Recent Development and Limitations of Laser Patterned Riblet-Structures

Abstract The extensive research over the last decades on riblet structures has proven their potential for vicious drag reduction in turbulent boundary layers. For an efficient application for technical use, however, riblet foils or riblets manufactured by grinding lack adaptability. In contrary laser processing technologies nowadays enable the production of riblet structures, whose dimensions can be adapted to the local flow conditions. As experiential laser structures differ process-related from ideal design specifications, however, a reliable prediction tool is necessary. Within this study, riblet structures were manufactured using a novel high-rate laser structuring technology. The viscous drag reduction is model based predicted solving the Navier-Stokes-Equations for a Coeutte flow over characteristic surface structures. Comparing own samples with samples manufactured by the national leading institutions in the fields, the achievable drag reduction shows an ongoing improvement with a currently achieved maximum wall shear stress reduction in the range of Δτ/τ0 ≈ –5,6%. This corresponds to about 70% of the viscous drag reduction achieved by geometric ideal structures. The authors will point out that further improvements are hardly feasible as manufacturable riblet dimensions, especially the riblet tip width, are limited by laser-physical and process-related restrictions.

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