Boundary layer receptivity to freestream disturbances

Abstract

Transition of fluid flow from a laminar to turbulent state has been studied extensively for over a century, beginning with experiments carried out by Osborne Reynolds in 1883. Theoretical, experimental and numerical techniques have advanced and ever more accurate measurements of the transition process have taken place; however, there still remains much to understand regarding the origins of boundary-layer instabilities - in other words, the ‘receptivity’ of the boundary layer to initial perturbations. The present work considers experimentally the receptivity process for the Blasius boundary layer formed over a flat plate at zero incidence. Two types of interaction are studied, both forced by a two-dimensional harmonic disturbance within the freestream. The first considers the receptivity of the boundary layer to the excitation of Tollmien-Schlichting waves at localised roughness while the second considers the excitation of streaky structures within the boundary layer at high-amplitudes of the freestream forcing. Two-dimensional Tollmien-Schlichting waves are shown to be excited only in the presence of a roughness element placed on the plate surface. Results of previous experiments regarding Tollmien-Schlichting wave excitation at localised roughness are corroborated and theoretical works are further validated. Although the forcing does not resemble the ‘optimal perturbations’ for exciting streaks within boundary layers, it is found that streaks are excited within the boundary layer for forcing amplitudes in excess of 0.5% and their growth and characteristics are found and compared with other experimental and theoretical results. It is shown that TS waves, once excited, cause transition at much smaller amplitudes but that they are noticeably more difficult to excite than streaks within the boundary layer.