In this paper, a comprehensive study has been performed to address the excitation of a separated boundary layer near the leading-edge due to the surface roughness. Experiments are performed on a model aerofoil with the semi-circular leading-edge at a Reynolds number of 1.6E05, where the free-stream turbulence (fts) being 1.2%. The characteristics roughness in the wall unit is 15, which is estimated from the velocity profile at a location far downstream of reattachment. The wall roughness results in an early transition and reattachment, leading to a reduction of the laminar shear layer length apart from the bubble length. It is worthwhile to note that although the large-amplitude pre-transitional perturbations are apparent from the beginning for the rough surface, the shear layer reflects the amplification of selected frequencies, where the fundamental frequency is almost the same as that of the smooth wall. The universal intermittency curve can be used to describe the transition of the shear layer, which exhibits some resemblance to the excitation of the boundary layer under fst and significance of the viscous effect.

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