Investigation of Freestream Turbulence Effect on Low-Pressure Turbine Horseshoe Vortex Dynamics

The low-pressure turbine, a key component of modern high-bypass turbofan engines, drives the fan that generates most of the thrust. Operating under low-Reynolds-number conditions, the flow through the low-pressure turbine is sensitive to freestream turbulence. Based on data from earlier large-eddy simulations of a linear low-pressure turbine cascade with front-loaded high-lift blades, a closer look is taken at the horseshoe vortex dynamics for freestream turbulence intensities of 0, 3.6, and 7.2% that are one axial chord upstream of the blades. Proper orthogonal decomposition analyses suggest a random back and forth motion of the 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 are 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’s low-frequency unsteadiness is caused by an intermittent breakup of the horseshoe vortex as a result of inherent vortex instabilities that are forced by the approach boundary layer.