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UBC Theses and Dissertations

Two causality correlation techniques applied to jet noise Rackl, Robert


The thesis describes two techniques for investigating experimentally the generation of noise by turbulent jets using a recently developed method of cross correlation (Siddon, 7th ICA, Budapest 1971), The work is motivated by the need to reduce further the exhaust noise of modern jet transport aircraft. The aim is to provide new information on noise generation mechanisms, to stimulate future theoretical research by finding the distribution and character of noise sources, and to provide diagnostic tools for making noise surveys of turbulent flows in general. Experiments were conducted in an anechoic chamber using a cold model air jet at a Mach number of about 0.32. The first method used, the image technique, cross correlates the pressure on a surface close to the jet with the radiated sound in the far field. This enables the deduction of the acoustic source strength per unit surface area which in turn gives an indication of the approximate location of the sound sources in the jet. The method is shown to be self consistent. The technique can be used in the investigation of supersonic jets as well; this is where its real power may lie. The second method directly cross correlates the hydrodynamic. pressure fluctuation in the turbulent jet flow with the far field sound allowing the deduction of the local acoustic source strength per unit volume and the associated power spectral density. A specially designed foil type sensor is used to measure the pressure fluctuations. A distribution of source strength over the jet for radiation at 45° to the jet axis is obtained, and is. not unexpectedly, similar to the distribution of mean velocity shear. The axial distribution from a "slice" of the jet results by radially integrating the source strengths; it is in qualitative agreement with the results of other researchers. However, difficulties are encountered in making the method self consistent, i.e., the radiated sound as predicted by integrating over all the sources in the jet differs widely from the directly measured sound. Reasons for this discrepancy are given and solutions to overcome it are proposed. A Fourier transform technique allows the deduction of the contribution to the spectrum of the radiated pressure from a unit volume of turbulence. Thus, the spectral character of the sound sources in the jet is obtained, although the above difficulties are reflected here as well. The- number of independent acoustic sources in the jet was estimated by calculating the average correlation volume. The number turns out to be of the order of 100 to 200; this lies between two widely different estimates of about 3 and about 2500 given by other researchers.

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