Abstract
A surface grooved with microscopic riblets aligned parallel to the flow is an effective means to reduce the turbulent skin friction up to 10% compared to a smooth surface. The maximum drag reduction is found for a dimensionless rib spacing s + in the range of 15–17. For s + < 10, a linear behaviour of the drag reduction curve is predicted by viscous theory. This linear slope of the drag reduction curve is in contradiction to Schlichting’s postulation of a hydraulically smooth behaviour of small-scale roughness in a turbulent flow. This regime of evanescent dimensionless rib spacings is investigated experimentally by direct wall shear stress measurements in a fully developed channel flow. Additionally, a numerical calculation of the viscous flow over riblets was carried out to predict the drag reducing behaviour. The experimental results show a linear drag reducing behaviour down to s + = 0.3, which is in good agreement with the numerical results of the viscous simulation. The postulation of Schlichting’s hydraulically smooth regime of a rough surface was not confirmed, neither for a riblet surface nor for a surface geometry with grooves oriented perpendicular to the flow. In the latter case, the drag increases as a quadratic function of the roughness height.
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Acknowledgments
This work was partly supported by Deutsche Forschungsgemeinschaft within the priority program SPP 1207 “Bio-inspired fluid mechanics” and partly by the Volkswagen Foundation within the funding program “Innovative methods for the manufacturing of multifunctional surfaces” and is gratefully acknowledged. We would like to thank Professor P. S. Virk for the encouragement to consider riblet behaviour outside their useful design parameters. This conversation was of invaluable assistance.
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Grüneberger, R., Hage, W. Drag characteristics of longitudinal and transverse riblets at low dimensionless spacings. Exp Fluids 50, 363–373 (2011). https://doi.org/10.1007/s00348-010-0936-7
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DOI: https://doi.org/10.1007/s00348-010-0936-7