Date of Award

Spring 1999

Project Type

Dissertation

Program or Major

Engineering

Degree Name

Doctor of Philosophy

First Advisor

David W Watt

Abstract

Fluid flows with turbulent characteristics are evident in many important engineering and scientific applications. For years researchers have investigated fluid flow turbulence but have not attained a complete physical understanding and quantitative description of the turbulent fluid motion. One approach has been to identify and classify the organized, or coherent, structures imbedded in the turbulence. Research in this area has been restricted by the lack of complete three-dimensional descriptions of the flow. In many studies, the velocity field is measured in either one or two dimensions using mechanical probes which can disrupt the downstream evolution of the flow and offer limited spatial resolution.

Tomographic reconstruction has been demonstrated as a method for obtaining three-dimensional scalar fields from a set of 2-D projections. In this study, a multiple-beam shearing interferometer was developed to obtain projection data from six viewing directions distributed over an angular range of 135°. The interferometric phase obtained was related to the two-dimensional projection increments. The Fourier transform method of carrier fringe removal was used to separate the desired incremental phase from the unwanted background irradiance variations. A tomographic reconstruction technique using a truncated Fourier-Bessel expansion was applied to obtain the three-dimensional turbulent field. A spatial "snapshot" Proper Orthogonal Decomposition (POD) was performed to examine the three-dimensional structure of three turbulent flows; the transition region of a neutrally buoyant, helium-argon jet, the mixing layer region of an underexpanded compressed air jet, and a helium jet.

The experimental results indicate the existence of three-dimensional structures imbedded in the turbulence. The eigenfunctions in the helium-argon jet indicate the presence of axisymmetric rings in the near field. The compressed air eigenfunctions indicate the presence of a helical structure in the mixing layer region of the compressed air jet. Several of the eigenfunctions in the helium jet contained a double helical structure.

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