INVESTIGATION OF BURST STRUCTURES IN TURBULENT CHANNEL FLOWS THROUGH SIMULTANEOUS FLOW VISUALIZATION AND VELOCITY MEASUREMENTS
Abstract
Experimental research into the fundamental nature of turbulent wall flows has in recent years centered on the various coherent structures present in these flows. In the near-wall region the coherent structure which is of major interest is the "burst" which is thought to be the predominant producer of Reynolds stress. A better understanding of the basic physical mechanisms of the burst is believed to be the key to improved modeling of turbulent wall flows. Experimental investigations of bursts have been conducted using flow visualization and/or probe measurements. Although flow visualization gives a good qualitative description of the burst, detailed characteristics are not obtained. Probe measurements have been hampered due to the difficulty of distinguishing bursts with single point measurements. Numerous burst detection algorithms have been postulated, but these have yielded contradictory results. In the present study of bursts, experiments were performed in a two-dimensional water channel using flow visualization and hot-film velocity measurements. Simultaneous dye-slot and hydrogen bubble flow visualization was used to identify the region of the flow over which dye effectively marks all bursts occurring. The various probe burst detection algorithms were used to analyze recorded velocity signals measured during simultaneous dye-slot flow visualization and hot-film measurements and the correlation of these probe detections with the individual dye marked ejections within the burst was evaluated. The optimum operational parameters for each of the detection techniques have been determined so that the maximum correlation with the flow visualization is obtained. Only one of the probe burst detection techniques was found to have a very good correspondence with the flow visualization on a one to one basis. Conditional sampling of velocity signals was done based on the observation of a dye marked ejection intersecting the probe. This analysis yielded detailed characteristics of the individual ejections within a burst.
Degree
Ph.D.
Subject Area
Mechanical engineering
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