An improved cross-correlation method for (digital) particle image velocimetry

An improved method that brings enhancement in accuracy for the interrogation of (digital) PIV images is described in this paper. This method is based on cross-correlation with discrete window offset, which makes use of a translation of the second interrogation window and rebuilds it considering rotation and shear. The displacement extracted from PIV images is predicted and corrected by means of an iterative procedure. In addition, the displacement vectors are validated at each intermediate of the iteration process. The present improved cross-correlation method is compared with the conventional one in accuracy by interrogation of synthetic and real (digital) PIV images and the interrogation results are discussed. The project supported by the National Natural Science Foundation of China (59936140 and 59876038)     

Theory of cross-correlation analysis of PIV images

To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.For the direct cross-correlation method of single-exposure, double-frame systems which model video array detector interrogation and of double-exposure single-frame systems which generalize earlier direct auto-correlation methods of interrogation of photographic recordings, optimal system parameters are recommended for a range of velocity fields in order to eliminate signal bias and to minimize loss of signal strength. The signal bias resulting from velocity gradients in auto-correlation analysis can be eliminated in cross-correlation interrogation by appropriate choice of the optimal parameters. Resolution, detection rate, accuracy and reliability are compared with direct auto-correlation methods for double- and multiple-pulsed systems.     

风沙两相流PIV测量算法研究

在风沙两相流图像特征的基础上，提出了一种基于模式识别动态聚类方法中$K$-均值 算法的数字面具(Digital Mask)自动生成算法来求解风沙两相流动. 简化了传统生成Digital Mask过程中手动设置参数的操作，减少了人为误差，为批量处理风沙两相流PIV图像提供 了一种安全快捷的方法. 并将该算法应用于风沙两相流的PIV实际测量，分别得到了不同流 动状态下的气流、沙粒以及风沙气固两相流的速度场.     

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.     

扭矩转化器（T／C）内部叶片流道流场的二维PIV实验研究

本文利用二维ＰＩＶ技术研究了汽车无级调连装置中扭矩转化器内部叶片流道流场的复杂流动,实验结果表明泵和透平在不同转速下,静子叶片流道中的流场体现出不同的流动特征,文中还对实验误差进行了分析,提出了减少误差的措施,通过叶栅流动理论分析说明了本文结果的可靠性。     

Stereoscopic PIV measurement of unsteady flows in an axial compressor stage

Stereoscopic particle-image velocimetry (SPIV) measurements are conducted in a Low-speed Large-scale Axial Compressor. During the experiment the two CCD cameras are placed at the different sides of the laser light sheet and it is proved that this configuration is more suitable for the investigation in multi-stage turbomachines. The measured results, including the overall performances of many typical flow structures near the rotor tip region and the phase locked unsteady flows inside the stator passage at both the design and near-stall conditions, are introduced. Some new features of the complicated flow structures, such as the breakdown of the tip leakage vortex, the formation of the compound corner vortex at the rotor suction tip corner, the interactions between the hub stall and the tip separation and the rotor wakes, and the evolutions of the tip corner anti-rotating streamwise vortices inside the stator passage, are revealed.     

Experimental study of the three-dimensional flow field in cross-flow fans

High-resolution PIV measurements of the flow field inside cross-flow fans have been performed in planes normal and parallel to the fan axis, both outside and inside the impeller. The well known difficulties in obtaining the optical access inside the impeller have been overcome by allowing the internal flow planes to be illuminated by the laser light sheet or shot by the CCD camera through the moving blade vanes. Measurements have been performed in two cross-flow fans having the same two-module impeller but casing geometries based on very different design concepts. PIV data in planes normal to the rotor axis show a strong correlation between vorticity distribution and turbulent shear stresses inside the eccentric vortex of each fan. Furthermore, they provide useful elements to explain the very different performance of the two fans evidenced by their characteristic curves. Measurements in planes parallel to the impeller axis show that wide three-dimensional recirculation structures develop near the casing end walls at the discharge of the fans. These mean flow structures are responsible for the backflow into the end portions of the impeller of part of the discharged fluid, which is then transported axially by the eccentric vortex towards the rotor central disc before being discharged once again outside the impeller. In the case of cross-flow fans including few rotor modules, the existence of significant axial velocity components inside the eccentric vortex can alter substantially the flow picture, common in the current literature, resulting from 2-D numerical models or measurements performed in a single transverse plane of the fan.     

Analysis of different POD methods for PIV-measurements in complex unsteady flows

Proper Orthogonal Decomposition (POD) is an effective tool in fluid dynamics for investigation of complex, transitional or turbulent flows. In POD the transient vector or scalar field (velocity, concentration, temperature, etc.) is decomposed into a sum of spatial modes multiplied with time coefficients (Fourier-splitting method). However, these spatial modes and time coefficients can in practice be obtained by different methods. Even if POD has been used in numerous fluid dynamical studies, there are only few publications describing the relationship between the different methods and comparing the results. In the present case the POD basis functions are calculated either by Singular Value Decomposition (SVD) or by the Snapshot-POD approach. The results are compared in order to understand similarities and differences between the methods, as well as advantages and drawbacks. Comparisons between the obtained spatial modes, time coefficients, required computational effort, and complexity of calculation are presented and discussed. The influence of the numerical settings is also investigated, in particular the impact of the number of snapshots on the results. Finally, the differences obtained when analyzing a vector field globally or component-wise are discussed in detail.     

大体积混凝土不稳定温度场求解的迭代算法

在大体积混凝土不稳定温度场计算中，无论是向前差分法、中点差分法还是向后差分法都会产生计算误差．根据拉格朗日中值定理及各节点温差和温度变化率的互不耦合假设，提出一种迭代算法，可以在适当增加差分步长的情况下，减少计算量并保证求解精度，或在保持常规差分步长的情况下，增加少许计算量而提高求解精度，使数值计算结果逼近理论解，是一个便于工程应用的有效计算方法．     

Experimental investigations of the unsteady rotating flow field in a cylindrical vessel

The rotating flow field in a cylindrical vessel — the so-called whirlpool — is widely used in food engineering as a method for separating particles out of a suspension (Cup-of-tea-method). However many of these whirlpools do not operate adequately or fail entirely. In order to solve this problem, the first step was to investigate the flow field and its time dependency which has not been sufficiently understood until now.The rotating flow in a cylindrical vessel — induced by a fluid jet during the filling period of this vessel — is slowed down by fluid friction after the closing of the inlet valve. The velocity fields to be found mainly near, and pressure distributions at the bottom of the vessel, are measured during this unsteady flow. The results, especially those which describe vortex systems, are used to improve the separation system. This paper is restricted to the hydrodynamic aspect. Therefore success in industrial applications can only be indicated.     

基于随机森林算法的非定常气动力建模研究

针对现有的非定常气动力建模方法对气动弹性预测的准确性和效率问题,将随机森林算法引入非定常气动力建模研究领域,构建了基于随机森林算法的非定常气动力降阶模型。将所得模型用于预测气动弹性,选择二维NACA0012翼型进行颤振边界的预测,选用NACA64A010翼型预测LCO特性,并说明了该降阶模型建模的详细过程,将其计算结果与CFD/CSD耦合计算结果及试验结果进行了对比。研究结果表明,该模型可行、高效且精确,可以快速准确地预测飞行器气动弹性特性。     

PIV measurement of the flow past a generic car body with wheels at LES applicable Reynolds number

Experiments by using 2D–2C Particle Image Velocimetry (PIV) were carried out and reported concerning the flow field past a generic car body (modified Ahmed body) which is equipped with wheels and wheel-arches. The Reynolds number was chosen to not exceed 2E+5 based on the height of the Ahmed body which makes it possible to investigate the same configuration by means of Large Eddy Simulation (LES). The wheels were rotating but the ground was stationary. The wheel-ground contact was realized by means of small rectangular openings below the wheels in the ground plane in which the wheels were immersed. The transition contour of the immersed wheels and the ground, as well as the rectangular openings below the wheels were properly sealed to prevent parasite flow and to provide well defined boundary conditions for an upcoming LES investigation.The flow field was measured in several planes with normal vectors pointing towards the directions normal to the free stream. Statistical characteristics of the flow are provided and discussed.     

基于流场定常化的桥梁颤振分析简化数值方法

基于数值方法实现快速有效地分析评价大跨桥梁颤振稳定性.针对强迫振动法识别颤振导数试验中的大跨桥梁二维节段模型.利用RNG κ-ε湍流模型并采用有限体积法求解桥梁模型绕流二维不可压缩流体Navier-Stokes方程.通过计算桥梁断面模型在周期运动中少数离散时刻的气动力,利用最小二乘法计算颤振导数,采用SCANLAN方法求解颤振临界风速,最终进行颤振稳定性分析.通过该方法计算出了丹麦Great Belt East桥和我国虎门大桥的颠振临界风速.计算结果与已有试验结果十分接近,进而验证了该数值方法的有效性和可靠性.     

Instantaneous structure and statistical feature of unsteady flow in a channel obstructed by a square rod

A two-dimensional numerical computation has been made for an unsteady flow in a channel obstructed by an inserted square rod. The results of the computation made for the flow with a parabolic inlet velocity profile at a specific value of channel Reynolds number are analyzed in detail. The obtained results reveal that momentum transfer is enhanced due to the apparent shear stress resulting from the nonzero value of cross-correlation between the streamwise and normal components of fluctuating velocity, , just as in turbulent shear flows, although the studied flow is quite different from turbulent flows in the sense that it is highly periodical and therefore free from randomness. This periodicity leads to a quick recovery of the velocity defect in some region of the wake of the rod. Special attention is paid to the time variation of flow structure. The crisscross motion of the Karman vortex previously found to occur is discussed again, and how it appears is explained in terms of the interaction between the Karman vortex and the disturbed wall shear layer. In the discussion of this relationship, wavering motion of the separation vorticity layers formed on both sides of the rod and the periodic formation of an isolated vortex island from the lifted tip of the wall vorticity layer are analyzed. The vortex island is found to play an important role not only for the occurrence of the crisscross motion of Karman vortex but also for the generation of the nonzero value of .     

Particle image velocimetry （PIV） measurements of tip vortex wake structure of wind turbine

Large-view flow field measurements using the particle image velocimetry (PIV) technique with high resolution CCD cameras on a rotating 1/8 scale blade model of the NREL UAE phase VI wind turbine are conducted in the engineering-oriented Φ3.2 m wind tunnel.The motivation is to establish the database of the initiation and development of the tip vortex to study the flow structure and mechanism of the wind turbine.The results show that the tip vortex first moves inward for a very short period and then moves out...     

An optical method for the measurement of unsteady film thickness

This paper describes a measurement technique to quantify temporal variations in the thickness of an unsteady liquid film with a resolution which is independent of the thickness. The optical transformation function has been derived for fringes of equal inclination and, for a temporally varying film, allows the unsteady component of film thickness to be measured in terms of frequency modulated signal analysis of light intensity variations. As it does not require calibration, the method is suited to in-situ measurements of complex and rapidly varying films as encountered in engineering two-phase flows. It requires the inversion of the frequency time-series of the light intensity observed by a photodetector which represents the absolute values of the time-derivative of the thickness variation.The technique has been used to measure the thickness of the film formed as a result of impingement of a pulsating two-phase jet onto a heated flat plate with surface temperatures of 150 °C and 240 °C and located 143 nozzle exit diameters downstream of the nozzle. The angle between the jet axis and the surface normal was 20 degrees and the injection frequency was 16.7 Hz corresponding to a flow rate of 7.2 mm³ per injection. The results along the line of incidence showed that the ensemble-averaged space-time structure of the film was qualitatively independent of the plate temperature with three peaks, two of which occurred at large radial distances and disappeared in less than 10 ms. The third peak was close to the impingement region and persisted for more than 50 ms due to the small velocities of the incoming two-phase jet as the nozzle needle closed and the low momentum wall jet which was unable to transport the droplets radially outwards. At the higher surface temperature, the rate of evaporation and the amplitude variation of the unsteady component of the overall film thickness increased, and the film covered a smaller area.List of symbols A amplitude of electric field - E electric field produced at a point - f _i (t) instantaneous frequency - f _PMT (t) frequency observed by photomultiplier tube - h thickness resolution - h _m minimum thickness that can be measured - h (t, x) unsteady film thickness - h _s (x) steady film thickness - h _t (t,x) overall film thickness - H nozzle-to-plate distance - i - I light intensity at a point - interference term - k wavenumber of the monochromatic point source - n refractive index of the air - refractive index of thin film material - r radial distance from the geometrical impingement point - rms root-mean-square - t time - t ₀ zero-crossing time defined in Fig. 2 - t _r rise-time defined in Fig. 2 - T _w wall temperature - x position vector in 3-D space Greek symbols angle of impingement - angle of reflection - integration constant defined in Eq. (15) - _c integration constant at r = 0 - phase angle - P optical path difference - P minimum optical path difference - azimuthal coordinate defined in Fig. 3 a - initial phase angle defined as = h _s (x) - wavelength of the illuminating source - initial phase of an electric field produced by a source - _f (t) ensemble-averaged rms series for f _PMT (t) - angle of incidence - angle of refraction - angular frequency of the electric field produced by a source - optical transformation function     

A multiphase,micro-scale PIV measurement technique for liquid film velocity measurements in annular two-phase flow

Prediction methods for two-phase annular flow require accurate knowledge of the velocity profile within the liquid film flowing at its perimeter as the gradients within this film influence to a large extent the overall transport processes within the entire channel. This film, however, is quite thin and variable and traditional velocimetry methods have met with only very limited success in providing velocity data. The present work describes the application of Particle Image Velocimetry (PIV) to the measurement of velocity fields in the annular liquid flow. Because the liquid is constrained to distances on the order of a millimeter or less, the technique employed here borrows strategies from micro-PIV, but micro-PIV studies do not typically encounter the challenges presented by annular flow, including very large velocity gradients, a free surface that varies in position from moment to moment, the presence of droplet impacts and the passage of waves that can be 10 times the average thickness of the base film. This technique combines the seeding and imaging typical to micro-PIV with a unique lighting and image processing approach to deal with the challenges of a continuously varying liquid film thickness and interface. Mean velocity data are presented for air–water in two-phase co-current upward flow in a rectangular duct, which are the first detailed velocity profiles obtained within the liquid film of upward vertical annular flow to the authors’ knowledge. The velocity data presented here do not distinguish between data from waves and data from the base film. The resulting velocity profiles are compared with the classical Law of the Wall turbulent boundary layer model and found to require a decreased turbulent diffusivity for the model to predict well. These results agree with hypotheses previously presented in the literature.     

Measurement of the flow field in a diesel engine combustion chamber after combustion by cross-correlation of high-speed photographs

A cellular cross-correlation technique is applied to high-speed photographs of the luminous phase of combustion in a high-speed direct-injection diesel engine. The method enables the velocity and vorticity distributions in the combustion chamber to be evaluated. The results obtained from the basic technique are refined to remove spurious results and to complete the definition of the flow field by applying data validation, interpolation, and smoothing. The velocity and vorticity fields evaluated at two swirl ratios show the way in which the basically solid body swirl motion interacts with the fuel jets in the combustion chamber. A better understanding of the post-combustion fluid motion is obtained, and this should be of help in validating CFD codes and also the design of engines.The combustion photographs which provided the data for this work were obtained by Mr. E. H. Clough, K. K. Rao, P. Gomes and R. Pruce. Funding for the programme was provided by the UK Science and Engineering Research Council. Financial support for one of the authors (J.-H. S.) was provided by the Sino-British Friendship Scholarship Scheme. To all these people and organisations the authors express their grateful thanks.     

Investigation of the unsteady flow velocity field above an airfoil pitching under deep dynamic stall conditions

The unsteady flow field above a NACA 0012 airfoil pitching under deep dynamic stall conditions has been investigated in a low-speed wind tunnel by means of particle image velocimetry. The measurements of the instantaneous flow velocity field show the characteristic features of the dynamic stall process: formation and development of an organized vortex structure for increasing incidences and the subsequent separation. Vorticity and divergence estimated from the measured data give a good insight into the complex flow behaviour during the downstroke motion. Furthermore, small-scale structures could be observed in the separated flow field and even within the dynamic stall vortex.The authors would like to thank Dr. Schäfer (ISL) for his support in organizing the cooperative measurements, Mr. Seyb (DLR) for his help during the recording of PIV images, Dr. Bretthauer (DLR) and Mr. Vollmers (DLR) for his assistance during the phase of evaluation and post processing of the PIV recordings and Dr. Geißler (DLR) for helpful discussions on the dynamic stall problem.