Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Professor Gregory Alan Kopp

Abstract

Investigations of separated and reattaching flows over three-dimensional bluff bodies in turbulent boundary layers are important because of the large aerodynamic loads that these flows cause. For example, roofs of low-rise buildings are vulnerable to this kind of wind loading. Turbulence in the upstream flow affects the pressure distributions and the mean size of separation bubbles formed on bluff body surfaces. Whereas a number of studies have focussed on two-dimensional separation bubbles and surface pressures, a comprehensive understanding of the surface pressures and the separating-reattaching flows in relation to the turbulence in the incident boundary layers for surface-mounted, three-dimensional bluff bodies have not been developed. In this study, the effects of turbulence intensities and length scales in the approaching boundary layer flows on surface-mounted three-dimensional bluff bodies are investigated. Particle Image Velocimetry measurements of the roof separation bubble, along with surface pressure measurements, were taken for six different upstream conditions. The results were analyzed to understand the responses of the separating-reattaching flows, along with the mean and fluctuating pressure fields, to the turbulence properties in the approach flow.

The mean reattachment length is found to be unaffected by the turbulence length scales (over the range examined), whereas turbulence intensity affects reattachment lengths significantly. The normalized mean pressure distribution was found to be a function of both the mean reattachment length and the upstream turbulence intensity. A method of estimating the mean reattachment lengths of the roof separation bubble from measured surface pressures and roof height turbulence intensity is proposed. Separating-reattaching flows exhibit self-similarity of the mean flow field, whereas the fluctuating flow fields do not exhibit similarity. The distributions of surface pressure fluctuations respond to both turbulence intensity and length scales in the upstream flow. Surface pressures near the leading edge are observed to be highly correlated with the velocity field just outside the separated shear layer near the leading edge and with the area-averaged swirling strength under the whole separation bubble. For surface-mounted three-dimensional bluff bodies, these findings provide some valuable insights on the fundamental features of the separated-reattaching flows and surface pressures.


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