Boundary-layer receptivity to transient convected disturbances

Previous work has shown that the boundary layer is receptive to acoustic and convected disturbances in regions where there is a rapid streamwise variation in the mean flow such as at the leading edge or at roughness elements or suction strips. Much of this work involved single-frequency, two-dimensional waves, and further studies are required to determine the effects of the broadband three-dimensional nature of turbulent fluctuations. An experimental investigation of the receptivity of a Blasius boundary layer to transient two-dimensional convected disturbances is presented here. A ribbon located upstream of a Rat plate and aligned parallel to the plate leading edge was used to introduce disturbances into the freestream. Two-dimensional roughness elements distributed on the surface of the plate provided a receptivity site. Measurements of the boundary-layer response to single-frequency waves, pulses, and random noise disturbances are reported. Similar receptivity was measured for each of these disturbance types, showing that the broadband nature of transient disturbances does not affect the receptivity characteristics of the boundary layer and that linear superposition may be used to extend theories for single-frequency disturbances to applications involving broadband disturbances.