Table of contents

The interest of today's research in fluid mechanics is increasingly directed to problems where unsteady and separated flows are predominant. For investigations of flow fields with pronounced spatial structures and/or rapid temporal or spatial changes, new experimental techniques, such as particle image velocimetry (PIV), are required in order to capture the instantaneous flow velocity field. From its basic physical principles, PIV is not really a new flow measurement technique. The great interest in PIV today is due to the fact that it has undergone rapid development during the last decade, receiving both a pull from the fluid mechanics community desiring spatially resolved velocity information about instantaneous flow fields, and a push from the technological advances of almost all of the hardware components involved, from lasers and cameras to computers. In its `classical form', i.e. the measurement of two components of the velocity vector in a plane of the flow, PIV has been developed to such a state that it can even be applied in the rough environment of industrial test facilities. Furthermore, the flow information obtained using PIV serves as a reliable basis for direct comparison with numerical calculations and, hence, for validation of computer codes. Nevertheless, there are still many aspects of PIV which are undergoing further development, for instance the measurement of all three components of the velocity vector, and thus further improvements can be expected in the near future.

This special issue is therefore an update on the state-of-the-art, intended to focus attention on new possibilities and outstanding problems with the PIV technique. First of all, the fundamentals of PIV are discussed in contributions dealing with the tracer particles required for seeding the flow, the optical principles for illumination of the flow, the recording of the light scattered off the tiny tracer particles, and the methods for the evaluation and post-processing of PIV recordings. Both the statistical and the tracking methods of data analysis are considered. Special attention is paid to an up- to-date description of the possibilities offered by modern video and computer equipment. A second group of papers deals with those aspects of PIV which are presently the subject of intensive research: measurement of three components of the velocity vector either in a plane (stereoscopic PIV) or in a volume (holographic PIV, scanning PIV); measurement of acceleration of the flow by means of multiple camera arrangements, as well as modern methods for analysis of the recordings by neural networks. The contributions also demonstrate the application of PIV in different areas of fluid mechanics, showing the advantages of this technique as well as some of the problems involved with its application.

No review of the historical development of PIV is given in this special issue. Those who are interested are referred to the Milestone Series Volume on particle image velocimetry [1]. With the increasing number of developers and users of PIV, the number of different approaches to recording and evaluation has also increased dramatically. The experience gained by the different research groups is widely spread and sometimes difficult to locate. Some initiatives have been taken to support the exchange of information about particle image velocimetry and to define some standards for recording and evaluation. Among those known to the editors of this special issue are the following.

· Center for Quantitative Visualization (CQV), jointly operated by the California Institute of Technology (CALTECH) and DLR (information available at http://www.cqv.caltech.edu)

· University of Tokyo, distribution of standard PIV images (http://sap.gen.u-tokyo.ac.jp/)

· EUROPIV project, supported by the European community. The database built up within this project will be distributed in a follow-up project i.e. a thematic network in PIV (PivNet) (http://www.es.go.dlr.de:8081:PivNet)

Measurement Science and Technology introduced special thematic issues in 1995. This issue on Particle Image Velocimetry is the sixth in the series, two of which also involved flow measurement techniques [2]. The normality of publishing a special issue should however not diminish the recognition of efforts made by the authors, the reviewers and IOP Publishing staff in assembling a large number of papers within a tight schedule. Therefore a word of thanks from the editors of this special issue is due and we hope that our readers will benefit from and appreciate these efforts.

J Kompenhans, Göttingen C Tropea, Darmstadt Guest Editors October 1997

References

[1] Grant I (ed) 1994 Selected Papers on Particle Image Velocimetry (Milestone Series, volume MS99) (Bellingham, WA: SPIE Int. Soc. Opt. Eng)

[2] Special sections on: Measurement in Fluids and Combustion Systems Meas. Sci. Technol.6 No 6 June 1995 Thermal Anemometry: Developments and Applications Meas. Sci. Technol.7 No 10 October 1996

The measurement principle of digital particle image velocimetry (PIV) is described in terms of linear system theory. The conditions for PIV correlation analysis as a valid interrogation method are determined. Limitations of the method arise as consequences of the implementation. The theory is applied to investigate the statistical properties of the analysis and to optimize and improve the measurement performance. The theoretical results comply with results from Monte Carlo simulations and test measurements described in the literature. Examples of both correct and incorrect implementations are given.

To observe instantaneous fields of vorticity and rate-of-strain using PIV (particle image velocimetry) measurements of the velocity field, it is necessary to achieve maximum dynamic ranges in space and in velocity while simultaneously making accurate, low-noise measurements for the differentiation process. In this paper the dynamic ranges achievable with PIV are established in the context of the super-resolution algorithm described previously which seeks to extract the maximum possible information from a particle image field. Bounds on the dynamic spatial range and the dynamic velocity range are established in terms of optical parameters, and simple rules are derived for optimum design of PIV optical systems for various types of image recording media.

The neural network method uses ideas developed from the physiological modelling of the human brain in computational mechanics. The technique provides mechanisms analogous to biological processes such as learning from experience, generalizing from learning data to a wider set of stimuli and extraction of key attributes from excessively noisy data sets. It has found frequent application in optimization, image enhancement and pattern recognition, key problems in particle image velocimetry (PIV). The development of the method and its principal categories and features are described, with special emphasis on its application to PIV and particle tracking velocimetry (PTV). The application of the neural network method to important categories of the PIV image analysis procedure is described in the present paper. These are image enhancement, fringe analysis, PTV and stereo view reconciliation. The applications of common generic net types, feed-forwards and recurrent, are discussed and illustrated by example. The key strength of the neural technique, its ability to respond to changing circumstances by self-modification or regulation of its processing parameters, is illustrated by example and compared with conventional processing strategies adopted in PIV.

The size specifications for suitable tracer particles for particle image velocimetry (PIV), particularly with respect to their flow tracking capability, are discussed and quantified for several examples. A review of a wide variety of tracer materials used in recent PIV experiments in liquids and gases indicates that appropriately sized particles have normally been used. With emphasis on gas flows, methods of generating seeding particles and for introducing the particles into the flow are described and their advantages are discussed.

The aim of the paper is to present some practical aspects of PIV recording set-ups which are not often discussed in the literature. After a short presentation of the different possible light sources available, emphasis is put on the Nd:YAG laser which has contributed to the strong evolution of PIV. Then the elements needed to realize a light sheet from a laser source are presented and discussed. Finally, the basic optics knowledge needed to record good particle images is recalled with emphasis on points of importance for PIV.

The goal of the present study is to quantify and reduce, when possible, errors in two-dimensional digital particle image velocimetry (DPIV). Two major errors, namely the mean bias and root-mean-square (RMS) errors, have been studied. One fundamental source of these errors arises from the implementation of cross correlation (CC). Other major sources of these errors arise from the peak-finding scheme, which locates the correlation peak with a sub-pixel accuracy, and noise within the particle images. Two processing techniques are used to extract the particle displacements. First, a CC method utilizing the FFT algorithm for fast processing is implemented. Second, a particle image pattern matching (PIPM) technique, usually requiring a direct computation and therefore more time consuming, is used. Using DPIV on simulated images, both the mean-bias and RMS errors have been found to be of the order of 0.1 pixels for CC. The errors of PIPM are about an order of magnitude less than those of CC. In the present paper the authors introduce a peak-normalization method which reduces the error level of CC to that of PIPM without adding much computational effort. A peak-compensation technique is also introduced to make the mean-bias error negligible in comparison with the RMS error. Noise in an image suppresses the mean-bias error but, on the other hand, significantly amplifies the RMS error. A digital video signal usually has a lower noise level than that of an analogue one and therefore provides a smaller error in DPIV.

Optical distortion due to inaccurate optical alignment, lens nonlinearity, and/or refraction by optical windows, fluid interfaces, and other optical elements of an experiment causes inaccuracy by introducing variable magnification. Since fractional changes in the magnification have a one-to-one effect on the accuracy of measuring the velocity, it is important to compensate for such distortions. A general experimental calibration procedure is described which determines the magnification matrix of a distorted imaging system, and an algorithm is presented to compute accurate velocity field displacements from measurements of distorted PIV images. These procedures form a basis for generalized stereoscopic PIV procedures which permit easy electronic registration of multiple cameras and accurate recombination of stereoscopic displacement fields to obtain the three-dimensional velocity vector field.

Experimental error analysis of a digital angular stereoscopic PIV system is presented. The paper firstly describes an experimental rig which includes the design of a novel PIV test block for in situ calibration. This allowed the user to set up a static seeded flow volume which was translated in and out of plane to record PIV images using two megapixel CCD cameras positioned for angular stereoscopic viewing. PIV data were collected for a range of camera angles up to and for a range of flow displacements and processed by cross correlation into a set of two-dimensional calibration and flow displacement vectors. These 2D data were then processed into three-dimensional data by the use of geometric and bicubic spline interpolation algorithms and an error analysis performed on the predicted displacements. Results from this analysis have shown optimum system performance will be obtained by using camera angles of between 20 and and f numbers of f16 and higher. The results have also shown a theoretical prediction of system performance derived in previous work, which considers the ratio of out of plane to in plane errors, matches to within 8 and 18% of the experimental system performance.

A particle image velocimetry system capable of accurately recovering the out-of-plane velocity component has been realized based on a stereoscopic viewing arrangement. To allow a large viewing angle with long focal length objective lenses, the angular displacement or Scheimpflug imaging configuration is employed in which the image, object and lens planes intersect in a common line. The varying magnification factor associated with this imaging configuration is accounted for using an accurate and simple-to-use calibration procedure based on solving the projection equations for each of the two cameras. A pair of high-resolution cameras, both capable of recording image pairs in the microsecond range, are synchronized to a pulsed Nd-YAG laser. By placing the cameras on either side of the light sheet the favourable light scattering characteristics of micron-sized seeding particles in forward scatter provide images at significantly higher illumination than at normal or backscatter viewing angles. Ultimately designed for use in industrial wind tunnels, the camera system is capable of working with non-symmetric arrangements. It has been successfully tested in a laboratory environment by imaging the unsteady flow field of a vortex ring passing through a laser light sheet.

Adaptive processing software capable of dynamically adjusting the sample location of the interrogation windows to the local displacement vector significantly improves data yield. The algorithm requires only the selection of the final window/overlap size. The hierarchical interrogation approach permits the processing of images whose displacement dynamic range exceeds the interrogation window size.

3D scanning particle image velocimetry (SPIV) provides particle images in a large set of parallel light-sheet planes using a rapid scanning light-sheet which samples the flow in depth from which the three-dimensional flow field can be reconstructed in the scanned volume. Together with a digital high-speed video technique and a high-speed rotating drum scanner for 3D beam scanning, first 3D SPIV measurements were carried out in a motored model engine with a transparent cylinder and piston crown. Attention was focused on the generation of small-scale structures during compression strokes. Using a spatial cross-correlation of particle images in the overlapping sheets, the 3D flow field could be recovered from the sheet-wise tomographic recorded flow volume. This demonstrates the potential of 3D SPIV in engine flows since 3D information can be obtained from a single view and therefore the method can be applied in actual research engines already in use without any additional modification. The first results of the feasibility study have shown complex flow at the very end of compression which is characterized here by meandering monopolar and dipolar vortices within the main swirling flow which interact with the centre. It is suggested that the observed vortex splitting and shearing is an important contribution to the creation of turbulence.

Due to the particular features that appear in the vector maps delivered by the PIV method, there are postprocessing steps that can substantially enhance its performance. These steps include: detection of false vectors, correction of these vectors and the calculation of derived flow magnitudes. Many derived magnitudes can be of interest but this work focuses on the calculation of the first spatial derivative, component of flow divergence or vorticity, on a two-dimensional flow configuration. New algorithms, developed for each step, are described, with the aim of jointly applying them in a coherent way. Where applicable, an analytical tool for filter analysis and design is explained. Application to synthetic and real PIV data is presented as well as a performance contrast with other conventional algorithms, in terms of accuracy, frequency response and error propagation, among others.

We describe an experimental investigation of various methods of using the non-intrusive particle image velocimetry (PIV) technology to obtain the acceleration field of fluid flows and particularly the force being exchanged between a fluid flow and its boundaries. Methods based on the Lagrangian and the Eulerian specifications respectively have been developed, applied to experiments and compared. The experiments were performed by using a four-CCD-camera system designed to acquire between two and four frames in a fast time sequence, to provide single- or multiple-exposure PIV images of general fluid flows. The experiments involved various types of surface waves impinging upon a vertical wall at the end of a flume. Simultaneous monitoring with various transducers provided the experimental evaluation of the force predictions, whereas numerical simulations and PIV measurements have been used to validate the techniques for determining accelerations.

This paper outlines the method by which the flow velocity and acoustic particle velocity can be measured instantaneously over an area using particle-image velocimetry techniques. Theoretical expressions for the power spectrum and autocorrelation function of the intensity distribution in the plane of the interrogation area when a flow and sound field is present are derived and it is shown that the flow velocity and the acoustic particle velocity can be measured. The technique is used to measure the velocities in a standing wave tube with a monotonic sound field of sufficient strength such that acoustic streaming is set up. The limitations of this technique are also discussed.

Combining LDA and PIV techniques to investigate unsteady swirling flows permits a more detailed interpretation of velocity fluctuations than that from LDA alone. This can be necessary when comparing measurements with numerical simulations which attempt to capture the unsteadiness. One possible method of combining the measurement data is introduced and discussed with application to swirling flowfields which exhibit a precessing vortex core. The analysis indicates that in such flows the unsteadiness accounts for up to 70% of the total energy in the velocity fluctuations.

This paper describes a method for the time reconstruction of continuous velocity and vorticity fields from velocity samples acquired in a sequence by means of PIV. It is also shown that the method can be applied to educe turbulent coherent structures in quasi-two-dimensional flows.

Three-dimensional particle tracking velocimetry (3D PTV) is a flow measurement technique for the determination of velocity vectors and trajectories within a three-dimensional observation volume. This makes this technique suitable not only for Eulerian but also for Lagrangian investigation of flow phenomena, especially in the field of turbulence and turbulent diffusion.

The principle and the application on open channel flow of 3D PTV are briefly described in the first part of this paper. By decomposition of the velocity vectors velocity profiles and turbulence intensities are derived and presented in the second part. Turbulent energy dissipation, Kolmogorov length and time scales and Taylor micro scales are computed from the measured data sets. In the Lagrangian analysis pdf's of the Lagrangian acceleration, correlation coefficients and Taylor micro scales are determined. The individual determination of the particle positions and displacements allows finally the application on the one and two particle approaches for turbulent diffusion from Taylor (1921), Richardson (1926) and Batchelor (1949, 1952).

The most common velocity measurement techniques, based on image analysis, calculate the velocity by cross-correlation of a portion of the digitized images, and give a Eulerian description of the investigated field. Particle tracking velocimetry (PTV), based on the recognition of trajectories of seeding particles, only furnishes a Eulerian description provided the trajectories are shorter than the characteristic scale of the phenomenon. If particles are tracked for a longer time, a Lagrangian description is obtained. Consequently, in order to successfully evaluate Lagrangian statistics, a long series of single-exposed images has to be acquired. PTV has been used to examine the pollutant dispersion in a laboratory simulation of the convective boundary layer of the atmosphere. The convective layer has been simulated by a water tank heated from below, where the atmospheric thermal stratification has been reproduced. Though the phenomenon was observed to be steady in the Eulerian reference frame, the same did not occur in the Lagrangian reference frame. From the analysis of particle motion, it is possible to determine the characteristic time scale of the turbulence and to describe the different behaviour of hot updraughts and cold downdraughts. The pollutant dispersion is described in detail by the transilient matrix representing the probability of transition of a particle from one level to another of the convective layer. From the information given by this matrix, it is possible, in principle, to estimate the concentration fields, due to a variety of concentrated and distributed pollutant sources.

The contribution of holographic techniques to particle velocity measurements is discussed. Two-dimensional holographic particle image velocimetry and three-dimensional holographic velocimetry are described and compared. Their advantages and limits are discussed and the latest progress in this domain is presented, together with perspectives for the near future.

Holographic particle image velocimetry instruments at both velocity extremes have been developed and applied. Except for the type of laser required, the fundamental recording and data processing methods are similar for both cases. This paper summarizes recording and reconstruction schemes, software for data reduction, and methods for data presentation. An example application focuses on the extreme low-velocity case in which a three-size matrix of tracer particles was added to a solution in a crystal-growth-in-space experiment to quantify and analyse microconvection in a microgravity environment. Over 1000 holograms of 1000 particles were produced in a Spacelab mission, leading to the need to process millions of images. Particle velocities ranged from 0.1 to . Reconstructed particle images were located with a computer traversed microscope imaged on a CCD array, and tracked with software designed to establish a reference frame in each hologram and precisely locate the particle centres. The measurements allow determination of the convective field, the residual gravitational field and gravity-jitter.

1998 February 25 - 26 5th International Symposium on Gas Analysis by Tunable Diode Lasers, Freiburg, Germany Details: VDI/VDE-GMA, PO Box 10 11 39, D-40002 Dusseldorf, Germany. Fax: +49 211 6214 215, WWW: http://www.vdi.de/pr/text/597/16697.htm March 31 - 2 April Second Scientific Instrument Association Conference and Exhibition, London, UK Details: SiA, PO Box 192, Watford, Herts WD1 8GF. WWW: http://www/interlab.co.uk/interlab 31 - 2 April Fifth International Conference on Adhesion and Surface Analysis, Loughborough, UK Details: D M Brewis, Institute of Surface Science and Technology, Loughborough University, Loughborough LE11 3TU, UK. Fax: +44 (0)1509 223986, e-mail: d.m.brewis@lboro.ac.uk April 6 - 8 Modeling and Simulation of Microsystems, Semiconductors, Sensors and Actuators, Santa Clara, CA, USA Details: MSM98 Conference Secretariat, 2394 Alsace Court, Livermore, CA 94550, USA. e-mail:msm@dmtsun.epfl.ch, WWW: http://www.iamcm.org/MSM98 (USA), WWW: http://dmtwww.epfl.ch/MSM98 (EUROPE) 16 - 17 Optical Methods and Data Processing in Heat and Fluid Flow, London, UK Details: Mrs U Otuonye, Conference and Events Department, Institution of Mechanical Engineers, 1 Birdcage Walk, London SW1H 9JJ. Telephone: +44 (0)171 973 1304, fax: +44 (0)171 222 9881, e-mail: u_otuonye@imeche.org.uk May 4 - 5 Metrology for Electronic Packaging and Interconnection, NIST, Gaithersburg, MD, USA Details: Michael Schen, NIST, Building 224, Room B320, Gaithersburg, MD 20899-0001, USA. Telephone: +1 301 975 6741, fax: +1 301 975 3239, e-mail: michael.schen@nist.gov June 14 - 17 8th European Symposium on Semiconductor Detectors, Schloss Elmau, Germany Details: http://www.hll.mpe-garching.mpg.de/sds98, e-mail: pxh@mpe-garching.mpg.de 14 - 19 9th CIMTEC Forum on New Materials: Solid State Chemical and Biochemical Sensors, Florence, Italy Details: 9th CIMTEC Forum on New Materials, PO Box 165, 48018 Faenza, Italy. Telephone: +39 546 22461, fax: +39 546 664138 16 - 19 3rd International Conference on Vibration Measurements by Laser Techiques and Short Cource on Laser Vibrometry, Ancona, Italy Details: Professor E P Tomasini, Dipartimento di Meccanica, Università di Ancona, Via Brecce Bianche, I-60131 Ancona, Italy. Telephone: +39 (0)71 2204489, fax: +39 (0)71 2204813, e-mail: aivela@mehpl.cineca.it, WWW: http://www.unian.it/html/congress/aivela/aivela.htm 24 - 26 Fifth International Conference on Structures Under Shock and Impact, Southampton, UK Details: Liz Kerr, Wessex Institute of Technology, Ashurst Lodge, Ashurst, Southampton SO40 7AA. Telephone: +44 (0)1703 293 223, fax: +44 (0)1703 292 853, e-mail: wit@wessex.ac.uk, WWW: http://www.wessex.ac.uk July 7 - 9 International Microscopy Conference and Exhibition, London, UK Details: Royal Microscopical Society, 37/38 St Clements, Oxford OX4 1AJ, UK. Telephone: +44 (0)1865 248768, Fax: +44 (0)1865 791237, e-mail: meetings@rms.org.uk (for conference), info@rms.org.uk (for exhibition), WWW: http://www.rms.org.uk 13 - 15 2nd European Conference on Magnetic Sensors and Actuators, Sheffield, UK Details: Conferences, The Institute of Physics, 76 Portland Place, London W1N 3DH. Telephone: +44 (0)171 470 4800, fax: +44 (0)171 470 4900, WWW: http://www.shef.ac.uk/~phys/emsa98.html http://www.iop.org/Confs/EMSA 13 - 16 9th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal Details: M V Heitor, Instituta Superior Técnico, Mechanical Engineering Department, 1096 Lisboa Codex, Portugal. Fax: +351 1 849 61 56 August 24 - 28 11th International Conference on Experimental Mechanics, Oxford, UK Details: BSSM, Surrey Technology Centre, Surrey Research Park, 40 Occam Road, Guildford GU2 5YG, UK. Telephone: +44 (0)1483 295 715, fax: +44 (0)1483 573 704 September 29 - 1 October 4th Optical Fibre Measurement Conference, Teddington, UK Details: Mrs Jane Tsakalou, Conference Office, National Physical Laboratory, Teddington TW11 0LW, UK. Telephone: +44 (0)181 943 5400, WWW: http://www.npl.co.uk/npl/news/ofmc97.html October 11 - 16 October 6th International Conference on Nuclear Microprobe Technology and Applications, Cape Town, South Africa Details: Mrs G Stone, National Accelerator Centre, PO Box 72, Faure 7131, South Africa. Telephone: +27 21 843 3820, Fax: +27 21 843 3543, e-mail: icnmta98@nac.ac.za, WWW: http://www.nac.ac.za/icnmta98 December 8 - 11 2nd Asian Meeting on Ferroelectrics, International (AMF-2), Singapore Details: Weiguang Zhu, fax: +65 792 0415, e-mail: ewzhu@ntuvax.ntu.ac.sg 1999 May 17 - 21 3rd CCM Conference on Pressure Metrology from Ultra High Vacuum to Very High Pressures (10-7 Pa to 109 Pa), Torino, Italy Details: M Bergoglio, Istituto di Metrologia `G Colonnetti', Strada delle Cacce 73, 10135 Torino, Italy. E-mail: bergoglio@imc.to.cnr.it