Measurement of fine scale structure in turbulence by time-resolved dual-plane stereoscopic PIV

To investigate fine scale structure of turbulent flows, time-resolved dual-plane stereoscopic particle image velocimetry (TRDPSPIV) has been developed using high-repetition-rate Nd:YAG lasers for industrial processing and high-speed CMOS cameras. This system provides all three velocity components and nine velocity gradients with high spatial and temporal resolution. The number of lasers used in this system is only two by adopting newly-developed parallel beam forming optics. The developed system was applied to velocity measurements of a turbulent jet with the same spatial resolution of general direct numerical simulation (DNS) of turbulence. The velocities obtained in both planes showed very similar pattern, and energy spectra in the both planes coincide very well because of the small distance between the planes. It is shown that probability density functions of the measured nine velocity gradients agree well with those obtained from DNS. From these velocity gradients, various flow properties such as vorticity vectors, second invariant of velocity gradient tensor and energy dissipation rate are obtained exactly. These properties allow an eddy identification, which has been used in the analysis of DNS, to investigate fine scale structure of turbulence. Velocity distributions around the experimentally-detected fine scale eddies have an elliptic feature, and the histograms of the diameter and maximum azimuthal velocity of the detected eddy show peaks at D/η≈10 and uθ,max/uk≈0.75. These characteristics of the fine scale eddy coincide with those obtained from DNS. Experimental evidence for the existence of the coherent fine scale eddy in turbulence is given in the present study.