Directional riblets as an airfoil passive flow control mechanism

Abstract

The effects of converging–diverging riblets (C–D riblets) on the surface of a flat-bottomed airfoil bump on the wind tunnel wall is investigated experimentally. Here long strips of C–D riblets with viscous height of $h^{+}$ $\approx$ 20–23) are applied and cover the surface of approximately 60% in chord percentage of the surface of a flat-bottomed airfoil bump resulting in counter rotating vortices under an adverse pressure gradient (APG) environment. The use of C–D riblets significantly affects the streamwise mean velocity profile and the thickness of the boundary layer $\delta$. Increased drag is observed above the APG converging regions, while drag decreases above the APG diverging regions, with distinct vertical shifts in the mean velocity profile. Compared the to zero pressure gradient (ZPG) riblet cases in the literature, these shifts are pushed further downwards for both APG riblets configurations. Premultiplied energy spectra also show notable differences from ZPG cases in the literatures. Here the results suggest that the adverse pressure gradient environment amplify the outer peak magnitude for both riblet configurations, indicating a higher occurrence of large-scale structure interactions (‘superstructure’) in the APG compared to the ZPG environments. Finally, scale decomposition analysis confirms that large-scales contribute to the outer peak of turbulence intensity across all surfaces, while small scales primarily influence the inner peak. Interestingly, for the APG converging riblet case, small scales also significantly contribute to the outer peak. These findings underscore the complex interplay of pressure gradient and riblet geometry in modulating turbulent boundary layer characteristics.