Bypass transition in a boundary layer past a localized strip of distributed roughness

In this work, we study two cases of bypass transition induced by surface roughness – one in which transition proceeds through the appearance of turbulent spots (‘spotty’ transition) and the other in which no distinct turbulent spots are seen (‘non-spotty’ transition). Velocity measurements are carried out in a boundary layer developing on a flat plate downstream of a localized strip of distributed roughness. The spotty transition is obtained at relatively low speeds and at sufficiently downstream distances from the roughness, whereas the non-spotty transition is observed at higher speeds and much closer the roughness. We use the Wavelet transform to contrast the frequency-time behavior of the two transition scenarios. Furthermore, we compare broad features of the velocity profiles representative of the two cases at similar Reynolds numbers. While the mean velocity and turbulence intensity profiles show similar variations, distinct differences are found in the shape of the energy spectrum – for the non-spotty transition, there is a single broad spectral hump, whereas for the spotty transition two humps are observed over a range of wall-normal distances. The implications of the present findings will be discussed during the talk.