Dynamics Of Non-spherical Particles Research Articles

Self-ordering and organization of a staggered oblate particle pair in three-dimensional square ducts

We numerically investigate the formation and ordering of staggered oblate particle pairs in three-dimensional straight ducts with a square cross section. The lattice Boltzmann method is employed to simulate rigid particle pairs in a Newtonian liquid. The effects of initial axial spacing, Reynolds number, blockage ratio, and particle aspect ratio on the formation process, migration behavior, and interparticle spacing are explored in detail. Current results indicate that the process from initial to final steady state can be divided into two stages. The first stage is rapid migration from initial positions toward equilibrium positions under shear-induced lift force and wall-induced repulsive force. The second stage is the slow self-assembly of stable particle pairs in the axial direction due to the interparticle interaction. Interestingly, initial axial spacing significantly affects the formation process of particle pairs but does not affect the final steady state. It is found that the equilibrium positions of staggered particle pairs move slightly toward the duct walls, and the axial spacing increases with increasing Reynolds number or particle aspect ratio, or decreasing blockage ratio. For a staggered particle pair, the second particle will occupy the eddy center induced by the first focusing particle. Based on the existing data, a correlation is put forward to predict the axial interparticle spacing of staggered oblate particle pairs in duct flows. The present results may give insights into manipulating and comprehending non-spherical particle dynamics in microfluidic applications.

Simulation of non-spherical particles stirring process in stirred tanks

There are few studies on the dependence of particle suspension and dispersion on particle shape in stirred tanks. Most of the non-spherical particle problems are converted into spherical particles in the simulation process. The influence of particle shape on particle suspension and dispersion is generally ignored. Therefore, combined with the RNG k-ɛ turbulence model, the DEM-VOF method is used to simulate the suspension and dispersion dynamics of non-spherical particles in liquid-solid stirred tanks. A four-blade 45° oblique blade turbine is used in a flat stirred tank, and the impeller rotation is simulated by the sliding grid method. The effectiveness of DEM-VOF coupling method is verified theoretically and experimentally. And the results show that the particle shape has a great influence on the interaction between the fluid and particles and the distribution of particles in stirred tanks. This study provides a useful foundation for further research on the influence of particle shape.

Memory-induced alignment of colloidal dumbbells

When a colloidal probe is forced through a viscoelastic fluid which is characterized by a long stress-relaxation time, the fluid is excited out of equilibrium. This is leading to a number of interesting effects including a non-trivial recoil of the probe when the driving force is removed. Here, we experimentally and theoretically investigate the transient recoil dynamics of non-spherical particles, i.e., colloidal dumbbells. In addition to a translational recoil of the dumbbells, we also find a pronounced angular reorientation which results from the relaxation of the surrounding fluid. Our findings are in good agreement with a Langevin description based on the symmetries of a director (dumbbell) as well as a microscopic bath-rod model. Remarkably, we find an instability with amplified fluctuations when the dumbbell is oriented perpendicular to the direction of driving. Our results demonstrate the complex behavior of non-spherical objects within a relaxing environment which are of immediate interest for the motion of externally but also self-driven asymmetric objects in viscoelastic fluids.

Configuration-dependent dynamics of non-spherical particles in a gas–solid fluidized bed

Configuration-dependent dynamics of non-spherical particles are investigated experimentally in a quasi-two-dimensional gas–solid fluidized bed. We show that the vertical velocity profiles are sensitive to the aspect ratio at low gas velocity, but not sensitive at high gas velocity. The velocity distribution, especially the rotational velocity distribution, deviates from the Maxwellian distribution. The joint probability density distribution of vertical and rotational velocity is asymmetric. We identify two high-probability particle configurations, two particles in a parallel or vertical arrangement, by calculating the radial distribution function, nematic correlation function and coordination number. We further explain the cause of this order appearance through entropy theory. By comparing the drag force of the particle configurations at various angles and Re, we demonstrate that it is necessary to put forward the configuration-dependent drag force for non-spherical particle fluidization.

Dynamics of non-spherical particles resting on a flat surface in a viscous fluid

The resting dynamics of non-spherical particles on a flat surface can be considered the last phase in settling a particle, which has yet to be fully investigated. This last phase for the non-spherical particle is numerically investigated, for the first time, using a sharp-interface immersed boundary method coupled with a kinematic-based collision model. The collision model guarantees a realistic, stable/settled position of non-spherical-shaped particles, contrary to alternative models that implement a repulsive penalty force. In the simulations, a single particle is released with a constant velocity downwards close to the wall until the collision occurs. Hydrodynamic moments alter the settling dynamics depending on the Reynolds number (Re) by opposing the gravity-driven motion of particles. It was observed that the settling trajectories/angles were generally not affected for each particle, but their rate of change, i.e., angular velocities, reduced as the Reynolds number decreased. A simplified model for the hydrodynamic moment was explored based on a modified Stokes drag moment for spherical particles, which includes a shape factor Kn for relating non-spherical particles to spherical ones. It was found that using the projected area of non-spherical particles provided the best overall scaling to find their equivalent spheres because it provided the lowest Kn values. In addition, Kn was found to deviate from the constant theoretical value because of the build-up pressure between the particle and the wall which changed with Re. A linear relation between the mean Kn and Re was found to be a good approximation. This work demonstrates how particle-resolved simulations can provide the data required for developing simplified models for non-spherical particles.

Dynamics of non-spherical particles in viscoelastic fluids flowing in a microchannel.

The migration and orientation dynamics of prolate spheroidal particles suspended in a viscoelastic liquid flowing in a square microchannel is experimentally investigated under inertialess flow conditions. The suspending fluid is an aqueous solution of PolyEthylene Oxyde at relatively high concentration characterized by a high level of elasticity and shear-thinning. Fluid viscoelasticity drives the spheroids towards the channel central region at relatively low flow rates when the particles explore the constant viscosity region of the fluid, without showing a preferential orientation. As the flow rate increases and the fluid enters in the shear-thinning region, a smaller fraction of particles migrates at the central channel region, reducing the focusing efficiency. The focused spheroids rotate sufficiently fast to attain a stable orientation with major axis aligned along the flow direction.

Height distribution and orientation of colloidal dumbbells near a wall.

Geometric confinement strongly influences the behavior of microparticles in liquid environments. However, to date, nonspherical particle behaviors close to confining boundaries, even as simple as planar walls, remain largely unexplored. Here, we measure the height distribution and orientation of colloidal dumbbells above walls by means of digital in-line holographic microscopy. We find that while larger dumbbells are oriented almost parallel to the wall, smaller dumbbells of the same material are surprisingly oriented at preferred angles. We determine the total height-dependent force acting on the dumbbells by considering gravitational effects and electrostatic particle-wall interactions. Our modeling reveals that at specific heights both net forces and torques on the dumbbells are simultaneously below the thermal force and energy, respectively, which makes the observed orientations possible. Our results highlight the rich near-wall dynamics of nonspherical particles and can further contribute to the development of quantitative frameworks for arbitrarily shaped microparticle dynamics in confinement.

Direct numerical simulation of rotating ellipsoidal particles using moving nonconforming Schwarz-spectral element method

We present application of a highly-scalable overlapping grid-based nonconforming Schwarz-spectral element method (Schwarz-SEM) to study the dynamics of rotating ellipsoidal particles. The current study is one of the first to explore the effect of rotation on ellipsoidal particles using direct numerical simulation. The rotating ellipsoidal particles show substantial difference in the dynamics of the flow, when compared against non-rotating particles. This difference is primarily due to periodic attachment and separation of the flow to the surface of the particle for the rotating cases, which results in a higher drag on the particle when compared to the corresponding non-rotating cases. The dynamics is also different from a rotating spherical particle, where a steady shear layer develops near the surface of the sphere. For the rotating ellipsoidal particles, we observe that there is a phase-difference between the position of observed maximum and minimum drag, and the position of expected maximum and minimum drag (i.e., maximum and minimum projected area). A similar phase-difference is also observed for the lift force acting on the rotating ellipsoidal particles. The results presented here demonstrate the importance of explicitly modeling the shape and rotation of particles for studying dynamics of non-spherical particles. Finally, this study also validates the use of non-conforming Schwarz-SEM for tackling problems in fully resolved particulate flow dynamics.

Numerical investigation on the mutual interaction between heat transfer and non-spherical particle dynamics in the blast furnace raceway

In this paper, the coupled computational fluid dynamics (CFD) and discrete element method (DEM) scheme is used to investigate the microstructure and heat transfer characteristics in a blast furnace (BF) raceway. The impact of gas velocity and particle shape on raceway evolution, microstructure characteristics, and particle temperature and voidage distributions is comprehensively explored based on the voidage change caused by the consumption of coke particles. Numerical results show that the oblate ellipsoidal particle system possesses higher average voidage in the original packing state and implements the faster burden descending rate than the prolate one under the same inlet velocity. For the non-spherical particle systems, larger contact forces exist in the oblate ellipsoidal particle system while the contact forces in the prolate ellipsoidal particle system are smaller. More uniform temperature distribution and higher average voidage can be caused by higher inlet gas velocities. Meanwhile, more prolate ellipsoidal particles are consumed than oblate ones under the same inlet velocity. These findings could be beneficial for understanding the kinetic and thermodynamic behaviors of particulate system within a BF raceway.

Influence of the quiescent core on tracer spheroidal particle dynamics in turbulent channel flow

Numerical studies into the dynamics of non-spherical particles in turbulent channel flow have, until now, been mostly confined to low Reynolds numbers. In this paper, we investigate the dynamics of tracer non-spherical particles in a channel flow at for the first time. Tracer spheroidal particles suspended in a turbulent channel flow are computed by means of direct numerical simulation coupled with a Lagrangian point-particle approach. We examine the rotational dynamics and alignment of tracer spheroids of different aspect ratio in both the near-wall region and, in particular, the quiescent core which is not observed at lower Reynolds numbers. In the near-wall region, the effect of Reynolds number on preferential alignment is negligible with particle rotation exhibiting a weak dependence for all aspect ratios investigated. However, the particle rotation is strongly damped in the quiescent core demonstrating that the motion of tracer spheroids becomes quiescent. The striking difference in spheroids' rotational dynamics observed inside and outside of the quiescent core is correlated with the local Kolmogorov time scale in the core and outside of it.

Orientation dynamics of nonspherical particles under surface gravity waves

Nonspherical particles in the ocean are relevant to phenomena such as microplastics, plankton and sediment. Experimental results of the orientation dynamics of nonspherical particles in waves find competition between the waves and the inertial effects on particle orientation.

Effect of lift force and hydrodynamic torque on fluidisation of non-spherical particles

The aim of many industrial processes is to manipulate solid particle aggregates within gas suspensions. Prime examples of such processes include fluidised bed reactors, cyclone separators, and dust collectors. In recent years, fluidised bed reactors have been used in the gasification of biomass particles. When fluidised, these particles are subject to various hydrodynamic forces such as drag, lift and torque due to interactions with the fluid. Computational approaches, which can be used to replicate laboratory and industrial scale processes, offer a crucial method for the study of reactor design and for the formulation of optimal operating procedures. Until now, many computer models have assumed particles to be spherical whereas, in reality, biomass feedstocks typically consist of non-spherical particles. While lift and torque are of minimal importance for spherical particles, non-spherical particles experience varying lift force and torque conditions, depending on particle orientation relative to the direction of the fluid velocity. In this study, we present a numerical investigation on the effect of different lift force and torque correlations on fluidised spherocylindrical particles. We find that lift force has a significant influence on particle velocities parallel to the direction of gravity. On the other hand, particle orientation is dependent on hydrodynamic torque. Results from this numerical study provide new insight with regards to the dynamics of non-spherical particles that can be of paramount importance for industrial processes involving non-spherical particles.

An investigation of CFD simulations capability in treating non-spherical particle dynamics in a rotary drum

A systematic study upon the Eulerian approach capability in predicting irregular particle (non-spherical) dynamics inside an unbaffled rotary drum was carried out through the investigation of different particle-particle interaction models and wall boundary conditions. The drum length effect on particle dynamics was also analyzed. The frictional viscosity, the wall movement method, and the particle boundary condition at the wall were shown to be of fundamental importance for irregular particles modelling in rotary drums. The moving mesh method did not represent well the experimental observation, regardless of the solid boundary condition used at the drum wall. The moving wall method combined with a no-slip boundary condition for the solid phase at the drum wall, and the frictional viscosity model using a critical particle concentration (αsc) of 0.4, satisfactory agreed with experiments. For the end-caps analyses, the higher the ratio L/ds, the lower the bed material surface slope and the lower the particle velocity values in the active region. From L/ds of 40.7 no differences were observed in particle dynamics behavior anymore.

Prolate spheroidal particles’ behavior in a vertical wall-bounded turbulent flow

Direct numerical simulations (DNSs) have been performed to examine the inertia, shape, and gravity field effects on the dynamics of ellipsoidal particles within a vertical turbulent channel flow. To investigate the effects induced by the particle inertia and shape, computations have been conducted for three aspect ratios and two response times. The influence of gravity has been examined through a comparison with DNS data provided in earlier studies without gravity. The originality of this study is that the prediction of the hydrodynamic force and pitching torque acting on the non-spherical particles has been carried out with recent expressions valid outside the Stokes flow regime. With the data extracted from the DNS, a statistical analysis of the particle spatial distribution, orientation, and translational and angular velocities is carried out. Results show that the presence of a significant mean relative velocity between the dispersed and continuous phases greatly modifies the dynamics of non-spherical particles. Without gravity, the dynamics of ellipsoids is close to that of spheres, whereas it becomes strongly dependent on the particle shape with gravity.

Anisotropic Particles in Turbulence

Anisotropic particles are common in many industrial and natural turbulent flows. When these particles are small and neutrally buoyant, they follow Lagrangian trajectories while exhibiting rich orientational dynamics from the coupling of their rotation to the velocity gradients of the turbulence field. This system has proven to be a fascinating application of the fundamental properties of velocity gradients in turbulence. When particles are not neutrally buoyant, they experience preferential concentration and very different preferential alignment than neutrally buoyant tracer particles. A vast proportion of the parameter range of anisotropic particles in turbulence is still unexplored, with most existing research focusing on the simple foundational cases of axisymmetric ellipsoids at low concentrations in homogeneous isotropic turbulence and in turbulent channel flow. Numerical simulations and experiments have recently developed a fairly comprehensive picture of alignment and rotation in these cases, and they provide an essential foundation for addressing more complex problems of practical importance. Macroscopic effects of nonspherical particle dynamics include preferential concentration in coherent structures and drag reduction by fiber suspensions. We review the models used to describe nonspherical particle motion, along with numerical and experimental methods for measuring particle dynamics.

非球形颗粒碰撞过程硬颗粒与软颗粒模型比较研究

在能源化工过程、矿业冶金及安全生产过程中均会涉及复杂的实际非球形颗粒体系，而颗粒的非球形碰撞动力学过程是对复杂颗粒体系进行研究必须要考虑的基本问题。离散元方法通过弹簧、阻尼、摩擦三种力学特性对颗粒-颗粒接触力进行建模，是目前国际上最为广泛使用的针对球形颗粒体系的模型。本文对典型三角形非球形碰撞动力学工程过程进行理论分析和模型比较研究，对国际上目前正在研究的硬颗粒和软颗粒非球形碰撞模型进行了比较。通过模型比较，证明了在工程上可以接受的计算误差范围内，软颗粒和硬颗粒计算结果吻合较好，两者在计算精度、稳定性、适用性方面各有优势，可以在不同的应用场合分别采用不同的计算模型。 Non-spherical particles and complex non-spherical particle systems are commonly encountered in energy chemical engineering, mineral engineering, and safety manufacturing engineering. The collision dynamics of non-spherical particles are fundamental issues in the investigations of complex particle systems. Discrete element method is the most prevalent models for particle-particle collision of spherical shapes, which incorporates a spring, a dashpot, and a frictional interaction. In this work, a comparative and analytical study on the mathematical models of collision dynamics for non-spherical particles is carried out. The hard non-spherical particle model and the soft-par- ticle model have been compared. It indicates that agreeable simulation results can be found for the two models. The two models have different advantages or disadvantages in accuracy, computational stability, and feasibility, with particular suitability for different cases.

Analysis of force acting on the non-spherical particle near a wall

Purpose To investigate the non-spherical particle dynamics near a wall in a vasculature, it is required that detailed force with respect to the particle angle and separation distance. The prismatic particles with different cross sections are employed to study the forces on the particle in linear shear flow.

Dynamics of single, non-spherical ellipsoidal particles in a turbulent channel flow

Using disk, spherical and needle-like particles with equal equivalent volume diameters, the orientational dynamics of non-spherical particles is studied in a turbulent channel flow. An Eulerian–Lagrangian approach based on large eddy simulation with a dynamic sub-grid scale model is used to predict a fully developed gas–solid flow at a shear Reynolds number Reτ=300. Particle shape and orientation are accounted for by the coupling between Newton׳s law of translational motion and Euler׳s law of rotational motion, both in a Lagrangian framework. The particle shapes are simulated using the super-quadrics form, with the dynamically relevant parameters being the particle aspect ratio, equivalent volume diameter and response time. The translational and orientational behaviour of single particles initially released at three different locations in the wall-normal direction are monitored, with analysis showing a clear distinction between the behaviour of the different particle shapes. The results show that turbulent dispersion forces non-spherical particles to have a broad orientation distribution. The orientational states observed include periodic, steady rotation, tumbling, precessing and nutating. Velocity gradient, aspect ratio and particle inertia all have an effect on the alignment of the particle principal axis to the inertial axes. Unlike spherical particles, the disk and needle-like particles display a transition from one orientational state to another, especially when their initial position is in the near-wall region, with the frequency of this transition highest for the disk-like particle. Overall, this study leads to an improved understanding of the significance of shape on particle behaviour which is of relevance to many practical flows.

Analysis of non-spherical particle transport in complex internal shear flows

Focusing on ellipsoidal particles of different aspect ratios, the motion characteristics, including critical angle and stable vs. unstable rotational periods, are computationally analyzed in developing and fully developed tubular flows. As an application of particle transport and deposition, the one-way coupled Euler-Lagrange method enhanced by Euler's rotation equations is then employed to simulate laminar-turbulent flow in a subject-specific lung-airway model. First, to gain some basic insight into the dynamics of non-spherical particles, tubular flow is considered where the trajectories of ellipsoidal fibers with randomly initialized incidence angles were released at different inlet-plane positions, computed and visualized. Local and overall particle deposition results are compared between spheres, ellipsoidal fibers, and sphere-equivalent particles for which a revised Stokes diameter was developed. Concerning non-spherical particle transport and deposition in a subject-specific respiratory system, the validated computer simulation model provides realistic and accurate particle-deposition results. Specifically, slender non-spherical particles (i.e., those with higher aspect ratios) are potentially more harmful than thicker ones due to their ability to penetrate into deeper lung regions when somewhat aligned with the major flow field. Furthermore, non-spherical particle deposition is enhanced as the breathing rate increases.

Effect of Shape on Inertial Particle Dynamics in a Channel Flow

Particle dynamics in a channel flow are investigated using large eddy simulation and a Lagrangian particle tracking technique. Following validation of single-phase flow predictions against DNS results, fluid velocities are subsequently used to study the behaviour of particles of differing shape assuming one-way coupling between the fluid and the particles. The influence of shape- and orientation-dependent drag and lift forces on both the translational and rotational motion of the particles is accounted for to ensure accurate representation of the flow dynamics of non-spherical particles. The size of the particles studied was obtained based on an equivalent-volume sphere, and differing shapes were modelled using super-quadratic ellipsoid forms by varying their aspect ratio, with their orientation predicted using the incidence angle between the particle relative velocity and the particle principal axis. Results are presented for spherical, needle- and platelet-like particles at a number of different boundary layer locations along the wall-normal direction within the channel. The time evolution and probability density function of selected particle translational and rotational properties show a clear distinction between the behaviour of the various particles types, and indicate the significance of particle shape when modelling many practically relevant flows.