Experimental Study of Low-Order Models of Highly-Irregular Roughness and Their Impact on Turbulent Boundary Layers
Mejia Alvarez, Ricardo
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https://hdl.handle.net/2142/87761
Description
Title
Experimental Study of Low-Order Models of Highly-Irregular Roughness and Their Impact on Turbulent Boundary Layers
Author(s)
Mejia Alvarez, Ricardo
Issue Date
2010
Doctoral Committee Chair(s)
Christensen, Kenneth T.
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)
Engineering, Mechanical
Language
eng
Abstract
The stereo-PIV measurements deep within the roughness sublayer at y = 0.047delta reveal a wealth of information about roughness-induced effects, including the tendency of the roughness to promote 'channeling' of the flow in the form of low- and high-momentum pathways as noted in contour maps of the mean velocity defect. Similarly, enhanced turbulent and vortical activity is observed both between and along the spanwise boundaries of these streamwise-elongated large-scale pathways. Taken together, these observations support the idea that these persistent low-momentum pathways might represent the statistical imprint of trains of hairpin vortex packets that are channeled along preferred paths over the roughness. Conditional averaging and two-point correlations of velocity further support these structural observations, particularly clear large-scale streamwise coherence of these motions. Of interest, while the M = 5 results show important differences from the full-surface results, the M = 16 results are virtually indistinguishable fromthose of the full surface, including in the single-point turbulence statistics as well as the analysis of the average spatial structure. This consistency is not simply qualitative but is indeed quantitative as the magnitudes of the M = 16 model single-point statistics mirror those of the full surface as do the spatial locations of the low- and high-momentum pathways identified in the mean velocity defect results as well as the enhanced turbulent and vortical activity along the spanwise boundaries of these large-scale motions. Hence, these observations provide significant evidence supporting the importance of the intermediate topographical scales in setting the flow conditions within the roughness sublayer, not only in a statistical sense but also in a structural sense.
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