Spatial resolution analysis of planar PIV measurements to characterise vortices in turbulent flows

The effects of spatial resolution of planar particle image velocimetry (PIV) on vortex size, swirling strength, circulation and population density characterisation are analysed using a series of experimental and numerical databases. The databases comprise a PIV database of an adverse-pressure-gradient turbulent boundary layer (APG TBL), a PIV database of a zero-pressure-gradient (ZPG) TBL in streamwise-wall-normal planes and streamwise-wall-normal slices of a direct numerical simulation (DNS) of a ZPG TBL. The effects of interrogation window and mesh sizes on the vortex parameters are analysed in the outer region of these flows using different qualitative and quantitative approaches. The quantitative analysis mainly capitalises on the possibility of mimicking the PIV data-sets with the DNS one. These approaches allow us to not only isolate the effects of mesh size and the interrogation window size but also to deduce the combined effects of other measurement errors in PIV. Typical values of mesh size and interrogation window size (0.01–0.03 of the boundary layer thickness) and typical levels of measurement uncertainties have significant effects on the vortex parameters. Moreover, each PIV error source affects the vortex parameters in different and frequently opposite manners. Hence, an optimal selection of measurement parameters such as the interrogation window size is indispensable in order to minimise the effects of spatial resolution and other measurement errors on the vortex parameters. Guidelines are presented in the Conclusions section of this paper. Finally, it is found that all the vortex parameters, when averaged across the outer region, are reasonably comparable in the ZPG and APG TBLs despite the fact that these are very different flows.     

Comparison of PIV parameters effects on displacement identification and cross correlation

An algorithm is presented to determine displacements thanks to the identification method. Its main properties are described: no link with the particle size, measurement of the velocity distribution. Determination of effects of PIV parameters on displacement identification is made. Parameters used are noise, bias, velocity distribution. Therefore, we can define a validity domain of PIV parameters for identification and compare it with the domain of cross correlation. The identification validity range is based on 70% of isolated particles, on a displacement norm and on displacement gradients corresponding to less than half the size of the interrogation cell and to 10% of the average velocity. The comparison with cross correlation domains indicates that the cross correlation is more robust. However, the identification method is interesting because of the possibility of displacement distribution measurement. We use it to measure the decreasing of the turbulence intensity for a grid-generated turbulence.     

A comparative study of the PIV and LDV measurements on a self-induced sloshing flow

Particle Imaging Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) measurements on a self-induced sloshing flow in a rectangular tank had been conducted in the present study. The PIV measurement result was compared with LDV measurement result quantitatively in order to evaluate the accuracy level of the PIV measurement. The comparison results show that the PIV and LDV measurement results agree with each other well in general for both mean velocity and fluctuations of the velocity components. The average disagreement level of the mean velocity between PIV and LDV measurement results was found to be within 3% of the target velocity for the PIV system parameter selection. Bigger disagreements between the PIV and LDV measurement results were found to concentrate at high shear regions. The spatial resolution and temporal resolution differences of the PIV and LDV measurements and the limited frames of the PIV instantaneous results were suggested to be the main reasons for the disagreement.     

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％。     

Application of PIV to over-expanded supersonic flows: Possibilities and limits

Two-dimensional velocity distributions outside a Mach 2.0 supersonic nozzle have been investigated using a digital particle im age velocimetry (PIV). Mean velocities , vor ticity field and volume dilatation field were obtained from PIV images using 0 .33 μm titanium dioxide (TiO₂) particle. The seeding particle of larger size , 1.4 μrn Ti0₂, was also used for the experimental comparison of velocity lag downstream of shock waves. The results have been compared and analyzed with schlieren photographs for the locations of shock waves and over-expanded shock structure to inspect possibilities and limits of a PIV technique to over-expanded supersonic flows. It is found that although the quantitative velocity measurement using PIV on over-expanded supersonic flows with large velocity and pressure gradients is limited, the locations of normal shock and oblique shock waves can be resolved by the axial/radial velocity fields, and over-expanded shock structure can be predicted by vorticity field and volume dilatation field which are acquired from the spatial differential of the velocity field.     

Review of LDA and PIV applied to the measurement of sound and acoustic streaming

A review of laser Doppler anemometry (LDA) and particle image velocimetry (PIV) with their application to the measurement of sound is presented. The fundamental principles behind LDA and PIV are discussed and extended to the application of sound measurement. Special attention is paid to analysis of LDA signals including the Hilbert transform, which enables amplitude information to be obtained about various frequency components of a signal and wavelet analysis, which allows non-stationary signals to be accurately analysed. The influence of the refractive index variations in a medium due to a sound wave on the laser beams of an LDA signal is discussed. Attention is also paid to acoustic streaming which arises due to high-intensity sound, and PIV results are presented to demonstrate the effect.     

Measurement of airflow of air-conditioning in a car with PIV

In the present study, a model experiment is performed in order to clarify the airflow characteristics of a car cabin. In addition, this study provides high precision data for a benchmark test using the CFD (Computational Fluid Dynamics) analysis method. Initially, the study focuses on the ventilation mode that describes the flow field in the car cabin obtained from an experiment with PIV. The car cabin model is made of transparent acrylic resin and measures 1,450 mm×700 mm×900 mm, almost half the size of a real car, and was installed in a thermostatic chamber. In the experiment, the cabin model was controlled by an orifice tube and a pressure gauge in order to confirm the airflow rate. The PIV measurement was performed at a total of 18 regions within the section. The analyzed PIV data provides the mean velocity profile, the standard deviation distribution and the turbulence intensity distribution at each region.     

Evaluation of the cross correlation method by using PIV standard images

Effects of various parameters for PIV image acquiring and processing on the final velocity field is studied by using PIV standard images (Okamoto et al., 1997) to evaluate the cross correlation method. The studied parameters include the size of interrogation window, the size of search window, the number of tracer particles, the diameter of tracer particles, out-of-plane velocity and average image velocity or the time interval between two images. In order to improve the PIV sub-pixel accuracy, the validity of the “sub-pixel interpolation” process also is discussed in the paper. Some useful conclusions are suggested for the optimal parameter selection for a final PIV result with high accuracy.     

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.     

A new super-resolution PIV accelerated by characteristic pixel selection

A new high-speed super-resolution PIV was proposed using characteristic pixel selection to accelerate the successive abandonment (SA) with recursive window subdivision. The performance and applicability of the proposed PIV were evaluated. In the SA calculation with the characteristic pixel selection, 1000 candidates are narrowed down to only one at over 50 % of the measurement points, and the number of error vectors is reduced because the difference between the cumulative intensities of a correct candidate and of other ones becomes clear due to the characteristics of selected pixels. In all recursive processes, error checks are carefully performed using the summation of the distribution of the cumulative intensity difference distribution, which is suitable for the SA method. In a comparison of the time per velocity vector, the present super-resolution PIV was shown to be 10 times faster than the former ordinary resolution PIV. Another feature of the present super-resolution PIV is that the velocity vectors are obtained in the region very close to the image boundaries and masked regions by using the recursive algorithm.     

High speed analogue autocorrelation for PIV transparency analysis using a ferroelectric liquid crystal spatial light modulator

This paper describes a novel automated high-speed optical autocorrelation system for particle image velocimetry (PIV) analysis. In this system, a bistable ferroelectric optically addressed spatial light modulator (BOASLM) is used for real time optical autocorrelation. Input data from a PIV transparency is scanned into the system by utilising two scanning mirrors and the output autocorrelation signals are detected with a two-dimensional scanning tracking system implemented by two acousto-optic modulators. This data extraction system is equivalent to a random access camera, which is used to track the autocorrelation signals from region to region in order to retrieve velocity data at the full frame rate of the BOASLM. The factors that influence the processing capacity of the optical system are discussed in detail and a critical comparison is made with digital analysis systems. A working prototype of an optical PIV analysis system is described and the measurement of 1296 velocity vectors from a PIV transparency in approximately five seconds is demonstrated.     

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.     

Stereoscopic PIV measurements of flow in and around axial flow fan

To analyze the complex three-dimensional flow structure of an axial flow fan and determine the validity of its application, PIV is used to provide detailed space and time resolved experimental data for understanding and control of flow field. The high resolution stereoscopic PIV system was successfully employed in this study for the investigation of flow structure around the axial flow fan. Using the once-per-revolution signal from the rotor, image fields were captured at a fixed position of the blades and hence provides the ability to do phase-averaging. The three-dimensional instantaneous velocity fields, phase-averaged velocity fields, instantaneous and mean vorticity distributions of the stereoscopic PIV measurement results were represented at typical planes of the flow field. Phaseaveraged velocity fields were calculated based on 200 frames of the instantaneous stereoscopic PIV measurement results. From the velocity distribution, the vorticity and turbulent intensity distribution, which are known to be major factors of fan noise, were calculated and its diffusion was discussed as they travel downstream. From the reconstructed three-dimensional velocity iso-surface at 8 cross planes of the outlet flow fields, the three-dimensional features can be seen clearly.     

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．     

A study on drag reduction of a rotationally oscillating circular cylinder at low Reynolds number

The flow field around a rotationally oscillating circular cylinder in a uniform flow is studied by using a particle image velocimetry to understand the mechanism of drag reduction and the corresponding suppression of vortex shedding in the cylinder wake at low Reynolds number. Experiments are conducted on the flow around the circular cylinder under rotational oscillation at forcing Strouhal number 1, rotational amplitude 2 and Reynolds number 2,000. It is found from the flow measurement by PIV that the width of the wake is narrowed and the velocity fluctuations are reduced by the rotational oscillation of the cylinder, which results in the drag reduction rate of 30%. The mechanism of drag reduction is studied by phase-averaged PIV measurement, which indicates the formation of periodic small-scale vortices from both sides of the cylinder. It is found from the cross-correlation measurement between the velocity fluctuations that the large-scale structure of vortex shedding is almost removed in the cylinder wake, when the small-scale vortices are generated at the unstable frequency of shear layer by the influence of rotational oscillation.     

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...     

水泵吸水池内部流动PIV试验的深入分析

ＰＩＶ技术是一种新型的流动量测手段，使用这种技术能够容易的得出流场中的速度分布情况，在此基础上进行深入分析，还可以得到许多十分重要的流动参数。这里，对水泵吸水池内部流动的ＰＩＶ试验进行深入分析，得出如下的流动参量：吸水口处的环量分布、测量面上的涡度分布、交叉面处的三维速度分布，另外，对试验中所拍摄的图片进行分析，可以得出吸入涡中气核大小的分布情况．     

An image-shifting technique based on grey-scale classification for particle image velocimetry

The image-shifting techniques are used to overcome the directional ambiguity of particle image displacement in the measurement of particle image velocimetry (PIV). This paper proposes an image-shifting technique based on grey-scale classification for PIV. By calculating the unified grey-scale statistical frequency of each interrogated unit, the directional ambiguity is resolved without any special requirement of the camera, and the particle image displacement is calculated synchronously. This image-shifting technique can be realized by controlling the difference in the light intensity of two lasers. Using this new technique, a PIV system was developed and used to measure the diesel spray flow. The displacement vector map of fuel particle in the spray flow was obtained, and the structure of the spray flow was investigated. The application confirmed that the image-shifting technique is viable and effective.     

An investigation on airflow in pathological nasal airway by PIV

Understanding of airflow characteristics in nasal cavity is closely related with the physiological functions, like air-conditioning and smelling, and pathological aspects in nasal breathing. Several studies have utilized physical models of the healthy nasal cavity to investigate the relationship between nasal anatomy and airflow. The next step on this topic is naturally studies for disordered nasal airways and this is the main purpose of this article. Airflows in the pathological nasal airways, including nasal cavity and upper pharynx, of Korean adults are investigated experimentally by PIV measurement technique and air resistance measurements. Quantitative data for normal and pathological nasal airway are obtained. Average and RMS velocity distributions are obtained for inspirational and expirational nasal airflows. The CBC PIV algorithm with window offset is used for PIV flow analysis. PIV measurements of nasal airflow for nasal cavities with 50% and 70% adenoid vegetation are conducted for the first time. The asymmetric nasal cavities, due to either congenital deformity or injury, are also investigated. Comparisons in nasal airflows for both normal and abnormal cases are also appreciated and airflow characteristics that are related with the abnormalities in nasal cavity are proposed.     

Target detection algorithm in side-scan sonar image based on pixel importance value measurement

Side-scan sonar detection application always combines with unstable results.A two-stage novel pixel importance value measurement algorithm is proposed to stabilize the detection ability and false alarm probability simultaneously.In first stage of the algorithm,a new feature defined as pixel importance value(PIV) is proposed in terms of distances between the target pixel and each other pixels.PIV measurement of current pixel is defined as the weighted sum of all remaining segmented pixels.The wei...