Date of Award

1998

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Sumanta Acharya

Abstract

Turbulent characteristics of a reattaching shear layer is important in gas turbine blade cooling, dump combustors, airfoil separation, internally ribbed heat exchangers, and in electronic cooling. In this study flow past a surface mounted rib encountered in the internal coolant channels of gas turbine blades and internally ribbed heat exchangers is studied. The main objectives of this investigation are: (1) to develop improved methods of coherent structures analysis, (2) to examine the spectral characteristics and length scale, (3) to investigate the types of turbulent motions using octant analysis, (4) to study the flow structures and kinetic energy budget for both excited and unexcited flows and (5) to investigate single and two frequency forcing on the mixing of the reattaching shear layer. This work proposes a pattern recognition based Fourier analysis method for computing the coherent structure magnitude and phase decorrelation. Using both simulated and reattaching shear layer data the new techniques are observed to provide better estimates of phase decorrelation and magnitude of coherent structures. A dominant frequency in the flow behind the rib is observed, in contrast to the findings of previous experimental investigations. The correct length scale for calculating the Strouhal number is the momentum thickness of the shear layer at the downstream face of the rib. The quadrant analysis technique modified here for reversed flow situation (termed as octant analysis) shows the importance of reversed outward interaction motion. The large scale structures are present well downstream of reattachment and the interaction motions are predominant there. The outer part of the shear layer is observed to be non-Gaussian indicated by high values of skewness and flatness, presumably due to the intermittency, originating from the turbulent and non-turbulent interface of the shear layer. The dominance of pressure transport term, the presence of negative production at the outer edge of the shear layer and the presence of reverse cascade mechanism are some of the important findings of this study. From the parametric evaluation of single frequency and two frequency forcing the fundamental mode forcing is observed to be most conducive to mixing enhancement.

ISBN

9780591766745

Pages

304

DOI

10.31390/gradschool_disstheses.6637

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