Abstract
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