Optical evaluation methods in particle image velocimetry

Application of particle image velocimetry (PIV) techniques for measurement of fluid velocities typically requires two steps. The first of these is the photography step, in which two exposures of a particle field, displaced between the exposures, are taken. The second step is the evaluation of the double-exposure particle pattern and production of appropriate particle velocities. Each of these steps involves optimization, which is usually specific to the experiment being conducted, and there is significant interaction between photographic parameters and evaluation characteristics. This paper will focus on the latter step, that of evaluation of the double-exposure photograph. In several parts of a PIV system, some performance advantage may be obtained by increasing use of optical processing over conventional digital image processing. Among the processes for which a performance advantage may be obtained are parallel or multiplex image interrogation and the evaluation of the Young's fringe pattern obtained from the scattered pattern from the double-exposure photograph. This paper will discuss parallel image interrogation and compare the performance of optical and numerical Fourier transform analysis of Young's fringes using speckle images.     

超声粒子图像测速技术及应用

心血管疾病的产生与动脉血流的流动状况密切相关。然而,目前普遍应用的超声多普勒成像技术不能精确测量复杂血流流场信息。本文提出了一种基于超声造影微泡的超声全流场粒子图像测速技术,能够获得多维流速速度信息,且不依赖于声束与速度向量之间的夹角。本文首先着重阐述了超声全流场粒子测速技术的基本原理以及系统组成,并对直管流和旋转流场流体动力学特性进行了实验测试研究,实验结果表明本技术能够测量全流场速度,并可作为表征复杂血流流场的有力手段。     

Echo particle image velocimetry technique and its applications

Developments of many cardiovascular problems have been shown to have a close relationship with arterial flow conditions.However,current ultrasound/Doppler imaging techniques cannot resolve the complex nature of arterial blood flow.We have recently developed a novel contrast-based echo particle imaging technique(Echo PIV) without angel dependence for non-invasively measuring multi-component flow vectors.This study introduces the Echo PIV principles,system characterization and utility examination to characterize hemodynamics in pipe laminar flow and rotating flow.Echo PIV measurement results show its capability to resolve the complex hemodynamics including proximal flow velocity vectors,and velocity mapping. The Echo PIV method provides an easy,direct and accurate means of quantitatively yet non-invasively characterizing the complex vascular hemodynamics.     

Validation of an instability growth model using particle image velocimetry measurements

A varicose-profile, thin layer of heavy gas ( SF6) in lighter gas (air) is impulsively accelerated by a planar, Mach 1.2 shock, producing the Richtmyer-Meshkov instability. We present the first measurements of the circulation in the curtain during the vortex-dominated, nonlinear stage of the instability evolution. These measurements, based on particle image velocimetry data, are employed to validate an idealized model of the nonlinear perturbation growth.     

Development of particle image velocimetry for multiphase flow diagnostics

This paper discusses the suitability of PIV for measuring simultaneously the velocity field of every phase and the size and concentration field of the disperse phases in multiphase flows. Velocity and disperse phase information are both inferred either directly from the Young’s fringe pattern (far field diffraction function) or from its 2-D Fourier transform (autocorrelation function). In the first case, the velocity is inferred from the orientation and spacing of the fringes while the disperse phase size is inferred from the size of the diffraction halo that modulates the fringes. In the second case, the velocity and particle shape are related to the position and shape of the strongest autocorrelation peaks, respectively. Particle sizes are used to discriminate between phases on the velocity measurements. The technique has been demonstrated on a high speed air-particle flow, where the potential for determining air velocities and particle size and velocities are shown.     

Effects of inclined impermeable bed on the turbulent characteristics of the flow using particle image velocimetry

In this study, the turbulent characteristics of the flow in an open channel with horizontal and inclined impermeable beds were studied experimentally using two-dimensional particle image velocimetry (PIV). The experiments were conducted in a channel of 6.5 m in length, 7.5 cm in width and 25 cm in height. The slope of the channel was S = 0 for the horizontal impermeable bed and for S = ?0.002, S = ?0.004 and S = ?0.006 for the inclined impermeable bed. Hydraulic characteristics such as distributions of velocities, turbulent intensities and Reynolds stress are investigated at a fine resolution using the PIV. Velocity is measured above the horizontal and inclined impermeable bed for the same different heights (h = 5, 7, 9, 11 and 13 cm) and for the same different discharges (Q = 0.735, 0.845 and 0.970 lt/s). Results show that the channel slope influences significantly near the impermeable bed but not near the free surface the variation of turbulent characteristics of the flow and also the alteration of the channel slope from ?0.002 to ?0.006 doesn't influence the variation of turbulent characteristics of the flow, which are the longitudinal turbulent intensity u′U_*, the vertical turbulent intensity v′/U_* and the turbulent kinetic energy. The channel slope doesn't influence the Reynolds stress.     

Numerical investigation of the accuracy of particle image velocimetry technique in gas-phase detonations

We numerically investigate the accuracy of the Particle Image Velocimetry (PIV) technique for the flow characterization in high-speed, compressible regimes, in particular in gas-phase detonations. We carry out synthetic PIV reconstruction of the flow field in a two-dimensional, planar detonation propagating under atmospheric conditions and modelled using single-step Arrhenius kinetics. The flow is uniformly seeded with monodispersed Al₂O₃ particles with sizes 50 and 200 nm, along with initially co-located massless Lagrangian tracer particles. The effect of massive particles on the detonation speed and thermodynamic state of the flow is investigated and is found to be negligible. We further assess the ability of massive particles to sample the flow field and while it is found that 50 nm particles sample the flow field better than the 200 nm ones, they also exhibit significant clustering. By comparing the trajectories of massive particles with those of massless tracers, it is shown that almost all massive particles rapidly diverge from the actual flow pathlines. Finally, we quantify the accuracy of the PIV reconstruction of the velocity field in comparison with the actual velocity field in the numerical simulations. It is shown that while PIV is generally capable of capturing the bulk flow features in the streamwise direction, its accuracy is not sufficient to characterize the transverse velocity component or velocity fluctuations.     

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.     

Experimental investigation of the flow inside a saxophone mouthpiece by particle image velocimetry

An experimental study of the flow inside a saxophone mouthpiece in playing conditions is carried out by means of particle image velocimetry at high acquisition rate. Planar velocity measurements on the midsection of a Plexiglas tenor saxophone mouthpiece are performed, respectively, in the mouthpiece baffle and in the reed channel. Sequences of velocity fields inside the mouthpiece baffle and around the reed tip are shown for one reed duty cycle. Maxima of the velocity fluctuations are observed at the upper surface of the mouthpiece at a distance between five and ten reed apertures from the tip. The proper orthogonal decomposition analysis reveals that almost 50% of the kinetic energy in the baffle is distributed in the first two modes displaying a periodic behavior at the fundamental frequency, the rest being turbulent flow behavior. The measured dynamical vena contracta coefficient at the inlet is reasonably constant around the value of 0.6 for reed positions far from closure. This is in agreement with existing steady flow analytical models and previous experimental results.     

Laser flow visualization and velocity fields by particle image velocimetry in an electrostatic precipitator model

Although improving electrostatic precipitator (ESP) collection of fine particles (micron and submicron sizes) remains of interest, it is not yet clear whether the turbulent flow patterns caused by the presence of electric field and charge in ESPs advance or deteriorate fine particle precipitation process. In this paper, results of the laser flow visualization and Particle Image Velocimetry (PIV) measurements of the particle flow velocity fields in a wire-to-plate type ESP model with seven wire electrodes are presented. Both experiments were carried out for negative and positive polarity of the wire electrodes. The laser flow visualization and PIV measurements clearly confirmed formation of the secondary flow (velocity of several tens of cm/s) in the ESP model, which interacts with the primary flow. The particle flow pattern changes caused by the strong interaction between the primary and secondary flows are more pronounced for higher operating voltages (higher electrohydrodynamic numbernehd) and lower primary flow velocities (lower Reynolds number Re). The particle flow patterns for the positive voltage polarity of the wire electrodes are more stable and regular than those for the negative voltage polarity due to the nonuniformity of the negative corona along the wire electrodes (tufts).     

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.     

Preliminary investigations of ultrasound induced acoustic streaming using particle image velocimetry

Particle image velocimetry was used to investigate ultrasound-induced acoustic streaming in a system for the enhanced uptake of substances from the aquatic medium into fish. Four distinct regions of the induced streaming in the system were observed and measured. One of the regions was identified as an preferential site for substance uptake, where the highest velocities in proximity to the fish surface were measured. A positive linear relationship was found between the ultrasound intensity and the maximum streaming velocity, where a unitless geometric factor, specific to the system, was calculated for correcting the numerical relationship between the two parameters. The results are part of a comprehensive study aimed at improving mass transdermal administrations of substances (e.g. vaccines, hormones) into fish from the aquatic medium.     

Holographic particle image velocimetry: analysis using a conjugate reconstruction geometry

Holographic recording techniques have recently been studied as a means to extend two-component, planar particle image velocimetry (PIV) techniques for three-component, whole-field velocity measurements. In a similar manner to two-component PIV, three-component, holographic PIV (HPIV) uses correlation-based techniques to extract particle displacement fields from double-exposure holograms. Since a holographic image contains information concerning both the phase and the amplitude of the scattered field it is possible to correlate either the intensity or the complex amplitude. In previous work we have shown that optical methods to compute the autocorrelation of the complex amplitude are inherently more tolerant to aberrations introduced in the reconstruction process, Coupland, Halliwell, Proc. Roy. Soc. 453 (1960) (1997) 1066. In this paper we introduce a new method of holographic recording and reconstruction that allows a constant image shift to be introduced to the particle image displacement. The technique, which we call conjugate reconstruction, resolves directional ambiguity and extends the dynamic range of HPIV. The theory of this method is examined in detail and a relationship between the image and object displacement is derived. Experimental verification of the theory is presented.     

An experimental study on swirling flow in a 90 degree circular tube by using particle image velocimetry

The study of swirl flow is of technical and scientific interest because it has an internal recirculation field, and its tangential velocity is related to the curvature of the streamline. The fluid flow for tubes and elbows of heat exchangers has been studied largely through experiments and numerical methods, but studies about swirl flow have been insufficient. Using the Particle Image Velocimetry method, this study found the time averaged velocity distribution, time averaged turbulence intensity with swirl and without swirl flow for Re=10,000, 15,000, 20,000 and 25,000 along longitudinal sections, and the results appear to be physically reasonable. In addition, streamwise mean velocity distribution was compared with those of Khodadai et al. and Jeong et al.     

The application of particle image velocimetry to the study of water waves

The application of Particle Image Velocimetry (PIV) to the measurement of velocity distributions under water waves is described. Two-dimensional water waves are generated in a laboratory wave flume with the measurement area illuminated by a powerful CW Argon ion laser. Photographic recording and subsequent electro-optical analysis of the film provides the displacement and hence velocity field for passing waves. The accuracy and reliability of the technique is illustrated by typical experimental results and a comparison with Linear wave theory.     

Characterization of acoustic streaming and heating using synchronized infrared thermography and particle image velocimetry

Real-time measurements of acoustic streaming velocities and surface temperature fields using synchronized particle image velocimetry and infrared thermography are reported. Measurements were conducted using a 20 kHz Langevin type acoustic horn mounted vertically in a model sonochemical reactor of either degassed water or a glycerin-water mixture. These dissipative phenomena are found to be sensitive to small variations in the medium viscosity, and a correlation between the heat flux and vorticity was determined for unsteady convective heat transfer.     

Simultaneous application of single-shot Ramanography and particle image velocimetry

A simultaneous two-dimensional determination of the concentration field and the velocity field in a turbulent mixing process is demonstrated for the first time to the best of our knowledge by using planar Raman scattering (Ramanography) and particle image velocimetry. An example of application of these techniques is tested by considering the mixing characteristic of a two-component nozzle for the injection of liquid ethanol into pure water.     

Acoustic streaming in lithotripsy fields: preliminary observation using a particle image velocimetry method

This study considers the acoustic streaming in water produced by a lithotripsy pulse. Particle image velocimetry (PIV) method was employed to visualize the acoustic streaming produced by an electromagnetic shock wave generator using video images of the light scattering particles suspended in water. Visualized streaming features including several local peaks and vortexes around or at the beam focus were easily seen with naked eyes over all settings of the lithotripter from 10 to 18 kV. Magnitudes of the peak streaming velocity measured vary in the range of 10-40 mm s(-1) with charging voltage settings. Since the streaming velocity was estimated on the basis of a series of the video images of particles averaged over 1/60s, the time resolution limited by the video frame rate which is 1-2 orders of magnitude larger than driving acoustic activities, measured velocities are expected to be underestimated and were shown a similar order of magnitude lower than those calculated from a simple theoretical consideration. Despite such an underestimation, it was shown that, as predicted by theory, the magnitude of the streaming velocity measured by the present PIV method was proportional to acoustic intensity. In particular it has almost a linear correlation with peak negative pressures (r=0.98683, p=0.0018).