一种改进的粒子图像测速混合算法研究

粒子图像测速技术作为一种新的流场测速方法能够在不干扰流场的情况下获得整个流场的速度信息。粒子图像测速技术最关键的步骤在于粒子匹配。针对粒子密度分布不均匀、流场不同等实际情况,提出了混合算法,即结合互相关和松弛算法能够更准确地搜索粒子,进而对粒子进行匹配。对3种匹配算法的匹配概率进行比较分析,发现混合算法能更准确地分析粒子的运动状态,减少错误矢量的产生;另外,对松弛算法进行改进,通过优化筛选加权因子发现改进的松弛算法在运行速度上相比原始算法有了较大提高,匹配率与原始算法基本一致。     

三维粒子示踪测速技术的开发及初步应用

开发的单颗粒三维运动轨迹测量系统的硬件由四部摄像机、一台PC机和内置的图像卡组成。软件系统包括4路图像实时同步采集、图像二值化和聚合程序，坐标校正和转换，以及各统计参数的计算公式。系统标定的空间误差小于0.3mm，速度误差约为1.0mm/s。开发的颗粒三维运动测量系统在测量颗粒运动更长的轨迹、实现较高的采样频率和保证每个摄像机的高分辨率、只进行一次标定而进行各种水力坡度的试验及实现极大样本的采集等方面都取得了明显的进展，为研究颗粒运动的基本规律提供了新的方法。试验可以得出颗粒三维运动的瞬时速度，初步试验得出颗粒的纵向平均速度遵循对数分布，颗粒速度小于相应水流的运动速度。     

实时流场测量系统在河工模型试验中的应用

流场实时测量系统VDMS(以下简称VDMS),是运用数字摄像与粒子跟踪测速技术(PTV)研制开发的表面流场大范围同步测速与监控系统。该系统可快速方便地得到模型试验范围研究区域内的流场、断面流速分布及单个或多个测点的流速矢量变化过程,提高了试验的精度和效率。     

基于回归分析的表面流场粒子匹配算法

针对表面流速图像测试中高浓度粒子条件下测量误差大的问题,改进了最邻近匹配算法,提出基于回归分析的表面流场粒子匹配算法,即多帧回归粒子匹配算法。该算法综合粒子距离、运动轨迹、运动趋势等因素,应用一元线性回归分析法拟合粒子中心坐标,通过相关系数完成粒子匹配,实现最佳同源粒子追踪。运用多帧回归粒子匹配算法开发了改进型大尺度模型表面流场图像测试系统(ILSPIV),并将ILSPIV应用到八卦洲河工模型,ILSPIV和旋桨流速仪测量的断面垂线平均流速分布形态及数值基本一致,表明多帧回归粒子匹配算法能提升高浓度粒子条件下流速测量精度,更好地满足工程实践研究需求。     

稀疏气泡流动的粒子跟踪测速技术研究

采用动态阈值技术实现了对气泡图像的正确分割，利用灰度加权方法提高了气泡颗粒的定位精度。根据稀疏气泡流动图像的特点，提出了一种新的基于相关的粒子跟踪测速算法（2-HPTV）。通过进行两时刻中围绕待研究气泡所取的两个小的诊断窗口的相关运算，成功实现了气泡颗粒的轨迹追踪。实验表明本文算法较传统的PIV技术在测速精度上有很大程度的提高，与4帧粒子跟踪测速算法相比，降低了对摄像机帧频的要求，具有较强的实用性。     

循环流化床环核结构速度分布PIV测试

利用粒子图像测速仪（PIV）对循环流化床中颗粒流场进行了系统的实验研究。运用适合于高粒子浓度的最小方差法（MQD）图像处理算法得到了截面内典型环核流动颗粒速度矢量场。采用速度矢量场的方法可以确定出环核结构分界。定量上获得了不同时刻颗粒轴向上升和下落平均速度大小及沿床径向分布特性。在统计颗粒轴向上升速度和下落速度代数和基础上，获得了总体上颗粒轴向速度沿床径向在整个截面内形成抛物线型的环核分布规律。     

表面流场的PyrLK“角点-质心”法在复杂流场中的应用

粒子匹配方法是准确获取表面流场的关键。本文详细阐述了新近提出的基于光流算法PyrLK"角点-质心"法的基本原理及具体实现过程,采用该方法对弯道水槽中较复杂的表面流场进行计算,并与目前较为先进的匹配几率法进行了对比。结果表明,对于本文所示的复杂流场,匹配几率法计算结果很大程度上依赖于对运动矢量允许偏差等参数的选取,且还会出现由于提取到非示踪粒子的质心并使其参与计算而产生的误匹配;而PyrLK"角点-质心"法只与运动图像特征信息相关,无示踪粒子特定匹配规则的假定,在复杂流场中能较好地实现粒子的跟踪与匹配。与匹配几率法相比,PyrLK"角点-质心"法对于复杂表面流场计算具有更好的适应性。     

基于OpenCV实现金字塔光流法对表面流场的计算

本文基于OpenCV视觉函数库和 VisualStudio2010 平台,采用 C++编程语言开发了基于金字塔光流法的 PTV(Particle Track Velocimetry)表面流场测量程序。程序计算结果显示:基于金字塔光流法实现 PTV 技术,需要在金字塔光流法对示踪粒子角点跟踪匹配的基础上进一步实现示踪粒子的质心匹配,该过程可使金字塔光流法准确、合理地应用于 PTV 技术,实现对示踪粒子运动速度的正确计算,同时还可剔除掉金字塔光流法对非匹配关系的角点产生的错误匹配。     

EXPERIMENT STUDY ON SEDIMENT INCIPIENCE IN BACKWARDFACING STEP FLOW

Flow over a backward-facing step was studied to investigate the effect of large-scale vortex structures on sediment incipience. The transient flow velocity field at the downstream of the backward-facing step was obtained using the technique of Particle Tracking Velocimetry (PTV). The optical amplification technique was employed to measure the instantaneous flow velocities near the bed and the instantaneous bed shear stress was given. The experimental observations revealed a new insight into the oscillation of the large-scale structure and the three-dimensional characteristics of the flow. In particular,very high turbulence intensity,instantaneous horizontal velocity near the bed and the bed shear stress near the reattachment point were observed. The sediment incipient probability obtained from the sequent images of sediment particles near the bed indicates that the critical instantaneous shear stress of the sediment incipience is independent of flow conditions.     

恒定流中泥沙运动速度实验

泥沙颗粒运动速度与水流运动速度是不一致的,进而会对水流输沙和河床演变产生影响。通过采用粒子追踪测速方法(Particle Tracking Velocimetry-PTV)和粒子图像测速技术(Particle Image Velocimetry-PIV)测量了水槽中2种粒径的泥沙颗粒运动速度及相应的水流运动速度。实验结果表明,泥沙颗粒运动速度沿垂线分布与水流速度不一致,在对数区,泥沙颗粒的运动速度略小于水流的速度,泥沙运动滞后于水流运动;在近壁区,泥沙颗粒的运动速度大于水流速度,且在本实验的粒径范围内,随着泥沙颗粒粒径增大,泥沙颗粒的运动速度也随之增大。     

AN IMPROVED ALGORITHM FOR DPIV CORRELATION ANALYSIS

In a Digital Particle Image Velocimetry (DPIV) system, the correlation of digital images is normally used to acquire the displacement information of particles and give estimates of the flow field. The accuracy and robustness of the correlation algorithm directly affect the validity of the analysis result. In this article, an improved algorithm for the correlation analysis was proposed which could be used to optimize the selection/determination of the correlation window, analysis area and search path. This algorithm not only reduces largely the amount of calculation, but also improves effectively the accuracy and reliability of the correlation analysis. The algorithm was demonstrated to be accurate and efficient in the measurement of the velocity field in a flocculation pool.     

Measurement of particle size based on digital imaging technique

To improve the analysis methods for the measurement of the sediment particle sizes with a wide distribution and of irregular shapes, a sediment particle image measurement, an analysis system, and an extraction algorithm of the optimal threshold based on the gray histogram peak values are proposed. Recording the pixels of the sediment particles by labeling them, the algorithm can effectively separate the sediment particle images from the background images using the equivalent pixel circles with the same diameters to represent the sediment particles. Compared with the laser analyzer for the case of blue plastic sands, the measurement results of the system are shown to be reasonably similar. The errors are mainly due to the small size of the particles and the limitation of the apparatus. The measurement accuracy can be improved by increasing the Charge-Coupled Devices (CCD) camera resolution. The analysis method of the sediment particle images can provide a technical support for the rapid measurement of the sediment particle size and its distribution.     

Probleme bei Planung und Betrieb von Absetzbecken für Straßenabwässer

Sedimentation basins are incorporated into runoff infiltration systems and serve as a retention system, in which the sedimentation of particles occurs as a result of gravitational force. The effectiveness of sedimentation basins in terms of removing particles is highly dependent on design parameters such as their dimensions and drainage times. To evaluate the flow distribution within sedimentation basins, a conceptual model was developed using a tracer and compared to the actual condition at the treatment system. Here, a significant difference in the height of the connecting tube between settling basin and filter basin was found. This leads to a drying out of the basin, which significantly affects the flow rates and hence the sedimentation there. Based on a model experiment with colour tracer, the flow paths were visually represented in the basin, and both the concrete variant and the effects of internal structures were considered. It was found that the incorporation of a baffle led to an improved utilization of the basin, and to water being retained for longer times. Due to the lower flow rate and these longer times, fine and medium-sized sediments could be easily settled. In a field study, the performance of a sedimentation basin for the removal of particles was evaluated based on the influent and effluent concentrations of total suspended solids (TSS). The street runoff treatment plant was monitored for one year and results showed that there was no particle accumulation in the sedimentation basin (there was no permanent retention of the particle loads). The mean TSS concentrations were 89 mg/L at the inlet of the sedimentation basin and 94 mg/L at the end of the settling tank, respectively. The structural design and operation of the system showed higher flow and turbulent flow conditions in the sedimentation basin, in which settlement of fine and medium sized particles was very low; as a result, the soil filter basin was loaded with runoff particles. Thus the particulate matter had to be removed either on the surface of the filter basin or within the porous space of the filter. The build-up of such particles can result in a significant increase of head loss due to clogging; therefore, filters must be maintained by removing the accumulated particles on a regular basis.     

图像处理技术及其在泥沙运动研究中的应用

总结了在悬移质泥沙运动特性试验研究中常用的量测手段，介绍了图像处理技术的基本原理和系统的组成，针对图像处理中的几个具体问题，特别是光片厚度和粒子浓度的选择，结合实践经验进行了讨论．利用开发的颗粒示踪与图像处理系统，对明槽悬移质颗粒的运动特性进行了量测．实践表明，图像处理技术为悬移质颗粒运动特性的研究提供了有力的手段．     

Experimental Research of Flow Structure in A Gas-Solid Circulating Fluidized Bed Riser by PIV

Particle Imaging Velocimetry (PIV) techniques were applied to investigate the particle motion and cluster properties in a gas-solid two-phase flow in a circulating fluidized bed riser. Visual images and micro-structure of various clusters were captured. After the boundary of clusters was determined by the gray level threshold method, clusters were classified by the distance between particles and the shape and position of clusters. In addition, the process of clusters forming and breaking up was described, and the sizes of clusters were also obtained. With the Minimum Quadric Difference (MQD) cross-correlation algorithm suitable for high-density particles, the axial velocities of the particles were obtained in the dilute phase section. The features of particle motion were revealed by investigating statistically the magnitude and distribution of particle axial velocity in the radial direction. At most radial cross-sections, there exists a parabola-shaped distribution of upward axial velocity of particles, namely, the magnitude of axial velocity in the core region is higher than that near the wall region of the riser.     

Modelling spatio‐temporal flow characteristics in groyne fields

The spatio‐temporal distribution of residence times and water ages in groyne fields plays an important role for a number of biotic processes, particularly conditioning nutrient dynamics and phytoplankton growth. Experimental determination of these parameters involves time‐consuming field work and is often limited to the water surface depending on the methodology applied. Computational fluid dynamics (CFD) can be used as efficient tool for direct modelling of residence times, limiting additional field work to collect data necessary for model calibration and validation. The objective of this study is to derive, test and apply a numerical particle‐tracing method to model these flow characteristics based on the output of a three‐dimensional hydrodynamic model for turbulent river flow. This allows for an evaluation of residence times and water ages at an arbitrary elevation within the water column. Virtual particles are released in a control volume, and their paths and travel times to and from the main stream are traced using a random‐walk approach based on the spatial distribution of turbulent kinetic energy to account for turbulent fluctuations. The modelling approach was successfully validated for the water surface using field data measured in a groyne field of the Danube River in Austria. Particle‐tracing experiments were carried out for several discharges to study the evolution of residence times and water ages with varying runoff. It was found that discharges at which overtopping of the groynes begins lead to significantly higher residence times and water ages than runoff higher or lower than this characteristic value. These findings are expected to be transferable for groyne fields of similar geometry and crest height. Copyright © 2008 John Wiley & Sons, Ltd.     

Water entry of decelerating spheres simulations using improved ISPH method

In this paper, we simulated the vertical impact of spheres on a water surface using three-dimensional incompressible smoothed particle hydrodynamics(3-D ISPH) method. The sphere motion is taken to be a rigid body motion and it is modeled by ISPH method. The governing equations are discretized and solved numerically using ISPH method. A stabilized incompressible SPH method by relaxing the density invariance condition is adopted. Here, we computed the motions of a rigid body by direct integration of the fluid pressure at the position of each particle on the body surface. The equations of translational and rotational motion were integrated in time domain to update the position of the rigid body at each time step. In this study, we improved the boundary treatment between fluid and fixed solid boundary by using virtual marker technique. In addition, an improved algorithm based on the virtual marker technique for the boundary particles is proposed to treat the moving boundary of the rigid body motion. The force exerted on the moving rigid boundary particles by the surrounding particles, is calculated by the SPH approximation at the virtual marker points. The applicability and efficiency of the current ISPH method are tested by comparison with reference experimental results.     

Numerical simulation of 3-D free surface flows by overlapping MPS

An overlapping moving particle semi-implicit(MPS) method is applied for 3-D free surface flows based on our in-house particle solver MLParticle-SJTU. In this method, the coarse particles are distributed in the whole domain and the fine particles are distributed in the local region of interest at the same time. With the fine particles being generated and removed dynamically, an algorithm of generating particles based on the 3-D overlapping volume is developed. Then, a 3-D dam break flow with an obstacle is simulated to validate the overlapping MPS. The qualitative comparison among experimental data and the results obtained by the VOF and the MPS shows that the shape of the free surface obtained by the overlapping MPS is more accurate than that obtained by the UNI-coarse and close to that obtained by the UNI-fine in the overlapping domain. In addition, the water height and the impact pressure at P1 are also in an overall agreement with experimental data. Finally, the CPU time required by the overlapping MPS is about half of that required by the UNI-fine.     

粒子图像测速技术在河工模型试验中的应用

近年来计算机图像处理技术有了很大的发展，已在许多生产、科研领域中得到广泛应用，粒子图像测速系统是该技术在河工模型表面流场测量中的应用，它将芯片技术、计算机技术、图像技术和数字信号技术等结合在一起，利用流体中跟随性较好的示踪粒子的成像来测量流体的速度，能实现对模型表面流场的实时采集、处理，最后生成流场矢量分布图，具有测量速度快、精度高、范围广的优点，该系统在南水北调中线穿黄河段河工模型流场测量中取得     

大型河工模型分布式表面流场测量系统研制及应用

在河工模型试验中, 粒子图像表面流场测量方法得到了广泛应用。研制了一种新型分布式表面流场测量系统,该系统采用局域网组网与光纤传输相结合,通过POE千兆交换机与高清智能一体化工业摄像机相连,显著降低了布线复杂度,具有系统传输距离远、布设简单、集成度高、可扩展性强等优点。系统具备可视化全自动采集、可视化错误矢量剔除、导出多种数据格式,生成流场等值线图、流线等功能。在系统研制基础上,提出了一种对粒子图像表面流场测量系统进行精度检测的新方法,通过精确控制匀速旋转平台模拟水流运动,将表面流场测量系统实测数据与旋转平台上各点精确数据进行对比检测,检测结果表明,研制的表面流场测量系统测量误差小于5%,已在长江河口模型等多个大型河工模型中得到成功应用。