Quantitative Measurements of Turbulence with Particel Image Velocimetry

• Main Authors: Bo-FanShih, 施柏帆
• Other Authors: Keh-Ching Chang
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/41325635487066716114

碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 101 === This thesis investigates the velocity distribution of turbulent flow over cylinder and grid turbulence by hot-wire anemometry (HWA) and particle image velocimetry (PIV). Using the characteristics of the flow over cylinder to highlight the limitation of spatial resolution in PIV： the characteristic frequency would be underestimated when the spatial resolution is insufficient； therefore， when we compare the spectrum with that measured with HWA，it shows a decay of the frequency. In contrast，the grid turbulence doesn’t have such a magnificent low frequency effect， the range of background noise arose from the PIV can be thus observed. The error spectrum by the synthetic PIV without considering the spatial correlation of the speed is consisted of bias error and random error. The bias error results in different characteristics with respect to the flow field， on the other hand， the random noise would lead to white noise. The change of local number density of the particles arose from the wake flow will affect the cross correlation in the PIV result. This research points out a fact that the correlation coefficient is related to the flow field. As a result ,it is not true to extend acceptable data from some regions of flow field to a criterion justifying the whole domain. Instead, it is necessary to set the threshold according to the characteristics of each flow region to avoid over-detecting the flow field information. Increasing the incoming light intensity increases the resolution of gray level， which can improve the ability of disturbance detecting；hence increasing the exposure time properly can lower down the high frequency noise. However， overexposing results in image blur；so does the error by increasing the integral scale. From the spectrum view， it shows that the high frequency noise increases apparently with the increasing exposure time. Thus, the exposure time should not exceed 1 pixel for the image blur. Synthetic PIV shows the two characters of real particle image: gathering and local uniformity. Using different parameters such as particle size or particle number to discuss the error and the integral scale of the image can be regarded as the quantitative parameters for the image quality. And from the simulation results， we can know that the gathering particles have major contribution to determination of particle velocity. Although the media filter can reduce the effect of the gathering， it still can’t eliminate the image of gathering particle and it may result in the uncertainty in calculation of particle velocity.