From drag-reducing riblets to drag-increasing ridges
نویسندگان
چکیده
Small drag-reducing riblets and larger drag-increasing ridges are longitudinally invariant laterally periodic surface structures that differ only in the details of their lateral periodicity size viscous units. Due to different drag behaviour, typically have been analysed separately. By studying experimentally trapezoidal-grooved surfaces sizes, we address systematically transition from riblet-like ridge-like behaviour a unified framework. The structure height wavelength varied both physically, by considering eight surfaces, viscous-scaled form, spanning wide range bulk Reynolds number $Re_b$ . effective skin-friction coefficient $C_f$ is determined via pressure-drop measurement turbulent channel flow facility designed for accurate measurements. An unexpectedly rich unveiled, which regimes distinguished depending on value $l_g^+$ , square root groove area. well-known regime spans up $l_g^+=17$ followed roughness function ${\rm \Delta} U^+$ increases logarithmically with indicating an apparent fully rough $l_g^+\approx 40$ Further increase leads clear departure regime, unexpected non-monotonic $50< l_g^+<200$ reported first time. For sufficiently large $l_g$ it shown single parameter, similar classical hydraulic diameter, sufficient describe ridges. We find appropriate definition crucial interpreting behaviour. When longitudinal protrusion structured accounted definition, laminar exhibits same $C_f(Re_b)$ relation known flat surfaces. This approach thus allows us discern modification induced turbulence. provide predictive correlations high become possible thanks chosen definition.
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ژورنال
عنوان ژورنال: Journal of Fluid Mechanics
سال: 2022
ISSN: ['0022-1120', '1469-7645']
DOI: https://doi.org/10.1017/jfm.2022.796