11. Tollmien-Schlichting Waves Artificially Inserted in Boundary Layer by Harmonic Point Source

The present work experimentally investigates the spatial evolution of controlled artificial disturbances inserted by a harmonic point source in a Blasius boundary layer over a flat plate. A small hole of 0.80 mm diameter is responsible to introduce a blowing and suction flow inside the boundary layer induced by an embedded loudspeaker. The model employed consists of a plexiglass plate with an 1800 mm chord, 1000 mm span, and 10 mm thickness, vertically assembled inside the test section of the Low Acoustic Noise and Turbulence (LANT) wind tunnel at EESC-USP. Aluminum leading edge, flap, and tab were attached to the model in order to promote a practically constant pressure distribution. Hot-Wire Anemometry was carried to measure base flow, turbulence level, and Tollmien-Schlichting profiles. Good agreement with Blasius was obtained in at least a 300 mm span range, on two different streamwise positions. Tollmien-Schlichting eigenfunction profiles were measured for different positions in a chordwise direction. The amplification region matches that predicted by Linear Stability Theory. Through spectral analysis, it was possible to identify the presence of fundamental and first harmonic oscillation, which led to the conclusion that the flow conditions corresponded to a weakly non-linear regime.