A displaced-line velocity diagnostic and its application in a visualization engine

A technique for measuring three-dimensional velocity by imaging the displacement of a marked fluid line is described, together with its use in an automotive visualization engine. In a flow seeded with 2–3 μ phosphorescing particles, a line is excited by a UV laser beam, deformed by the local velocity field, and detected by stereo low-light-level video cameras. The derivation of velocity from digitized images is discussed and capabilities of the diagnostic are assessed. Some image data taken in the engine are shown and quantitative two-component velocity plots along the line are presented.     

Measurement of instantaneous Eulerian acceleration fields by particle image accelerometry: method and accuracy

Acceleration is a fundamental quantity in fluid mechanics because it reflects the sum of all forces (pressure and viscous) present within the flow. However, measurements of acceleration have been difficult to achieve relative to the ease with which fluid velocity can be measured. A particle image accelerometer (PIA) has been developed to measure Eulerian acceleration fields by time-differencing successive measurements of the Eulerian velocity field as measured by particle image velocimetry (PIV). The measurements can also be made in uniformly translating frames. With current video camera technology, it is often not possible to measure the two velocity fields with a time separation sufficiently small enough to permit accurate finite difference approximation of the time derivative. A two-CCD-camera system has been developed to alleviate this limitation. Polarization filtering is utilized to separate the particle images viewed by each camera. The polarization filtering is achieved using cross-polarized light-sheets and a polarization filter just upstream of the imaging optics of the cameras. In this manner, PIV measurements can be achieved easily at time delays several orders of magnitude smaller than the shutter-time of the CCD cameras. The accuracy of the acceleration measurements is determined by numerical finite differencing errors and random noise and bias errors associated with the measurement of velocity. These errors, and methods of compensating for them, are studied.     

Simultaneous velocimetry/accelerometry measurements in a turbulent two-phase pipe flow

A two-color digital particle image velocimetry and accelerometry (DPIV and DPIA) measurement technique is described that records the velocity and acceleration fields of both the solid and liquid phases simultaneously. Measurements were taken at turbulent conditions of a vertical pipe flow using glass spheres as the solid phase and fluorescent particles to indicate fluid phase motion. Nd-YAG pulse lasers acted as illumination sources and images were recorded by two monochrome CCD cameras. The two-color aspect of the technique was realized by placing optical filters in front of the cameras to discriminate between the phases. Cross-correlations and auto-correlations were applied to determine velocity and acceleration fields of the two phases. Results showing some of the capabilities of the technique as applied to a two-phase pipe flow experiment are provided. For the condition studied, it was found that there was turbulence suppression due to the solid phase and that the statistics associated with the acceleration probability distribution were different for the solid and fluid phases.     

Experimental investigation of the unsteady structure of a transitional plane wall jet

 A laminar wall jet undergoing transition is investigated using the particle image velocimetry (PIV) technique. The plane wall jet is issued from a rectangular channel, with the jet-exit velocity profile being parabolic. The Reynolds number, based on the exit mean velocity and the channel width, is 1450. To aid the understanding of the global flow features, laser-sheet/smoke flow visualizations are performed along streamwise, spanwise, and cross-stream directions. Surface pressure measurements are made to correlate the instantaneous vorticity distribution with the surface pressure fluctuations. The instantaneous velocity and vorticity field measurements provide the basis for understanding the formation of the inner-region vortex and the subsequent interactions between the outer-region (free-shear-layer region) and inner-region (boundary-layer region) vortical structures. Results show that under the influence of the free-shear-layer vortex, the local boundary layer becomes detached from the surface and inviscidly unstable, and a vortex is formed in the inner region. Once this vortex has formed, the free-shear-layer vortex and the inner-region vortex form a vortex couple and convect downstream. The mutual interactions between these inner- and outer-region vortical structures dominate the transition process. Farther downstream, the emergence of the three-dimensional structure in the free shear layer initiates complete breakdown of the flow. Received: 8 November 1995/Accepted: 6 November 1996     

Direct measurement of local instantaneous laminar burning velocity by a new PIV algorithm

This paper presents a new experimental approach using PIV technique to measure the local instantaneous laminar burning velocity of a stretched premixed flame. Up to now, from experimental techniques, this physical property was only accessible in average and the instantaneous interactions of flame with flow structures, mixture variations and walls could not be considered. In the present work, the local burning velocity is measured as the difference between the local flame speed and the local fresh gas velocity at the entrance of the flame zone. Two original methods are proposed to deduce these quantities from pair of particle images. The local flame speed is measured from the distance between two successive flame positions. For the flame localization, a new extraction tool combined with a filtering technique is proposed to access to the flame front coordinates with sub-pixel accuracy. The local fresh gas velocity near the flame front is extracted from the maximum of the normal velocity profile, located within 1 mm ahead of the flame front. To achieve the required spatial resolution, a new algorithm based on adaptive interrogation window scheme has been developed by taking into account the flow and flame front topologies. The accuracy and reliability of our developments have been evaluated from two complementary approaches based, respectively, on synthetic images of particle and on the well-established configuration of outwardly propagating spherical flames. In the last part of the paper, an illustration of the potentials of our new approach is shown in the case of a laminar flame propagating through a stratified mixture.     

二维粒子图像测速系统的研制

研制了一套二维粒子图像测速系统,该系统采用CCD对流场中的示踪粒子视频图像进行采集,以拓扑映射的新方法完成料粒子像对的匹配;整个过程无需人工干预,处理结果声速、准确、可靠,如每处理一幅图像仅需5分钟,匹配准确度达95％以上,其结果可给出被测流场的速度分量、速度矢量、等流函数线和等涡线等。该系统与业已建成的多相分离实验模拟系统相配套,可用来对多种设备内流场进行多参数、多工况实验诊断,为揭示设备工作机理、优化设备结构等提供了有效手段。     

A study of the flow characteristics in micro-abrasive jets

An experimental study of particle velocities in micro-abrasive jets by using the particle image velocimetry (PIV) technique is presented. It has been found that the particle jet flow has a nearly linear expansion downstream. The particle velocities increase with air pressure, and the increasing rate increases with nozzle diameter within the range considered. The instantaneous velocity profile of the particle flow field in terms of the particle velocity distribution along the axial and radial directions of the jets is discussed. For the axial profile in the jet centerline downstream, there exists an extended acceleration stage, a transition stage, and a deceleration stage. For the radial velocity profiles, a relatively flat shape is observed at a jet cross-section near the nozzle exit. Mathematical models for the particle velocities in the air jet are then developed. It is shown that the results from the models agree well with experimental data in both the variation trend and magnitude.     

Stereoscopic PIV on multiple color-coded light sheets and its application to axial flow in flapping robotic insect wings

Non-scanning volume flow measurement techniques such as 3D-PTV, holographic and tomographic particle image velocimetry (PIV) permit reconstructions of all three components (3C) of velocity and vorticity vectors in a fluid volume (3D). In this study, we present a novel 3D3C technique termed Multiple-Color-Plane Stereo Particle-Image-Velocimetry (color PIV), which allows instantaneous measurements of 3C velocity vectors in six parallel, colored light sheets. We generated the light sheets by passing white light of two strobes through dichroic color filters and imaged the slices by two 3CCD color cameras in Stereo-PIV configuration. The stereo-color images were processed by custom software routines that sorted each colored fluid particle into one of six gray-scale images according to its hue, saturation, and luminance. We used conventional Stereo PIV cross-correlation algorithms to compute a 3D planar vector field for each light sheet and subsequently interpolated a volume flow map from the six vector fields. As a first application, we quantified the wake and axial flow in the vortical structures of a robotic insect (fruit fly) model wing. In contrast to previous findings, the measured data indicate strong axial flow components on the upper wing surface, including axial flow in the leading-edge vortex core. Collectively, color PIV is robust against mechanical misalignments, avoids laser safety issues, and computes instantaneous 3D vector fields in a fraction of the time typical for other 3D systems. Color PIV might thus be of value for volume measurements of highly unsteady flows.     

Instantaneous, three-component planar Doppler velocimetry using imaging fibre bundles

This paper describes a planar Doppler velocimetry (PDV) technique that is capable of measuring the three, instantaneous and time average components of velocity over two spatial dimensions using a single pair of signal and reference cameras. The three views required to obtain three-component velocity information are guided from the collection optics to a single imaging plane using flexible fibre imaging bundles. These are made up of a coherent array of single fibres and are combined at one end as the input plane to the measurement head. Measurements of the velocity field of a rotating disk are used in the development of the technique and initial results of the instantaneous velocity field of a jet are presented.     

Navier–Stokes simulations in gappy PIV data

Velocity measurements conducted with particle image velocimetry (PIV) often exhibit regions where the flow motion cannot be evaluated. The principal reasons for this are the absence of seeding particles or limited optical access for illumination or imaging. Additional causes can be laser light reflections and unwanted out-of-focus effects. As a consequence, the velocity field measured with PIV contains regions where no velocity information is available, that is gaps. This work investigates the suitability of using the unsteady incompressible Navier–Stokes equations to obtain accurate estimates of the local transient velocity field in small gaps; the present approach applies to time-resolved two-dimensional experiments of incompressible flows. The numerics are based on a finite volume discretization with partitioned time-stepping to solve the governing equations. The measured velocity distribution at the gap boundary is taken as time-varying boundary condition, and an approximate initial condition inside the gap is obtained via low-order spatial interpolation of the velocity at the boundaries. The influence of this I.C. is seen to diminish over time, as information is convected through the gap. Due to the form of the equations, no initial or boundary conditions on the pressure are required. The approach is evaluated by a time-resolved experiment where the true solution is known a priori. The results are compared with a boundary interpolation approach. Finally, an application of the technique to an experiment with a gap of complex shape is presented.     

Detection of Passive Scalar Interface Directly from PIV Particle Images in Inhomogeneous Turbulent Flows

This article proposes a technique to estimate the cross-sectional scalar interface (outer boundary) in an inhomogeneous turbulent flow from a conditioned particle image velocimetry (PIV) experiment, which is suitable for medium to high Reynolds numbers. The scalar interface is estimated directly by using conditioned PIV particle images which have distinguishably high particle seeding density in the area of interest, whereas conventionally in water based experiments, scalar interface is often determined from planar laser induced fluorescence (PLIF) or equivalent dye images. By comparing quantities in the vicinity of this scalar interface, it also shows that in terms of separate turbulent and non-turbulent regions, this technique could also replace the function of PLIF images in water experiments, with slightly lower spatial resolution. At the same time, if velocity information is also required simultaneously then the cost of a separate camera-laser system can be saved. The effect of particle field inhomogeneity on the PIV accuracy can be well reduced to an insignificant level by an image local normalisation treatment. This article shows that the interfacial layer could be detected fairly accurately by enhancing the particle images by wavelet based thresholding methods. The degree of detection accuracy is quantified by synthetic particle image analyses, where a scalar interface can be artificially pre-defined. The proposed technique is tested in two water based experiments but is expected to be particularly useful in gas-phase based experiments or some combustion applications, where liquid-phase dye cannot be applied.     

Investigation of internal pressure gradients generated in electrokinetic flows with axial conductivity gradients

Field amplified sample stacking (FASS) is used to increase sample concentrations in electrokinetic flows. The technique uses conductivity gradients to establish a non-uniform electric field that accumulates ions within a conductivity gradient, and can be readily integrated with capillary electrophoresis. Conductivity gradients also cause gradients in near-wall electroosmotic flow velocities. These velocity gradients generate internal pressure gradients that drive secondary, dispersive flows. This dispersion leads to a significant reduction in the efficiency of sample stacking. This paper presents an experimental investigation of internally generated pressure gradients in FASS using micron-resolution particle image velocimetry (μPIV). We measure velocity fields of particles seeded into an electrokinetic FASS flow field in a glass microchannel with a single buffer–buffer interface. μPIV allows for the direct quantification of local, instantaneous pressure gradients by analyzing the curvature of velocity profiles. Measurements show internally generated pressure-driven velocities on the order of 1mm/s for a typical applied electric field of 100 V/cm and a conductivity ratio of 10. A one-dimensional (1D) analytical model for the temporal development of the internal pressure gradient generation is proposed which is useful in estimating general trends in flow dynamics.

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PIV measurements in a weakly separating and reattaching turbulent boundary layer

A high Reynolds number flat plate turbulent boundary layer was studied in a wind-tunnel experiment using particle image velocimetry (PIV). The flow is subjected to an adverse pressure gradient (APG) which is designed such that the boundary layer separates and reattaches, forming a weak separation bubble. With PIV we are able to get a more complete picture of this complex flow phenomenon. The view of a separation bubble being composed of large scale coherent regions of instantaneous backflow occurring randomly in a three-dimensional manner in space and time is verified by the present PIV measurements. The PIV database was used to test the applicability of various velocity scalings around the separation bubble. We found that the mean velocity profiles in the outer part of the boundary layer, and to some extent also the Reynolds shear-stress, are self-similar when using a velocity scale based on the local pressure gradient. The same can be said for the so called Perry–Schofield scaling, which suggests that the two velocity scales are connected. This can also be interpreted as an experimental evidence of the claimed relation between the latter velocity scale and the maximum Reynolds shear-stress.     

On the interaction between two consecutive elongated bubbles in a vertical pipe

The motion of elongated air bubbles in a vertical pipe filled with water is studied quantitatively using video imaging of the flow and subsequent digital image processing of the recorded sequence of images. Experiments are carried out to determine the influence of the separation distance between two consecutive bubbles (liquid slug length) upon the behavior of the trailing bubble in vertical slug flow. The details of the trailing bubble acceleration and merging process are observed and the instantaneous parameters of the trailing bubble, such as its shape, velocity, acceleration, etc., are measured as a function of the separation distance. The leading bubble is found to be unaffected by the trailing elongated bubble.     

Three-component velocity field measurements of propeller wake using a stereoscopic PIV technique

A stereoscopic PIV (Particle Image Velocimetry) technique was used to measure the three-dimensional flow structure of the turbulent wake behind a marine propeller with five blades. The out-of-plane velocity component was determined using two CCD cameras with an angular displacement configuration. Four hundred instantaneous velocity fields were measured for each of four different blade phases, and ensemble averaged in order to find the spatial evolution of the propeller wake in the region from the trailing edge up to one propeller diameter (D) downstream. The influence of propeller loading conditions on the wake structure was also investigated by measuring the velocity fields at three advance ratios (J=0.59, 0.72 and 0.88). The phase-averaged velocity fields revealed that a viscous wake formed by the boundary layers developed along the blade surfaces. Tip vortices were generated periodically and the slipstream contracted in the near-wake region. The out-of-plane velocity component and strain rate had large values at the locations of the tip and trailing vortices. As the flow moved downstream, the turbulence intensity, the strength of the tip vortices, and the magnitude of the out-of-plane velocity component at trailing vortices all decreased due to effects such as viscous dissipation, turbulence diffusion, and blade-to-blade interaction.     

Measurement of velocity and velocity derivatives based on pattern tracking in 3D LIF images

Pattern tracking in consecutive 3D LIF images based on least squares matching (LSM) of grey levels has been developed recently for velocity and velocity gradient measurements. The shortcomings of this method are clearly shown. The present article presents an improvement on this method by introducing a local multi-patch (LMP) technique through the LSM approach. The method is validated using the flow field of a turbulent channel flow obtained by direct numerical simulation (DNS) and a synthetic image with grey-level patterns. The results show that LMP matching allows the determination of the velocity and the velocity gradient fields with high accuracy including the second derivatives. Measurements of a round non-buoyant jet are presented which demonstrate the good performance of the method when applied under laboratory conditions. This method can also be applied on two-dimensional images provided that the flow is strictly two-dimensional. Received: 7 July 1999/Accepted: 13 November 1999     

Double pulsed particle image velocimeter with directional resolution for complex flows

An automated particle image velocimeter has been constructed to determine fluid velocity fields by directly measuring particle image separations. A new image shifting technique eliminates directional ambiguity from velocity measurements. Theoretical and practical considerations in implementing the PIV system are presented. Using a simple, known flow field to investigate system performance, RMS errors of less than 0.7% of full scale are achieved, and at low seeding density fewer than 10% of velocity measurements produce bad data. Applications of system measurements to the instantaneous velocity field of an axisymmetric jet are presented.A version of this paper was presented at the 10th Symposium on Turbulence, University of Missouri-Rolla, September 22–24, 1986.     

Digital particle image accelerometry

A high-resolution video-based technique for obtaining two-dimensional fluid acceleration field data has been developed. The algorithm uses a combination of cross-correlations and autocorrelations on doubly exposed images of particle-seeded flows. Autocorrelations of individual video frames in an image pair yield two instantaneous velocity fields from which accelerations can be computed. Cross-correlations between successive images in the pair are used to resolve directional ambiguity associated with the autocorrelation. Time intervals are made arbitrarily small through the use of a laser sheet generator circuit which is synchronized with the framing rate of the camera. The technique is validated using a fluid-filled Petri dish subject to a known periodic motion. Ongoing development, uncertainties, and limitations of the technique are discussed. Received: 22 October 1998/Accepted: 27 September 2000     

Micro-PIV技术--粒子图像测速技术的新进展

Micro-PIV是近年来发展起来的一种微尺度流动测速技术.它是传统PIV测量与光学显微技术相结合的一种整场、瞬态、定量测量方法, 其基本测速原理与传统PIV相同, 但在示踪粒子选择、图像获取和处理等方面两者存在较大差别.Micro-PIV突破了传统微尺度流体力学测量手段的局限性, 使得对微尺度流动元件的研究从过去只能给出流量、阻力特性等有限信息逐步转向对全流场内流结构的直接测量上, 并且达到了相当高的分辨率和测量精度.本文对近几年Micro-PIV技术发展状况进行了总结和分析, 论述了Micro-PIV技术与传统PIV的主要区别以及具体的处理技术, 反映了其在科学与工程中的应用,并对此项技术的发展作了展望.