A novel particle tracking velocimetry method for complex granular flow field

Particle tracking velocimetry(PTV)is one of the most commonly applied granular flow velocity measurement methods.However,traditional PTV methods may have issues such as high mismatching rates and a narrow measurement range when measuring granular flows with large bulk density and high-speed contrast.In this study,a novel PTV method is introduced to solve these problems using an optical flow matching algorithm with two further processing steps.The first step involves displacement correction,which is used to solve the mismatching problem in the case of high stacking density.The other step is trajectory splicing,which is used to solve the problem of a measurement range reduction in the case of high-speed contrast The hopper flow experimental results demonstrate superior performance of this proposed method in controlling the number of mismatched particles and better measuring efficiency in comparison with the traditional PTV method.

Determination of diffusion coefficient by image-based fluorescence recovery after photobleaching and single particle tracking system implemented in a single platform

Fluorescence recovery after photobleaching(FRAP)and single particle tracking(SPT)techni-ques determine the diffusion coefficient from average diffusive motion of high-concentration molecules and from trajectories of low-concentration single molecules,respectively.Lateral dif-fusion coefficients measured by FRAP and SPT techniques for the same biomolecule on cell membrane have exhibited inconsistent values across laboratories and platforms with larger dif-fusion coefficient determined by FRAP,but the sources of the inconsistency have not been investigated thoroughly.Here,we designed an image-based FRAP-SPT system and made a direct comparison between FRAP and SPT for diffusion coefficient of submicron particles with known theoretical values derived from Stokes-Einstein equation in aqueous solution.The combined iFRAP-SPT technique allowed us to measure the diffusion coefficient of the same fluorescent particle by utilizing both techniques in a single platform and to scrutinize inherent errors and artifacts of FRAP.Our results reveal that diffusion coefficient overestimated by FRAP is caused by inaccurate estimation of the bleaching spot size and can be corrected by simple image analysis.Our iFRAP-SPT technique can be potentially used for not only cellular membrane dynamics but also for quantitative analysis of the spatiotemporal distribution of the solutes in small scale analytical devices.

Hydrodynamic characteristics and particle tracking of 90° lateral intakes at an inclined river slope

Lateral intakes are common in rivers.The pump effciency and sediment deposition are determined by the local hydrodynamic characteristics and mainstream division width.The hydraulic characteristics of lateral withdrawal from inclined river slopes at different intake elevations should be investigated.Meanwhile,the division width exhibits significant vertical non-uniformity at an inclined river slope,which should be clarified.Hence,a three-dimensional(3-D)hydrodynamic and particle-tracking model was developed with the Open Source Field Operation and Manipulation(Open FOAM),and the model was validated with physical model tests for 90°lateral withdrawal from an inclined side bank.The flow fields,withdrawal sources,and division widths were investigated with different intake bottom elevations,withdrawal discharges,and main channel velocities.This study showed that under inclined side bank conditions,water entered the intake at an oblique angle,causing significant 3-D spiral flows in the intake rather than two-dimensional closed recirculation.A lower withdrawal discharge,a lower bottom elevation of the intake,or a higher main channel velocity could further strengthen this phenomenon.The average division width and turbulent kinetic energy were smaller under inclined side bank conditions than under vertical bank conditions.With a low intake bottom elevation,a low withdrawal discharge,or a high main channel velocity,the sources of lateral withdrawal were in similar ranges near the local inclined bank in the vertical direction.Under inclined slope conditions,sediment deposition near the intake entrance could be reduced,compared to that under vertical slope conditions.The results provide hydrodynamic and sediment references for engineering designs for natural rivers with inclined terrains.

Ensuring adequate statistics in particle tracking experiments

Particle tracking techniques such as magnetic particle tracking,radioactive particle tracking and positron emission particle tracking are widely used in academia and industry to image the dynamics of particulate and multiphase systems.These techniques can provide detailed data concerning a range of important,whole-field quantities based only on the time-averaged dynamics of a small number of tracer particles.However,in order for this data to be reliable,the duration over which these time-averages are taken must be suitably long.Further,the‘minimum averaging time’required to produce good statistics depends sensitively on the system in question and,at present,cannot be determined a priori in advance of an experiment.In this paper,we take a step toward resolving this issue,using discrete element method simulations of a simple vibrofluidised granular bed to develop a series of scaling laws relating said minimum averaging time to a variety of key system variables.The scaling laws developed may be used by future experimentalists to predict the required averaging time for each given system during an experimental campaign,thus improving both the efficiency with which particle tracking techniques may be applied,and the reliability of the data produced thereby.

Double-plume Lagrangian particle tracking model and its application in deep water oil spill

Due to the density stratification of sea water,the dispersed oil droplets and gas bubbles with small diameters,as well as the dissolved components,may remain in some specific depths.The double-plume Lagrangian particle tracking model for bubbly plumes in vertical density stratified environments is improved and applied to predict the underwater pollutants in a blowout.This model considers the different properties and dissolution processes of components in crude oil and focuses on their behavior and stratification differences in the plume.The crude oil components are divided into several groups and the dissolution of oil and gas is also considered.The model is applied to simulate the“Deepwater Horizon”oil spill accident in the Gulf of Mexico in 2010.The results show several enrichment layers of oil and gas at different depth and the differences in concentration between components,which corresponds to the distribution of petroleum pollutants in the in-situ observation.

A Parallel Boundary Element Formulation for Tracking Multiple Particle Trajectories in Stoke’s Flow for Microfluidic Applications

A new formulation for tracking multiple particles in slow viscous flow for microfluidic applications is presented.The method employs the manipulation of the boundary element matrices so that finally a system of equations is obtained relating the rigid body velocities of the particle to the forces applied on the particle.The formulation is specially designed for particle trajectory tracking and involves successive matrix multiplications for which SMP(Symmetric multiprocessing)parallelisation is applied.It is observed that present formulation offers an efficient numerical model to be used for particle tracking and can easily be extended for multiphysics simulations in which several physics involved.

Modelling ignition probability with pairwise mixing-reaction model for flame particle tracking

Reduced order models for ignition analysis can offer insights into ignition processes and facilitate the combustor optimization.In this study,a Pairwise Mixing-Reaction(PMR)model is formulated to model the interaction between the flame particle and the surrounding cell mixture during Lagrangian flame particle tracking.Specifically,the model accounts for the two-way coupling of mass and energy between the flame particle and the surrounding shell layer by modelling the corresponding turbulent mixing,chemical reaction and evaporation process if present.The state of a flame particle,e.g.,burnt,hot gas or extinguished,is determined based on particle temperature.This model can properly describe the ignition process with a spark kernel being initiated in a nonflammable region,which is of practical importance in certain turbine engines and has not been rigorously accounted for by the existing models based on the estimation of local Karlovitz number.The model is integrated into an ignition probability analysis platform and is demonstrated for a methane/air bluff-body flame with the flow and fuel/air mixing characteristics being extracted from a non-reacting simulation.The results show that for the spark location being at the extreme fuellean outer shear layer of the recirculation zone,PMR can yield ignition events with a significant number of active flame particles.The mechanisms for the survival of the initial flame particles and the entrainment of the survived flame particles into the recirculation zone are analyzed.The results also show that the ignition probability map from PMR agrees well with the experimental observation:a high ignition probability in the shear layer of the recirculation zone near the mean stoichiometric surface,and low ignition probabilities inside the recirculation zone and the top stagnation region of the recirculation zone.The parametric study shows that the predicted shape of the ignition progress factor and ignition probability is in general insensitive to the model parameters and the model is adequate for quantifying the regions with high ignition probabilities.

A forced ignition probability analysis method using kernel formation analysis with turbulent transport and Lagrangian flame particle tracking

A forced ignition probability analysis method is developed for turbulent combustion,in which kernel formation is analyzed with local kernel formation criteria,and flame propagation and stabilization are simulated with Lagrangian flame particle tracking.For kernel formation,the effect of turbulent scalar transport on flammability is modelled through the incorporation of turbulenceinduced diffusion in a spherically outwardly propagating flame kernel model.The dependence of flammability limits on turbulent intensities is tabulated and serves as the flammability criterion for kernel formation.For Lagrangian flame particle tracking,flame particles are tracked in a structured grid with flow fields being interpolated from a Computational Fluid Dynamics(CFD)solution.The particle velocity follows a Langevin model consisting of a linear drift and an isotropic diffusion term.The Karlovitz number is employed for the extinction criterion,which compares chemical and turbulent timescales.The integration of the above two-step analysis approach with non-reacting CFD is achieved through a general interpolation interface suitable for general unstructured CFD grids.The method is demonstrated for a methane/air bluff-body flame,in which flow and fuel/air mixing characteristics are extracted from a non-reacting simulation.Results show that the computed ignition probability map agrees qualitatively with experimental results.A reduction of the ignition probability in the recirculation zone and a high ignition probability on the shear layer of the recirculation zone near the mean stoichiometric surface are well captured.The tools can facilitate optimization of spark placement and offer insights into ignition processes.

A simulation study on spatial and time resolution for a cost-effective positron emission particle tracking system

This work is the second part of a simulation study investigating the processing of densely packed and moving granular assemblies by positron emission particle tracking (PEPT).Since medical positron emission tomography (PET) scanners commonly used for PEPT are very expensive,a PET-like detector system based on cost-effective organic plastic scintillator bars is being developed and tested for its capabilities.In this context,the spatial resolution of a resting positron source,a source moving on a freely designed model path,and a particle motion given by a discrete element method (DEM) simulation is studied using Monte Carlo simulations and the software toolkit Geant4.This not only extended the simulation and reconstruction to a moving source but also significantly improved the spatial resolution compared to previous work by adding oversampling and iteration to the reconstruction algorithm.Furthermore,in the case of a source following a trajectory developed from DEM simulations,a very good resolution of about 1 mm in all three directions and an average 3D deviation between simulated and reconstructed events of 2.3 mm could be determined.Thus,the resolution for realistic particle motion within the generic grate system (which is the test rig for further experimental studies) is well below the smallest particle size.The simulation of the dependence of the reconstruction accuracy on tracer particle location revealed a nearly constant efficiency within the entire detector system,which demonstrates that boundary effects can be neglected.

Trajectory Analysis of Particle Motions in Superfluid Helium-4 Using PTV Method

This paper describes the use of particle tracking velocimetry to analyze the Lagrangian acceleration of small particles in superfluid helium with varying time increments, . The probability density of acceleration exhibits Gaussian properties for , but displays a lognormal distribution for , where is the migration time characterizing the particle motion. The particle trajectories are well characterized by the Hurst exponent H. For smaller time scales than , the trajectories exhibit linear motion (), but have certain fractal properties with for time scales larger than .

A simulation study for a cost-effective PET-like detector system intended to track particles in granular assemblies

Since many industrial applications rely on the processing of densely packed and moving granular ma-terial,obtaining bulk internal information on the particle movement inside the reactors is of great importance.Such information can be delivered by Positron Emission Particle Tracking(PEPT).By marking pellets with a positron-emitting radioisotope,the position of these tracer particles can be determined via the time-of-flight differences of the emitted gamma-ray pairs.The current paper proposes a PET-like detector system based on cost-effective organic plastic scintillators instead of the more common but expensive inorganic scintillators.This system is currently under construction and was tested for its resolution and efficiency in this simulation study.Using Monte Carlo simulations and the software toolkit Geant4,three different geometries(an empty glass box,a generic grate system,and a cubic box of 1 m3 completely filled with pellets)were investigated,leading to a spatial resolution in the millimeter range and an efficiency,defined as the ratio of reconstructed decay locations to simulated decays,of 2.7%,1.4%,and 0.3%.

Impact of normal stress-induced closure on laboratory-scale solute transport in a natural rock fracture

The impact of normal stress-induced closure on fluid flow and solute transport in a single rock fracture is demonstrated in this study.The fracture is created from a measured surface of a granite rock sample.The Bandis model is used to calculate the fracture closure due to normal stress,and the fluid flow is simulated by solving the Reynold equation.The Lagrangian particle tracking method is applied to modeling the advective transport in the fracture.The results show that the normal stress significantly affects fluid flow and solute transport in rock fractures.It causes fracture closure and creates asperity contact areas,which significantly reduces the effective hydraulic aperture and enhances flow channeling.Consequently,the reduced aperture and enhanced channeling affect travel time distributions.In particular,the enhanced channeling results in enhanced first arriving and tailing behaviors for solute transport.The fracture normal stiffness correlates linearly with the 5th and 95th percentiles of the normalized travel time.The finding from this study may help to better understand the stress-dependent solute transport processes in natural rock fractures.

Surface water exchanges in the Luzon Strait as inferred from Lagrangian coherent structures

This study presents a Lagrangian view of upper water exchanges across the Luzon Strait based on the finite-time Lyapunov exponents(FTLE)fields computed from the surface geostrophic current.The Lagrangian coherent structures(LCSs)extracted from the FTLE fields well identify the typical flow patterns and eddy activities around the Luzon Strait.In addition,they reveal the intricate transport paths and fluid domains,which are validated by the tracks of satellite-tracked surface drifters and cannot be visually recognized in the velocity maps.The FTLE fields indicate that there are mainly four types of transport patterns near the Luzon Strait;among them,the Kuroshio northward-flowing"leaping"pattern and the clockwise rotating"looping"pattern occur more frequently than the"leaking"pattern of the direct Kuroshio branch into the SCS and the"outflowing"pattern from the SCS to the Pacific.The eddy shedding events of the Kuroshio at the Luzon Strait are further analyzed,and the importance of considering LCSs in estimating transport by eddies is highlighted.The anticyclonic eddy(ACE)shedding cases reveal that ACEs mainly originate from the looping paths of Kuroshio and thus could effectively trap the Kuroshio water before eddy detachments.LCSs provide useful information to predict the positions of the upstream waters that finally enter the ACEs.In contrast,LCS snapshots indicate that during the formation of cyclonic eddies(CEs),most CEs are not connected with the pathways of Kuroshio water.Hence,the contribution of CEs to the surface water exchanges from the Pacific into the SCS is tiny.

Numerical study of short-circuiting flow and particles in a gas cyclone

Short-circuiting flow is an important secondary flow in gas cyclones, which has a negative impact on the separation performance. To improve the understanding of the short-circuiting flow and guide the optimization of gas cyclones, this paper presents a numerical study of a cyclone using computational fluid dynamics. Based on the steady flow field, three methods were adopted to investigate the formation mechanism and characteristics of the short-circuiting flow and particles. The temporal variation of the tracer species concentration distribution reveals that the formation mechanism of the short-circuiting flow is the squeeze between the airflows entering the annular space of the gas cyclone at different times. The short-circuiting flow region, distinguished through the spatial distribution of the moments of age, is characterized by a small mean age and a large coefficient of variation. The proportion of the short-circuiting particles increases with the increase of the inlet velocity only for small particles. But with the increase of particle size, the proportion of the short-circuiting particles decreases faster at higher inlet velocities, resulting in significant differences in collection efficiency curves.

Adjoint Method-Based Algorithm for Calculating the Relative Dispersion Ratio in a Hydrodynamic System

Relative dispersion ratio(RDR)can be used to quantify the deviation behavior of a water parcel’s trajectory caused by a disturbance in a hydrodynamic system.It can be calculated by using a standard method for determining relative dispersion(RD),which accounts for the growth of the deviation of a cluster of particles from a specific initial time.However,the standard method for computing RD is time consuming.It involves numerous computations on tracing many water parcels.In this study,a new method based on the adjoint method is proposed to acquire a series of RDR fields in one round of tracing.Through this method,the continuous variation in the RDR corresponding to a time series of the disturbance time t can be obtained.The consistency and efficiency of the new method are compared with those of the standard method by applying it to a double-gyre flow and an unsteady Arnold-Beltrami-Childress flow field.Results show that the two methods have good consistency in a finite time span.The new method has a notable speedup for evaluating the RDR at multiple t.

Rotational dynamics of bottom-heavy rods in turbulence from experiments and numerical simulations

We successfully perform the three-dimensional tracking in a turbulent fluid flow of small axisymmetrical particles that are neutrally-buoyant and bottom-heavy,i.e.,they have a non-homogeneous mass distribu-tion along their symmetry axis.We experimentally show how a tiny mass inhomogeneity can affect the particle orientation along the preferred vertical direction and modify its tumbling rate.The experiment is complemented by a series of simulations based on realistic Navier-Stokes turbulence and on a point-like particle model that is capable to explore the full range of parameter space characterized by the gravi-tational torque stability number and by the particle aspect ratio.We propose a theoretical perturbative prediction valid in the high bottom-heaviness regime that agrees well with the observed preferential ori-entation and tumbling rate of the particles.We also show that the heavy-tail shape of the probability distribution function of the tumbling rate is weakly affected by the bottom-heaviness of the particles.

Reveal heterogeneous motion states in single nanoparticle trajectory using its own history

Single particle tracking(SPT)has long been utilized for investigation of complex system dynamics such as nanoparticle-cell interaction,however,the analysis of individual particle motions is always a difficult issue.Existing methods treat each data point or fragment on the recorded trajectory as an isolated"atom"and determine their relationship based on externally predefined models or physical states,which inevitably lead to oversimplification of the associated spatiotemporal complexity.Herein,inspired by the historical analysis in social science,we propose a modeless preprocessing framework for SPT analysis based on the"history"of the particle.This new strategy consists of 3 steps:(1)assign a"history"to each data point and construct successive overlapped historical vectors;(2)perform unsupervised clustering in the vector space to find their relative differences;(3)project differences back to the trajectory by coloring each point accordingly for visualization.As a result,the inner heterogeneity of the particle motion self-emerges as a colored trajectory,exhibiting a global picture of the local state transitions and providing valuable information for further model-based analysis.Since the complexity issues at various spatiotemporal scales have attracted increasing attention,and individual objects such as single molecules,cells,vehicles and even stars in the universe could all be treated as"single particles",this presuppositionless data preprocessing approach could help the investigations of many complex systems in fundamental research.

Single virus tracking of Ebola virus entry through lipid rafts in living host cells

Ebola virus(EBOV)is one of the most pathogenic viruses in humans which can cause a lethal hemorrhagic fever.Understanding the cellular entry mechanisms of EBOV can promote the development of new therapeutic strategies to control virus replication and spread.It has been known that EBOV virions bind to factors expressed at the host cell surface.Subsequently,the virions are internalized by a macropinocytosis-like process,followed by being trafficked through early and late endosomes.Recent researches indicate that the entry of EBOV into cells requires integrated and functional lipid rafts.Whilst lipid rafts have been hypothesized to play a role in virus entry,there is a current lack of supporting data.One major technical hurdle is the lack of effective approaches for observing viral entry.To provide evidence on the involvement of lipid rafts in the entry process of EBOV,we generated the fluorescently labeled Ebola virus like particles(VLPs),and utilized single-particle tracking(SPT)to visualize the entry of fluorescent Ebola VLPs in live cells and the interaction of Ebola VLPs with lipid rafts.In this study,we demonstrate the compartmentalization of Ebola VLPs in lipid rafts during entry process,and inform the essential function of lipid rafts for the entry of Ebola virus.As such,our study provides evidence to show that the raft integrity is critical for Ebola virus pathogenesis and that lipid rafts can serve as potential targets for the development of novel therapeutic strategies.

A novel statistically tracked particle swarm optimization method for automatic generation control

Particle swarm optimization(PSO)is one of the popular stochastic optimization based on swarm intelligence algorithm.This simple and promising algorithm has applications in many research fields.In PSO,each particle can adjust its‘flying’according to its own flying experience and its companions’flying experience.This paper proposes a new PSO variant,called the statistically tracked PSO,which uses group statistical characteristics to update the velocity of the particle after certain iterations,thus avoiding localminima and helping particles to explore global optimum with an improved convergence.The performance of the proposed algorithm is tested on a deregulated automatic generation control problem in power systems and encouraging results are obtained.

Evaluation of coarse-grained CFD-DEM models with the validation of PEPT measurements

Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) is a commonly used numerical method to model gas-solid flow in fluidised beds and other multiphase systems. A significant limitation of CFD-DEM is the feasibility of the realistic simulation of large numbers of particles. Coarse-graining (CG) approaches, through which groups of multiple individual particles are represented by single, larger particles, can substantially reduce the total number of particles while maintaining similar system dynamics. As these three CG models have not previously been compared, there remains some debate, however, about the best practice in the application of CG in CFD-DEM simulations. In this paper, we evaluate the performance of three typical CG methods based on simulations of a bubbling fluidised bed. This is achieved through the use of a numerical validation framework, which makes full use of the high-resolution 3D positron emission particle tracking (PEPT) measurements to rigorously validate the outputs of CFD-DEM simulations conducted using various different coarse-graining models, and various different degrees of coarse-graining. The particle flow behaviours in terms of the particle occupancy field, velocity field, circulation time, and bubble size and velocity, are comprehensively analysed. It is shown that the CG simulation starts to fail when the size ratio between the bed chamber and the particles decreases to approximately 20. It is also observed, somewhat surprisingly, that the specific CG approach applied to interparticle contact parameters does not have a substantial effect on the simulation results for the bubbling bed simulations across a wide range of CG factors.