Properties of Conditional Eddies in Free Shear Flows
Tung, Andrew Train-Chao
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https://hdl.handle.net/2142/71671
Description
Title
Properties of Conditional Eddies in Free Shear Flows
Author(s)
Tung, Andrew Train-Chao
Issue Date
1982
Department of Study
Theoretical and Applied Mechanics
Discipline
Theoretical and Applied Mechanics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Fluid and Plasma
Abstract
Large structures in two types of free turbulent flows, namely, grid turbulence and two-stream, plane mixing flow, have been investigated through studies of conditional averages. The conditional averages of interest were expected values of the instantaneous velocity and the second-order velocity moment around a point where the velocity was specified.
Fourth-order estimation of the conditional velocity as a truncated Taylor's series expansion in terms of the given velocity was derived explicitly by using properties of isotropic turbulence. Typical grid turbulence data of higher-order velocity moments and space correlations were used to derive estimated flow patterns. It has been found that the linear estimation is as good, in studying the conditional eddies in isotropic turbulence, as higher-order estimation for most probably fluctuation intensities.
A well-defined two-stream, plane mixing layer was produced with a low-speed wind tunnel for the experimental study. The conditional averages were measured in the self-preserving region with two x-sensor, hot-wire anemometers. A conditional trigger module checked satisfaction of preset conditions by signals from one anemometer and triggered sampling of signals from the other anemometer accordingly. The resulting conditional eddies represented large structures depicted from unconditional observations in an averaged sense. Also, linear estimates were evaluated from measured and modeled space correlations, and their adequacy in approximating measured conditional velocity was confirmed.
The measured conditional second-order moments were compared with two linear models. One was in terms of conditional velocities and the other in terms of unconditional second-order moments. The second model was demonstrated to be a promising approximation, but the need for further investigation was recognized.
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