In-Flight Investigations of Tollmien-Schlichting Waves

Abstract

Flight tests were performed in order to learn more about the technically important case of naturally occurring Tollmien-Schlichting waves in boundary layer transition. A multi-element hot-film array, placed on the right hand wing of the flying test bed LFU-205, was used to sense skin friction fluctuations provoked by Tollmien-Schlichting disturbances propagating in the laminar boundary layer. Fourier analysis in time and space of the experimental data revealed that a broadband spectrum of 2d and 3d Tollmien-Schlichting waves is involved in the signals, typical for wave packets. Occurrence of these wave packets happens stochastically and it was found that they vary in intensity as well as in their spanwise extent. Examination of signal time histories from sensors distributed in chordwise direction showed that breakdown to turbulence, indicated by spikes in the signal, starts from within the wave packets with the strongest amplitudes.