开式轴流风扇叶尖流动的PIV实验研究

粒子成像测速仪(PIV)技术是一种测量叶轮机械内部流场的有效手段。本文利用对开式前缘弯掠轴流风扇进行PIV实验,捕捉到了开式轴流风扇的叶尖涡。实验结果表明,叶尖涡产生于叶顶前缘的吸力面,其涡核沿一条与旋转方向相反的斜线延伸,一直到转子下游出口。PIV实验数据和CFD内流计算结果吻合良好。本实验为改善风扇性能和认识其内流机制提供了可靠的实验依据。     

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...     

Combined two-dimensional velocity and temperature measurements using a high-speed camera and luminescent particles

This paper proposes a combined method for two-dimensional temperature and velocity measurements using temperature sensitive particles (TSParticles), a pulsed ultraviolet (UV) laser and a single high-speed camera. TSParticles were synthesized using ion-exchange particles and Eu(TTA) luminescent dye. The size and material of the particles for synthesizing TSParticles are selectable. TSParticles respond to temperature changes in a flow and can also serve as tracers for the velocity field. TSParticles were seeded into a heated water flow in a complex-shaped channel constructed of MEXFLON resin, which has a refractive index exactly equal to that of water. Particle images of flow beyond the structure can be recorded without any distortion. The TSParticles were excited by the UV pulsed laser and the luminescence from the TSParticles were recorded at 40,000 frames per second as sequential images for a lifetime-based temperature analysis. Another advantage of our approach is that high time-resolved PIV can be carried out without a high-frequency laser. The recorded images were also used for the particle image velocimetry (PIV) calculation.     

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.     

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.     

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.     

Enhancement of measurement accuracy of X‐ray PIV in comparison with the micro‐PIV technique

The X‐ray PIV (particle image velocimetry) technique has been used as a non‐invasive measurement modality to investigate the haemodynamic features of blood flow. However, the extraction of two‐dimensional velocity field data from the three‐dimensional volumetric information contained in X‐ray images is technically unclear. In this study, a new two‐dimensional velocity field extraction technique is proposed to overcome technological limitations. To resolve the problem of finding a correction coefficient, the velocity field information obtained by X‐ray PIV and micro‐PIV techniques for disturbed flow in a concentric stenosis with 50% severity was quantitatively compared. Micro‐PIV experiments were conducted for single‐plane and summation images, which provide similar positional information of particles as X‐ray images. The correction coefficient was obtained by establishing the relationship between velocity data obtained from summation images (V_S) and centre‐plane images (V_C). The velocity differences between V_S and V_C along the vertical and horizontal directions were quantitatively analysed as a function of the geometric angle of the test model for applying the present two‐dimensional velocity field extraction technique to a conduit of arbitrary geometry. Finally, the two‐dimensional velocity field information at arbitrary positions could be successfully extracted from X‐ray images by using the correction coefficient and several velocity parameters derived from V_S.     

Direct measurement of fluid velocity gradients at a wall by PIV image processing with stereo reconstruction

Velocity gradient is typically estimated in Particle Image Velocimetry (PIV) by differentiating a measured velocity field, which amplifies noise in the measured velocities. If gradients near a boundary are sought, such noise is usually greater than in bulk fluid, because of small tracer displacement, uncertainty in the effective positions of velocity vectors, intense deformation of tracer patterns, and laser reflection. We consider here a modified form of the Particle Image Distortion (PID) method todirectly calculate velocity gradients at a fixed wall, and refer it as “PIV/IG” (“Interface Gradiometry”). Results from synthetic 2D PIV images suggest our method achieves higher SNR and accuracy than velocity differentiation. Also, we have developed a procedure to reconstruct three-dimensional velocity gradient at a fixed wall the two non-zero components from PIV/IG data obtained in stereo views; these equations simplify considerably thanks to the no-slip condition. Experimental data from the bottom wall of turbulent open channel flow appear to suffer from a form of pixel locking. As with standard PIV, this underlines the importance of adequate tracer diameter in the images, sufficient seeding density, and of dynamic range of the camera sensor.     

Performance of a PIV system for a combusting flow and its application to a spray combustor model

The performance of PIV system for combusting flow was evaluated by using artificial images generated from computer graphics and experimental data. The influences of shutter speed, filter, laser power and the PIV algorithms on the measurement uncertainty were studied for optimizing the performance of the PIV system. This system was applied to the spray combustor model for boiler, and the flow patterns with and without combustion were elucidated. Results showed that the burner flow generates complex three-dimensional flow pattern, which contributes to highly mixed fuel flow in the combustor. Although the flow pattern with and without combustion is similar, the growth of burner flow area and an increase in velocity magnitude are found in the flow field by the influence of chemical reactions in combustion.     

Noninvasive quantification of fluid mechanical energy losses in the total cavopulmonary connection with magnetic resonance phase velocity mapping

A major determinant of the success of surgical vascular modifications, such as the total cavopulmonary connection (TCPC), is the energetic efficiency that is assessed by calculating the mechanical energy loss of blood flow through the new connection. Currently, however, to determine the energy loss, invasive pressure measurements are necessary. Therefore, this study evaluated the feasibility of the viscous dissipation (VD) method, which has the potential to provide the energy loss without the need for invasive pressure measurements. Two experimental phantoms, a U-shaped tube and a glass TCPC, were scanned in a magnetic resonance (MR) imaging scanner and the images were used to construct computational models of both geometries. MR phase velocity mapping (PVM) acquisitions of all three spatial components of the fluid velocity were made in both phantoms and the VD was calculated. VD results from MR PVM experiments were compared with VD results from computational fluid dynamics (CFD) simulations on the image-based computational models. The results showed an overall agreement between MR PVM and CFD. There was a similar ascending tendency in the VD values as the image spatial resolution increased. The most accurate computations of the energy loss were achieved for a CFD grid density that was too high for MR to achieve under current MR system capabilities (in-plane pixel size of less than 0.4 mm). Nevertheless, the agreement between the MR PVM and the CFD VD results under the same resolution settings suggests that the VD method implemented with a clinical imaging modality such as MR has good potential to quantify the energy loss in vascular geometries such as the TCPC.     

High accuracy measurement of unsteady flows using digital particle image velocimetry

The analysis of digital PIV data, either derived from CCD technology or through film and then scanned, typically involves two quantization steps: spatial and intensity quantization. The all-optical systems do not introduce these sources of error. For systems which make use of digital technology however, it is of crucial importance to have reliable error bounds and a sufficiently accurate estimate of particle position, taking into consideration both types of quantization. The accuracy demanded by aerodynamicists from PIV has been a major barrier to its practical application in the past. The more recent approach of using the Gaussian profile of the particle images to yield sub-pixel accurate position estimates has resulted in robust measurements being taken to an accuracy of 1/10th pixel and 1% in velocity for the in-plane velocity, in hostile industrial environments. A major problem for 3D PIV estimation has historically been that the out-of-plane velocity error was of the order of 3–4 times larger than in-plane. The out-of-plane velocity estimate can be derived from the change in the ratio of amplitude to variance—known as the depth factor—of the Gaussian form, as a particle traverses the beam profile. However, such measurements are crucially dependent not only on an accurate position estimate but also on an equally accurate estimate of the amplitude and variance. The accuracy of the Gaussian profile fit using a Nelder–Meade optimisation method as developed until now however, is not capable of providing the required accuracies. Therefore, this paper presents a development of the “locales” approach to position estimation to achieve the desired objective of high accuracy PIV measurements. This approach makes use of the fact that by considering all the possible digital representations of the Gaussian particle profile, regions of indistinguishable position can be derived. These positions are referred to as “locales”. By considering the density, distribution, and shape of these locales, the available precision can be estimated and an accurate (no worse than 0.5% error for a typical PIV image) in-plane velocity, accuracy can be obtained; while at the same time providing estimates of the depth factor with an error of approximately 0.8%. This paper describes the implementation of an efficient algorithm to provide velocity estimates to an accuracy of at least 0.5% in-plane, together with a discussion of the required constraints imposed on the imaging. The method was validated by creating a synthetic PIV image with CCD-type noise. The flow being analysed is that of flow past the near wake of a cylinder at a Reynolds Number of 140,000. This image was then analysed with the new method and the velocity estimates compared to the CFD data for a range of signal-to-noise ratios (SNR). For a realistic SNR of 5, the accuracy of the method is confirmed as being at least 0.5% in-plane. Finally, the algorithm was used to map an experimental transonic flow field of the stator trailing edge region of a full-size annular cascade with an estimated error of 0.5%. The experimental results are found to be in good agreement with a previously reported steady state viscous calculation and PIV mapping.     

交变流动中压力速度之间相位差对突变截面损失的影响分析

从弱非线性热声理论出发,给出交变流动中突变截面阻力系数的定义以及考察方法。通过PIV(粒子成像测速仪)测量与CFD计算结果的对比,验证了CFD计算结果的可靠性。进而利用CFD模拟考察了交变流动中压力与速度之间相位差对突变截面局部损失的影响,观察到产生这种影响的流场内部流动机制。分析表明,阻力特性确实对声场压力与速度之间的相位差存在依赖性,但这种依赖性会随着非线性的增强而减少。     

Three-dimensional bubbly flow measurement using PIV

The experimental flow visualization tool, Particle Image Velocimetry (PIV), is being extended to determine the velocity fields in three-dimensional, two-phase fluid flows. In the past few years, the technique has attracted quite a lot of interest. PIV enables fluid velocities across a region of a flow to be measured at a single instant in time in the whole volume (global) of interest. This instantaneous velocity profile of a given flow field is determined by digitally recording particle (microspheres or bubbles) images within the flow over multiple successive video frames and then conducting flow pattern identification and analysis of the data. This paper presents instantaneous velocity measurements in various three-dimensional, bubbly two-phase flow situations. This information is useful for developing or improving existing computer constitutive models that simulate this type of flow field. It is also useful for understanding the detailed structure of two-phase flows.     

Qualitative comparison between numerical and experimental results of unsteady flow in a radial diffuser pump

Comparison between numerical simulation and experimental results for unsteady flow field in a radial diffuser pump is presented for the design operating point. The numerical result is obtained by solving three-dimensional, unsteady Reynolds-averaged Navier-Stokes equations by the commercial CFD code CFX-10 withk-ω based shear stress transport turbulence model. Two-dimensional PIV measurements are conducted to acquire the experiment result. The phase-averaged velocity and turbulent kinetic energy fields are compared in detail between the results by the two methods in the impeller, diffuser and return channel regions. The qualitative comparison between CFD and PIV results is quite good in the phase-averaged velocity field. Although the turbulence level by PIV is higher than that by CFD generally, the main turbulence features are nearly the same. Furthermore, the blade orientation effect and other associated unsteady phenomena are also examined, in order to enhance the understanding on impeller-diffuser interaction in a radial diffuser pump.     

旗面周期摆动的实验研究

用实验方法研究了旗面周期摆动的运动过程,采用改进的算法优化了粒子图像测速仪测量结果,定量得了水洞中摆动旗面的近壁流场信息.通过选定旗面包络上的一个拐点,将其振幅作为特征长度重新计算旗面运动的Strouhal数.多组实验结果中,新的Strouhal数均为0.21左右,这与相同Reynolds数下圆柱绕的Strouhal数结果相近.     

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.     

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％。     

Measurement of pressure distribution from PIV experiments

In this study, a non-staggered grid SIMPLER pressure solution algorithm, which is able to produce correct pressure distribution directly if correct velocities are given, is proposed to solve the pressure distribution for PIV experiments. The cell face pseudo velocity required in the pressure equation is approximated by a simple linear average of the adjacent nodal pseudo velocities so that the velocity and pressure are collocated without causing the checkerboard pressure distribution problem. In addition, the proposed pressure solution algorithm has the features that upwind effects of the convective terms are considered, boundary conditions are not required, and the pressure distribution obtained can be used to correct the velocity field so that the continuity equation is satisfied. These features make the present algorithm a superior method to calculate the pressure distribution for PIV experiments. The pressure field solved is realistic and accurate. The proposed pressure equation solver is first calibrated with a two-dimensional cavity flow. It is found that the results are almost identical to the exact solution of the test flow. The algorithm is then applied to analyze a uniform flow past two side-by-side circular cylinders in a soap film channel. With the velocity and pressure distributions successfully measured, the structures of the complex shedding flow patterns are clearly manifested.     

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.     

Flowfield vorticity calculation using PIV data

The simultaneous velocity measurement at different flowfield locations is one of the key advantages of a PIV system. This allows a straightforward calculation of derived flow magnitudes including spatial correlations. Thus, postprocessing techniques need further attention in order to assure maximum feature extraction with minimum error, among other issues. This paper is devoted to expand the capability of calculating vorticity in a PIV sampled flow field. The methodology proposed is based on linear algorithms (FIR filters) able to obtain the first spatial derivative of a grid sampled magnitude containing random noise. Generalization to other flow magnitudes based on spatial derivatives is immediate. Been this a widely used method, the main objective of the study is to develop new filters from families already documented. The relevant performance parameters of these filters are evaluated and commented. Synthetic data fields are used to test the basic metrological attributes in a controlled way. As a result of the study, algorithms with better performances than the usual ones are proposed and strong points are highlighted. Finally, results of the application to real PIV data are exhibited and commented.