Dynamics Of Non-spherical Particles Research Articles

Anisotropic particles in turbulence: status and outlook

Our motivation to publish a special journal issue focusing on various aspects of anisotropic particles in turbulent flows arose during the EUROMECH Colloquium 513 on Dynamics of non-spherical particles in fluid turbulence held at CISM (International Centre for Mechanical Sciences) in Udine in April 2011. The 42 participants came from rather diverse fields of research all along the route from curiosity-driven research on generic phenomena to investigations addressing industrial needs. The practical applications were intentionally biased towards fibre suspension flows since the colloquium was co-organized by ERCOFTAC’s Special Interest Group (SIG43) focussing on the several issues involved in fibre suspensions. Besides conventional paper manufacturing [1], wood crushed down into fibres and transported as a water suspension in pipes is part of the process of producing new products from forest fibres, such as micro-fibrillated cellulose (MFC), nano-crystalline cellulose (NCC), and ethanol. We, as the colloquium organizers, appreciated the benefits of bringing together scientists concerned with the dynamics of non-spherical particles in fluid flow, irrespective of their driving motivation and independent of their approach, be it modelling, experimental or simulation-based research. In accordance with the traditions of EUROMECH colloquia, no proceedings were published. Nevertheless, we found it timely to make a collection of current research achievements in the field of anisotropic particles in turbulence available to a wider audience. In 2012, invitations were sent to several research groups with a well-established activity in the field to contribute a paper to this special issue of Acta Mechanica. To our great pleasure, almost all of the invitees were able to accept the invitation and submitted their works in the late fall. All manuscripts were then subject to evaluation by anonymous reviewers as per the regular publication procedure in Acta Mechanica. Altogether, 13 reviewed and revised papers are published in this special issue. The papers focus on the different physical, modelling and measuring issues emerging when examining the behaviour of anisotropic particles in a turbulent environment. The dynamics of spherical (isotropic) particles has been extensively studied over the years. Studies of anisotropic particles are more scarce although dilute or

Dynamics of non-spherical particles in a rotating drum

Considerable amount of experimental work has been performed to elucidate the behavior of granular flow inside a rotating drum and it has yet to be clearly understood. However, a majority of these investigations have involved spherical or nearly spherical particles. The extent of the experiments involving non-spherical particles previously carried out was limited by the experimental technique used for the investigation or the inability to single out the effect of the particle shape. In this work, the radioactive particle tracking technique (RPT) is adapted to follow large non-spherical particles inside a rotating drum. The particles consist of pharmaceutical tablets containing a suitable compound, thus enabling their use as a tracer particle. Three crucial aspects of particle dynamics inside a rotating drum are studied: residence time in the active and passive layers, mixing and segregation, as well as axial dispersion. The results obtained for non-spherical particles are compared to those which would be predicted using models developed for spherical of nearly spherical particles. For the different shapes studied in this work, it is found that particles having an aspect ratio greater than two can lead to significant deviations in velocity profile and residence time. In addition, the mixing of different shaped particles is observed to lead to unexpected core segregation patterns. Lastly, it is found that the non-spherical particle higher degree of spatial orientation in the active layer leads to a lower axial dispersion coefficient than the ones obtained with spherical particles.

A Computer Model for the Simulation of Nonspherical Particle Dynamics in the Human Respiratory Tract

In the study presented here deposition of spheres and nonspherical particles with various aspect ratios (0.01–100) in the human respiratory tract was theoretically modeled. Shape of the nonspherical particles was considered by the application of the latest aerodynamic diameter concepts. Particle deposition was predicted by using a stochastic model of the lung geometry and simulating particle transport trajectories according to the random-walk algorithm. Concerning fibers total deposition is significantly enhanced with respect to that of spheres for μm-sized particles, whereby at normal breathing conditions peripheral lung compartments serve as primary deposition targets. In the case of oblate disks, total deposition becomes mostly remarkable for submicron particles, with the bronchioli and alveoli being targeted to a high extent. Enhancement of the aerodynamic diameter and/or flow rate generally causes a displacement of deposition maxima from peripheral to more proximal lung regions. From these findings, it can be concluded that these particle classes may represent tremendous occupational hazards, especially if they are attached with radioactive elements or heavy metals.

Study of In-Flight and Impact Dynamics of Nonspherical Particles from HVOF Guns

High velocity oxygen fuel thermal spray has been widely used to deposit hard composite materials such as WC-Co powders for wear-resistant applications. Unlike gas atomized spherical powders, WC-CO powders form a more complex geometry. The knowledge gained from the existing spherical powders on process control and optimization may not be directly applicable to WC-Co coatings. This paper is the first to directly examine nonspherical particle in-flight dynamics and the impingement process on substrate using computational methods. Two sets of computational models are developed. First, the in-flight particles are simulated in the CFD-based combusting gas flow. The particle information prior to impact is extracted from the CFD results and implemented in a FEA model to dynamically track the impingement of particles on substrate. The morphology of particles is examined extensively including both spherical and nonspherical powders to establish the critical particle impact parameters needed for adequate bonding.

Stokesian dynamics of nonspherical particles, chains, and aggregates

The microstructure and properties of suspensions of nonspherical particles are influenced by the specific particle shapes through hydrodynamic interactions, but here traditional numerical approaches of solving the Stokes equations are limited to small systems by computational cost, and often to special particle arrangements by symmetry requirements. On the other hand, the analytical development of a hydrodynamic mobility algorithm for Stokesian dynamics (SD) simulations of rigid nonspherical particles is mathematically involved, must be derived for each distinct particle shape needed, and cannot handle deformable particles. Hence we present algorithms for SD simulations of arbitrary shape particles, rigid or flexible, constructed with appropriate constraints among rigid spherical particles whose hydrodynamic mobility is computable by various available schemes, including ours [J. Chem. Phys. 112, 2548 (2000)]. The optimal algorithm also provides for rigid attachment among particles during simulation, by aggregation for example. Its implementation for a system with internal coordinate constraints is tested in simulations of aggregation of spheres and sedimentation of spheroids and chains in bounded and unbounded geometries.

A simple bimodal model for the evolution of non-spherical particles undergoing nucleation, coagulation and coalescence

A simple and efficient particle dynamics model is developed accounting for simultaneous nucleation, coagulation, and coalescence or sintering of non-spherical particles. In this model two discrete monodisperse modes are used to represent the non-spherical particle size distributions approximately: a size-fixed nucleation mode and a moving accumulation mode. The size-fixed nucleation mode accounts for the introduction of newly generated particles and the moving accumulation mode characterizes the particle growth by coagulation and coalescence. The simulation results for titania particle formation and growth using the proposed bimodal model are compared with those using the previous monodisperse non-spherical particle dynamics model and non-spherical polydisperse sectional model. The present bimodal model results in a very good agreement with the polydiserse sectional model even when particle nucleation coexists with coagulation process while the monodisperse model shows significant differences. It successfully predicts the morphological change of the non-spherical particles by coalescence. The present model is also shown to be capable of predicting the polydispersity of non-spherical particle distribution. The present non-spherical bimodal model requires the same level of the computation time that the simple monodisperse model does.

A Simple Model for the Evolution of the Characteristics of Aggregate Particles Undergoing Coagulation and Sintering

A simple model describing the evolution of particle morphology, size, and number concentration by coagulation and sintering is presented that neglects the spread of the polydispersity of aggregate and primary particles. The influence of irregular/fractal structure on the collision kernel is accounted for, from the free molecular to the continuum regime. The model predictions compare well to those of a detailed two-dimensional sectional model of nonspherical particle dynamics. After a theoretical evaluation of the main sintering mechanism, the proposed model was applied to laser synthesis of silicon in an aerosol reactor.

4918386 Method for simultaneously obtaining three-dimensional NMR angiograms and stationary tissue NMR images

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.