A series of experiments have been conducted to investigate turbulent flow structures when it is exposed to a highly directional riblet-type surfaces roughness (converging-diverging/herringbone pattern) at a relatively low Reynolds number (Reτ). These experiments show that even at a low Reτ, the surface pattern is able to modify the turbulent boundary layer. Over the diverging region, we observe a decrease in drag penalty, while over the converging region there is an increase of drag penalty, which is indicated by the shift in the mean velocity profiles. The surface roughness also influences the turbulence production, indicated by the elevated turbulence intensities profiles for both the converging and diverging regions. The result seems to deviate from early investigations that show an increase in turbulence intensities above the converging region and a lowered turbulence intensities above the diverging region. The discrepancy may be caused by the lower Reτ Ret in the current report. Other important statistics such as skewness and flatness are also reported.

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