Application of large scale PIV in river surface turbulence measurements and water depth estimation

Large-Scale Particle Image Velocimetry (LSPIV) has emerged as a reliable technology to measure river surface flow velocity distribution and can be applied to estimate river discharge. Fewer studies have explored the capability of surface turbulence measurements using LSPIV. In this paper, LSPIV is applied to evaluate statistics of surface turbulence of a natural river. Turbulence measurements including velocity fluctuation, velocity spectra and the dissipation rate of turbulent kinetic energy (TKE) are validated by comparing with those measured by an Acoustic Doppler Velocimeter (ADV). Traditionally, estimation of stream discharge through LSPIV needs a secondary measurement to determine river bathymetry and water depth. A new method is presented here to demonstrate that for a fully developed and channel-controlled flow, the cross section geometry can be estimated from the combined measurements of surface mean velocity and the dissipation rate, following the Manning-Strickler formula. Therefore, river discharge can be estimated with LSPIV along with a calibrated Manning's roughness, without additional bathymetry survey. The proposed new method is applied to measure discharge in Milwaukee River (Milwaukee, Wisconsin, U.S.A.), which agreed well with data obtained from a nearby streamgage station.     

Measurement of the surrounding liquid motion of a single rising bubble using a Dual-Camera PIV system

A new type of Particle Image Velocimetry technique, called “Dual-Camera PIV System”, was developed in order to achieve higher-accuracy measurement at a high time resolution. It is very difficult to measure precisely a complex flow field such as a gas–liquid two phase flow using PIV, because of the existence of a strong turbulence. In the conventional dynamic PIV, a time interval of two images required for analysis depends basically on a camera frame rate. A time interval of a set of PIV images affects the measurement accuracy significantly. Therefore, it is necessary to shorten the time interval of a set of PIV images as well as to achieve high frame rates. For this specific purpose, we developed a measurement system composed of two high speed cameras. The interval of two images obtained from each camera was controlled arbitrarily. Furthermore, a recursive cross-correlation method was adopted as PIV algorithm in order to achieve high spatial resolution. The interrogation areas were determined from the number density of PIV particles. The developed system was evaluated by cross-correlation coefficient and signal–noise (S/N) ratio. As the demonstration, the surrounding liquid motion in the vicinity of a single rising bubble was measured via this measurement system.     

粒子图像测速(PIV)技术的发展

在流场显示测量技术中，粒子图像测速(PIV)技术占有相当重要的地位，本文重点介绍了PIV的测试原理及应用要求，并以TSI公司开发的PIV系统为例对DPIV系统的构成作了概括，同时指出了PIV技术的未来发展方向。     

磁力搅拌湍流场的PIV测量及流场演变的POD分析

为了解磁力搅拌过程湍流特性,对其开展了激光粒子图像测速(PIV)试验,并进行了本征正交分解(POD)分析。分析表明,在测量平面转子外部区域,除平均流动,含能量较高的流动结构为循环剪切流和转子旋转的尾涡结构。在搅拌槽底部具有较高的旋涡强度,有利于固体颗粒悬浮。流体与转子之间的流-固耦合作用使转子产生振荡,随转速升高,系统稳定性下降,平均流动含能降低,小尺度结构含能增加,涡流扩散增强,有利于混合和反应进行。     

Flow characteristics of back supported V-cone flowmeter (wafer cone) using PIV

The present experimental study has been carried out to evaluate the performance and flow characteristics of the Wafer cone flowmeter using Particle Image Velocimetry (PIV). Two equivalent diameters (β) of 0.62 and 0.72 with combination of two vertex angles (ϕ) namely 30°and 45°are used for the evaluation of the performance of the flowmeter in the range of Reynolds number of 3 × 10³ to 8.19 × 10⁴. The investigation shows that the coefficient of discharge seems to be independent of β-value with the increase in vertex angle. Further, the appropriate location of the downstream pressure tap is also estimated for the cone configuration of β = 0.62 and ϕ = 30°. It is observed that the downstream pressure tap location of 0.8D distance gives a higher value of discharge coefficient compared to 0.0D distance with the error being also lower marginally. PIV data has been analysed for the cone configuration of β = 0.62 and ϕ = 30°at four Reynolds numbers of 3028, 6057, 52755 and 74488 in terms of axial velocity and turbulent intensity. The measurements reveal an interesting phenomenon in terms of the rapid decay of turbulent kinetic energy on the downstream of the cone. This may be due to the interference of the cone wake with the support wake resulting in fast decay. This unique phenomenon leads to the reduction in the requirement of the downstream straight length for the Wafer cone flow meter, unlike other obstruction type flowmeters.     

Structure analysis of bubble driven flow by time-resolved PIV and POD techniques

In this paper, the recirculation flow motion and turbulence characteristics of liquid flow driven by air bubble stream in a rectangular water tank are studied. The time-resolved Particle Image Velocimetry (PIV) technique is adopted for the quantitative visualization and analysis. 532nm Diode CW laser is used for illumination and orange fluorescent (λ_ex = 540nm, λ_em = 584nm) particle images are acquired by a 1280×1024 high-speed camera. To obtain clean particle images, 545nm long pass optical filter and an image intensifier are employed and the flow rate of compressed air is 3ℓ/min at 0.5MPa. The recirculation and mixing flow field is further investigated by time-resolved Proper Orthogonal Decomposition (POD) analysis technique. It is observed that the large scale recirculation resulting from the interaction between rising bubble stream and side wall is the most dominant flow structure and there are small scale vortical structures moving along with the large scale recirculation flow. It is also verified that the sum of 20 modes of velocity field has about 67.4% of total turbulent energy.     

Effects of different porous beds on turbulent characteristics in an open channel above the porous bed

An analysis of the turbulent characteristics of the flow in an open channel with horizontal porous beds using two-dimensional (2D) Particle Image Velocimetry (PIV) is presented. Hydraulic characteristics such as velocity distributions, turbulent intensities and Reynolds stress are investigated at a fine resolution using the PIV. Experiments were carried out, with four types of porous bed with the same porosity ε=0.70 and the same porous thickness (3 cm): (a) porous filter, (b) rods bundle, (c) grass vegetation and (d) gravel bed. Turbulent characteristics are measured above the porous bed for the same different water heights (h΄=3, 5, 7 and 9 cm) and for the same slope S=1.5‰. Main emphasis is given to the effects of relative porous thickness s΄/h (s΄=porous thickness, h=total flow height) on the flow characteristics over the porous beds. The relative thickness s΄/h varies between 0.250 and 0.500. The discharge ranges between 3.0 and 24.6 lt/s. Measurements of mean velocity and turbulence characteristics indicate the effects of the bed material used on the flow characteristics. The presence of rods bundle influences with different way the turbulent characteristics of the flow as regards the other porous beds.     

When,what and how image transformation techniques should be used to reduce error in Particle Image Velocimetry data?

Particle Image Velocimetry is commonly used to compute velocity fields in several areas including fluid mechanics, hydraulics and geophysics. However, acquired images often contain deformations caused either by camera lenses or placement. In this work the most popular digital transformation methods used to remove/reduce these deformations are benchmarked and suggestions tailoring specific transformations to different types of deformations are made. This article also shows the reduction of the error associated to the first and second order statistics, in the case of two-dimensional Particle Image Velocimetry, when the transformation techniques are applied to the computed velocity fields, and not the raw images, a common option in available commercial software.     

A hybrid phase boundary detection technique for two-phase-flow PIV measurements

Particle Image Velocimetry (PIV) measurement accuracy is lower along the phase boundaries of two-phase-flows, because the interrogation windows contain information from both phases. Different seeding density, background intensity, velocity magnitude and flow direction conditions often exist across the boundary, and the cross-correlation-based PIV algorithm selects only the highest correlation peak. The highest correlation peak is either influenced by the wrong phase (across the boundary), or the correctly calculated displacement is erroneously detected as an outlier at a later stage and is subsequently replaced. Phase-separated PIV measurements minimize this problem, and increase accuracy along the boundary by treating each phase separately. This type of measurement requires for each time step; (i) the accurate detection of the phase boundary in consecutive frames, (ii) generation of dynamic phase masks, (iii) an accurate PIV evaluation of each phase and (iv) recombination of the flow fields. In this article, we focus on the first step and test a hybrid phase boundary detection (PBD) technique in three different two-phase-flow configurations which manifest different challenges: The first configuration is the mixing of two liquids in a magnetic micromixer, the second is a combustion experiment where a turbulent, pre-mixed, low-swirl, lifted flame is investigated, and the third is a bubble column reactor where air bubbles are rising in a water tank. The PBD implementation uses a three-step procedure: approximate global thresholding, local Otsu thresholding, and discrimination of image gradients. Comparison of results with and without the use of PBD and phase separation indicate that there are significant measurement accuracy improvements along the boundary.     

Application of video imagery techniques for low cost measurement of water surface velocity in open channels

Developments in digital video recording technology make the video imagery tools more popular for velocity measurement in water flows. This has especially been of large interest due to its inherent advantage of non-contact nature which is quite handy in extreme flow conditions. Particle Image Velocimetry (PIV), Particle Tracking Velocimetry (PTV) and Large Scale Particle Tracking Velocimetry (LSPTV) are applied to free surface channel flow for water surface velocity measurement. Experiments are conducted to measure either a single point velocity applying PTV or velocity profiles across the channel width applying PIV on the water surface in a rectang typical velocities of nearly 1 andular tilting flume for various flow conditions. Technical issues regarding tracer particle size and type, travel distance, lighting, recording speed, camera position, image distortion and state of flow are discussed. Measured data is compared to computational results obtained from a numerical model involving a non-linear turbulence model capable of predicting turbulence driven secondary flows. Confirmation of reasonable match between computational and experimental results whereby applying mutual collaboration of them for discharge measurement has been attested. In addition to discharge, boundary roughness has also been predicted as an outcome of the numerical solution.     

Low reynolds number κ-ε modeling with improved ε equation

A series of κ-ε model modification has been carried out using DNS data to include near wall effects. Though these methods aided by DNS data open new ways of turbulence modeling, the κ-ε turbulence models still have shortcomings in predicting turbulent flows for various Reynolds numbers and various geometric conditions. Therefore a new κ-ε models with improved dissipation rate of turbulent kinetic energy equation and the damping function for eddy viscosity model is proposed. The new dissipation rate equation is based on the energy spectrum and magnitude analysis. The damping function for eddy viscosity is also based on the dissipation rate length scale distribution near the wall and the DNS data. The new κ-ε model is applied to fully developed turbulent flows in channels and pipes with a various Reynolds numbers. Predictions show that the proposed model represents properly the turbulence properties in all turbulent regions over a wide range of Reynolds numbers.     

An experimental study on flow characteristics of erf between two parallel-plate electrodes by using piv technique

An experimental investigation was performed to study the characteristics of ER(Electro-Rheological) fluid flow in a horizontal rectangular tube with or without D. C voltage. To determine some characteristics of the ER flow, 2D PIV (Particle Image Velocimetry) technique was employed for velocity measurements. This research found the mean velocity distributions with OkV/mm, 1.0kV/mm and 1.5kV/mm for Re=0, 0.62, 1.29 and 2.26. When the strength of the electric field increased, the cluster of ERF was clearly strong along the test tube and the flow rate decreased. The present results will contribute to the economical and compact design of ER fluids system.     

Eulerian particle flamelet modeling for combustion processes of bluff-body stabilized methanol-air turbulent nonpremixed flames

The present study is focused on the development of the RIF (Representative Interactive Flamelet) model which can overcome the shortcomings of conventional approach based on the steady flamelet library. Due to the ability for interactively describing the transient behaviors of local flame structures with CFD solver, the RIF model can effectively account for the detailed mechanisms of NOx formation including thermal NO path, prompt and nitrous NOx formation, and reburning process by hydrocarbon radical without any ad-hoc procedure. The flamelet time of RIFs within a stationary turbulent flame may be thought to be Lagrangian flight time. In context with the RIF approach, this study adopts the Eulerian Particle Flamelet Model (EPFM) with mutiple flamelets which can realistically account for the spatial inhomogeneity of scalar dissipation rate. In order to systematically evaluate the capability of Eulerian particle flamelet model to predict the precise flame structure and NO formation in the multi-dimensional elliptic flames, two methanol bluffbody flames with two different injection velocities are chosen as the validation cases. Numerical results suggest that the present EPFM model has the predicative capability to realistically capture the essential features of flame structure and NOx formation in the bluff-body stabilized flames.     

三相旋流抛光磨粒运动的测量与微气泡补偿

研究了气液固三相旋流流场抛光机理和规律。设计了三入口的抛光加工流道,对气液固三相旋流抛光流场进行了数值模拟。基于模拟结果设计了气液固三相磨粒流旋流流场测量平台,并通过粒子图像测速法(PIV)测量了微气泡补偿条件下气液固三相旋流抛光的流场参数,获得了微气泡补偿区域流场的运动图像、速度矢量图和涡量图。PIV测量试验数据显示:在微气泡补偿区域,磨粒速度主要集中在30m/s到80m/s,同一测量点高速磨粒出现频率明显增加,少数磨粒速度达到100m/s以上;磨粒平均速度从33.8m/s增大到44.2m/s,经4h抛光后硅片表面最大粗糙度从10.4μm下降到1.3μm。理论和试验研究表明,气液固三相旋流抛光流场中微气泡溃灭引发的空化冲击效应可增大磨粒动能,提高抛光效率,实现B区域的均匀化抛光。     

DPIV技术及其在流场测量中的应用

本文对流动显示技术和全流场测量技术作了简单介绍,重点介绍了数字 粒子图像测试技术DPIV（Digial Particle Image Velocimetry）及其在流场测量中的应用,并给出矩形腔内流场的测试结果,指出DPIV技术是流场可视化测试中的重要发展趋势之一。     

PSV and marker detection techniques for investigating the wake of an oscillating cylinder in a large wind tunnel

Although Particle Image Velocimetry techniques are widely used in the experimental investigation of the flow structure around obstacles, the application of these techniques in wind tunnels is frequently limited by the dimension of the areas involved, which make the image acquisition set-up expensive.This paper examines the use of Particle Streak Velocimetry (PSV) and marker detection techniques to measure the flow field around bodies with relatively large dimensions. In spite of its minimal requirements in terms of acquisition set-up, the method permits the investigation of complex phenomena.The tests described in the paper were carried out in air using a rigid cylinder, free to oscillate in a wind tunnel. Coupling PSV and marker detection techniques with the phase average of the measured velocity fields, the evolution of vortex shedding behind a cylinder with a diameter of 200 mm was measured with an acquisition area of about 1000×1200 mm.     

Diagnosis of the behavior characteristics of the evaporative diesel spray by using images analysis

In this study, the effects of change in injection pressure on spray structure have been investigated on the high temperature and pressure field. To analyze the structure of evaporative diesel spray is important in speculation of mixture formation process. Also emissions of diesel engines can be controlled by the analyzed results. Therefore, this study examines the evaporating spray structure in a constant volume chamber. The injection pressure is selected as the experimental parameter, is changed from 72 MPa to 112 MPa with a high pressure injection system (ECD-U2). The PIV (Particle Image Velocimetry) technique was used to capture behavior variation of the evaporative diesel spray. Analysis of the mixture formation process of diesel spray was executed by the results of flow analysis in this study. Consequentially the large-scale vortex flow could be found in downstream spray and the formed vortex governs the mixture formation process in diesel spray.     

PIV measurements of the time-averaged flow velocity downstream of flow conditioners in a pipeline

The flow downstream of three different flow conditioners, a tube bundle and two perforated plates, was investigated by measuring the time-averaged, axial velocity component with Particle Image Velocimetry (PIV). The conditioners were exposed to the flow disturbed by a 90° out-of-plane double-bend. The experiments were performed with air flow through a pipeline of 100 mm i.d. and at Reynolds numbers between 100 000 and 200 000. The axial development of the velocity profiles, without and with conditioner, is documented, and the performance of the three devices in conditioning the disturbed flow can be compared. Particular attention is given to the determination of time-averaged velocity values by means of PIV.     

6 km自容式湍流观测剖面仪设计与试验研究

目前大部分湍流观测剖面仪的观测范围在1 km以浅。为获取深海湍流数据,探究深海动力过程演变机制与能量耗散过程,研制6 km自容式湍流观测剖面仪。从湍流观测应用的翼型剪切流传感器测量原理出发,分析约束深海剖面仪设计的关键技术,开展了深海剖面仪稳定水动力布局设计、耐压6 km的轻量化结构设计、下潜速度分析与控制设计。观测传感器集成微结构的双PNS系列翼型剪切流传感器与FP07快速温度传感器,实现了深海湍流两个维度下湍动能耗散与热耗散的同步、高速观测,采用模块化的设计思想完成自容式湍流观测硬件系统的设计。通过性能检测试验,剖面仪可满足6 km应用,下潜速度线性可调范围0.4~0.9 m/s,验证了剖面仪结构设计的可行性;海上试验,并与MSS90进行比测,剖面仪测量湍流脉动剪切变化趋势与MSS90相同,计算的湍流剪切波数谱与Nasmyth理论谱在截止波数前非常吻合,相同深度的湍动能耗散率与MSS90为同一数量级,验证了深海6 km自容式湍流观测剖面仪湍流测量的准确性。     

Qualitative and quantitative measurements of horseshoe vortex formation in the junction of horizontal and vertical plates

Horseshoe vortex formation around a vertical flat plate mounted in the water channel is experimentally investigated. In order to demonstrate the development of the horseshoe vortex system in the junction of the horizontal and vertical plates a Particle Image Velocimetry (PIV) and dye visualization techniques were used. Instantaneous and time-averaged velocity vector fields, corresponding streamline topology, vorticity values were analyzed. Boundary layer separation, developing vortices, secondary or counter-rotating vortices, merging of developing and counter-rotating vortices which results in a primary horseshoe vortex system and corner vortex were demonstrated qualitatively and quantitatively.