Abstract
ABSTRACTKnowledge of hydraulic resistance of single-valued self-affine fractal surfaces remains very limited. To advance this area, a set of experiments have been conducted in two separate open-channel flumes to investigate the effects of the spectral structure of bed roughness on the drag at the bed. Three self-affine fractal roughness patterns, based on a simple but realistic three-range spectral model, have been investigated with spectral scaling exponents of −1, −5/3 and −3, respectively. The different widths of the flumes and a range of flow depths also afforded an opportunity to consider effects of the flow aspect ratio and relative submergence. The results show that with all else equal the friction factor increases as the spectral exponent decreases. In addition, the relationship between the spectral exponent and effective slope of the roughness is demonstrated, for the first time. Aspect ratio effects on the friction factor within the studied range were found to be negligible.
Highlights
Most natural and industrial flows encounter and are influenced by the effects of bed surface roughness
A set of experiments were carried out in two separate openchannel facilities to investigate the effects of bed roughness structure, flow submergence, and channel aspect ratio on hydraulic resistance
Longitudinal scans of the bed roughness installed in the Aberdeen Open-Channel Facility (AOCF) flume verified the validity of the design and manufacturing process
Summary
Most natural and industrial flows encounter and are influenced by the effects of bed surface roughness. A fundamental part of the problem exists around properly quantifying the surface roughness. The popularity of the discrete approach lies in its simplicity and stems from the early work of Nikuradse (1933), who extensively studied roughness effects of densely-packed uniform sand in pipes. He found that the single parameter (sand diameter) can serve as a sufficient descriptor of such surface roughness. For more complex rough surfaces, that need multiple parameters to be properly described, it was proposed that
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