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Quantitative methods for the measurement and monitoring of mixing flows using a computer vision system

其他書名	thesis
出版	M. Novak, 2001
URL	http://books.google.com.hk/books?id=tJSlQwAACAAJ&hl=&source=gbs_api

註釋In numerous production processes, such as mineral wool and paper production process, there exists a need for the direct quantitative visualization of turbulent flows. Assessment of quantitative results obtained using such methods requires, however, comparison with other state-of-the-art measurement methods. In the present work the performance of a computer-aided visualization and digital image-processing method was investigated by application to a turbulent mixing flow in the wake of a prismatic bluff body in a non-return subsonic wind tunnel and compared with more conventional techniques. A high-speed camera was used to capture smoke visualization images in the wake of a bluff body. Qualitative analysis as well as quantitative analysis of digitised sequences at different Reynolds numbers was performed on time series, obtained by simultaneous digitisation of the grey level in several small areas (windows) of the overall image. The flow visualization and analysis technique employed has the advantage that it enables simultaneous observation and analysis of separate regions of the flow field, and the size, location, and number of these regions can be readily selected. These advantages were demonstrated in practise. The study formed part of a co-operative EU programme (Noviscam) designed to compare such the visualization systems with conventional measures, and to test different chaotic measures obtained from digital image-processing signals. To verify the computer-aided visualization method against conventional methods of the flow field analysis, two types of analysis were performed: comparative non-simultaneous analysis using results of FFT analysis and turbulence profile intensity analysis, and cross-correlation analysis of simultaneous measurements using both a hot-film anemometer probe and the digital image-processing technique. In the second part of the study, concerned with chaotic measures, correlation dimension of the chaotic attractor, the degree of average local deformation of the trajectories in phase portraits of the light intensity time series, box-count, and Hölder exponent in various windows were investigated. The results confirmed that power spectra from both computer-aided visualization, and hot-film anemometry methods identified the expected vortex structures in the bluff body wake. Small scale characteristics were also detected and the average local deformation of the trajectories in the phase portraits was found to increase in connection with increasing correlation dimension. This is linked with increased flow complexity near the axis zone behind the bluff body. The -5/3 power law in power spectra was well defined insome regions. The turbulence intensity, calculated either from digital image analysis concentration fluctuations or velocity fluctuation analysis, reaches a maximum at a downstream position in the wake and asymptotically falls to a lower value further downstream. The results of cross-correlation analysis agreed well for both methods. Several other processes were examined quantitatively using the visualization system and methods of analysis developed in this work.