Large scale PIV-measurements at the surface of shallow water flows

To measure the flow dynamics at the surface of shallow water flows over a large measuring field, a simple and reliable method has been developed using the advantages of Particle Image Velocimetry (PIV). Besides the determination of mean flow conditions and turbulent flow characteristics, this method makes it possible to track two-dimensional large coherent structures, which are the dominating flow phenomena in many shallow flow applications. As basic equipment, a commercial PIV software system has been used. The measurements are carried out at the water surface, which means that no laser light sheet is needed. Depending on the time scales of the flow and camera characteristics, it is even possible to work with a constant light source. A particle dispenser to provide a homogeneous distribution of particles on the water surface is also presented. Because floating particles have a strong tendency of sticking together, different types of particles and special coatings have been tested to reduce this problem. A laboratory application of this method is presented to analyze the effects of shallow dead-water zones on exchange processes in rivers where large coherent two-dimensional flow structures in the mixing layer dominate the flow characteristics.     

A modified cross-correlation algorithm for PIV image processing of particle-fluid two-phase flow

PIV (Particle Image Velocimetry) technique for flow field measurement has achieved popular self-identify through over ten years development, and its application range is becoming wider and wider. PIV post-processing techniques have a great influence on the success of particle-fluid two-phase flow field measurement and thus become a hot and difficult topic. In the present study, a Phase Respective Identification Algorithm (PRIA) is introduced to separate low-density solid particles or bubbles and high-density tracer particles from the PIV image of particle-fluid two-phase flow. PTV (Particle Tracking Velocimetry) technique is employed to calculate the velocity fields of low-density solid particles or bubbles. For the velocity fields of high-density solid particles or bubble phase and continuous phase traced by high-density smaller particles, based on the thought of wavelet transform and multi-resolution analysis and the theory of cross-correlation of image, a delaminated processing algorithm (MCCWM) is presented to conquer the limitation of conventional Fourier transform. The algorithm is firstly testified on synthetic two-phase flows, such as uniform steady flow, shearing flow and rotating flow, and the computational results from the simulated particle images are in reasonable agreement with the given simulated data. The algorithm is then applied to images of actual bubble-liquid two-phase flow and jet flow, and the results also confirmed that the algorithm proposed in the present study has good performance and reliability for post-processing PIV images of particle-fluid two-phase flow.     

Application of video imagery techniques for low cost measurement of water surface velocity in open channels

Developments in digital video recording technology make the video imagery tools more popular for velocity measurement in water flows. This has especially been of large interest due to its inherent advantage of non-contact nature which is quite handy in extreme flow conditions. Particle Image Velocimetry (PIV), Particle Tracking Velocimetry (PTV) and Large Scale Particle Tracking Velocimetry (LSPTV) are applied to free surface channel flow for water surface velocity measurement. Experiments are conducted to measure either a single point velocity applying PTV or velocity profiles across the channel width applying PIV on the water surface in a rectang typical velocities of nearly 1 andular tilting flume for various flow conditions. Technical issues regarding tracer particle size and type, travel distance, lighting, recording speed, camera position, image distortion and state of flow are discussed. Measured data is compared to computational results obtained from a numerical model involving a non-linear turbulence model capable of predicting turbulence driven secondary flows. Confirmation of reasonable match between computational and experimental results whereby applying mutual collaboration of them for discharge measurement has been attested. In addition to discharge, boundary roughness has also been predicted as an outcome of the numerical solution.     

The development of an automated PIV image processing software—SmartPIV

Since the popularity of digital particle image velocimetry technique (DPIV), many PIV image processing algorithms have been proposed. Amongst them, fast Fourier transform (FFT) Cross Correlation, Discrete Window Offset Cross Correlation, Iterative Multigrid Cross Correlation, Iterative Image Deformation Cross Correlation and cross correlation based particle tracking methods are widely used algorithms and have been extensively studied by researchers. All of these algorithms have their advantages and disadvantages in terms of computational load and measurement accuracy. To choose a suitable algorithm, researchers not only need to understand the complex principles of these algorithms, but also need to find out their applicable flow conditions. This could greatly increase work load for PIV users who focus more on flow structure itself instead of PIV algorithms. It is therefore necessary to develop a method which can choose PIV algorithms wisely according to the input PIV images. This paper firstly reviews the development of PIV algorithm with mainly focus on analysing advantages and disadvantages of six widely used algorithms. By using both synthetic and real PIV images, comparative studies are then carried out among these algorithms. The tests give a rate for the performance of the algorithms and provide a parameter to automatically separate pattern match and particle tracking algorithms. Based on qualitative and quantitative analysis, an automated PIV image processing method—SmartPIV is proposed and tested by both synthetic and real PIV images. For all the three test cases, the SmartPIV successfully picks the most suitable algorithm and gives very promising results.     

Simultaneous measurement of particle and fluid velocities in particle-laden flows

For the velocity measurement in a particle-laden fluid flow, the fluid velocity and the inherently dispersed particle velocity can be analyzed by using PIV and PTV, respectively. Since the PIV result statistically represents the average displacement of all the particles in a PIV image, it is inevitable that the PIV result includes the influence of the dispersed particles’ displacement if a single CCD camera is used to simultaneously measure the fluid velocity and the dispersed particle velocity. The influence of dispersed particles should be excluded before the PIV analysis in order to evaluate the fluid velocity accurately. In this study, the optimum replacement brightness of dispersed particles to minimize the false influence of dispersed particles on the PIV analysis was theoretically derived. Simulation results show that the modification of dispersed particle brightness can significantly reduce the PIV error caused by the dispersed particles. This modification method was also verified in the analysis of an actual experimental case of the particle-laden fluid flow in a triangular grooved channel.     

Measurement of particle/bubble motion and turbulence around it by hybrid PIV

Characteristics of bubble flow are influenced by bubble motion, liquid flow and interactions between bubbles, and between a bubble and liquid phase. Thus because behavior of a single bubble and liquid around it is regarded as one of the basic elements characterizing bubble flow, the single bubble motion in stagnant water was investigated experimentally by using flow visualization and image processing methods. The bubble motion is influenced by several factors, that is, bubble size, density difference between gas and liquid, bubble shape and deformation in motion. In order to separate the effect of each factor, some solid particles with different size, shape and/or density were also measured and the characteristic of each factor was discussed. Two-dimensional water velocity field and the motion of a rising particle/bubble in the water were simultaneously measured by PIV (Particle Image Velocimetry) and PTV (Particle Tracking Velocimetry), respectively (Hybrid PIV). The experimental results showed that the large density difference between a particle and water caused high relative velocity and induced zigzag motion of the particle. Furthermore, the turbulence intensity of a bubble was about twice in the case of the spherical solid particle of similar diameter.     

离心泵叶轮内水流相对速度的实验研究

该研究应用PIV系统(粒子成像测速仪)这种先进的流场测试技术在不干扰流场的情况下,进行高精度的测量,即利用撒在流体中的粒子对光的散射作用,用光学的方法记录下粒子在不同时刻的位置,从而得到粒子的位移,基于粒子对流场的跟随性,测出水流在离心泵叶片流道内的绝对速度分布,并利用软件进行数据处理,得到离心叶轮内部从吸力边到压力边相对速度的分布,为离心泵的设计提供了更为可靠的理论依据。     

Application of modern optical methods for detecting the spatial structure of turbulent flames

A number of modern optical methods used for diagnostics of reactive flows are described. Various aspects of using advanced modifications of particle image velocimetry (Stereo-PIV, High-repetition PIV, and Tomo-PIV) for measuring instantaneous velocity fields in reactive flows are discussed in detail. Capabilities of PIV and spectroscopy of flame radiation (CH* radical) in obtaining data on the spatial flow structure and the flame are demonstrated by an example of studying a swirled turbulent propane-air flame and an isothermal jet.     

Note: development of a compact x-ray particle image velocimetry for measuring opaque flows. II. Three-dimensional velocity field reconstruction

An x-ray particle image velocimetry (PIV) system using a cone-beam type x-ray was developed. The field of view and the spatial resolution are 36 × 24.05 mm(2) and 20 μm, respectively. The three-dimensional velocity field was reconstructed by adopting the least squares minimum residue and simultaneous multiplicative algebraic reconstruction techniques. According to a simulation study with synthetic images, the reconstructions were acceptable with 7 projections and 50 iterations. The reconstructed and supplied flow rates differed by only about 6.49% in experimental verification. The x-ray tomographic PIV system would be useful for 3D velocity field information of opaque flows.     

Application of the Method of Particle Image Velocimetry for Analyzing Turbulent Flows with a Periodic Component

The foundations of the field method for measuring the flow velocities—the particle image velocimetry (PIV) method—are presented. An approach is described that ensures measurements of conventionally averaged characteristics in flows with pronounced periodicity. Using the high-resolution PIV and conditional averaging methods, detailed measurements of the velocity fields in an axisymmetrical impact jet have been performed for the first time. The flow characteristics are analyzed using a triple decomposition of fluctuating quantities.     

Measurement of the surrounding liquid motion of a single rising bubble using a Dual-Camera PIV system

A new type of Particle Image Velocimetry technique, called “Dual-Camera PIV System”, was developed in order to achieve higher-accuracy measurement at a high time resolution. It is very difficult to measure precisely a complex flow field such as a gas–liquid two phase flow using PIV, because of the existence of a strong turbulence. In the conventional dynamic PIV, a time interval of two images required for analysis depends basically on a camera frame rate. A time interval of a set of PIV images affects the measurement accuracy significantly. Therefore, it is necessary to shorten the time interval of a set of PIV images as well as to achieve high frame rates. For this specific purpose, we developed a measurement system composed of two high speed cameras. The interval of two images obtained from each camera was controlled arbitrarily. Furthermore, a recursive cross-correlation method was adopted as PIV algorithm in order to achieve high spatial resolution. The interrogation areas were determined from the number density of PIV particles. The developed system was evaluated by cross-correlation coefficient and signal–noise (S/N) ratio. As the demonstration, the surrounding liquid motion in the vicinity of a single rising bubble was measured via this measurement system.     

Particle image velocimetry of a flow at a vaulted wall

The assessment of flow along a vaulted wall (with two main finite radii of curvature) is of general interest; in biofluid mechanics, it is of special interest. Unlike the geometry of flows in engineering, flow geometry in nature is often determined by vaulted walls. Specifically the flow adjacent to the wall of blood vessels is particularly interesting since this is where either thrombi are formed or atherosclerosis develops. Current measurement methods have problems assessing the flow along vaulted walls. In contrast with conventional particle image velocimetry (PIV), this new method, called wall PIV, allows the investigation of a flow adjacent to transparent flexible surfaces with two finite radii of curvature. Using an optical method which allows the observation of particles up to a predefined depth enables the visualization solely of the boundary layer flow. This is accomplished by adding a specific dye to the fluid which absorbs the monochromatic light used to illuminate the region of observation. The obtained images can be analysed with the methods of conventional PIV and result in a vector field of the velocities along the wall. With wall PIV, the steady flow adjacent to the vaulted wall of a blood pump was investigated and the resulting velocity field as well as the velocity fluctuations were assessed.     

A flow field characterization in a circular channel along a side weir

Side weirs have been used for centuries in urban drainage and flood control for their ability to divert high incoming flow rates. The flow along a side weir may be studied by means of different approaches. However, although the question of side weir has been debating for many decades, there are few studies on the local hydraulic characteristics of the flow along these structures in circular channels.An experimental study of the flow field in a circular channel along a side weir using a commercial TSI Particle Image Velocimetry (PIV) system is shown in this paper. Weakly supercritical flows running into different geometric configurations of the weir were investigated, with Froude numbers varying from 1.1 to 1.6. Free surface profiles have been obtained by an image processing technique. An empirical equation has been proposed for their representation. Longitudinal velocity profiles along the side weir can be well predicted by an entropic approach. Local outflow along the side weir may be represented by an asymmetric curve. The peak outflow generally occurs between the 30% and the 50% of the weir length. The elementary discharge coefficient significantly increases from upstream to downstream. An energy head reduction was observed under the investigated configurations. Most of the head variation occurs in the central part of the weir. Finally, the flow power decreases along the weir according to a non-linear function.A thorough knowledge of the flow field features should allow to improve side weir analysis and design, whatever approach is used for the study.     

Experimental investigation of particle distribution in a flow through a stenosed artery

The particle distribution of a dilute solid-liquid suspension through a stenosed arterial geometry was investigated. Particle image velocimetry (PIV) was used to determine the velocity as well as to acquire the flow images. The light intensity scattered by particles was evaluated to determine the particle distribution. Flow separation exists where the flow emerges from the stenosis throat. From the PIV images, the particle density distribution exhibited differing non-uniform characteristics which vary with flow rate, particle size and particle concentration. At low flow rates, a particle-free layer is formed. As the flow rate is increased, particles accumulate in concentric recirculation orbits within the downstream vortex. Particles with larger size and higher concentration tend to accumulate more towards the center of the vortex.     

The μPIVOT: an integrated particle image velocimeter and optical tweezers instrument for microenvironment investigations

A novel instrument to manipulate and characterize the mechanical environment in and around microscale objects in a fluidic environment has been developed by integrating two laser-based techniques: micron-resolution particle image velocimetry (μPIV) and optical tweezers (OT). This instrument, the μPIVOT, enables a new realm of microscale studies, yet still maintains the individual capabilities of each optical technique. This was demonstrated with individual measurements of optical trap stiffness (～70 pN μm(-1) for a 20 μm polystyrene sphere and a linear relationship between trap stiffness and laser power) and fluid velocities within 436 nm of a microchannel wall. The integrated device was validated by comparing computational flow predictions to the measured velocity profile around a trapped particle in either a uniform flow or an imposed, gravity-driven microchannel flow (R(2) = 0.988, RMS error = 13.04 μm s(-1)). Interaction between both techniques is shown to be negligible for 15 μm to 35 μm diameter trapped particles subjected to fluid velocities from 50 μm s(-1) to 500 μm s(-1) even at the highest laser power (1.45 W). The integrated techniques will provide a unique perspective toward understanding microscale phenomena including single-cell biomechanics, non-Newtonian fluid mechanics and single particle or particle-particle hydrodynamics.     

3D Particle image velocimetry test of inner flow in a double blade pump impeller

The double blade pump is widely used in sewage treatment industry,however,the research on the internal flow characteristics of the double blade pump with particle image velocimetry(PIV) technology is very little at present.To reveal inner flow characteristics in double blade pump impeller under off-design and design conditions,inner flows in a double blade pump impeller,whose specific speed is 111,are measured under the five off-design conditions and design condition by using 3D PIV test technology.In order to ensure the accuracy of the 3D PIV test,the external trigger synchronization system which makes use of fiber optic and equivalent calibration method are applied.The 3D PIV relative velocity synthesis procedure is compiled by using Visual C++ 2005.Then absolute velocity distribution and relative velocity distribution in the double blade pump impeller are obtained.Test results show that vortex exists in each condition,but the location,size and velocity of vortex core are different.Average absolute velocity value of impeller outlet increases at first,then decreases,and then increases again with increase of flow rate.Again average relative velocity values under 0.4,0.8,and 1.2 design condition are higher than that under 1.0 design condition,while under 0.6 and 1.4 design condition it is lower.Under low flow rate conditions,radial vectors of absolute velocities at impeller outlet and blade inlet near the pump shaft decrease with increase of flow rate,while that of relative velocities at the suction side near the pump shaft decreases.Radial vectors of absolute velocities and relative velocities change slightly under the two large flow rate conditions.The research results can be applied to instruct the hydraulic optimization design of double blade pumps.     

粒子图像测速(PIV)技术的发展

在流场显示测量技术中，粒子图像测速(PIV)技术占有相当重要的地位，本文重点介绍了PIV的测试原理及应用要求，并以TSI公司开发的PIV系统为例对DPIV系统的构成作了概括，同时指出了PIV技术的未来发展方向。     

Application of PIV measurement techniques to study the characteristics of flame–acoustic wave interactions

The present paper introduce the development of the experimental setup and measurement methodologies to study the structure of acoustic wave and the interaction between the acoustic fields and flame flow inside a square tube by using a time-resolved Particle Image Velocimetry (PIV). The design of signal synchronisation system, the selection of seeding particles, the test of seeding methods and the statistical analysis of the PIV data were introduced. The titanium dioxide (TiO₂) was selected as the seeding particle for the PIV. The accuracy of synchronisation system was checked by a simple experiment. A time-resolved PIV results showed that the acoustic velocity difference was less than 2.8% at specific phase angle over 1000 excitation cycles at an excitation frequency of 385 Hz. For the case of hot flame gas, the largest difference is 4.4% over 100 excitation cycles at an excitation frequency of 10 Hz. Results proves that the present experimental system has high reliability to measure and analyse the characteristics of flame–acoustic interactions.     

Phase discrimination and a high accuracy algorithm for PIV image processing of particle–fluid two-phase flow inside high-speed rotating centrifugal slurry pump

PIV technology is an efficient and powerful measurement method to investigate the characteristics of fluid flow field. But for PIV particle image post-processing, some problems still exit in two-phase particles discrimination and velocity field algorithm, especially for high-speed rotating centrifugal slurry pump. In this study, through summarization and comparison of the various phase discrimination methods, we proposed a two-phase identification method based on statistics of gray-scale level and particle size. The assessment of performance through experimental PIV images shows that a satisfying effect for particle identification. For high speed rotation of the impeller, a combination of adaptive cross-correlation window deformation algorithm and multistage grid subdivision is presented. The algorithm is applied to experimental PIV images of solid–liquid two-phase flow in a centrifugal slurry pump, the results show that the algorithm in the present study has less pseudo vector number and more matching particle pairs than those of fixed window and window translation methods, having the ability to remove pseudo vector efficiently. It confirmed that the algorithm proposed in the present study has good performance and reliability for PIV image processing of particle–fluid two-phase flow inside high-speed rotating centrifugal slurry pump.     

On sampling bias in multiphase flows: Particle image velocimetry in bubbly flows

Measuring the liquid velocity and turbulence parameters in multiphase flows is a challenging task. In general, measurements based on optical methods are hindered by the presence of the gas phase. In the present work, it is shown that this leads to a sampling bias. Here, particle image velocimetry (PIV) is used to measure the liquid velocity and turbulence in a bubble column for different gas volume flow rates. As a result, passing bubbles lead to a significant sampling bias, which is evaluated by the mean liquid velocity and Reynolds stress tensor components. To overcome the sampling bias a window averaging procedure that waits a time depending on the locally distributed velocity information (hold processor) is derived. The procedure is demonstrated for an analytical test function. The PIV results obtained with the hold processor are reasonable for all values. By using the new procedure, reliable liquid velocity measurements in bubbly flows, which are vitally needed for CFD validation and modeling, are possible. In addition, the findings are general and can be applied to other flow situations and measuring techniques.