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https://hdl.handle.net/2142/70884
Description
Title
Turbulent Flow Over Rough Surfaces
Author(s)
Nourmohammadi, Khosrow
Issue Date
1982
Department of Study
Nuclear Engineering
Discipline
Nuclear Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Nuclear
Abstract
The objective of the present study was to experimentally obtain a better understanding of the turbulent flow structure in repeated-rib geometry rough-walled surfaces in two size pipe systems as a function of the ratio of roughness height to spacing distance between the elements (P/K), the axial variation of the flow, and the flow rates. The variations in P/K geometries caused the flow structure to change from quasi-smooth flow regime (in low P/K values) to hyperturbulent flow regime (in medium P/K) to isolated-roughness-element flow regime at high P/K geometries. The repeated-rib geometries examined included P/K values of 2, 5, 7, 10, 13, 16, 19, 22, and 25 in the small pipe system and 2, 13, and 24 in the large pipe system.
The friction factors were small in value in the quasi-smooth flow regime where the flow behaves like a smooth pipe with small wall roughness. The maximum friction factor occurred for the P/K of 13 geometry (in the hyperturbulent flow regime) in the small pipe system and in the large pipe system. The friction factor then decreased as P/K was further increased in the isolated-roughness-element flow regime.
The center of pipe mean axial velocity varied sinusoidally in the axial direction within a cycle of geometry. This oscillation was observed for P/K (GREATERTHEQ) 10 in the small pipe system and P/K (GREATERTHEQ) 13 in the large pipe system.
The mean axial velocity profiles were examined in several ways. First these profiles were described using the concept of error in origin and the roughness function. Secondly, the concept of error in origin and the roughness height as the similarity parameter was used. Thirdly, using the power law, a simple correlation was fit to the mean axial velocity profiles. In the inner layer, a confined jet formulation described the mean flow data.
The turbulent intensities were normalized with their respective center of pipe values, and the Reynolds shear stress was normalized with the shear velocity. In the outer layer, the turbulence parameters for the rough wall results were essentially the same as the smooth wall. Generally, as the wall was approached the turbulence parameters increased in magnitude, and, as the flow moves downstream, the effect of the step reduces and the parameters increase less sharply. Again, the confined jet nondimensionalization of the turbulence parameters was best suited for the present data. The turbulent eddy diffusivity results are presented for all the geometries and axial locations. Their general characteristics follow the same pattern as the turbulence results.
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