Two-dimensional roughness and Tollmien-Schlichting waves: a study on transition

Abstract: Two-dimensional (2D) roughnesses induce premature transition to turbulence in laminar boundary layers by amplifying the disturbances in the flow and promoting the development of non-linearities. The present thesis aims at furthering our understanding of this phenomenon's underlying mechanisms and elucidating the effects of the different geometrical parameters of the roughness' cross section. Semi-circular and semi-elliptical 2D roughnesses were placed in turns on a flat plate in a low-subsonic, low-disturbance laminar flow and transition was detected by means of cumputing the hot-wire velocity signal's intermittency. It is shown that increasing the roughness element's aspect ratio delays the onset of transition. On the other hand, enlargning the roughness' size does not necessarily result in the promotion of transition: the data suggests that the most destabilizing size depends on the free-steam velocity and the disturbance environment. Single-frequency Tollmien-Schilchting (TS) waves were then forced into the boundary layer by means of the periodic suction and blowing produced by a loudspeaker's membrane. The critical amplitude of the TS waves impining of the roughness at sub-critical free-steam velocities was determined and a series of velocity profiles were measured and then phase-averaged. Different growth rates are observed for different roughtnesses and the amplitude at which TS wave breakdown occurs increases with increasing aspect ratio and diminishes with growing roughness height.