粒子图像测速技术研究进展

粒子图像测速技术(PIV)作为一种全新的无扰、瞬态、全场速度测量方法,在流体力学及空气动力学研究领域具有极高的学术意义和实用价值.本文对PIV技术的原理、分类作了简要地介绍,详细归纳和评述了现有的各种速度信息的提取方法,并对拓扑图论、神经网络、遗传算法、模糊聚类等新技术在PIV中的应用以及三维PIV技术、两相流PIV测试技术进行了介绍.指出当前PIV技术除了向三维和多相流方向发展外,如何提高PIV的测量精度以及缩短计算时间仍然是目前研究的主要目标.PIV技术随着计算机技术、激光技术和CCD性能的发展,必将取得更大的发展与突破      

风沙两相流测量技术研究进展

围绕风沙两相流的测量, 归纳了过去几十年来在风沙动力学研究中所使用的风速测量技术和输沙率测量装置.着重讨论了高频测量在目前风沙动力学研究中的必要性, 分析了传统风速和输沙率测量装置的优缺点.对新一代光学测量技术------PIV在风沙两相流测量中的应用进行了较为详细的探讨.指出PIV测速技术在风沙两相流研究中具有广泛的应用前景, 使用PIV测速技术可以得到风沙流结构、两相速度场等宏观信息, 同时也可以进行单个颗粒运动状态的研究.      

数字全息粒子图像测速技术(DHPIV)研究进展

全息粒子图像测速技术(DHPIV)是当前非常具有发展潜力的非定常三维流场测量技术,是一种具有点空间分辨力的三维空间三维速度场和时间历程的实验观测方法和技术. 本文介绍了该项技术(数字全息DH和粒子图像测速PIV)的发展背景和近20年来的研究进展,并介绍了已测得的非定常复杂流动的初步结果. 详细论述了DHPIV技术所面临的关键性问题和应用基础问题以及相应的进展:粒子空间场的重建与再现的空间分辨率问题、粒子定位或位移精度问题、信噪比和数字再现的光学与快速算法以及测量空间的扩展等问题.同时讨论了数字离轴全息等有关技术的潜力, 介绍了进一步的研究发展方向.      

多普勒全场测速技术的进展

多普勒全场测速(Doppler global velocimetry)是一种基于分子滤波原理来测量散射光多普勒频移, 从而测量平面内流动速度场的技术, 主要应用于流体力学、空气动力学和燃烧学实验研究中, 尤其适用于较高马赫数流场测量. 研究人员也称其为平面多普勒测速(planar Doppler velocimetry)、吸收-滤波平面多普勒测速(Absorption filtered planarDoppler velocimetry), 滤波瑞利散射技术(filtered Rayleigh scattering)等. 本文对多普勒全场测速技术的工作原理、结构组成、数据处理、发展趋势等进行了比较全面的介绍.      

高超声速流场激光测速技术研究进展

高超声速气流条件下飞行器内/外部流动中存在强湍流及脉动、边界层转捩、激波-边界层干扰和高温真实气体效应等耦合效应,表征该非定常流动现象对飞行器气动力、气动热以及目标光电特性等产生的影响是高超声速流动研究中的前沿课题.速度作为表征流动过程最重要的参数之一,准确的速度测量对于深入理解上述复杂流动-传输机理以及高超声速飞行器设计具有重要指导意义.文章针对高超声速流场速度测量中几种常用的非接触式激光测试技术进行了综述,主要包括基于空间法的粒子图像测速,基于激光吸收光谱、激光诱导荧光和瑞利散射的多普勒测速,基于飞行时间法的分子标记测速,以及基于流场折射率的聚焦激光差分干涉测速技术.首先简要介绍每种激光测速技术的基本原理,然后进一步介绍该技术在高超声速自由流、层/湍流边界层、激波/边界层干扰、尾流或其他复杂流动区域的速度及其脉动度测量等方面的典型应用,分析各种技术环境适用性及面临的局限性和挑战.最后对基于激光技术的高超声速流场速度测量进行了总结及发展趋势展望.     

基于NPLS技术的可压缩湍流机理实验研究新进展

可压缩湍流机理的实验研究是一件难度很大的工作, 其主要的难度在于高时空分辨率的可压缩湍流结构非接触精细测试技术和低噪声的高速风洞设备技术. 近几年来, 由于在低噪声的超声速、高超声速风洞研究和可压缩流动精细结构测量技术研究方面取得的重要进展及其在可压缩湍流机理研究方面的应用, 超声速流动转捩与湍流的机理研究取得了较大的进展. 本文介绍了最近几年高速流动非接触精细测试技术, 尤其是基于纳米粒子的平面激光散射技术(nano-tracer planar laser scattering, NPLS)、背景导向纹影技术(background oriented schlieren, BOS) 和超声速流场的粒子图像测速技术(particle image velocimetry, PIV)的研究进展和发展前景, 以及基于这些技术, 在可压缩湍流机理实验研究方面的进展和发展前景, 其中包括在超声速混合层转捩、超声速绕流与尾流结构、超声速边界层转捩、激波边界层干扰等典型流场的机理研究方面, 以及气动光学机理研究方面的研究进展. 最后, 展望了目前湍流机理实验研究对湍流工程模型研究的可能贡献.      

气液两相流中气泡运动速度场的PIV分析与研究

粒子图像测速技术（PIV）作为一种无扰、瞬态、全场速度测量方法，已被广泛应用于液体或气体的单相流流速场测定。对于两相流PIV技术，目前还处于起步阶段，本文应用PIV技术的基本原理，对静止液体中的气泡运动速度进行了分析，并对有关气液两相流测量问题进行了探讨。     

应用PIV对角区非定常马蹄涡结构的实验研究

利用PIV技术研究了柱体与平板层流边界层角区的非定常流动结构,流动显示和PIV测量均 表明角区存在3种非定常的马蹄涡模态,即绕合模态、脱落-绕合模态以及脱落-耗散模态, 一定$Re$数下主涡脱落后既可能表现为脱落-绕合模态,也可能表现为脱落-耗散模态. 这主 要取决于模型头部形状对涡轴造成的拉伸以及耗散和扩散程度. PIV测量表明,随雷诺数增 加主涡下方从壁面喷发的反向二次涡逐步增大形成强度和尺度较大的``涡舌', 该``涡舌' 将突入整个涡系所在的边界层,最终将主涡与上游涡系隔离并使其从旋涡生成区涡系脱落. 马蹄涡非定常摆动时具有较复杂的奇点形态组合和演化,反映涡轴受到了交替的拉伸和压缩 作用.     

微纳结构超疏水表面的湍流减阻机理研究

超疏水表面的优异性质使其在现代生活和工业生产中具有十分广泛的潜在应用价值. 本文采用了碳纳米管缠绕技术和聚氟硅氧烷疏水化处理方法制备了具有二级微纳米结构的超疏水表面. 测量了由该超疏水表面构建的槽道中的流动压降,将其与普通表面构建的槽道内的流动压降进行比较,发现在层流情况下,流动阻力减小最多达到了22.8%. 在湍流的情况下,超疏水表面的减阻比例约为53.3%,减阻效果比层流更加明显.利用PIV (particle image velocimetry) 技术测量了具有超疏水表面的槽道内的速度场,通过超疏水表面速度滑移和湍动脉动场信息,分析了湍流减阻效果比层流更加明显的物理机制.     

PIV速度场坏矢量的本征正交分解处理技术

介绍了一种针对粒子图像测速(PIV)基于本征正交分解(POD)的速度场后处理技术.该技术改变了现在后处理技术将速度场坏矢量识别和修正分开实现的局面,通过迭代方法有效地实现了速度场坏点统一的识别和修复算法.算法利用POD分解的低阶模态信息重构出可以用于坏矢量识别的参考速度场,利用该参考速度场对全流场进行坏点识别并完成修正.通过对一套光滑的PIV速度场数据引入高斯分布的随机误差,测试验证了该POD方法的优越性.在坏矢量识别方面新方法较归一化中值检验有更高的正确性,能识别大面积出现的坏矢量区域.在坏矢量修补的插值算法中,新方法的计算效率又高于传统Gappy POD方法,且计算精度优于常见的矢量场内插数学方法.特别是在数据缺失的大连通区域,该方法对物理流场有很好的预测效果.     

Flow Characterization Through a Network Cell Using Particle Image Velocimetry

Particle image velocimetry (PIV) with refractive index matching was developed to map pore-scale fluid flow through a clear, acrylic two-dimensional network flow cell. A microscope objective lens was incorporated in the PIV set up so that flow in micro-scale throats could be measured. The flow cell consists of 20 × 20, equal-size cylindrical pore bodies, 2.5mm in diameter and 1.0mm in height, connected on a diamond lattice by 2.5 mm long, square cross-section throats of widths that varied randomly among 0.2, 0.6, and 1.0 mm. Micro-PIV data was used to obtain the two-dimensional streamline pattern of fluid flow and the velocity field over the field of view (FOV) by periodically illuminating seed particles following the flow and cross correlating particle positions to determine displacements over time. Refractive index matching of the flow cell and test fluid minimizes extraneous scattering of light at solid--liquid interfaces improving image resolution. Experimentally determined velocity vectors for single-phase flow through three pore bodies and their adjoining throats as well as for the outlet of the flow cell were compared with numerical simulations of flow through the cell.     

A method for automatic estimation of instantaneous local uncertainty in particle image velocimetry measurements

The uncertainty of any measurement is the interval in which one believes the actual error lies. Particle image velocimetry (PIV) measurement error depends on the PIV algorithm used, a wide range of user inputs, flow characteristics, and the experimental setup. Since these factors vary in time and space, they lead to nonuniform error throughout the flow field. As such, a universal PIV uncertainty estimate is not adequate and can be misleading. This is of particular interest when PIV data are used for comparison with computational or experimental data. A method to estimate the uncertainty from sources detectable in the raw images and due to the PIV calculation of each individual velocity measurement is presented. The relationship between four error sources and their contribution to PIV error is first determined. The sources, or parameters, considered are particle image diameter, particle density, particle displacement, and velocity gradient, although this choice in parameters is arbitrary and may not be complete. This information provides a four-dimensional “uncertainty surface” specific to the PIV algorithm used. After PIV processing, our code “measures" the value of each of these parameters and estimates the velocity uncertainty due to the PIV algorithm for each vector in the flow field. The reliability of our methodology is validated using known flow fields so the actual error can be determined. Our analysis shows that, for most flows, the uncertainty distribution obtained using this method fits the confidence interval. An experiment is used to show that systematic uncertainties are accurately computed for a jet flow. The method is general and can be adapted to any PIV analysis, provided that the relevant error sources can be identified for a given experiment and the appropriate parameters can be quantified from the images obtained.     

Development and validation of echo PIV

The combination of ultrasound echo images with digital particle image velocimetry (DPIV) methods has resulted in a two-dimensional, two-component velocity field measurement technique appropriate for opaque flow conditions including blood flow in clinical applications. Advanced PIV processing algorithms including an iterative scheme and window offsetting were used to increase the spatial resolution of the velocity measurement to a maximum of 1.8 mm×3.1 mm. Velocity validation tests in fully developed laminar pipe flow showed good agreement with both optical PIV measurements and the expected parabolic profile. A dynamic range of 1 to 60 cm/s has been obtained to date.     

Quantitative evaluation of PIV peak locking through a multiple Δ<Emphasis Type="Italic">t</Emphasis> strategy: relevance of the rms component

The possibility of using different times between laser pulses (Δt) in a PIV (Particle Image Velocimetry) measurement of the same real flow field for error assessment has already been proposed by the authors in a recent paper Nogueira et al. (Meas Sci Technol 20, 2009). It is a simple procedure that is available with the usual PIV setup. In that work, peak locking was considered basically as a bias error. Later measurements indicated that, using appropriate processing algorithms, this error is not the main peak-locking effect. Scenarios with the rms (root mean square) error due to peak locking as the most relevant contribution are more common than initially expected and require a differentiated approach. This issue is relevant due to the impact of the rms error in the evaluation of flow quantities like turbulent kinetic energy. The first part of this work is centred on showing that peak-locking error in PIV is not always a measurement bias towards the closest pixel integer displacement. Insight in the subject indicates that this is the case only for algorithm-induced peak locking. The peak locking coming out of image acquisition limitations (i.e. resolution) is not ‘a priory’ biased. It is a random error with a peculiar probability density function. Discussion on the subject is offered, and a particular approach to use a simple multiple Δt strategy to asses this error is proposed. The results reveal that in real images where amplitude of the peak-locking bias error is assessed to be as small as 0.02 pixels, rms errors can be in the order of 0.1 pixels. As PIV approaches maturity, providing a quantitative confidence interval by estimating measurement error seems essential. The method developed is robust enough to quantify these values in the presence of turbulence with rms up to ~0.6 pixels. This proposal constitutes a relevant step forward from the traditional histogram-based considerations that only reveal whether strong peak-locking error is present or not, without any information on its magnitude or whether its origin is bias or rms.     

The challenge of stereo PIV measurements in the tip gap of a transonic compressor rotor with casing treatment

This contribution is aimed at summarizing the effort taken to apply stereoscopic PIV (SPIV) measurements in the tip clearance of a transonic compressor rotor equipped with a casing treatment. A light sheet probe was placed downstream of the stator and aligned to pass the light sheet through a stator passage into the blade tip clearance of the rotor. A setup with three cameras has been used in order to record the entire 2C velocity field and the smaller area of 3C field of view at the same time instance for comparison with earlier 2C PIV results. A homogeneous seeding distribution was achieved by means of a smoke generator. The main emphasis of the SPIV measurement was to establish a data set with high spatial resolution close to the compressor casing, where the aerodynamic effects of a CT are known to be strong. The paper will discuss some major aspects of the utilized PIV data processing and point out a variety of frequently underestimated error sources that influence the overall quality of the recovered data in spite of the fact that the individual PIV recordings seemed to be of very good quality. Thus, the authors will not focus on the PIV results and related interpretation of the flow field, but on the optimization and procedures applied during setup of the experiment and data processing, respectively.     

基于光流算法的粒子图像测速技术研究和验证

光流测量技术作为一种新的空气动力学实验技术,以其像素级分辨率的矢量场测量优势获得广泛的应用。光流测量技术使用光流约束方程,配合平滑限定条件,可以进行速度场测量,获得高分辨率的全局矢量场。本文首先通过研究积分最小化光流测速理论和算法,采用C++编写光流速度测量程序;然后通过三种典型的人工位移图像对光流计算程序进行了验证,并将结果和标准位移分布进行比对分析,以指导如何在实际应用中获得高精度光流速度场;最后进行小型风洞后向台阶实验,利用高速相机拍摄示踪粒子图像,使用光流计算程序获得速度矢量场,同采用互相关算法的粒子图像测速计算结果相比较,体现出光流计算方法像素级分辨率的矢量场测量优势。     

Acoustic streaming measurements in standing wave resonator using Particle Image Velocimetry

This paper experimentally investigates the measurement of acoustic streaming in a 7 m long-standing wave air-filled acoustic resonator. One can describe the acoustic streaming as a second-order steady flow, which is superimposed on the dominant acoustic velocity. It is induced by the nonlinearities of the acoustic propagation inside the resonator. The exploration of the acoustic velocity field by the synchronized PIV (stands for Particle Image Velocimetry) technique enabled to highlight and quantify these secondary flows. The PIV measurements of the acoustic velocity fields at different phases over the excitation signal period gave information on streaming profiles and the post processing applied allowed plotting the acoustic velocity over time. These results were compared to the outcome of a 2D numerical study performed with the commercial software Fluent, where good agreements were found. It indicates the ability of this method to accurately measure second order steady flow variations of the acoustic velocity field.     

粒子图像测速技术互相关算法研究进展

粒子图像测速技术(particle image velocimetry, PIV)中采用的互相关算法就是需要从独立 存在的两幅图像通过一定的判别方法得到流场中各点的流速矢量的计算方法. 互相关算法的具体实现步骤包括图像前处理、区域离散、 匹配原则选取、搜索方法选取和变形预测, 最后对结果进行后处理. 文中从上述几个方面总 结了国内外近年来互相关算法的发展过程, 并通过对各种方法精度和效率的比较对其应用发 展进行了归纳.     

Digital interferometry: techniques and trends for fluid measurement

Interferometric based measurement techniques have long played a significant role in enabling insight and understanding into the flow and heat transfer processes in a wide range of fluid flow situations. In recent years, with the advent of digital processing techniques, full field quantitative data can be obtained from interferometric data with relative ease and a high degree of accuracy. However, these developments seem not to have percolated through to experimental fluids practitioners. This paper sets out to review the various processing and acquisition techniques for quantitative and qualitative measurement using interferometry. Full field phase measurement interferometry (PMI) and differential interferometry (DI) are discussed. PMI is demonstrated for diffusing mixing in a mini-channel and DI is demonstrated for heat transfer measurement in a convecting fluid.