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Reorganisation of turbulence by large and spanwise-varying riblets

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Endrikat, Sebastian 
Newton, Ryan 
Modesti, Davide 
Garcia Mayoral, Ricardo 
Hutchins, Nick 


We study the flow above non-optimal riblets, specifically large drag-increasing and two-scale trapezoidal riblets. In order to reach large Reynolds numbers and large scale separation while retaining access to flow details, we employ a combination of boundary-layer hot-wire measurements and direct numerical simulation (DNS) in minimal-span channels. Although the outer Reynolds numbers differ, we observe fair agreement between experiments and DNS at matched viscous–friction-scaled riblet spacings s+ in the overlapping physical and spectral regions, providing confidence that both data sets are valid. We find that hot-wire velocity spectra above very large riblets with s+ ≳ 60 are depleted of near-wall energy at scales that are (much) greater than s. Large-scale energy likely bypasses the turbulence cascade and is transferred directly to secondary flows of size s, which we observe to grow in strength with increasing riblet size. Furthermore, the present very large riblets reduce the von Karman constant κ of the spanwise uniform mean velocity in a logarithmic layer and thus reduce the accuracy of the roughness-function concept, which we link to the near-wall damping of large flow structures. Half-height riblets in the groove, which we use as a model of imperfectly repeated (spanwise varying) riblets, impede in-groove turbulence. We show how to scale the drag optimum of imperfectly repeated riblets based on epresentative measurements of the true geometry by solving inexpensive Poisson equations.



drag reduction, turbulent boundary layers

Journal Title

Journal of Fluid Mechanics

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Cambridge University Press