Editorial

Abstract

Particle image velocimetry (PIV) is a well-established and powerful technique for performing whole-field velocity measurements under a broad range of flow conditions, and is widely used in fluid mechanics laboratories.The aim of this feature issue of Measurement Science and Technology is to provide the reader with an overview of some recent progress in PIV applications developed in the EUROMECH 411 colloquium, which was held at CORIA, Rouen University, France from 29 to 31 May 2000.The topics of the meeting organized by M Trinité and B Lecordier were focused on three main aspects: Advanced PIV techniques (2D2C, 2D3C or 3D) used to study turbulent flows. Accuracy and limitations of PIV treatments for measurement of turbulence properties. Relevant post-processing tools for PIV data to extract information on turbulent flows.In total, 24 papers were presented on these topics and four keynote lectures were given by recognized specialists. Seven papers have been selected by the EUROMECH Scientific Committee for publication in this feature issue.If 3C, 3D (4D with time) is the ultimate goal of PIV developments, then holography is the only practical way to extend sheet-oriented PIV to measuring a volume of considerable depth in 3D. However, one major problem is the pollution from particles located at different depths: image quality is spoiled by noise from out-of-focus particles. Digital holography (Coëtmellec et al) using a 2D wavelet algorithm is a very simple promising new method in this area despite poor resolution.Stereoscopic PIV, using two cameras viewing the flow field from different angles, is one technique for measuring all three components in a fluid plane. To reconstruct the three-dimensional velocity field, a calibration stage is necessary since the imaging geometry causes significant aberrations. A 3D vector is calculated at a physical point in the object plane. The first step consists in computing two 2D vectors from both cameras at the physical point. Two methods are discussed (warping and mapping) in the paper by Coudert and Schon.Cross-correlation based PIV algorithms have a limited spatial resolution, which is determined by the size of the measurement window. The study and development of an integrated PIV-PTV method is described by Stitou and Riethmuller. This combines the robustness of Particle Image Velocimetry with the spatial resolution of Particle Tracking Velocimetry for super resolution analysis.Different experimental studies using known flow fields such as grid turbulence or pipe flows have been used to try to show the capability of the PIV technique to extract the information from turbulent vector maps. Nevertheless, from these studies, it always seems difficult to evaluate which treatment is the most accurate to measure a given flow. This is mainly due to the difficulty in knowing the exact characteristics of the flows. A solution can be to numerically impose the properties of the flow and to generate synthetic images (Lecordier et al). This approach is now possible by using Direct Numerical Simulation (DNS).If PIV has now evolved from its embryonic state to reach a certain level of confidence, we are now facing a problem directlylinked to the high spatial resolution and frequency ofacquisition that can nowadays be achieved by manufactured CCDcameras: the overabundance of information, which has to beappropriately reduced if one desires to draw conclusions fromthe data set. Two recent tools can now be used to resolve theproblem of unsteady turbulent flows and to try to separate thefluctuation intensity due to the unsteadiness and the turbulenceitself: Proper Orthogonal Decomposition (POD) and wavelettransform analysis. Promising results are obtained using POD inrotating flows (Graftieaux et al, Patte-Rouland et al). With regard to spatial correlation and patternrecognition this tool is also promising in evaluating thecontribution of coherent structures to the generation andself-sustaining of wall turbulence in boundary layers. Wavelettransform analysis can be applied to analyse temporal evolutionof coherent structures and to characterize the leading vortexgenerated in a starting flow (Schram and Riethmuller).We believe that this special feature provides a valuable overview of the proceedings at the colloquium and we would like to express our thanks to all the participants and to the contributing authors and reviewers who have made this special feature possible.