Reorganisation of turbulence by large and spanwise-varying riblets

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 largescale 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 riblet swith s(+)(sic)60 are depleted of near-wall energy at scales that are (much) greater thans. Large-scale energy likely bypasses the turbulence cascade and is transferred directly to secondary flows of sizes, which we observe to grow in strength with increasing riblet size. Furthermore, the present very large riblets reduce the von K & aacute;rm & aacute;n constant kappa of the span wise 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 (span wise-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.