DEVELOPMENT OF A TWO-DIMENSIONAL TURBULENT JET UNDER NATURAL AND EXCITED CONDITIONS
Abstract
An experimental study was performed to document the development of both the natural and acoustically excited two-dimensional turbulent jet. The acoustic excitation consisted of a symmetric pressure perturbation imposed upon the flow. Two representative excitation conditions (in addition to the natural jet) were considered in detail. One at St(,D) = 0.35 (based on exit velocity and nozzle width) was found to lead to greatly enhanced widening and longitudinal fluctuation intensities. The other, at St(,D) = 0.66 exhibited widening comparable to the natural jet and suppression of fluctuation intensities near the jet centerline. Particular emphasis was placed upon studying the development of the large-scale structure of the natural and excited flows. Structural patterns characteristic of the development were derived from longitudinal fluctuation spectra, lateral correlation (phase angle) and correlation based two-dimensionality measurements. Results of the experimental program indicate that the large-scale structural patterns in the excited flows are typically quite different (in terms of phase angle, characteristic wavelength and two-dimensionality) than occur naturally. The greatest differences occur in the early jet development, where excitation induced organization effects on the coherent structure are greatest (and where other researches have typically generalized results based on excitation to corresponding natural flows). With sufficient downstream distance, measurements suggest that all cases achieve a similar underlying structure. Based on preliminary measurements, this appears to take the form of an extended structure possessing high lateral inclination and aligned approximately with the direction of mean shear. The increased widening and fluctuation intensities associated with the St(,D) = 0.35 case are shown to originate due to the ability of an excitation induced organized coherent structure to efficiently extract energy from the mean flow and pass it to the production of turbulence. On the other hand, excitation at St(,D) = 0.66 leads to competition among multiple modes that prevents the formation of a single dominant structure. The resulting structures are inefficient in laterally transferring fluctuation energy from the regions of shear, resulting in centerline turbulence suppression.
Degree
Ph.D.
Subject Area
Mechanical engineering
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