Reorganisation of turbulence by large and spanwise-varying riblets

Journal of Fluid Mechanics - Tập 952 - 2022
Sebastian Endrikat1, R. Newton1, Davide Modesti1,2, Ricardo García-Mayoral3, Nicholas Hutchins1, Daniel Chung1
1Department of Mechanical Engineering, University of Melbourne, Victoria 3010, Australia
2Faculty of Aerospace Engineering, Delft University of Technology, 2629 HS Delft, Netherlands
3Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK

Tóm tắt

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 $s^+ \gtrsim 60$ 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 Kármán constant $\kappa$ 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 representative measurements of the true geometry by solving inexpensive Poisson equations.

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Journal of Fluid Mechanics

Tập 952

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