Flow Coefficient and Reduced Frequency Effects on Low Pressure Turbine Unsteady Losses

Particle image velocimetry measurements have been carried out in a low-pressure turbine cascade operating under unsteady inflow to deeply investigate reduced frequency and flow coefficient effects on flow dynamics, and, consequently, on loss generation in the boundary layer and in the core flow region. Two independent measuring setups have been used for the purpose. The first one captured a large view of the entire blade passage, thus allowing the observation of the incoming wakes and related large-scale vortices developing in the core flow region. The second setup was instead focused on the rear part of the blade suction side to analyze the boundary layer development and to observe the mechanisms dominating the wake-boundary-layer interaction. Tests were performed for four flow cases, varying the reduced frequency and the flow coefficient independently. Proper orthogonal decomposition has been applied to quantify the turbulent kinetic energy production in the core flow, due to wake dilatation and distortion, and in the boundary-layer region. Upstream wake migration and boundary-layer-related losses are consequently quantified from particle image velocimetry data and compared with total pressure measurements for the different combinations of the inflow parameters, providing a clear view of the different loss sources affecting the unsteady operation of low-pressure turbine cascades.