Abstract
This paper presents experimental studies on bypass transition of separated boundary layer on low-pressure turbine airfoils, focusing on the effects of freestream turbulence on the transition process. Hot-wire probe measurements are performed on the suction side of an airfoil in the low-pressure linear turbine cascade at several Reynolds number conditions. Freestream turbulence is enhanced by use of turbulence grid located upstream of the cascade. The results of this experimental study show that the location of boundary layer separation does not strongly depend on the freestream turbulence level. However, as the freestream turbulence level increases, the size of separation bubble becomes small and the location of turbulent transition moves upstream. The size of separation bubble becomes small as the Reynolds number increases. At low freestream turbulence intensity, the velocity fluctuation due to Kelvin-Helmholtz instability is observed clearly in the shear layer of the separation bubble. At high freestream turbulence intensity, the streak structures appear upstream of the separation location, indicating bypass transition of attached boundary layer occurs at high Reynolds number.
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