Thesis. 1977. Sc.D.--Massachusetts Institute of Technology. Dept. of Civil Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Vita. / Bibliography : leaves 209-217. / Sc.D.

An experimental investigation of axial and near - axial flow over long slender cylinders, which involved both flow visualisation and hot - wire anemometry, is detailed. The investigation of this type of flow was instigated by the current interest in towed underwater sonar arrays. The need to discriminate between background noise of mechanical origin and the flow - induced noise generated on a moving underwater soundrecording device has produced a requirement for a greater understanding of the larger scale, lower frequency, turbulent flow processes in the wake and the boundary layer of a cylinder in both axial and near - axial flow. Of particular interest are any regular periodic fluid - dynamic processes. Thick axisymmetric boundary layers with the ratio of outer - layer length scale ( the boundary - layer thickness δ ) to cylinder radius a in the range 31 [approximately equal to or less than] δ / a [approximately equal to or less than] 38 and the corresponding ratio of cylinder radius to the inner - layer length scale ( the viscous length v / U [subscript τ] ) in the range 22 [approximately equal to or less than] aU [subscript τ] / v = a [superscript +] [approximately equal to or less than] 41 have been investigated. In accord with previous experimental results their mean - flow and turbulence properties are found to be strongly influenced by transverse curvature and to diverge significantly from those of flat - plate boundary layers. A characteristic feature of such thick axisymmetric layers is the occurrence of " spots " of low - speed fluid which are attributed to displacement of inner - layer fluid by large - scale turbulent cross - flows. A front of low - speed fluid which propagates radially across the boundary layer is identified as the primary large - scale, low - frequency, coherent structure within the boundary layer turbulence. A flow mechanism that describes the process by which these fronts are formulated on the basis of the experimental evidence formed from low - speed spots is obtained. The stripping of low - speed fluid from the cylinder surface by large - scale crossflows within the turbulent boundary layer is seen as an additional vorticity - and turbulence - generating mechanism, which cannot occur in a flat - plate layer. When the cylinder is yawed to the free - stream, an attached boundary layer persists over a small range of yaw angle, before flow separation occurs. In this range the boundary layer becomes extremely asymmetric, even at yaw angles less than 1 °. The asymmetry and mean - flow properties of such layers have been investigated for yaw angles of 0.25 ° and 0.5 ° at several Reynolds numbers in the range 300 [approximately equal to or less than] Re [subscript a] [approximately equal to or less than] 600. At somewhat larger yaw angles, a new regime of regular vortex - shedding in near - axial flow has been identified. From the experimental results, an empirical relation for the vortex - shedding frequency ( in terms of yaw angle, vortex - shedding angle, and a Reynolds number based on the component of free - stream velocity normal to the vortex axes ) has been derived as an extension of the Roshko formula for the frequency of vortex shedding from cylinders with their axes normal to the flow. The results presented advance the current understanding of the fundamental fluid mechanics of cylinders in axial and near - axial flow, and thereby have the potential to contribute to the advancement of the signal - processing techniques applied to towed underwater sonar arrays. / Thesis (Ph.D.)--School of Mechanical Engineering, 2005.

<p>Boundary layer separation is usually an unwanted phenomenon in most technical applications as for instance on airplane wings, on ground vehicles and in internal flows such as diffusers. If separation occurs it leads to loss of lift, higher drag and results in energy losses. It is therefore important to be able to find methods to control and if possible avoid separation altogether without introducing a too heavy penalty such as increased drag, energy consuming suction etc.</p><p>In the present work we study one such control method, namely the use of vortex generators (VGs), which are known to be able to hinder turbulent boundary layer separation. We first study the downstream development of streamwise vortices behind pairs and arrays of vortex generators and how the strength of the vortices is coupled to the relative size of the vortex generators in comparison to the boundary layer size. Both the amplitude and the trajectory of the vortices are tracked in the downstream direction. Also the influences of yaw and free stream turbulence on the vortices are investigated. This part of the study is made with hot-wire anemometry where all three velocity components of the vortex structure are measured. The generation of circulation by the VGs scales excellently with the VG blade height and the velocity at the blade edge. The magnitude of circulation was found to be independent of yaw angle.</p><p>The second part of the study deals with the control effect of vortex generators on three different cases where the strength of the adverse pressure gradient (APG) in a turbulent boundary layer has been varied. In this case the measurements have been made with particle image velocimetry. It was found that the streamwise position where the VGs are placed is not critical for the control effect. For the three different APG cases approximately the same level of circulation was needed to inhibit separation. In contrast to some previous studies we find no evidence of a universal detachment shape factor<i> H</i>_12,that is independent of pressure gradient.</p>

Thesis (M.S.)--Mississippi State University. Department of Aerospace Engineering. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Also issued in print. Includes bibliographical references. Available via ProQuest Digital Dissertations.

Not available / text

published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

No description available.

Thesis (M. App. Sc.)--Carleton University, 2005. / Includes bibliographical references. Also available in electronic format on the Internet.

Thesis (M. Eng.), Memorial University of Newfoundland, 1998. / Bibliography: leaves 118-122.