Improrement of PIV Measurement Accuracy and Identification of Structures for Turbulent Flows

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A turbulent natural-convection boundary layer induced along a vertical heated surface made of a 1 m wide and 4 m high copper plate was measured with a particle image velocimetry (PIV). The boundary layer flow was seeded with smoke and illuminated by light sheets discharged from a double pulse Nd : YAG laser system. Time-series video images captured by a CCD camera for several minutes were directly recorded with a mass-storage device purchased by Grant-in AID for Scientific Research, and instantaneous velocity vectors and turbulence statistics were calculated with a cross-correlation method and a sub-pixel algorism. In consequence, it is founded that turbulent statistics with the PIV were approximately consistent with those obtained with a hot-and cold-wires due to the acquirement of adequate time-series data, which had not been realized without a mass-storage device. However, incorrect velocity vectors were observed and frequency appearance of the incorrect vectors increased in the int … More ermittent outer region of the boundary layer, and thus the advanced improvement of measurement accuracy was required.In addition to the actual visualization measurement, database of visualized images were established with a direct numerical simulation (DNS) for a time-developing turbulent boundary layer on a flat plat to confirm the accuracy of PIV measurement. Benchmark images were created by looking upon the concentration distribution derived from the simulation as the luminance distribution of seeded particles, and were processed to calculate velocity vectors. Then, the calculated vectors ware compared with those obtained directly from the DNS and the uncertainty of PIV measurement was accessed for the effects of laser-sheet width, pulse injection interval and image processing algorism. As the result of such comparisons, it is ascertained for the three-dimensional turbulent behavior in the natural-convection boundary later that incorrect velocity vectors certainly apt to appear intermittent region, the laser-sheet width has no effect on the calculated velocity vectors, the optimum pulse injection interval was 3-4 seconds and the size of interrogation windows should be relatively enlarged to calculate accurate velocity vectors with a cross-correlation method. These results will be submitted to the international journal concerning flow measurement and visualization in the near term. Less