LAMINAR BOUNDARY-LAYER OVER RIBLETS

Laser doppler anemometry (LDA) measurements and numerical calculations have been made for a laminar boundary layer on triangular riblets. Ca lculated mean velocity distributions along the riblet contour are in g ood agreement with the measured ones. The results show that no transve rsal motion exists above and within the riblet valleys (e.g., no secon dary motion). It is found that despite the large wetted area increase, the frictional drag is not increased on riblets relative to a smooth wall. This result suggests that the viscous effects are at play in the drag reduction for a turbulent boundary layer, in the sense that they compensate for the increase in wetted area.