An experimental study of receptivity of acoustic waves in laminar boundary layers

The receptivity of a laminar boundary layer developing on a flat plate was studied with two- and three-dimensional roughness elements. The layer was subjected to acoustic waves from speakers orientated perpendicular to the surface of the plate. Visualization of the transition patterns were obtained by heating temperature sensitive liquid crystals on the plate and observing the cooling patterns associated with the different flow regimes. Hot-wire data showed that the most amplified Tollmien-Schlichting waves dominated the downstream growth when the roughnesses were placed within the linearly unstable regime. The receptivity depended upon the size and aspect ratio of the three-dimensional roughness as predicted by Choudhari and Kerschen 1990. This research was partially funded by the Office of Naval Research under Contract N00014-89-J-1400. Their support is gratefully acknowledged. We also thank one of the reviewers for his helpful comments.