Transitional boundary layer over a pitching airfoil at low Reynolds number (Re = 2:7 105) is experimentally investigated using space-frequency and time-frequency analyses of hot-lm signals. Boundary layer events are visualized based on the spacefrequency and time-frequency plots. The precursor phenomenon for turbulent and fully separated ows is presented based on the time-frequency analysis. A new technique based on time-frequency analysis of hot-lm signals is introduced to measure the transition onset and relaminarization locations. This technique functions based on the analysis of highfrequency disturbances of the measured data. Signicant attention has been drawn to the spatial/temporal progression of the transition onset and relaminarization points rather than to the static values for dierent oscillation frequencies and amplitudes. Investigations are performed prior to, within, and beyond the static stall angle of attack conditions. The results obtained from this new technique are discussed and compared with the observations of previous investigators.

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