INVESTIGATION OF TRANSITION TO TURBULENCE USING WHITE-NOISE EXCITATION AND LOCAL ANALYSIS TECHNIQUES

Weak free-stream turbulence excites modulated Tollmien-Schlichting (T- S) waves in a laminar boundary layer that grow in magnitude with downs tream distance and ultimately lead to the formation of turbulent spots and then fully turbulent flow. Hot-wire experiments have indicated th at the development of localized large-amplitude 'events' in the veloci ty records are the essential precursor to the eventual formation of tu rbulent spots in the flow field. Traditional global Fourier techniques are unable to resolve the localized nature of these events and hence provide little useful information concerning the physical processes re sponsible for this breakdown process. This investigation used sequence s of computer-generated deterministic white noise to excite a laminar boundary layer via a loudspeaker embedded in a flat-plate model. This form of excitation generated the modulated disturbance waves of intere st a short distance downstream from the source in a repeatable and det erministic manner. Further downstream the pattern of flow breakdown an d subsequent generation of turbulent spots was similar to that observe d in naturally excited situations. By repeatedly exciting the boundary layer with a single white-noise sequence it was possible to examine t he highly nonlinear stages of 'event' development and breakdown with a single hot-wire probe. Two local analysis techniques, the wavelet tra nsform (WT) and singular spectrum analysis (SSA), were used in conjunc tion with the white-noise excitation technique to examine the highly n onlinear flow mechanisms responsible for the localized formation of ev ents that lead to the eventual breakdown to turbulence.