LINEAR AND NONLINEAR EVOLUTION OF BOUNDARY-LAYER INSTABILITIES GENERATED BY ACOUSTIC-RECEPTIVITY MECHANISMS

Experimental results are presented concerning the evolution of instabi lities generated by the interaction between low-level broad-band acous tic waves and small two-dimensional roughness elements. Streamwise per turbation velocity spectra are measured and it is found that on the sm ooth plate, naturally-occurring Tollmien-Schlicting (T-S) waves grow i n a manner consistent with a resonant subharmonic wave interaction. Ho wever, in the presence of small two-dimensional roughness elements, a strong primary T-S mode is forced by an interaction with the backgroun d acoustic field. This leads to a K-type of nonlinear interaction char acterized by the generation of harmonics (up to six harmonics are obse rved) at the expense of the subharmonic. The scaling of the T-S waves and their harmonics with the number and amplitude of the wall roughnes s is also considered. (C) 1996 American Institute of Physics.