hard-sphere system (rather than the standard ideal gas). We show that this leads to nonstandard factorization rules for diagrams and use this to construct a closed-form expression for the free energy at low densities. We then compare this construction to several models previously developed for the hard-sphere dipole system in the regime where chain structures dominate and argue that the comparison provides evidence in favor of one model over the others. We also use this construction to incorporate some finite-density effects though the hard-sphere radial distribution function and analyze how these effects impact chain length and the equation of state.

Pressure fluctuation instability in vertical plug formation of coarse particles with a non-mechanical feeder

Plug conveying receives attention due to its advantages such as low particle attrition, low pipeline wear, and low energy consumption. Based on the background of pneumatic transportation of absorber spheres in the small absorber sphere system of pebble bed HTGR, a novel non-mechanical feeder for plug formation of coarse particles has been proposed in our previous work. We further investigate the pressure fluctuation instability in the vertical plug formation of coarse particles with the non-mechanical feeder. Experiments for plug formation are conducted with glass beads of three kinds of particle diameter (dp = 6, 4, and 2 mm). The results show that micro-scale instabilities along with macro-scale instabilities are observed for the pressure fluctuations of feeder gas inlet in the vertical plug formation for the three particle diameters. It is interesting to find that the micro-scale instabilities decrease first and then increase with superficial gas velocity increasing for dp = 6 and 4 mm glass beads, respectively. The present study provides a further understanding of the pressure fluctuation instabilities in vertical plug formation of coarse particles with the non-mechanical feeder, which can contribute for on-line monitoring and operation optimization of dense-phase gas–solid flows with coarse particles.

Phase diagram for ensembles of random close-packed Ising-like dipoles as a function of texturation

We study random close packed systems of magnetic spheres by Monte Carlo simulations in order to estimate their phase diagram. The uniaxial anisotropy of the spheres makes each of them behave as a single Ising dipole along a fixed easy axis. We explore the phase diagram in terms of the temperature and the degree of alignment (or texturation) among the easy axes of all spheres. This degree of alignment ranges from the textured case (all easy axes pointing along a common direction) to the non-textured case (randomly distributed easy axes). In the former case we find long-range ferromagnetic order at low temperature but, as the degree of alignment is diminished below a certain threshold, the ferromagnetic phase gives way to a spin-glass phase. This spin-glass phase is similar to the one previously found in other dipolar systems with strong frozen disorder. The transition between ferromagnetism and spin-glass passes through a narrow intermediate phase with quasi-long-range ferromagnetic order.

Mean-field stability map of hard-sphere glasses.

The response of amorphous solids to an applied shear deformation is an important problem, both in fundamental and in applied research. To tackle this problem, we focus on a system of hard spheres in infinite dimensions as a solvable model for colloidal systems and granular matter. The system is prepared above the dynamical glass transition density, and we discuss the phase diagram of the resulting glass under compression, decompression, and shear strain, expanding on previous results [Urbani and Zamponi, Phys. Rev. Lett. 118, 038001 (2017)PRLTAO0031-900710.1103/PhysRevLett.118.038001]. We show that the solid region is bounded by a "shear jamming" line, at which the solid reaches close packing, and a "shear yielding" line, at which the solid undergoes a spinodal instability towards a liquid flowing phase. Furthermore, we characterize the evolution of these lines upon varying the glass preparation density. This paper aims to provide a general overview on yielding and jamming phenomena in hard-sphere systems by a systematic exploration of the phase diagram.

МОДЕРНІЗАЦІЯ СИСТЕМИ ВІЙСЬКОВОЇ ОСВІТИ УКРАЇНИ ТА ФОРМУВАННЯ ЗАГАЛЬНОГО ОБРИСУ ОФІЦЕРА МАЙБУТНЬОГО: РЕАЛІЇ І ПЕРСПЕКТИВИ

Introduction. The article deals with modern state and prospects for modernization of educational sphere and military education system of Ukraine under conditions of globalization, transition to informational and technological society, endorsement of its priorities, constant development.Purpose. The purpose of the article is to create a general image officer of the future image, to create an idea of his professional, personal, ideological, civic, psycho-physiological qualities and value orientations, based on the realities of the state of military education national system and the prospects for its modernization.Methods. The author used system analysis and synthesis, induction and deduction, comparison, classification, generalization and systematization methods.Results. Due to the modernization of education and eurointegrational processes Professional, personal, philosophical, civic psycho-physiological qualities and value orientations of the officer of the future image are defined in the article.Originality. Professional, personal, philosophical, civic psycho-physiological qualities and value orientations of the officer of the future image (2030) are presented for the first time .Conclusion. The article presents the general theoretical approaches to the formation of the officer of the future image as a holistic representation of his professional, personal, philosophical, civic psycho-physiological qualities and value orientations .

Relationship between optical properties and soluble sugar contents of apple flesh during storage

Soluble solids (SS) in fruit are mainly composed of soluble sugars. This research aims to further the understanding of the detection mechanism of soluble solid content (SSC) based on optical technology by exploring the relationship between optical properties and soluble sugar contents. The total reflectance and total transmittance at 400–1050 nm of Fuji apple flesh stored at 25 °C for 50 d and 0 °C for 150 d were collected by an automatic integrating sphere system. The absorption coefficient (μa) and reduced scattering coefficient (μ's) were obtained by iteratively solving the radiative transfer equation using the inverse adding doubling algorithm. The relationship of μa and μ's with the contents of SS, total soluble sugars, fructose, glucose and sucrose were quantitatively analyzed at different wavelengths, and prediction models were established by partial least squares regression (PLSR). The results showed that the changes in μa, μ's, SSC and soluble sugar content presented similar trends during storage at the two test temperatures. As the storage time increased, the decreases in μa and μ's were accompanied by declines in SSC and soluble sugar content. In addition, μa and μ's at 550–1050 nm were both positively correlated with SSC and soluble sugar content, with correlation coefficients (r) of 0.834-0.992 and 0.737-0.981, respectively. Compared with the correlations at 550–780 nm, the correlations at 780–1050 nm between μa and SSC and soluble sugar content were enhanced, while the corresponding correlations with μ's were gradually weakened. In addition, SS was most strongly correlated with sucrose among the three types of soluble sugars. SS and sucrose had closer relationship with μa and μ's than fructose and glucose with μa and μ's. Moreover, their prediction models also performed better than the models for fructose and glucose, with Rp2 values of 0.731-0.804. Thus, the prediction of SSC based on Vis-NIR optical technology may be related to the high correlations between the absorption and scattering properties and the sucrose content.

Coincidence of the freezing and the onset of caging in hard sphere and Lennard-Jones fluids.

In this article, we examine the collective particle dynamics, as expressed by the time correlation function of the longitudinal particle current density, of several different fluids in the vicinity of their freezing points/lines. We consider and compare results obtained by dynamic light scattering for a suspension of hard spheres and by molecular dynamics for fluids with hard sphere and Lennard-Jones interactions. The latter are performed along both an isotherm and an isochore. In all cases, we find a qualitative change in the collective dynamics, within the resolution of the data, when their respective freezing lines are crossed. We associate this change with the onset of caging. The new results for the Lennard-Jones fluid reported here confirm that the occurrence of caging, found previously for systems of hard spheres, is a more general feature that distinguishes a metastable fluid from one in thermodynamic equilibrium.

Convergence to self-similarity for ballistic annihilation dynamics

We consider the spatially homogeneous Boltzmann equation for ballistic annihilation in dimension d⩾2. Such model describes a system of ballistic hard spheres that, at the moment of interaction, either annihilate with probability α∈(0,1) or collide elastically with probability 1−α. Such equation is highly dissipative in the sense that all observables, hence solutions, vanish as time progresses. Following a contribution, by two of the authors, considering well-posedness of the steady self-similar profile in the regime of small annihilation rate α≪1, we prove here that such self-similar profile is the intermediate asymptotic attractor to the annihilation dynamics with explicit universal algebraic rate. This settles the issue about universality of the annihilation rate for this model brought in the applied literature.

Photoluminescence Quantum Yield of Fluorescent Silicon Carbide Determined by an Integrating Sphere Setup.

The excitation-dependent photoluminescence quantum yield (PL-QY) of strong n-type nitrogen–boron codoped 6H fluorescent silicon carbide (f-SiC) at room temperature is experimentally determined for the first time. The PL-QY measurements are realized by an integrating sphere system based on a classical two-measurement approach. In particular, in accordance to the difference between our in-lab setup and the standard setup of the two-measurement approach, we have technically modified the experimental design, the data processing algorithm, and the estimation of relative uncertainty. The measured highest PL-QY of f-SiC samples is found to reach above 30%. We compare the PL-QYs at a certain excitation power of all f-SiC samples by considering their intrinsic defect densities. Finally, the evolution of the excitation power-dependent PL-QY of f-SiC is attributed to both band-to-band and impurity-assisted Auger recombination.

Indicative Conditionals and Graded Information

I propose an account of indicative conditionals that combines features of minimal change semantics and information semantics. As in information semantics, conditionals are interpreted relative to an information state in accordance with the Ramsey test idea: “if p then q” is supported at a state s iff q is supported at the hypothetical state s[p] obtained by restricting s to the p-worlds. However, information states are not modeled as simple sets of worlds, but by means of a Lewisian system of spheres. Worlds in the inner sphere are considered possible; worlds outside of it are ruled out, but to different degrees. In this way, even when a state supports “not p”, it is still possible to suppose p consistently. I argue that this account does better than its predecessors with respect to a set of desiderata concerning inferences with conditionals. In particular, it captures three important facts: (i) that a conditional is logically independent from its antecedent; (ii) that a sequence of antecedents behaves like a single conjunctive antecedent (the import-export equivalence); and (iii) that conditionals restrict the quantification domain of epistemic modals. I also discuss two ways to construe the role of a premise, and propose a generalized notion of entailment that keeps the two apart.

New Media and the Dalit Counter-public Sphere

The recent surge of online mobilization among Dalits, who belong to India’s most oppressed caste groups, signals a remarkable trend in Internet activism and political engagement. Analyzing Dalit mobilization online, this article argues that technological affordances and distinct cultural practices associated with digital media have enabled certain sections of Dalits to resist the dominant caste narrative, thereby contributing to mobilization against caste-based discrimination. However, multiple “counter-narratives” within the subaltern digital sphere and hybrid media systems have placed limitations on unified action, signaling the need to address mediated factional positions and caste Hindu narratives online in efforts to strengthen the Dalit social movement. This article forwards the argument by examining the expansion of web forums organized by Dalits, with a closer focus on a nationwide online agitation that drew reference to the Black Lives Matter movement in the United States.

Inelastic collapse of perfectly inelastic particles

One of the most intensively discussed subjects in the dynamics of dissipative hard sphere systems is the effect of inelastic collapse, where the entire kinetic energy of the relative motion of a set of particles is dissipated in finite time due to an infinite sequence of collisions. The known collapse scenarios imply two preconditions: inertia of the particles and at least some degree of elasticity. For completely inelastic particles, collapse scenarios degenerate to a single sticky contact. By considering the overdamped motion of a frictional particle along the steepest descent in a rigid landscape, here we show that there exist collapse scenarios of novel type even if neither of these preconditions hold true. By means of numerical simulations we show that such collapses are no rare events due to particular particle shape and/or initial conditions and, thus, may be considered as an alternative scenario of granular cluster formation.

Vibrational properties of two-dimensional dimer packings near the jamming transition.

Jammed particulate systems composed of various shapes of particles undergo the jamming transition as they are compressed or decompressed. To date, sphere packings have been extensively studied in many previous works, where isostaticity at the transition and scaling laws with the pressure of various quantities, including the contact number and the vibrational density of states, have been established. Additionally, much attention has been paid to nonspherical packings, and particularly recent work has made progress in understanding ellipsoidal packings. In this work, we study the dimer packings in two dimensions, which have been much less understood than systems of spheres and ellipsoids. We first study the contact number of dimers near the jamming transition. It turns out that packings of dimers have "rotational rattlers," each of which still has a free rotational motion. After correcting this effect, we show that dimers become isostatic at the jamming, and the excess contact number obeys the same critical law and finite-size scaling law as those of spheres. We next study the vibrational properties of dimers near the transition. We find that the vibrational density of states of dimers exhibits two characteristic plateaus that are separated by a peak. The high-frequency plateau is dominated by the translational degree of freedom, while the low-frequency plateau is dominated by the rotational degree of freedom. We establish the critical scaling laws of the characteristic frequencies of the plateaus and the peak near the transition. In addition, we present detailed characterizations of the real space displacement fields of vibrational modes in the translational and rotational plateaus.

Non-equilibrium hybrid insertion/extraction through the 4th dimension in grand-canonical simulation

The process of inserting/deleting a particle during grand-canonical Monte-Carlo (MC) simulations is investigated using a novel, original technique: the trial event is made of a short nonequilibrium molecular dynamics (MD) trajectory during which a coordinate w along a 4th dimension is added to the particle in the course of insertion/deletion and is forced to decrease from large values down to zero (for insertion) or increased from 0 up to large values (for extraction) at imposed vw velocity. The probability of acceptation of the whole MC move is controlled by the chemical potential and the external work applied during the trajectory. Contrary to the standard procedures which create/delete suddenly a particle, the proposed technique gives time to the fluid environment to relax during the gradual insertion/extraction before the acceptation decision. The reward for this expensive trial move is a gain of many orders of magnitude in the success rate. The power and wide domain of interest of this hybrid "H4D" algorithm which marries stochastic MC and nonequilibrium deterministic MD flavors are briefly illustrated with hard sphere, water, and electrolyte systems. The same approach can be easily adapted in order to measure the chemical potential of a solute particle immersed in a fluid during canonical or isobaric simulations. It then becomes an efficient application of the Jarzynski theorem for the determination of solvation free energy.

Effect of a plant growth regulator on optical properties of kiwifruit during growth in wavelength range of 950-1650 nm

The absorption coefficient (μa) and reduced scattering coefficient (μ's) of flesh and skin of kiwifruit samples, which were treated with N-(2-chloro-4-pyridyl)-N′-phenylurea (CPPU, a plant growth regulator) at the concentrations of 0, 5, 10, and 15 mg/L, were measured using an integrating sphere system in 950–1650 nm and 4–16 weeks after full bloom. The results showed that at the same growth stage, the flesh's μa of CPPU treated kiwifruit were higher than that of CPPU untreated ones at the absorption peaks of 970 nm, 1190 nm and 1390 nm. Over the whole growth stage, the skin's μa of CPPU treated kiwifruit was higher than that of CPPU untreated ones at the absorption peak of 1440 nm. The μ's of flesh and skin almost increased with the increase of CPPU concentration. Both CPPU concentration and growth stage had significant influence on μa and μ's of kiwifruit's flesh and skin at absorption peaks. Moreover, the CPPU treated kiwifruit had higher soluble solids content and moisture content, lower firmness, and larger cell than untreated ones. The study provides useful information for knowing the influence of CPPU on optical properties of kiwifruit, and suggests that the optical properties has great potential in discriminating CPPU treated kiwifruits from untreated ones and even in identifying CPPU concentration.

Monte Carlo simulation for a gas of hard spheres in d-dimensional space: Equilibrium structure and state equations

In this work, we emphasize that it is possible using a personal computer to perform a MonteCarlo simulations in a reasonable computing time, and find the equilibrium structure of a hardsphere gas for a Euclidean multi-dimensional spaces. We study the properties of equilibrium and determine the equation of state of gas of hard spheres in Euclidean spaces from two to seven dimensions. The results show that the pressure is in agreement with different theoretical models based on virial expansion in spaces from two to five dimensions, also our results are extended for seven dimensions. As expected, it was found that the system of hard spheres loses its structure and the pressure of the system decreases when the dimension of the space increases.

Simulation of solid propellant microstructures by combining the collective rearrangement method with the discrete element method

The polydisperse particulate components in solid propellant are incompact and randomly packed, which determines the microstructural features of the propellants. A packing method, combining the discrete element method (DEM) and collective rearrangement method, was applied to model propellant microstructures. The validity of this method was investigated by comparing the calculated and experimental properties of the monodisperse, bidisperse, and polydisperse random close packed sphere systems. The propellant models were generated using a stepwise approach, and their homogeneity, local randomness, and long-range pattern were analyzed. A statistical study of aluminum (Al) particle distribution was also conducted. The results indicated that this packing method can effectively determine the microscopic characteristics of random close packed monodisperse spheres. The maximum packing fraction of bidisperse and polydisperse spheres had similar trends to those reported in experimental studies and using other packing algorithms. In addition, this method was capable of generating non-compacted propellant structures with uniformly distributed polydisperse particles. The radial distribution functions (RDFs) for Al-Al particles provided information about the Al distribution, but this was mainly related to the size and content of the large particle components.