Local flowfield about large distributed roughness elements in a laminar boundary layer
Winkler, John Francis
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https://hdl.handle.net/2142/21513
Description
Title
Local flowfield about large distributed roughness elements in a laminar boundary layer
Author(s)
Winkler, John Francis
Issue Date
1996
Doctoral Committee Chair(s)
Bragg, Michael B.
Department of Study
Aerospace Engineering
Discipline
Aerospace Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Aerospace
Engineering, Mechanical
Language
eng
Abstract
The local flowfield about large roughness elements placed in a flat plate laminar boundary layer was studied. Flowfield measurements were taken upstream, over, and downstream of an isolated hemispherical roughness element and distributed hemispherical roughness elements. The height of the roughness elements was large compared to the local undisturbed boundary-layer thickness. The roughness flowfields were scaled such that the roughness Reynolds number, $Re\sb{k}$, and the ratio of the roughness height to boundary-layer thickness, $k$/$\delta$, were typical of those found in initial glaze-ice accretions. For the isolated roughness case, $k$/$\delta$ = 2.5, and at the leading edge of the distributed roughness, $k$/$\delta$ = 3.4. The nominal value of $Re\sb{k}$ for both cases was 3700. The large physical size of the elements improved the spatial resolution of the measurements and helped facilitate the flow visualization. Velocity measurements were obtained with a 2-D laser Doppler velocimeter (LDV). Two-dimensional velocities, time-averaged three-dimensional velocities, turbulence intensities, and vorticity were all calculated from the measured velocities. Fluorescent-oil flow and laser-sheet/smoke-wire flow visualization data were obtained in order to help qualitatively explain the flowfield.
Results showed that the horseshoe vortex system in front of the isolated roughness element contained three primary vortices and demonstrated the characteristics of an unsteady amalgamation phenomena. Distinct spiral-type vortices and a hairpin-type vortex structure formed behind the isolated element. In comparison, the horseshoe vortex system that formed in front of each element on the front row of the distributed roughness contained only two primary vortices, but it also appeared to be unsteady. Although a vortical structure was observed immediately behind the last row of the distributed roughness, it was weaker than, and did not appear to have the same structure as, the hairpin-type vortex behind the isolated roughness element. The distributed roughness wake was thicker and more persistent than the isolated roughness wake. This was attributed partially to the stronger vortex systems behind the isolated roughness element enhancing the mixing of the wake with the outer inviscid flow, thereby re-energizing and thinning the isolated roughness wake.
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