Loading Distribution Effects on Separated Flow Transition of Ultra-High-Lift Turbine Blades

The suction side boundary-layer evolution in two ultra-high-lift low-pressure turbine blade cascades, characterized by the same Zweifel number but two different aerodynamic loading distributions, has been experimentally analyzed under steady and unsteady incoming flows. For the steady inflow case, a suction side boundary-layer separation has been detected for both cascades. Time-mean velocity and unresolved unsteadiness distributions have been exploited to survey the dynamics of the separated flow transition mode. The spectral analysis reveals that only the midloaded cascade is affected by a Kelvin-Helmholtz instability that induces the separated shear layer rollup, which provokes high losses. Results obtained for the unsteady case reveal that linear stability mechanisms drive the amplification of velocity fluctuations carried by wakes with dynamics similar to that characterizing the steady inflow condition. A rollup vortex has been found to be generated for both cascades as a consequence of the wake shear-layer interaction process. Different vortex dimensions have been recognized in the two cases, due to the different shear layer thicknesses at separation. The stronger and larger roll-up vortex characterizing the front-loaded cascade allows explaining the higher losses of this cascade, when it operates with unsteady inflow, as compared with the midloaded one.