Aerofoil wake-induced transition characteristics on a flat-plate boundary layer

This paper presents an experimental investigation of the characteristics of laminar– turbulent transition occurring on a flat-plate boundary layer due to the interaction with a non-impinging aerofoil wake. Previous studies have tended to focus on transition induced by free-stream turbulence or by the wake of a circular cylinder, both of which exhibit different forcing characteristics to the present experimental arrangement. A tripped NACA 0014 aerofoil was used to generate a fully turbulent wake, upstream of and at various heights above a laminar, flat-plate boundary layer, in the UK National Low-turbulence Wind Tunnel at City, University of London. Hot-wire measurements conducted in the pre-transitional region reveal the wall-normal and spanwise structure of the disturbances induced within the boundary layer and the rate of growth of disturbance energy. Disturbance profiles generally (but not uniquely) follow the non-modal distribution obtained from transient growth theory, but energy growth rates are mainly exponential rather than algebraic. Energy spectra demonstrate the existence of mixed transitional features (both natural and bypass) in the boundary layer. Two-point spatial correlations reveal the presence of a streaky structure, but with spanwise scale much larger than the boundary layer thickness, in contrast to the trends seen in free-stream turbulence-induced bypass transition and cylinder wake-induced transition. The gap between aerofoil and flat plate affects both the evolution of non-modal disturbance profile and the appearance of the streaky structure; the spacing of the streaks was also found to scale with the vertical gap between aerofoil and flat plate. Overall, the combination of observed characteristics is quite different from the forced transition mechanisms previously reported in the literature.