Virtual Particles Research Articles

Shape characteristics of granular materials through realistic particle avatars

The micro- and macroscale mechanical behaviors of granular materials strongly depend on the morphological features of the grains. Imaging techniques, such as X-ray computed tomography, have been used to scan realistic particle geometries and obtain shape characteristics. However, these methods are expensive, laborious, and only applicable to limited grain sizes. A simpler alternative would be to estimate 3D shape features from 2D digital photographs in the field, but they might be affected by image resolution and particle orientation. This study aims to assess the effect of image quality for shape characterization and obtain relationships between widely used 2D and 3D shape descriptors through randomly generated computational particle geometries termed ‘avatars.’ Additionally, X-ray μ-CT scans are performed on beach sand grains with irregular shapes. Particle avatar-based 2D-3D form relationships are compared with μ-CT data of 38 beach sand grains and at least 2300 particles of other material types from literature. Changes in the resolution of the images affected both geometrical and morphological features of the particles. Volume and surface area measurements are affected by the 3D image quality, and low-quality 2D digital images influenced shape characterization. From the observed relationships between 2D and 3D shape descriptors, some 3D shape characteristics such as true sphericity and convexity can be reliably approximated from 2D digital images of randomly oriented particles.

An improved SPH method for simulating crack propagation and coalescence in rocks with pre-existing cracks

An improved smoothed particle hydrodynamics (SPH) method is proposed to simulate the failure process of rock samples with pre-existing cracks under compression loads. By improving the kernel function of SPH, the brittle fracture characteristics of rock materials are captured. In the improved SPH, a prefabricated node segment (PNS) method is used to generate arbitrarily complex open or closed cracks, avoiding the kernel truncation caused by digging out particles. In order to improve numerical stability, the tension instability control (TIC) technique is embedded in the momentum equation. Moreover, the improved SPH adopts the coupled dynamic buffer solid boundary treatment algorithm, which reduces the configuration of boundary virtual particles, makes the application of boundary conditions more efficient. The accuracy of the improved SPH method and the feasibility of TIC technique are verified by a benchmark example. Then, the effect of the center dislocation spacing of the pre-existing cross crack on the failure mode and compressive strength of rock samples is studied. In addition, the effects of confining pressure on the coalescence type and penetration failure mode of cracks in a rock sample with two pre-existing cracks are also investigated. The numerical results agree well with the experimental results, which verifies the accuracy and validity of the improved SPH method.

Efficient two-dimension particle element method of interior ballistic two-phase flow

Reducing the calculation cost is of great importance as the demand for interior ballistic calculation is increasing. The key factor of interior ballistic simulation is the quest to develop a more efficient method. In this paper, an efficient two-dimension particle element method (PEM) is proposed to simulate the detailed multidimensional flow of the gas and propellant particle in the chamber. Firstly, several real particles are packed as the particle element to reduce the calculation of the solid phase. In particular, the particle element matrix is established to describe the distribution of particle parameters. Secondly, the particle element boundary is adjusted according to the particle’s movement to reduce the computational cost of grid generation. Besides, the dynamic self-adapting mesh map method is adopted to realise the coupling computation between gas phase and the particle element. The application of a standard virtual gun as a standard benchmark for interior ballistic codes is used to validate the accuracy and reliability with 1.68% error. The particle element model accurately describes the distribution of flow field in the chamber. Compared with the two-fluid method, the PEM significantly improves the computational efficiency by 21.7%. The PEM may be promising for the rapid simulation of two-phase flow in interior ballistic.

Structure of swollen hollow polyelectrolyte nanogels with inhomogeneous cross-link distribution

Hypothesis: Recently, it has become possible to synthesize hollow polyelectrolyte nano- and microgels. The shell permeability can be controlled by external stimuli, while the cavity can serve as a storage place for guest molecules. However, there is a lack of a detailed understanding at the molecular level regarding the role of the network topology, inhomogeneities of the distribution of cross-links, and the impact of the electrostatics on the structural response of hollow microgel to external stimuli. To bridge these gaps, molecular dynamics (MD) of computer simulations are used.Experiments: Here, we propose a fresh methodology to create realistic hollow microgel particles in silico. The technique involves a gradual change in the average local length of subchains depending on the distance to the center of mass of the microgel particles resulting in the microgels with a non-uniform distribution of cross-linking species. In particular, a series of microgels with (i) a highly cross-linked inner part of the shell and gradually decreased cross-linker concentration towards the periphery, (ii) microgels with loosely cross-linked inner and outer parts, as well as (iii) microgels with a more-or-less homogeneous structure, have been created and validated. Counterions and salt ions are taken into account explicitly, and electrostatic interactions are described by the Coulomb potential.Findings: Our studies reveal a strong dependence of the microgel swelling response on the network topology. Simple redistribution of cross-links plays a significant role in the structure of the microgels, including cavity size, microgel size, fuzziness, and extension of the inner and outer parts of the microgels. Our results indicate the possibilities of qualitative justification of the structure of the hollow microgels in the experiments by measuring the relative change in the size of the sacrificial core to the size of the cavity and by estimation of a power law function, RH∼csα, of the hydrodynamic radius of the hollow microgels as a function of added salt concentration.

Utilizing Machine Learning to Greatly Expand the Range and Accuracy of Bottom-Up Coarse-Grained Models through Virtual Particles.

Coarse-grained (CG) models parametrized using atomistic reference data, i.e., "bottom up" CG models, have proven useful in the study of biomolecules and other soft matter. However, the construction of highly accurate, low resolution CG models of biomolecules remains challenging. We demonstrate in this work how virtual particles, CG sites with no atomistic correspondence, can be incorporated into CG models within the context of relative entropy minimization (REM) as latent variables. The methodology presented, variational derivative relative entropy minimization (VD-REM), enables optimization of virtual particle interactions through a gradient descent algorithm aided by machine learning. We apply this methodology to the challenging case of a solvent-free CG model of a 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid bilayer and demonstrate that introduction of virtual particles captures solvent-mediated behavior and higher-order correlations which REM alone cannot capture in a more standard CG model based only on the mapping of collections of atoms to the CG sites.

Plasmonic Properties of Individual Gallium Nanoparticles.

Gallium is a plasmonic material offering ultraviolet to near-infrared tunability, facile and scalable preparation, and good stability of nanoparticles. In this work, we experimentally demonstrate the link between the shape and size of individual gallium nanoparticles and their optical properties. To this end, we utilize scanning transmission electron microscopy combined with electron energy loss spectroscopy. Lens-shaped gallium nanoparticles with a diameter between 10 and 200 nm were grown directly on a silicon nitride membrane using an effusion cell developed in house that was operated under ultra-high-vacuum conditions. We have experimentally proven that they support localized surface plasmon resonances and their dipole mode can be tuned through their size from the ultraviolet to near-infrared spectral region. The measurements are supported by numerical simulations using realistic particle shapes and sizes. Our results pave the way for future applications of gallium nanoparticles such as hyperspectral absorption of sunlight in energy harvesting or plasmon-enhanced luminescence of ultraviolet emitters.

Quantum field theory with ghost pairs

We explicitly show that general local higher-derivative theories with only complex conjugate ghosts and normal real particles are unitary at any perturbative order in the loop expansion. The proof presented here relies on integrating the loop energies on complex paths resulting from the deformation of the purely imaginary paths, when the external energies are continued from imaginary to real values. Contrary to the case of nonlocal theories, where the same integration path was first proposed, for the classes of theories studied here the same procedure is not analytic, but the resulting theory is unitary and unique when the complex ghosts are present in pairs. As an explicit application, a special class of higher-derivative super-renormalizable or finite gravitational and gauge theories turns out to be unitary at any perturbative order if we exclude the complex ghosts from the spectrum of the theory, as it is normally accepted for Becchi-Rouet-Stora-Tyutin (BRST) ghosts. Finally, we propose an analogy between confined gluons in quantum Yang-Mills theory and classical complex pairs in local higher-derivative theories. According to such interpretation, complex ghosts will not appear on shell as asymptotic states because confined in what is natural to name “ghostballs.”

Optimized synthesis characterization and protective activity of quercetin and quercetin–chitosan nanoformula against cardiotoxicity that was induced in male Wister rats via anticancer agent: doxorubicin

The study’s goal was to look into the protective properties of quercetin (QU) in natural form and QU nanoparticles-loaded chitosan nanoparticles (QU-CHSNPs) against cardiotoxicity. The ionotropic gelation approach was adopted to form QU-CHSNPs. The characterizations were performed using advanced techniques. In vitro, the release profile of QU was studied. Cardiotoxicity was induced by doxorubicin (DOX) and protected via concurrent administration of QU and QU-CHSNPs. The heart's preventive effects of QU and QU-CHSNPs were manifested by a decrease in elevated serum activities of cardiac enzymes, as well as an improvement in the heart's antioxidant defence system and histological changes. The findings substantiated QU-CHSNPs' structure with an entrapment efficiency of 92.56%. The mean of the zeta size distribution was 150 nm, the real average particle size was 50 nm, and the zeta potential value was − 27.9 mV, exhibiting low physical stability. The percent of the free QU-cumulative release was about 70% after 12 h, and QU-CHSNPs showed a 49% continued release with a pattern of sustained release, reaching 98% after 48 h. And as such, QU and QU-CHSNPs restrained the induced cardiotoxicity of DOX in male Wistar rats, with the QU-CHSNPs being more efficient.

Selection of Relative DEM Time Step for Modelling Fast Fluidized Bed of A-Type FCC Particles

In chemical production processes, the most suitable operation regime for A-Type powders such as typical FCC particles is high-speed fast fluidization owing to their uniquely advantageous properties. Discrete element method (DEM) for modelling fast fluidization with A-Type powders has rarely been reported. How to appropriately select the DEM time step and the stiffness coefficient is one of the most critical problems for stable and accurate calculation. This article mainly discusses the effect of the stiffness coefficient and DEM time step on simulations of A-type FCC particles. In order to describe the effect of both parameters and their complex interaction, a dimensionless relative DEM time step is introduced. In total, nine cases with different numbers of relative time steps are adopted for modelling a microfluidized bed of A-Type FCC particles, the regime of which is proved to be fast fluidization. Results show that three bifurcations occur in all the simulations. Only the moderate relative time step possesses the capability of modelling the process of particle collision and thus predicts the right flow regime with asymmetric and heterogeneous typical fast fluidized structures. When the relative time step increases to large rank, simulations predict untrue fluidization regimes with symmetric and homogeneous structures. Moreover, both over-large and over-low relative time steps cause excessive particle overlap and thus a divergence of simulation. The further optimization of moderate relative DEM time step in relation to real particle property is unidentifiable and is thus an outstanding issue. That the range of the moderate relative time step is limited indicates that the common soft-sphere model is poor at modelling fast fluidization of A-Type particles. It is suggested that possible future work should be focused on improving the simulation frame and employing the molecular dynamics model to more appropriately deal with particle contact.

Morphology characterization and discrete element modeling of coral sand with intraparticle voids

Coral sand is the main geomaterial on tropical reefs and its constituent particles are featured with complex shapes and abundant intraparticle voids. This work presents a discrete element method (DEM)-based numerical modeling of coral sand in consideration of both the irregular shape and intraparticle voids of particles. To develop the DEM model for coral sand, the acquisition and characterization of coral sand particles are first introduced. A signed distance field-based DEM applicable to arbitrarily irregular-shaped particles is then presented, in conjunction with two versatile particle models, namely spherical harmonics (SH) and level set (LS). To generate virtual coral sand particles that conform to target morphology characteristics, the combined random field and SH approach and the remedy to elongated particles are also presented. The developed DEM model is calibrated and validated against laboratory column collapse tests, where the agreements between numerical simulations and laboratory experiments demonstrate the good validity and accuracy of DEM model for coral sand. The performance of SH and LS particle models are also compared, providing a useful reference for the selection of appropriate particle models for coral sand in practice.

Pathways and Timescales of Connectivity Around the Antarctic Continental Shelf

AbstractThe Antarctic Slope Current (ASC) and Antarctic Coastal Current advect heat, freshwater, nutrients, and biological organisms westward around the Antarctic margin, providing a connective link between different sectors of the continental shelf. Yet the strength and pathways of connectivity around the continent, and the timescales of advection, remain poorly understood. We use daily velocity fields from a global high‐resolution ocean‐sea ice model, combined with Lagrangian particle tracking, to shed light on these timescales and improve our understanding of circumpolar connectivity around Antarctica. Virtual particles were released along vertical transects over the continental shelf every 5 days for a year and were tracked forward in time for 21 years. Analysis of the resulting particle trajectories highlights that the West Antarctic sector has widespread connectivity with all regions of the Antarctic shelf. Advection around the continent is typically rapid with peak transit times of 1–5 years for particles to travel 90° of longitude downstream. The ASC plays a key role in driving connectivity in East Antarctica and the Weddell Sea, while the Coastal Current controls connectivity in West Antarctica, the eastern Antarctic Peninsula, and along the continental shelf east of Prydz Bay. Connectivity around the shelf is impeded in two main locations, namely, the tip of the Antarctic Peninsula and Cape Adare in the Ross Sea, where significant export of water from the continental shelf is found. These findings help to understand the locations and timescales over which anomalies, such as meltwater from the Antarctic Ice Sheet, can be redistributed downstream.

Metaball-Imaging discrete element lattice Boltzmann method for fluid–particle system of complex morphologies with case studies

Fluid–particle systems are highly sensitive to particle morphologies. While many attempts have been made on shape descriptors and coupling schemes, how to simulate particle–particle and particle–fluid interactions with a balance between accuracy and efficiency is still a challenge, especially when complex-shaped particles are considered. This study presents a Metaball-Imaging (MI) based Discrete Element Lattice Boltzmann Method (DELBM) for fluid simulations with irregular shaped particles. The major innovation is the MI algorithm to capture the real grain shape for DELBM simulations, where the Metaball function is utilized as the mathematical representation due to its versatile and efficient expressiveness of complex shapes. The contact detection is tackled robustly by gradient calculation of the closest point with a Newton–Raphson based scheme. The coupling with LBM is accomplished by a classic sharp-interface scheme. As for refiling, a local refiling algorithm based on the bounce back rule is implemented. Validations on the Jeffery orbit of ellipsoidal particles and three settling experiments of irregular-shaped natural cobblestones indicate the proposed model to be effective and powerful in probing micromechanics of irregular-shaped granular media immersed in fluid systems. The potential of this model on studies of shape-induced physical processes is further investigated with numerical examples that consider the drag and lift forces experienced by realistic particles, as well as the “drafting, kissing and tumbling” process of pairs of non-spherical particles.

Interaction effect of second particles on the stress concentration in ZK60 magnesium alloy

AbstractSecond particles play a crucial role in influencing the performance of magnesium alloys. A previous work showed that during the fracture initiation process a competitive relationship exists between micron MgZn2 particles and submicron Mg4Zn7 particles of ZK60 magnesium alloy. Especially, some of the MgZn2 particles were found to be gathered in pairs, which resulted in the interaction effect. In this work, three‐dimensional MgZn2 particles were characterized in terms of size, morphology, and distribution using computed tomography technology. Based on the visualization and quantification results of MgZn2 particles, the typical structural models were established. The relationship between stress concentration and shape ratio induced by a single particle was investigated using finite element computations. Furthermore, empirical equations were proposed to quantify the interaction effect induced by two adjacent particles. The validity of the equations was verified by models based on real particle. Comparing the stress concentration between micron MgZn2 particles and submicron Mg4Zn7 particles, the fracture initiation was confirmed to be caused by Mg4Zn7 particles considering the interaction effect of micron particles. The obtained quantitative information would be useful for understanding the crack initiation of engineering materials.

Generation of irregular particle packing with prescribed statistical distribution, spatial arrangement, and volume fraction

A method for packing irregular particles with a prescribed volume fraction is proposed. Furthermore, the generated granular material adheres to the prescribed statistical distribution and satisfies the desired complex spatial arrangement. First, the irregular geometries of the realistic particles were obtained from the original particle images. Second, the Minkowski sum was used to check the overlap between irregular particles and place an irregular particle in contact with other particles. Third, the optimised advance front method (OAFM) generated irregular particle packing with the prescribed statistical distribution and volume fraction based on the Minkowski sum. Moreover, the signed distance function was introduced to pack the particles in accordance with the desired spatial arrangement. Finally, seven biaxial tests were performed using the UDEC software, which demonstrated the accuracy and potential usefulness of the proposed method. It can model granular material efficiently and reflect the mesostructural characteristics of complex granular materials. This method has a wide range of applications where discrete modelling of granular media is necessary. • A method for packing irregular particles with the prescribed volume fraction is proposed. • The generated granular material can respect the prescribed statistical distribution. • The generated granular material can satisfy the desired complex spatial arrangement. • The packing speed is fast, and the volume fraction is better than most packing methods.

Percolation through voids around toroidal inclusions.

In the case of media comprised of impermeable particles, fluid flows through voids around impenetrable grains. For sufficiently low concentrations of the latter, spaces around grains join to allow transport on macroscopic scales, whereas greater impenetrable inclusion densities disrupt void networks and block macroscopic fluid flow. A critical grain concentration ρ_{c} marks the percolation transition or phase boundary separating these two regimes. With a dynamical infiltration technique in which virtual tracer particles explore void spaces, we calculate critical grain concentrations for randomly placed interpenetrating impermeable toroidal inclusions; the latter consist of surfaces of revolution with circular and square cross sections. In this manner, we study continuum percolation transitions involving nonconvex grains. As the radius of revolution increases relative to the length scale of the torus cross section, the tori develop a central hole, a topological transition accompanied by a cusp in the critical porosity fraction for percolation. With a further increase in the radius of revolution, as constituent grains become more ringlike in appearance, we find that the critical porosity fraction converges to that of high-aspect-ratio cylindrical counterparts only for randomly oriented grains.

A novel implicit meshless particle method: NURBS-based particle hydrodynamics (NBPH)

In this work, a new strong form-based implicit meshless particle method, NURBS based Particle Hydrodynamics (NBPH) is proposed. The kernel approximation is replaced by NURBS basis function interpolation. Thus, the calculation of physical field function relies on NURBS basis function-based interpolation, rather than weighted contributions from neighboring particles. Pseudo particles and real particles are configured in the algorithm to accomplish the physical field solution and geometry description. And the gradient operators and Laplacian operators are evaluated based on the partial derivatives of the NURBS basis functions combined with control variables. The update of the physical field functions is completed by an implicit time integration without pre-setting more constraints or intermediate intervention. Generally, the number of pseudo particles is less than the number of real particles, and the value of this difference is related to the order of NURBS basis functions. In addition, surface tension is achieved by cohesion and surface area minimization, tensile instability is solved by shifting particles. Based on the good performance of NBPH in solving numerical simulations of transient conduction and several incompressible flows, it is demonstrated that NBPH is a feasible meshless method. The proposed NBPH scheme provides an alternative computer method for nonlinear transient analysis, such as transient heat transfer and incompressible flow simulation in engineering.

Scheduling optimization for UAV communication coverage using virtual force-based PSO model

When the ground communication base stations in the target area are severely destroyed, the deployment of Unmanned Aerial Vehicle (UAV) ad hoc networks can provide people with temporary communication services. Therefore, it is necessary to design a multi-UAVs cooperative control strategy to achieve better communication coverage and lower energy consumption. In this paper, we propose a multi-UAVs coverage model based on Adaptive Virtual Force-directed Particle Swarm Optimization (AVF-PSO) strategy. In particular, we first introduce the gravity model into the traditional Particle Swarm Optimization (PSO) algorithm so as to increase the probability of full coverage. Then, the energy consumption is included in the calculation of the fitness function so that maximum coverage and energy consumption can be balanced. Finally, in order to reduce the communication interference between UAVs, we design an adaptive lift control strategy based on the repulsion model to reduce the repeated coverage of multi-UAVs. Experimental results show that the proposed coverage strategy based on gravity model outperforms the existing state-of-the-art approaches. For example, in the target area of any size, the coverage rate and the repeated coverage rate of the proposed multi-UAVs scheduling are improved by 6.9–29.1%, and 2.0–56.1%, respectively. Moreover, the proposed scheduling algorithm is high adaptable to diverse execution environments.© 2022 Published by Elsevier Ltd.

A Modified MPS Method with a Split-Pressure Poisson Equation and a Virtual Particle for Simulating Free Surface Flows

As a Lagrangian mesh-free method, the moving particle semi-implicit (MPS) method can easily handle complex incompressible flow with a free surface. However, some deficiencies of the MPS method, such as inaccurate results, unphysical pressure oscillation, and particle thrust near the free surface, still need to be further resolved. Here, we propose a modified MPS method that uses the following techniques: (1) a modified MPS scheme with a split-pressure Poisson equation is proposed to reproduce hydrostatic pressure stably; (2) a new virtual particle technique is developed to ensure the symmetrical distribution of particles on the free surface; (3) a Laplacian operator that is consistent with the original gradient operator is introduced to replace the original Laplacian operator. In addition, a two-judgment technique for distinguishing free surface particles is introduced in the proposed MPS method. Four free surface flows were adopted to verify the proposed MPS method, including two hydrostatic problems, a dam-breaking problem, and a violent sloshing problem. The enhancement of accuracy and stability by these improvements was demonstrated. Moreover, the numerical results of the proposed MPS method showed good agreement with analytical solutions and experimental results.

An efficient approach for computing the finite time Lyapunov exponent in complex three-dimensional flow based on the discrete phase model

We propose a method for computing the finite-time Lyapunov exponent (FTLE) based on the discrete phase model (DPM), which is integrated with a CFD solver and has several advantages as follows. Unlike the conventional method that calculates the fine velocity field first and then calculates the particle trajectory based on the velocity field, the particle position and velocity field in the resulting method are solved simultaneously in the CFD solver, which saves memory and improves the computational efficiency. The resulting method can be applied to arbitrarily complex three-dimensional flows, which helps to promote Lagrangian analysis in the complex flows in aerospace. The discrete phase model used in the resulting method is flexible and easy to be extended. For example, it can be extended from massless tracer particles to real particles with mass. The finite-time Lyapunov exponent is also used to evaluate the uniformity of scalar convection mixing, it is found that the larger the finite-time Lyapunov exponent at the mixture contact line, the longer the contact line for the next time period, indicating more uniform convective mixing.

Real world ultrafine particle emission factors for road-traffic derived from multi-year urban flux measurements using eddy covariance

Mixed fleet particle number emission factors as derived from 3 years of size-resolved particle flux observations show about 2/3 of emission in the nucleation mode <30 nm. Long-term monitoring helps to understand variation in emission factors.