Analysis of Freestream Turbulence Effect on Low-Pressure Turbine Junction Flow

The low-pressure turbine, a key component of modern high-bypass turbofan engines, drives the fan which generates most of the thrust. Operating under low-Reynolds number conditions, the flow through the low-pressure turbine is sensitive to freestream turbulence. Large-eddy simulations of the flow through a linear low-pressure turbine cascade with front-loaded high-lift blades were carried out for zero and 7.2% freestream turbulence intensity one axial chord upstream of the blades. For the 0% case, freestream turbulence was only introduced near the endwall to trip the endwall boundary layer. Instantaneous flow visualizations of the junction region reveal considerable horseshoe vortex unsteadiness. Proper orthogonal decomposition analyses suggest a random back and forth motion of the leading edge horseshoe vortex that is weakly correlated to low-frequency approach boundary layer unsteadiness at dimensionless frequencies of 0.25 and 0.5. A spectral proper orthogonal decomposition confirms the dominant low-frequency content for the interaction region. Biglobal stability analyses were performed to identify instabilities of the horseshoe vortex. The spanwise wavelength of the most unstable mode is close to that of the turbulent approach boundary layer streaks and observed spanwise modulations of the horseshoe vortex. The present results suggest that the junction region low-frequency unsteadiness is caused by an intermittent breakdown of the horseshoe vortex as a result of inherent instabilities that are forced by the approach boundary layer.