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.

Estimation of particle dynamics in 2-D fluidized beds using particle tracking velocimetry

The experimental characterization of particle dynamics in fluidized beds is of great importance in fostering an understanding of solid phase motion and its effect on particle properties in granulation processes, Commonly used techniques such as particle image velocimetry rely on the cross-correlation of illumination intensity and averaging procedures. It is not possible to obtain single particle velocities with such techniques. Moreover, the estimated velocities may not accurately represent the local particle velocities in regions with high velocity gradients. Consequently, there is a need for devices and methods that are capable of acquiring individual particle velocities. This paper describes how particle tracking velocimetry can be adapted to dense particulate flows. The approach presented in this paper couples high-speed imaging with an innovative segmentation algorithm for particle detection, and employs the Voronoi method to solve the assignment problem usually encountered in densely seeded fows. Lagrangian particle tracks are obtained as primary information, and these serve as the basis for calculating sophisticated quantities such as the solid-phase flow field, granular temperature, and solid volume fraction. We show that the consistency of individual trajectories is sufficient to recognize collision events.

LIQUID PHASE FLOW ESTIMATION IN GAS-LIQUID TWO-PHASE FLOW USING INVERSE ANALYSIS AND PARTICLE TRACKING VELOCIMETRY

An inverse analysis algorithm is proposed for estimating liquid phase flowfield from measurement data of bubble motion. This kind of technology will be applied in future forvarious estimation of fluid flow in rivers, lakes, sea surface flow, and also microscopic channelflow as the problem-handling in civil, mechanical, electronic, and chemical engineering. Therelationship between the dispersion motion and the carrier phase flow is governed and expressed bythe trans-lational motion equation of spherical dispersion. The equation consists of all the forcecomponents including inertia, added inertia, drag, lift, pressure gradient force and gravity force.Using this equation enables us to estimate the carrier phase flow structure using only the data ofthe dispersion motioa Whole field liquid flow structure is also estimated using spatial or temporalinterpolation method. In order to verify this principle, the Taylor-Green vortex flow, and theKarman vortex shedding from a square cylinder have been chosea The results show that the combinationof the inverse analysis and Particle Tracking Velocimetry (PTV) with the spatio-temporalpostprocessing algorithm could reconstruct well the carrier phase flow of the gas-liquid two-phaseflow.

Determination of particle exchange rates at over-flow weirs in horizontal fluidised beds by particle tracking velocimetry

Residence time distributions （RTDs） in horizontal fluidised beds have a huge effect on solid product properties and are infuenced by the internal design of the apparatus, e.g. the separation into different compartments by weirs. Weirs can be passed in or against the overall solid transport direction, with the back-flow resulting in axial dispersion, which is a measure of the spread of the RTD. Therefore, the ratio of exchange rates at weirs under different fluidisation conditions provides information on axial dispersion. In this work, a methodology based on particle tracking velocimetry is presented to obtain information on the exchange rates of particles at weirs in horizontal fluidised beds. The internal recirculation is studied for over-flow weirs with respect to different fluidisation conditions, providing a first step towards determining the effects of weirs and fluidisation conditions on axial dispersion and RTDs in horizontal fluidised beds.

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 .

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.

Adaptive-optical 3D microscopy for microfluidic multiphase flows

Measurements based on optical microscopy can be severely impaired if the access exhibits variations of the refractive index.In the case of fluctuating liquid-gas boundaries,refraction introduces dynamical aberrations that increase the measurement uncertainty.This is prevalent at multiphase flows(e.g.droplets,film flows)that occur in many technical applications as for example in coating and cleaning processes and the water management in fuel cells.In this paper,we present a novel approach based on adaptive optics for correcting the dynamical aberrations in real time and thus reducing the measurement uncertainty.The shape of the fluctuating water-air interface is sampled with a reflecting light beam(Fresnel Guide Star)and a Hartmann-Shack sensor which makes it possible to correct its influence with a deformable mirror in a closed loop.Three-dimensional flow measurements are achieved by using a double-helix point spread function.We measure the flow inside a sessile,oscillating 50-μl droplet on an opaque gas diffusion layer for fuel cells and show that the temporally varying refraction at the droplet surface causes a systematic underestimation of the flow field magnitude corresponding to the first droplet eigenmode which plays a major role in their detachment mechanism.We demonstrate that the adaptive optics correction is able to reduce this systematic error.Hence,the adaptive optics system can pave the way to a deeper understanding of water droplet formation and detachment which can help to improve the efficiency of fuels cells.

AN IMPROVED PTV SYSTEM FOR LARGE-SCALE PHYSICAL RIVER MODEL

To measure the surface flow in a physical river model, an improved system of Large-Scale Particle Tracking Velocimetry （LSPTV） was proposed and the elements of the PTV system were described. Usually the tracer particles of a PTV system seeded on water surface tend to form conglomerates due to surface tension of water. In addition, they can not float on water surface when water flow is shallow. Ellipsoid particles were used to avoid the above problems. Another important issue is particle recognition. In order to eliminate the influence of noise, particles were recognized by the processing of multi-frame images. The kernel of the improved PTV system is the algorithm for particle tracking. A new 3-frame PTV algorithm was developed. The performance of this algorithm was compared with the conventional 4-frame PTV algorithm and 2-frame PTV algorithm by means of computer simulation using synthetically generated images. The results show that the new 3-frame PTV algorithm can recover more velocity vectors and have lower relative error. In addition, in order to attain the whole flow field from individual flow fields, the method of stitching individual flow fields by obvious marks was worked out. Then the improved PTV system was applied to the measurement of surface flow field in Model Yellow River and shows good performance.

QUADRANT ANALYSIS OF BUBBLE INDUCED VELOCITY FLUCTUATIONS IN A TRANSITIONAL BOUNDARY LAYER

Our previous study showed that the frictional drag decreases with increasing void fraction at Re〉1300, while it increases at Re 〈 1000. Decomposition of the Reynolds shear stress also implied that bubbles induce isotropy of turbulence. In order to confirm our previous analysis and to further investigate flow fields in the vicinity of bubbles, we analyze velocity fluctuations on the quadrant space in the streamwise and transverse directions （u′-v′ plane）. Here, we focus on two specific Reynolds numbers （at Re≈900 and ≈1410, which are close to the laminar-to-turbulent transition regime） and discuss bubble effects on sweep （u′〉 0, v′〈 0 ） and ejection （u′〈 0, v′〉 0） events as a function of the Reynolds number. We also illustrate velocity fluctuations in the vicinity of an individual bubble and a swarm of bubbles on the u′- v′ coordinates. The results show that a bubble swarm suppresses the velocity fluctuations at Re≈1410.

EXPERIMENT STUDY ON SEDIMENT INCIPIENCE IN BACKWARD-FACING STEP FLOW

Flow over a backward-facing step was studied to investigate the effect of large-scale vortex structures on sediment incipience. The transient flow velocity field at the downstream of the backward-facing step was obtained using the technique of Particle Tracking Velocimetry （PTV）. The optical amplification technique was employed to measure the instantaneous flow velocities near the bed and the instantaneous bed shear stress was given. The experimental observations revealed a new insight into the oscillation of the large-scale structure and the three-dimensional characteristics of the flow. In particular, very high turbulence intensity, instantaneous horizontal velocity near the bed and the bed shear stress near the reattachment point were observed. The sediment incipient probability obtained from the sequent images of sediment particles near the bed indicates that the critical instantaneous shear stress of the sediment incipience is independent of flow conditions.

The effect of bubble plume on oxygen transfer for moving bed biofilm reactor

The movement of the bubble plume plays an important role in the operation of a moving bed biofilm reactor （MBBR）, and it directly affects the contact and the mixture of the gas-liquid-solid phases in the aeration tank and also the oxygen transfer from the gas phase to the liquid phase. In this study, the velocity field is determined by a 4-frame PTV as well as the time-averaged and timedependent velocity distributions. The velocity distribution of the bubble plume is analyzed to evaluate the operating efficiency of the MBBR. The results show that the aeration rate is one of the main factors that sway the velocity distribution of the bubble plumes and affect the operating efficiency of the reactor.

Investigation of the Surface Vortex in a Spillway Tunnel Intake

The surface vortex in a spillway tunnel intake was investigated in a physical model of the Xiluodu hydropower station.The velocity fields were measured using the particle tracking velocimetry technique.The tangential velocity formula of the surface flow field was derived based on the Navier-Stokes equations,and this formula greatly improved the consistency between the numerical and experimental data.Also,the formula has the advantage of describing the tangential velocity more accurately than previous formulas.The current research is based on established engineering practices,and the results provide a valuable reference for actual projects designed to prevent and eliminate surface vortexes.

Scattering Characteristics of Liquid Droplets Spun Off from Rotating Disk Edge

Scattering characteristics of liquid droplets spun off from a rotating disk edge are experimentally investigated. In the present research, aluminum disks are utilized and ethanol is employed for liquid. Scattering phenomena of the droplets are captured by the high-speed digital camera. Frequency distribution of the droplet diameter is evaluated from these images and distributions of horizontal flying velocity and angle of the droplets were measured by PTV. Liquid filaments are stretched outward from the stagnant liquid layer by centrifugal force and skew complicatedly by aerodynamic force. Some peaks appear in the distribution of the scattered droplet diameter and they are origi- nated from large terminal droplets and small droplets generated from filamentwise breakup. Most of the scattered droplets fly slightly inside in the tangential direction of the disk edge. The droplets spun off from the thin disk scatter widely compared with that from the thick one.