By-pass transition of flat plate boundary layers on the surfaces near the limit of admissible roughness

Results of the experimental investigation on the development of boundary layers on flat plates with the smooth surface and with the surfaces covered by sandpapers 60-grit, 80-grit and 100-grit under external turbulent flows of various grid turbulence scales are presented. The displacement thickness Reynolds number was at the most 2000 during experiments. The investigated boundary layers belong to the class of layers close to the lower limit of admissible roughness region, k⁺ = 4.6, 5.7 and 8.7 respectively. It was certified that both the wall roughness and the free stream turbulence accelerate individually the boundary layer development from the laminar state of boundary layer to turbulence. Next it was ascertained that their joint effect amplifies the development of boundary layers so, that the surface roughness impact is predominating but the actions of intensity and length scale of the free stream turbulence disturbances are also significant. With the increasing roughness number the initial region with a pseudo-laminar flow structure and the transitional region become shorter.