Effects of perforation on turbulent boundary layers over riblets

Drag-increasing riblets modify near-wall turbulence by forming crest-centred shear layers, which influence turbulent structures across multiple spatial scales. Previous studies indicate that in certain drag-increasing riblet geometries, these shear layers trigger KH rollers when sufficient wall-normal permeability is present. The presence of KH rollers over drag-increasing riblets is reported to break down the Reynolds analogy favourably. However, increasing permeability by enlarging the groove weakens the inflectional shear layer at crest and disrupts conditions required to sustain coherent KH rollers, as reflected in weakened KH-related spectral signatures. This study examines a novel configuration in which perforations are introduced within the riblet grooves and connected to a backing cavity to increase permeability without directly modifying riblet geometry and investigates the effect of perforation on the turbulent boundary layer over riblets. Smooth, riblet, and perforated riblet surfaces were investigated experimentally in a wind tunnel at various free-stream velocities. Hot-wire anemometry was employed to analyse mean streamwise velocity profiles, turbulence intensity, and premultiplied energy spectra. The results show that groove-localised perforation alters near-wall turbulence relative to the riblet surface. Perforated riblets exhibit an upward shift in the mean velocity profile and elevated turbulence intensity close to riblet crest and within the logarithmic region, indicating enhanced wall-normal exchange. However, the amplitude of riblet-induced KH roller spectral signatures is reduced across all investigated velocities, indicating this configuration does not promote coherent structures associated with favourable Reynolds-analogy breakdown under the present geometric and Reynolds-number constraints and the adopted methodology of varying viscous-scaled parameters by changing free-stream velocity.