Holographic particle image velocimetry: analysis using a conjugate reconstruction geometry

Holographic recording techniques have recently been studied as a means to extend two-component, planar particle image velocimetry (PIV) techniques for three-component, whole-field velocity measurements. In a similar manner to two-component PIV, three-component, holographic PIV (HPIV) uses correlation-based techniques to extract particle displacement fields from double-exposure holograms. Since a holographic image contains information concerning both the phase and the amplitude of the scattered field it is possible to correlate either the intensity or the complex amplitude. In previous work we have shown that optical methods to compute the autocorrelation of the complex amplitude are inherently more tolerant to aberrations introduced in the reconstruction process, Coupland, Halliwell, Proc. Roy. Soc. 453 (1960) (1997) 1066. In this paper we introduce a new method of holographic recording and reconstruction that allows a constant image shift to be introduced to the particle image displacement. The technique, which we call conjugate reconstruction, resolves directional ambiguity and extends the dynamic range of HPIV. The theory of this method is examined in detail and a relationship between the image and object displacement is derived. Experimental verification of the theory is presented.     

气液两相流速度及粒径分布激光干涉测量方法的研究

为了实现对气液两相流的粒子粒径、空间分布及其速度测量。对激光干涉气液两相流测量技术（ILIDS）进行了深入研究,该技术是一种应州于气液两相流测量的新技术,其主要优点是不干扰流场和颗粒粒径、位置测量精度高。基于该技术所开发的图像自动处理方法可以利用普通粒子成像测量技术系统拍摄气液两相流的激光散射干涉图像。并利用图像卷积定位、傅里叶变换频率分析及其图像互相关测速等图像处理手段从干涉图像中自动提取粒子的位置、直径和速度信息。为了验证该方法的测量精度,对喷嘴生成的气水两相流进行了测量实验,得到了喷嘴出口处不同区域的粒径、速度矢量的空间分布,并将测得的速度矢量与用粒子成像测量技术方法测得的结果进行对比,证明两种方法测量的平均速度差别仅为0．38％。     

Velocity and density field measurements by digital speckle method

Velocity and density field measurements based on image processing of laser speckle or pseudo-speckle pattern have been developed. Laser speckle velocimetry (LSV) or white-light speckle velocimetry (WSV), which corresponds to a high-image-density PIV, gives a local velocity vector map or whole field velocity contour map of a two-dimensional flow field seeded densely with fine particles. This technique has an advantage in high-speed flow measurement without limit of frame rate except for directional ambiguity. New techniques of laser speckle photography and laser speckle interferometry by means of digital image processing have been developed recently for density field measurements. In laser speckle photography, a local density gradient vector map is reconstructed by cross-correlation evaluation between the reference and the object speckle patterns. In laser speckle interferometry, an equi-density contour map is reconstructed by image subtraction between the reference and the object interferometric speckle patterns.     

A real-time video encoded particle imaging tracking technique for velocity measurement

A real-time video encoded particle imaging tracking technique (VPIT) for velocity measurement has been developed. It can currently capture images of a seeded particle flow field at up to a video rate of 25 pictures per second. The method as shown in this paper is suitable for measuring a slow sparsely seeded flow. A VPIT image presents a triplet image pattern. The image has been encoded into a single video frame with the time history of three events. This is achieved by synchronising the video (CCIR) signal from a CCD (charge coupled device) camera, operating in frame integration mode with a suitable light source. The principle of VPIT demonstrates how the direction and the magnitude of the velocity can be recorded for a sequence or track of particles. The VPIT triplet images resolve several common difficulties associated with the application of PIV. Firstly, the time history of the laser pulse can be ‘labeled’ on an individual particle image. Secondly, there is no velocity direction ambiguity in the VPIT image. Thirdly, it is possible to extract the acceleration of the particle from a single VPIT frame. Finally, for a sequence of captured frames, the problems of particle path tracking are simplified, because each VPIT image has a video encoded time sequence ‘labelled’ on it.     

Time resolved particle image velocimetry

The temporal evolution of the velocity field of an unsteady fluid flow can be tracked by combining Particle Image Velocimetry (PIV) and high speed photography. Two alternative techniques for PIV are discussed: the classical light sheet technique and a method which makes use of the light scattered in forward direction. We applied time resolved PIV to investigate the flow around cavitation bubbles during their collapse near a solid boundary. The light source was an argon laser with an external acousto-optic deflector which produces series of short pulses. Using a drum camera for high speed photography, we achieved a temporal resolution of10 kHz and a spatial resolution of better than2 points/mm². Velocities could be determined without directional ambiguity in a range from2 m/s to30 m/s.     

Time‐resolved X‐ray PIV technique for diagnosing opaque biofluid flow with insufficient X‐ray fluxes

X‐ray imaging is used to visualize the biofluid flow phenomena in a nondestructive manner. A technique currently used for quantitative visualization is X‐ray particle image velocimetry (PIV). Although this technique provides a high spatial resolution (less than 10 µm), significant hemodynamic parameters are difficult to obtain under actual physiological conditions because of the limited temporal resolution of the technique, which in turn is due to the relatively long exposure time (～10 ms) involved in X‐ray imaging. This study combines an image intensifier with a high‐speed camera to reduce exposure time, thereby improving temporal resolution. The image intensifier amplifies light flux by emitting secondary electrons in the micro‐channel plate. The increased incident light flux greatly reduces the exposure time (below 200 µs). The proposed X‐ray PIV system was applied to high‐speed blood flows in a tube, and the velocity field information was successfully obtained. The time‐resolved X‐ray PIV system can be employed to investigate blood flows at beamlines with insufficient X‐ray fluxes under specific physiological conditions. This method facilitates understanding of the basic hemodynamic characteristics and pathological mechanism of cardiovascular diseases.     

3D imaging of evaporating fuel droplets by stereoscopic PIV

A gun-type burner is a widely used oil burner for industrial and domestic applications. The oil is pressure-atomized and mixed with air generating a recirculating, swirling flow. Because of the surrounding flame, fuel droplets evaporate, being difficult to obtain information on droplets’ dynamics. Several laser techniques have been applied to this burner for spray diagnosis. PDA provides information about droplet size and velocity but can say little about the instantaneous spatial structures in the flow. Planar laser techniques as PIV can describe the 2D instantaneous spatial structures, but cannot provide information about the 3D structures in the flow. Then Stereoscopic PIV was applied. This technique allows us to measure the full 3D velocity vector map in a whole fluid plane. This paper has a double purpose. Firstly, to visualize the 3D structures which are present in the burner; secondly, to show that Stereoscopic PIV is an applicable technique for the diagnosis of an evaporating spray.     

Modern developments in particle image velocimetry

This paper summarises the development of particle image velocimetry (PIV) over the last decade and describes its applications as an effective measuring technique for mapping complex flow fields. A review of work in the field is given. Current aerodynamic and hydrodynamic investigation by the authors exploiting the method are described and used as illustration.     

多光谱成像的粒子图像测速

基于时间分辨的粒子图像测速技术(time-resolved particle image velocimetry, TR-PIV)是一种广泛应用的非接触式二维瞬时流场可视化测量技术。为了得到流场精细的瞬态空间结构和演变过程,提出了一种利用多光谱成像技术来提高流场测量的时间分辨率的方法。利用多个不同波长的脉冲激光照明流场中的同一测量区域,使用多光谱成像系统采集不同波长的粒子图像,经过图像分离,判决计算产生速度矢量场。为了验证这一原理的可行性,使用三种不同波长(488,532和632.8 nm)的单色光谱脉冲搭建了一套基于多光谱成像的TR-PIV系统,通过多波长激光脉冲之间时序的精确控制,将两帧图像之间的时间间隔从10 ms缩短至3.4 ms,时间分辨率提高了3倍。结果表明基于多光谱的TR-PIV测量系统在保持PIV技术瞬时全场测量特点的同时,时间分辨率大为提高。     

Flow measurement in a transonic centrifugal impeller Using a PIV

Laser velocimetries, such as LDV or laser-2-focus (L2F) velocimetry, have been widely used for a flow measurement in a high-speed rotating impeller. A particle image velocimetry (PIV) is one of the popular velocity measurement techniques for the ability to measure a velocity field. And a PIV offers an extensive velocity field in an extremely shorter measurement time than the laser velocimetries. In the present experiment, a PIV was applied to a flow measurement in a transonic centrifugal impeller. A phase locked measurement technique every 20% blade pitch was performed to obtain a velocity field over one blade pitch of the inducer. The measured velocity field at the inducer of impeller clearly showed a shock wave generated on the suction surface of a blade. The validity of the present technique was also discussed.     

多相等离子体气动激励流动特性

 等离子体激励器电极组相位不同便产生多相等离子体气动激励,建立了粒子图像测速仪流场参数测试系统,利用粒子图像测速仪技术,研究了非对称布局等离子体气动激励诱导空气流动特性,分析了多相等离子体气动激励对诱导空气流动速度的影响。结果表明：粒子图像测速仪流场测试系统能够准确地反映等离子体气动激励诱导空气流动的流场空间结构,等离子体气动激励诱导空气流动是平行于激励器的近壁面射流,多相等离子体气动激励能够增大等离子体气动激励诱导气流速度,或者使等离子体气动激励影响流场区域增大。粒子图像测速仪系统是深入研究等离子体气动激励的流场结构最佳的方式之一。     

Particle image velocimetry measurements of wave-current interaction in a laboratory flume

The hydrodynamics of wave-current interaction is of interest to those concerned with marine and offshore structures. In particular the fluid loading characteristic may be radically altered in a sea state consisting of a mean current flow mixed with freely propagating gravity waves.

The present paper describes water flume experiments, using Particle Image Velocimetry (PIV), executed to examine hydrodynamics of wave-current interaction. A variety of wave and current conditions were investigated to determine the major influences on the combined flow.

This paper describes the experimental procedures used to obtain simultaneous measurements of the resulting wave velocity vectors over an extended region of the wave. It also describes how the directional ambiguity inherent in the basic PIV method was resolved by ‘pulse tagging’ technique.

Velocity vectors under waves at various phase points for different current and wave conditions are presented and compared in some cases with measurements derived using Laser Doppler Anemometry (LDA). The resulting velocity vectors are used to estimate how the mutual interaction, between wave and current, effect the calculation of structural loads using Morrison's equation.     

Simultaneous flow field and fuel concentration imaging at 4.8 kHz in an operating engine

High-speed particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) techniques are combined to acquire flow field and fuel concentration in a spray-guided spark-ignited direct-injection (SG-SIDI) engine under motored and fired operation. This is a crucial step to enable studies that seek correlations between marginal engine operation (misfires or partial burns) and local, instantaneous mixture and flow conditions. Correlated flow and fuel data are extracted from a 4 mm×4 mm sub-region directly downstream the spark plug to characterize the in-cylinder conditions next to the spark plug during the spray and ignition event. Values of equivalence ratio, velocity magnitude, shear strain rate, and vorticity all increase during the spray event and decrease an order of magnitude during the duration of the spark event.     

Investigation on the flow characteristics inside an automotive HVAC system with varying ventilation mode

The ventilation flow in a heating, ventilation and air conditioning (HVAC) module of a passenger car was investigated experimentally. Three different ventilation modes with varying temperature mode were tested to study the effect of ventilation mode on the velocity field inside the HVAC module. For each mode, more than 450 instantaneous velocity fields were measured using a particle image velocimetry (PIV) velocity field measurement technique. The instantaneous velocity fields were ensemble averaged to obtain the spatial distribution of mean velocity and spanwise vorticity. The present work highlights the usefulness of the PIV technique for the analysis of the flow inside an HVAC module. The experimental results can be used not only to understand and improve the ventilation flow of an HVAC module but also to validate numerical predictions.     

水平槽道内湍流变动的PTV实验研究

本文采用PIV测量技术研究充分发展水平槽道内的两相湍流的变动规律(Re=590)。首先将单相湍流的测量结果与文献中DNS的结果进行了比较,证明了PIV测量湍流脉动的可行性,并通过引入PTV算法获得了近壁对数边界层内的湍流量。对两相流动的测量结果表明,即使在1％的低颗粒质量载荷下,气体湍流已有明显的变动,并且壁面附近和槽道中心的变动规律不同。     

水洞流场PIV显示与分析系统离轴测试校正

粒子图像测速技术(PIV)通过测量被测流场截面上每一位置点的速度,获得整个被测流场的信息.在PIV一般应用中所使用的照明激光片光与成像CCD装置的拍摄方向是垂直的,在某些应用场合受测试条件的限制,需要采用离轴方式进行测量,此时CCD成像方向与照明的激光片光不垂直,而是有一定夹角.离轴测试方式将对PIV系统的光学成像系统、示踪粒子选择和粒子图像处理带来影响.实验采用Scheimpflug离轴聚焦的方法对表面镀银高反射率的示踪粒子进行成像,通过调整成像透镜与CCD像面的夹角可获得清晰的粒子成像,并利用网格校正板和软件计算处理等方法有效校正了由于离轴测试带来的影响.     

二维及三维流场的光学测量方法

对于复杂的非定常流动 ,流场的测量往往要求无干扰、非接触 ,并且能够瞬时记录流场的二维甚至三维信息。对近年来流场测量领域发展快速、应用广泛的几种光学测量方法 ,如 PIV技术及其由此发展而来的 DPIV和 HPV技术 ,做一些介绍和比较。     

Particle Image Velocimetry Applied to a Spray Jet

Experiments are described in which particle image velocimetry (PIV) is applied to the measurement of liquid droplets in a spray jet. The two velocity components in planes formed by the light sheet originating from a double-pulsed ruby laser are determined. The PIV records are evaluated with the method of Young's fringes. It is shown that this procedure allows the simultaneous measurement of the droplet size within a certain size range.     

PIV技术在气体界面不稳定性实验研究中的应用

采用粒子图像测速(Particle Image Velocimetry,PIV)技术,以乙二醇烟雾作为示踪粒子,实验测量了平面激波作用下SF6气柱-空气界面Richtmyer-Meshkov不稳定性演化图像和二维速度场.测量结果揭示了不稳定性流场的典型特征和细微结构,与高速摄影和数值模拟结果符合,说明建立的PIV技术适...     

用PIV技术测量跨音压气机转子内流的激波结构

在跨音压气机试验台上进行了用ＰＩＶ技术测量内流激波结构的试验研究。乙二醇微小液滴成功地被用作示踪粒子；自行设计制造的激光潜望镜成功地将双脉冲激光器发出的激光束导入机匣。用数字ＰＩＶ技术测量到的跨音压气机叶片流道内的二维瞬态绝对速度场分布被转化为相对速度分布，发现的激波结构测量结果与静压分布测量结果进行了比较，得到了比较满意的符合。本文讨论了目前试验装置和测量系统所存在的问题，提出了进一步工作的要求．