Patterned Surface Roughness for Passive Transition Delay

Surface roughness is demonstrably detrimental to boundary-layer stability in many scenarios; it is now known that sensibly chosen roughness can also delay the onset of transition, resulting in a drag reduction. The latest part of an ongoing research effort\footnote{Shahinfar et al. \emph{Phys. Rev. Lett.} 109, 074501 (2012).} exploring the use of streamwise streaks to attenuate growth of forced disturbances, the present experiments employ a spatially periodic surface pattern to modify the flow in a flat plate boundary layer. With respect to conventional cylindrical surface roughness, the critical roughness-height-based Reynolds number of the surface pattern is improved. Tollmien--Schlichting waves are excited via suction and blowing at the wall, to form a well-controlled disturbance. A parametric study reveals that patterned roughness inhibits the growth of these \mbox{T--S} waves and increases the transition Reynolds number by 70\% compared with the smooth plate reference case. Systematic changes to the pattern spacing demonstrate that the roughness can also accelerate the onset of transition.