Investigation of time-frequency analysis and transitional boundary layer over a pitching airfoil

Transitional boundary layer over a pitching airfoil at low Reynolds number (Re = 2.7×10^5) is experimentally investigated through the space-frequency and time-frequency analyses of hot-film signals. Boundary layer events are visualized based on the space-frequency and time-frequency plots. The precursor phenomena for turbulent as well as fully separated flows are presented based on the time-frequency analysis. A new technique based on the time-frequency analysis of hot-film signals is presented to measure the transition onset as well as the relaminarization locations. This technique is based on the analysis of high-frequency disturbances of the measured data. Special attention is focused on the spatial/temporal progression of the transition onset and the relaminarization points, compared to the static values, for different oscillation frequencies and amplitudes. Investigations are performed prior to, within and beyond the static stall angle of attack conditions. The results obtained by the new technique will be discussed and compared with the observations from the previous investigators.