超声粒子图像测速技术及应用

心血管疾病的产生与动脉血流的流动状况密切相关。然而,目前普遍应用的超声多普勒成像技术不能精确测量复杂血流流场信息。本文提出了一种基于超声造影微泡的超声全流场粒子图像测速技术,能够获得多维流速速度信息,且不依赖于声束与速度向量之间的夹角。本文首先着重阐述了超声全流场粒子测速技术的基本原理以及系统组成,并对直管流和旋转流场流体动力学特性进行了实验测试研究,实验结果表明本技术能够测量全流场速度,并可作为表征复杂血流流场的有力手段。      

A high-speed all-digital technique for cycle-resolved 2-D flow measurement and flow visualisation within SI engine cylinders

Particle image velocimetry (PIV) is now a well-established planar flow measurement technique for the study of in-cylinder flow fields in internal combustion engines. Here the authors describe a turnkey, high-speed digital imaging system that provides combined real-time flow visualisation and rapidly processed PIV data in an industrial optical research engine facility. The system is based on commercially available, high-speed imaging and laser technology and conventional digital cross-correlation processing to provide cycle-resolved PIV data and flow visualisation within timescales appropriate for engine development. A simple variation on the synchronisation scheme also allows the acquisition of tens to thousands of flow visualisation sequences and PIV maps at the same crank angle, thus giving the potential for the study of cycle-to-cycle flow variability and its effect on combustion stability in a suitably instrumented optical engine. The technique may also find applications in other unsteady or oscillatory flows of importance in aerodynamics, acoustics, mixing, and heat transfer.     

Three-dimensional particle image velocimetry: a low-cost 35 mm angular stereoscopic system for liquid flows

A low-cost 35 mm PIV stereoscopic system for liquid flows is presented which has an imaging component cost under US$9000. The system uses an angular configuration, rotating mirror image shifting and in-situ calibration techniques. Image processing algorithms based on cross correlation and bicubic interpolation are also used to calculate the 3D data from the PIV images. Results from an error analysis have shown the system to have in plane errors ranging from 4.15 to 5.95% and out of plane errors of 7.01% providing an f-number of f2 is fixed for all imaging. Subsequent application of the system to a flow field generated by a free falling sphere in wheat syrup have produced results which when compared to previous flow visualisation give good qualitative agreement. Suggested improvements to the PIV system costing US$1300 would allow operation at f-numbers down to f by modifying the cameras for the Scheimpflug condition and using a corrective liquid prism.     

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.     

Optical coherence tomography based particle image velocimetry (OCT-PIV) of polymer flows

The measurement of spatially resolved velocity distributions is crucial for modelling flow and for understanding properties of materials produced in extrusion processes. Traditional methods for flow visualization such as particle image velocimetry (PIV) rely on optically transparent media and cannot be applied to turbid polymer melts. Here we present optical coherence tomography as an imaging technique for PIV data processing that allows for measuring a sequence of time resolved images even in turbid media. Time-resolved OCT images of a glass-fibre polymer compound were acquired during an extrusion process in a slit die. The images are post-processed by ensemble cross-correlation to calculate spatially resolved velocity vector fields. The results compared well with velocity data obtained by Doppler-OCT. Overall, this new technique (OCT-PIV) represents an important extension of PIV to turbid materials by the use of OCT.     

Flow visualization and PIV measurement of flow field around a darrieus rotor in dynamic stall

The flow field around a Darrieus rotor in dynamic stall is studied by flow visualization and PIV measurements. The visualization is carried out by dye injection technique while the phase averaged velocity distributions around the blade are measured by PIV combined with a conditional imaging technique. The results indicate the appearance of dynamic stall phenomenon due to the shedding of two pairs of vortices from the blade during one rotation of the rotor. These stall vortices are produced by the separation of flow over the inner surface of the blade and the formation of roll-up vortices from the outer surface. The second stall vortices develop along the blade and strongly interact with the flow field near the blade, affecting the aerodynamic performance of the rotor.     

Volume-resolved flame chemiluminescence and laser-induced fluorescence imaging

There is significant need for optical diagnostic techniques to measure instantaneous volumetric vector and scalar distributions in fluid flows and combustion processes. This is especially true for investigations where only limited optical access is available, such as in internal combustion engines, furnaces, flow reactors, etc. While techniques such as tomographic PIV for velocity measurement have emerged and reached a good level of maturity, instantaneous 3D measurements of scalar quantities are not available at the same level. Recently, developments in light field technology have progressed to a degree where implementation into scientific 3D imaging becomes feasible. Others have already demonstrated the utility of light field technology toward imaging high-contrast particles for PIV and for imaging flames when treated as single-surface objects. Here, the applicability and shortcomings of current commercially available light field technology toward volumetric imaging of translucent scalar distributions and flames are investigated. Results are presented from imaging canonical chemiluminescent and laser-induced fluorescent systems. While the current light field technology is able to qualitatively determine the position of surfaces by locating high-contrast features, the correlation-based reconstruction algorithm is unable to fully reconstruct the imaged objects for quantitative diagnostics. Current analysis algorithms are based on high-contrast correlation schemes, and new tools, possibly based on tomographic concepts, will have to be implemented to reconstruct the full 3D structure of translucent objects for quantitative analysis.     

PIV水下流场测试系统试验研究

基于粒子图像测速技术,设计研制了一套适用于水洞洞体内高速、封闭试验条件的流场测量与显示设备.该系统由光源、成像、图像处理与分析、控制和粒子投放等分系统组成，具有复杂环境下全流场的无接触、无扰动、高准确度测量和显示能力，流场测试误差小于2%，可为水下航行器外形设计及其改进提供相关的实测数据.     

Evaluation of the 3D-PIV standard images (PIV-STD project)

Particle Imaging Velocimetry (PIV) offers many advantages for studies of fluid flows. Lots of PIV techniques have been developed and applied to various flow fields. However, there are no standard tools for evaluation of the PIV system effectiveness and accuracy. To popularize the PIV for practical use, the PIV system should have some standard.     

Study on concurrent measurements of velocity and density distributions for high-speed CO2 flow

Velocity and density distributions of a high-speed and initial CO₂ jet flow have been analyzed concurrently by a developed three-dimensional digital speckle tomography and a particle image velocimetry (PIV). Two high-speed cameras have been used for the tomography and one for the PIV since a shape of a nozzle for the jet flow is bilaterally symmetric and the starting flow is fast and unsteady. The speckle movements between no flow and CO₂ jet flow have been obtained by a cross-correlation tracking method so that those distances can be transferred to deflection angles of laser rays for density gradients. The three-dimensional density fields for the high-speed CO₂ jet flow have been reconstructed from the deflection angles by the real-time tomography method, and the two-dimensional velocity fields have been calculated by the PIV method concurrently and instantaneously.     

Simultaneous PIV/PTV/OH PLIF imaging: Conditional flow field statistics in partially premixed turbulent opposed jet flames

A combination of particle imaging velocimetry (PIV), particle tracking velocimetry (PTV) and planar laser-induced fluorescence (PLIF) was employed to measure conditional flow field statistics in partially premixed turbulent opposed jet flames. These flames were observed to be very sensitive to excessive seeding of particles. Since flames close to extinction were studied, very low seeding densities were required to prevent impact on the extinction behavior of the flame, and conventional PIV algorithms would have resulted in poor spatial resolution. An improved PIV algorithm was developed, in connection with a PTV procedure used in high-temperature regions of low seed density, and revealed high in-plane resolution up to 300 μm. The PIV/PTV algorithm slightly under-resolved the Kolmogorov scales for the present cases, whereas Batchelor scales were fully resolved in-plane by the simultaneous OH PLIF. In the data processing, transient OH contours obtained from single-shots were used to define flame-fixed coordinates. Conditional velocities, out-of-plane vorticity, 2D dilatation, and both axial and radial strain were processed from the data. The conditional statistics show that vorticity is preferably generated close to the reaction zone, particularly at off-centerline positions. Hence, flow-chemistry interactions could be identified directly in the region of the reaction zone. This finding was also supported by qualitative high speed Mie scattering/chemiluminescence imaging that permitted temporally resolved visualization of the formation of eddies just upstream of the luminous flame areas.     

Acquisition of detailed laryngeal flow measurements in geometrically realistic models

Characterization of laryngeal flow velocity fields is important to understanding vocal fold vibration and voice production. One common method for acquiring flow field data is particle image velocimetry (PIV). However, because using PIV with models that have curved surfaces is problematic due to optical distortion, experimental investigations of laryngeal airflow are typically performed using models with idealized geometries. In this paper a method for acquiring PIV data using models with realistic geometries is presented. Sample subglottal, intraglottal, and supraglottal PIV data are shown. Capabilities and limitations are discussed, and suggestions for future implementation are provided.     

Application of DPIV to study both steady state and transient turbomachinery flows

Digital particle imaging velocimetry (DPIV) is a powerful measurement technique, which can be used as an alternative or complementary approach to laser doppler velocimetry (LDV) in a wide range of research applications. The instantaneous planar velocity measurements obtained with PIV make it an attractive technique for use in the study of the complex flow fields encountered in turbomachinery. The planar nature of the technique also significantly reduces the facility run time over point-based techniques. Techniques for optical access, light sheet delivery, CCD camera technology and particulate seeding are discussed. Results from the successful application of the PIV technique to both the blade passage region of a transonic axial compressor and the diffuser region of a high speed centrifugal compressor are presented. Both instantaneous and time-averaged flow fields were obtained. The averaged flow field measurements are used to estimate the flow turbulence intensity. The instantaneous velocity vector maps obtained during compressor surge provide previously unobtainable insight into the complex flow field characteristics occurring during short lived surge events. These flow field maps illustrate the true power of the DPIV technique.     

Study of interactions between liquid vortices and bubbles by simultaneous particle image velocimetry and laser-induced fluorescence tracking

Extensive experimental research has been conducted using the particle image velocimetry (PIV), laser-induced fluorescence (LIF) imaging and backlit photographic recordings to study the complex interactions between coherent vortex structures created in the shear layer of jets and the bubbles. Triggering of the naturally-developing instabilities of the shear layer by a thin, pulsed annular flow surrounding the jets allowed the creation of large, orderly structures with controllable frequency and phase. Synchronization of the triggering with data acquisition permitted phase averaging of the data and revealed several interesting phenomena. In particular, the evolution of large vortices and bubble fields could be tracked and the interactions could be studied. The horizontal and vertical velocity components of the liquid and bubble fields and the vertical velocity of both the vortex and bubble rings that were created were measured by the PIV. LIF and image recordings have been combined to visualize bubble trapping inside large eddy structures.     

Correlated displacement-T2 MRI by means of a Pulsed Field Gradient-Multi Spin Echo Method

A method for correlated displacement-T2 imaging is presented. A Pulsed Field Gradient-Multi Spin Echo (PFG-MSE) sequence is used to record T2 resolved propagators on a voxel-by-voxel basis, making it possible to perform single voxel correlated displacement-T2 analyses. In spatially heterogeneous media the method thus gives access to sub-voxel information about displacement and T2 relaxation. The sequence is demonstrated using a number of flow conducting model systems: a tube with flowing water of variable intrinsic T2's, mixing fluids of different T2's in an "X"-shaped connector, and an intact living plant. PFG-MSE can be applied to yield information about the relation between flow, pore size and exchange behavior, and can aid volume flow quantification by making it possible to correct for T2 relaxation during the displacement labeling period Delta in PFG displacement imaging methods. Correlated displacement-T2 imaging can be of special interest for a number of research subjects, such as the flow of liquids and mixtures of liquids or liquids and solids moving through microscopic conduits of different sizes (e.g., plants, porous media, bioreactors, biomats).     

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.     

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。     

Applications of Nuclear Magnetic Resonance Imaging in Particle Technology

During the past decade, the application of Nuclear Magnetic Resonance (NMR) imaging techniques to problems of relevance to the process industries has been identified. In the context of particle technology, NMR imaging, in addition to the more routinely used techniques of Pulsed Gradient Spin Echo (PGSE) NMR and NMR spectroscopy, offer new methods of characterising pore structure, adsorption and diffusion processes within particles and packed beds of particles, as well as enabling time-resolved in-situ study of processes such as twophase flow, aggregation, polymerisation, crystallisation and phase separation phenomena. This paper reviews recent work in these areas, and also highlights the new insights NMR imaging can give us regarding the characterisation of porous materials, and the influence of the structure of the pore space on the transport processes occuring within a given porous solid.     

交变流动中突变截面局部损失特性分析

从突变截面流道内流体满足的方程组出发,给出交变流动中突变截面阻力系数的定义以及考察方法,采用量纲分析法获得影响局部阻力特性的四个无量纲影响参数：动态雷诺数与幅值雷诺数之比、幅值雷诺数、变截面面积比、声场压力、速度相位差。通过PW（粒子成像测速仪）测量,分析了流场结构特征,并与CFD计算结果对比,验证了CFD计算结果的可...     

Particle image velocimetry in aerodynamics: Technology and applications in wind tunnels

Particle image velocimetry (PIV) is increasingly used for aerodynamic research and development. The PIV technique allows the recording of a complete flow velocity field in a plane of the flow within a few microseconds. Thus, it provides information about unsteady flow fields, which is difficult to obtain with other experimental techniques. The short acquisition time and fast availability of data reduce the operational time, and hence cost, in large scale test facilities. Technical progress made in the last years allowed DLR to develop a reliable, modular PIV system for use in industrial wind tunnels. The features of this system are summarized and results of recent PIV applications are presented.