EXPERIMENTS ON LOCALIZED DISTURBANCES IN A FLAT-PLATE BOUNDARY-LAYER - PART 1 - THE RECEPTIVITY AND EVOLUTION OF A LOCALIZED FREE-STREAM DISTURBANCE

The receptivity of a laminar boundary layer to free stream disturbance s has been experimentally investigated through the introduction of det erministic localized disturbances upstream of a hat plate mounted in a wind tunnel. Hot-wire measurements indicate that the spanwise gradien t of the normal velocity component (and hence the streamwise vorticity ) plays an essential role in the transfer of disturbance energy into t he boundary layer. Inside the laminar boundary layer the disturbances were found to give rise to the formation of longitudinal structures of alternating high and low streamwise velocity. Similar streaky structu res exist in laminar boundary layers exposed to free stream turbulence , in which the disturbance amplitude increases in linear proportion to the displacement thickness. In the present study the perturbation amp litude of the streaks was always decaying for the initial amplitudes u sed, in contrast to the growing fluctuations that are observed in the presence of free stream turbulence. This points out the importance of the continuous influence from the free stream turbulence along the bou ndary layer edge. (C) Elsevier, Paris.