Aspherical Particle Research Articles

High-Resolution Two-Phase Velocimetry Of Aspherical Particles Using Wavelet-Based Optical Flow Velocimetry (WOFV) Benchmarked With Fully Resolved Direct Numerical Simulations

Several techniques exist to measure the carrier and dispersed phases in multi-phase flows. Particle image velocimetry (PIV) analyzes the carrier phase by cross-correlating particle images, but its resolution is limited by the size of the interrogation window, making it difficult to resolve fine-scale turbulent structures. Particle tracking algorithms capture translational motion of dispersed particles well, but struggle with rotational motion, especially for irregular and aspherical particles. Currently, there is no method that universally measures both the carrier phase velocity field and the translational and rotational motions of dispersed particles. This study evaluates wavelet-based optical flow velocimetry (wOFV) for motion estimation in multi-phase flows, focusing on dispersed ellipsoidal particles and their surrounding turbulent carrier flow using synthetic image data. The research proceeds in two phases: first, the rigid motion of ellipses, including translational and rotational components, is analyzed using wOFV-generated dense motion fields. The results highlight the critical role of the regularization weighting parameter λ. Higher λ values improve translational motion estimation, while an optimal λ avoids under-regularization and non-physical structures in rotational motion. wOFV maintains accuracy in combined motion scenarios with optimal λ values. In the second phase, wOFV captures the turbulent carrier flow around an aspherical particle, benchmarked against DNS data and compared to PIV. wOFV outperforms PIV in resolving finer structures near the particle surface and in accurately representing the wake region. Error analyses confirm wOFV’s superior performance, with optimal results within a specific λ range. In conclusion, wOFV is a highly effective tool for the analysis of multi-phase flow dynamics, providing greater resolution and accuracy than PIV, especially in complex scenarios involving aspherical particles.

Effect of sand particle shape on micromechanical modeling in direct shear testing

Soil representation in discrete element method (DEM) simulations is a complex process due to the intrinsic variability in grain shape. Many methods have been proposed to include shape effects in DEM, from rolling friction to aspherical particle representations such as ellipsoids or polyhedra. In this paper the micromechanical differences between two commonly used methods of shape representation, rolling friction applied to spherical particles and ellipsoids, are investigated for direct shear testing. This simulation study is supported by dry direct shear tests in the laboratory using a poorly graded sand and three vertical confining pressures: 50, 100 and 200 kPa. While the bulk behavior observed in the laboratory can be matched using both shape representation approaches, the micromechanics show significant differences. The average cumulative rotation of the ellipsoidal particles is an order of magnitude lower than for the spheres with rolling friction. The addition of rolling friction significantly affects the mechanical redundancy of the system, making the spherical-particle samples more hyperstatic than their ellipsoidal equivalents. Although using ellipsoids enables a better representation of the micromechanical behavior, this increases the computational cost five-fold compared to using rolling friction and spheres.

Stress analysis of aspherical TRISO-coated particle with X-ray computed tomography

Stress analysis of aspherical TRISO-coated particle with X-ray computed tomography

PEPT validated CFD-DEM model of aspherical particle motion in a spouted bed

Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) has been used to model the dynamics of aspherical particles in a spouted bed. CFD-DEM results were rigorously validated against high-resolution, three-dimensional experimental data acquired using Positron Emission Particle Tracking (PEPT). A workflow facilitating the direct, detailed, quantitative comparison of experimental and numerical data, and thus validation of CFD-DEM simulations, was developed. Sensitivity analysis considering a wide range of particle properties and drag correlations was carried out. It was observed that the Young’s modulus, Poisson’s ratio, and restitution coefficient do not have a significant effect on the system’s dynamics across the range of parameters explored, and that Gidaspow’s correlation using equivalent intrinsic particle density is an appropriate choice for modelling drag forces. Analysis shows PEPT-validated CFD-DEM models quantitatively capture key features including particle circulation time, particle velocity and occupancy distributions, and the proportion of residence time in the bulk-bed and freeboard regions, all of which are crucial to understanding mass and heat transfer within the system. Our results also demonstrated that models using simple, spherical particles were – with suitably modified friction and drag models – capable of quantitatively reproducing the dynamics of a range of highly aspherical materials.

Asphericity Can Cause Nonuniform Lithium Intercalation in Battery Active Particles

Uniform intercalation is desired to enable next-generation Li-ion batteries. While we expect nonuniformity in materials undergoing a phase change, single-phase intercalation materials such as nickel manganese cobalt oxide are believed to lithiate uniformly at the particle/electrolyte interface. However, recent imaging reveals nonuniform lithiation. Motivated by this discrepancy, we examine if aspherical particle shape can cause such nonuniformity since the conventional belief is based on spherical particle theory. We obtain real particle geometries using rapid lab-based X-ray computed tomography and subsequently perform physics-based calculations accounting for electrochemical reactions at the particle/electrolyte interface and lithium transport inside the particle bulk. The aspherical geometry breaks the symmetry and causes nonuniform reaction distribution. Such nonuniformity is exacerbated as the particle becomes more aspherical. The proposed mechanism represents a fundamental limit on achievable lithiation uniformity in aspherical particles in the absence of other mechanisms causing inhomogeneity, such as grain structure, nonuniform carbon-binder coating, etc.

TRISO particle fuel performance and failure analysis with BISON

Because of its widespread use in multiple advanced reactor concepts, the ability to accurately simulate tri-structural isotropic (TRISO) fuel performance is essential for ensuring the safe operation of these reactors. To that end, the BISON fuel performance code has undergone significant recent development to expand its TRISO particle fuel performance modeling capabilities. This includes the development of material models, such as elastic, creep, swelling, thermal expansion, thermal conductivity, and fission gas release models. The capability to perform statistical failure analysis on large sets of samples has also been developed, utilizing a Monte Carlo scheme to execute fast-running 1-D spherically symmetric models. Stress adjustments are made in those 1-D models to account for multi-dimensional failure phenomena. Stress correlation functions are extracted from multi-dimensional failure simulation results, such as from a particle with cracked inner pyrolytic carbon (IPyC) and an aspherical particle. This paper provides a detailed description of the models used by BISON for TRISO fuel, along with a set of problems that test these models by (favorably) comparing them both with another code and experimental data. These include simulations of the Advanced Gas Reactor (AGR)-2 and AGR-5/6/7 experiments, with predictions for fuel performance parameters, failure probability, and fission product transport.

Effect of particle size and shape on liquid–solid fluidization in a HydroFloat cell

The separation of coarse particles by flotation can be maximized by the support of fluidized bed hydrodynamics. Here, we investigated the effect of particle size and shape on the liquid-solid fluidization in a HydroFloat cell that was developed for the special separation process using CFD results and experimental data. Specifically, a 3D CFD model was developed to study the liquid and aspherical quartz particle flow behaviors in a fluidized bed in the cell. The Eulerian–Eulerian formulation with the RNG k-ε turbulence model was applied for the liquid phase, while the kinetic theory of granular flow was utilized for the particle phase. The Gidaspow with Haider and Levenspiel equations were used for the drag force coupling between the liquid and the aspherical particles. The CFD results agreed with the experimental data for the bed voidage and pressure drop. The CFD model was further applied to investigate the effect of particle size (dS) and particle shape factor (ψ) on the bed expansion, pressure drop, and voidage of the fluidization for Re (particle Reynolds number) < 11.6. The Richardson-Zaki equation was modified to predict the voidage, within 12% of relative errors. The prediction was further improved by our semi-empirical correlation that reduced the errors to 8%. The particle size and shape were also found to significantly affect the time-averaged radial distribution of the axial velocity and volume fraction of the particles, granular temperature, dissipation rate, and viscosity of turbulence. The outcomes of this paper are useful for optimizing the operation of the HydroFloat cell.

Aspherical particle models for molecular dynamics simulation

In traditional molecular dynamics (MD) simulations, atoms and coarse-grained particles are modeled as point masses interacting via isotropic potentials. For studies where particle shape plays a vital role, more complex models are required. In this paper we describe a spectrum of approaches for modeling aspherical particles, all of which are now available (some recently) as options within the LAMMPS MD package. Broadly these include two classes of models. In the first, individual particles are aspherical, either via a pairwise anisotropic potential which implicitly assigns a simple geometric shape to each particle, or in a more general way where particles store internal state which can explicitly define a complex geometric shape. In the second class of models, individual particles are simple points or spheres, but rigid body constraints are used to create composite aspherical particles in a variety of complex shapes. We discuss parallel algorithms and associated data structures for both kinds of models, which enable dynamics simulations of aspherical particle systems across a wide range of length and time scales. We also highlight parallel performance and scalability and give a few illustrative examples of aspherical models in different contexts.

Coefficient of restitution of aspherical particles.

We consider the motion of an aspherical inelastic particle of dumbbell type bouncing repeatedly on a horizontal flat surface. The coefficient of restitution of such a particle depends not only on material properties and impact velocity but also on the angular orientation at the instant of the collision whose variance is considerable, even for small eccentricity. Assuming random angular orientation of the particle at the instant of contact we characterize the measured coefficient of restitution as a fluctuating quantity and obtain a wide probability density function including a finite probability for negative values of the coefficient of restitution. This may be understood from the partial exchange of translational and rotational kinetic energy.

Stress analysis of aspherical coated particle with inner pressure

Coated particles used in HTR fuel element sustain the inner pressure during irradiation as a pressure vessel. In actually the coated particle is not real spherical but with some asphericity, the stress distribution in the vessel is not uniform, coated layer in aspherical particle sustain more additional stress due to the asphericity. In this paper, the geometric shape distribution is summarized based on actual coated particle statistic. A mechanical analysis model is proposed for SiC layer by geometric combinations, and stress distribution of coated particle with a flat is calculated. The results show that the local maximum stress of aspherical particle increased two times than that of ideal spherical coated particle, which increase the failure probability under irradiation and high temperature.

Interfacial Hydrodynamic Drag on Nanowires Embedded in Thin Oil Films and Protein Layers

We investigate the motion of ferromagnetic nanowires confined to nanometer-scale oil films at an air/aqueous interface in response to the application of external magnetic fields and field gradients. By varying the oil viscosity, film thickness, and wire length, we cover two regimes of response suggested by theory: one where the surface viscosity is expected to dominate the wire's motion and one where the subphase viscosity is expected to dominate [Levine, A. J.; Liverpool, T. B.; MacKintosh, F. C. Phys. Rev. E 2004, 69, 021503]. For wire motion parallel to the long axis of the wire, the observed drag agrees reasonably with theoretical predictions. However, the drag on wires moving perpendicular to their long axis or rotating about a short axis is unexpectedly insensitive to the film properties over the full range of measurements. This behavior is in contrast to the rotational and translational drag on nanowires in molecularly thin protein layers, which follow theoretical expectations. The observations in the oil films, which are explained in terms of the manner in which the wire immerses dynamically in the film and subphase, demonstrate how the effective drag viscosity of an aspherical particle confined to a fluid interface can depend on its direction of motion.

Probing ice clouds by broadband mid-infrared extinction spectroscopy: case studies from ice nucleation experiments in the AIDA aerosol and cloud chamber

Abstract. Series of infrared extinction spectra of ice crystals were recorded in the 6000–800 cm−1 wavenumber regime during expansion cooling experiments in the large aerosol and cloud chamber AIDA of Forschungszentrum Karlsruhe. Either supercooled sulphuric acid solution droplets or dry mineral dust particles were added as seed aerosols to initiate ice formation after having established ice supersaturated conditions inside the chamber. The various ice nucleation runs were conducted at temperatures between 237 and 195 K, leading to median sizes of the nucleated ice particles of 1–15 µm. The measured infrared spectra were fitted with reference spectra from T-matrix calculations to retrieve the number concentration as well as the number size distribution of the generated ice clouds. The precise evaluation of the time-dependent ice particle number concentrations, i.e., the rates of new ice particle formation, is of particular importance to quantitatively analyse the ice nucleation experiments in terms of nucleation rates and ice activation spectra. The ice particles were modelled as finite circular cylinders with aspect ratios ranging from 0.5 to 3.0. Benefiting from the comprehensive diagnostic tools for the characterisation of ice clouds which are available at the AIDA facility, the infrared retrieval results with regard to the ice particle number concentration could be compared to independent measurements with various optical particle counters. This provided a unique chance to quantitatively assess potential errors or solution ambiguities in the retrieval procedure which mainly originate from the difficulty to find an appropriate shape representation for the aspherical particle habits of the ice crystals. Based on these inter-comparisons, we demonstrate that there is no standard retrieval approach which can be routinely applied to all different experimental scenarios. In particular, the concept to account for the asphericity of the ice crystals, the a priori constraints which might be imposed on the unknown number size distribution of the ice crystals (like employing an analytical distribution function), and the wavenumber range which is included in the fitting algorithm should be carefully adjusted to each single retrieval problem.

Linking snowpack microphysics and albedo evolution

Snow aging causes reflectance to vary significantly on timescales of days. This variability influences the strength of snow albedo feedback and can affect the timing of snowmelt. However, climate models have yet to incorporate important controls on snow aging and albedo evolution. We develop a physically based model that predicts evolution of dry, pure snow specific surface area, and apply aspherical ice particle theory to link these results with albedo evolution. This is the first theoretical study to quantify the relative roles of initial size distribution, vertical temperature gradient, and snow density in snow albedo evolution. Vapor diffusion caused by curvature differences causes rapid albedo decay in the first day following snowfall. Vertical temperature gradient generally dominates grain growth processes afterward but is modulated by snow density, irregularity in particle spacing, and temperature. These processes operate as a coupled system, which we uniquely represent without abrupt transitions between regimes. Model results agree very well with measurements of isothermal snow evolution and are within reasonable range of temperature gradient observations. We show that different snow state regimes cause albedo of nonmelting snow surfaces with identical initial albedo to vary by 0.12 or more after 14 days. Lack of quality observational data illuminates the need for well‐controlled snow studies that simultaneously monitor specific surface area, temperature gradient, and albedo. Accounting for snow aging processes, especially temperature gradient, will improve understanding and assessment of snow albedo feedback and climate sensitivity. The modeling framework we develop will also be useful for air‐snow chemistry studies that consider specific surface area.

Aspherical particle orientation in the atmosphere

Phase states of aspherical particles in the atmosphere are examined. The orientation mechanisms at work in particles in the form of ellipsoids of revolution moving in a viscous medium on exposure to aerodynamic forces and in interactions of the induced dipole moment of such a particle with an external electric field are considered.

Calculation of Thermal Stresses in Glass-Ceramic Composites

Opto-electronics make intensive use of composite materialsbased on amorphous materials, which can be considered as smart materialssince they are capable of high performances in their final state.Particularly, glass-ceramic composites involved in welding operationsfor microelectronics applications are subjected to important thermalstresses during their production, which can deteriorate their propertiesat room temperature, until the failure stage is reached. It is thenessential to be able to predict the evolution of the internal stressesgenerated during the cooling. We have performed finite-elementsimulations in order to quantify the stress evolutions for differentcomposite geometries: a ceramic fiber embedded within a glass matrix, aspherical particle located at the center of a spherical glass matrix,and a dispersion of spherical ceramic particles, this last case beingthe most representative of reality. The thermomechanical modeling of theglassy matrix takes into account its viscoelastic behavior, and theglass transition is described by the decrease during cooling of the freevolume as a function of the temperature history. The combined effect ofthe differential thermal strain during the transition and mechanicalrelaxation of glass on stress evolutions is evidenced. It is shown thatthe consideration of a periodical or random distribution of sphericalceramic particles leads to similar stress profiles.

Liquid lenses at fluid/fluid interfaces

In a previous study we have explored how the wettability of a spherical solid particle resting in a fluid/fluid interface can be influenced by effects of line tension (τ) acting in the circular three-phase contact line around the particle. Here, we extend that study to consider possible effects of line tension on a small liquid lens resting in a liquid surface; the deformability of the lens predictably adds some new features. In the case of a lens we are interested in the way in which τ influences both the complete wetting of the subphase by the lens and the complete wetting of the lens by the subphase (i.e. engulfment of the lens). We propose a definition of a spreading coefficient (for the lens material on the subphase) which incorporates line tension. Negative τ favours spreading and it is shown that it is possible in systems in which large lenses would not spread (e.g. dodecane on water at room temperature), the operation of negative line tension could cause spreading in lenses below a critical radius. For positive τ the behaviour of a lens broadly mirrors that of a spherical solid particle. For line tensions below a critical value τ c the lens can assume a thermodynamically stable configuration in the interface. This happens when the Gibbs energy of the system with the lens at the interface is more negative than that for the system in which the lens exists (in the form of a spherical droplet) in the more wetting of the contiguous phases. For line tensions between τ c and τ m only metastable configurations are possible; τ m is the line tension above which no stable configuration for the lens at the interface is possible. We consider possible effects of line tension in surfactant systems of potential practical interest, and allude to the role of liquid oil droplets in the rupture of thin liquid films and hence in foam breaking.

Observation of Aspherical Particle Rotation in Poiseuille Flow via the Resistance Pulse Technique: I. Application to Human Erythrocytes

The resistance pulse detector (Coulter counter) has been widely applied to the problem of determining the volumes of insulating particles in electrolyte solutions. This technique is based on the simple relationship, DeltaR/R = fv/V, between the fractional resistance change DeltaR/R and the ratio of particle volume v to pore volume V. The proportionality constant f is a function of particle shape and orientation. Direct observation of the expected resistance anisotropies for aspherical particles is reported here. As predicted by simple hydrodynamic theory each individual resistance pulse has a periodically varying amplitude as it traverses a long pore in the shear field of Poiseuille flow. Characteristics of the particle motion allow improved volume distribution determinations by properly accounting for the shape factor. Application is made to normal human erythrocytes and a gaussian volume distribution with a coefficient of variation approximately 19% is found. The electrical shape effect for erythrocytes is consistent with an oblate ellipsoidal particle with a diameter-to-thickness ratio of 4. Analysis of the data indicates that the convergent entrance flow orients the cells so that they enter the pore with their axis of symmetry perpendicular to the pore axis.