Spectral analysis was used to investigate boundary layer separation, transition and reattachment under low-pressure turbine airfoil conditions. Cases with Reynolds numbers ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) have been considered at low (0.5%) and high (9% inlet) free-stream turbulence levels. Spectra of the fluctuating streamwise velocity and the turbulent shear stress are presented. The spectra for the low free-stream turbulence cases are characterized by sharp peaks. The high free-stream turbulence case spectra exhibit more broadband peaks, but these peaks are centered at the same frequencies observed in the corresponding low turbulence cases. The frequencies of the peaks suggest that a Tollmien-Schlichting instability mechanism drives transition, even in the high turbulence cases. The turbulent shear stress spectra proved particularly valuable for detection of the early growth of the instability. The predictable nature of the instability may prove useful for future flow control work.

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