EXPERIMENTAL STUDY ON QUANTITATIVE MEASUREMENT OF THREE-DIMENSIONAL STRUCTURE OF HAIRPIN VORTEX BY STEREO-PIV 1)

Hairpin vortex is one of the most concerned paradigm in the study of turbulent coherent structures. The quantitative experimental measurement and hydrodynamic analysis of the three-dimensional hairpin vortex structures~is of great significance for further~applications in the turbulent flow control.~In this study, with an optimized arrangement of the synthetic jet device, a~series of~regular artificial hairpin vortex structures~are~produced in the laminar boundary layer, and then the three-dimensional shapes of~hairpin vortex structures are quantitatively revealed by stereoscopic particle image velocimetry (Stereo-PIV) using phase-locking technique. The three-dimensional flow field of the hairpin vortex within a complete period is obtained. The quality of the reconstructed three-dimensional hairpin vortex structure is reliable. The results are in line with the general knowledge of hairpin vortex, low/high~speed streaks and~ejection/sweeping~events. In addition, we acquire a more detailed understanding of the near-wall secondary vortex, the spanwise~vortex head and the area of strong shear in the spanwise~vorticity~concentration region, and the convergent/divergent flow related to the ejection~event. We also discuss the reconstruction of the three-dimensional flow field of hairpin vortice based on the fluctuating characteristics of two-dimensional flow field. It provides an idea for the quantitative study of wall turbulence coherent structure in the formation and evolution of the hairpin vortex structure, the fusion of adjacent vortex structures and the secondary induced vortex.