14th European Conference on Turbomachinery Fluid dynamics & Thermodynamics
The paper presents a detailed analysis of particle image velocimetry (PIV) measurements performed in a turbine cascade representative of a high speed low deflection low pressure module. Two cameras have simultaneously been used to observe with the highest possible spatial resolution a great portion of the suction side boundary layer, from the peak suction position to the blade trailing edge, thus allowing to solve in detail the interaction process between impinging upstream wakes and boundary layer. Four unsteady inflow conditions, characterized by different incoming wake reduced frequency and flow coefficient, have been examined at fixed Reynolds number. This allows us to explore the effects due wake trajectory and distance on the large scale structures carried by wakes, and their effects on the boundary layer. At the same time, hot-wire anemometer velocity data have been acquired at the inlet of the cascade to characterize the wakes entering in the blade passage and, additionally, to support the phasing procedure properly elaborated to sort the PIV snapshots in the wake period.Phase averaged velocity snapshots from PIV provide a direct visualization of the deterministic part of the flow fluctuations, showing the effects of the two unsteady parameters on dimension, shape and evolution of the flow structures generated as consequence of the wake impingement on the blade suction side. The highly resolved flow field has been inspected by means of proper orthogonal decomposition (POD) to identify coherent flow structures generated in the boundary layer. For an even deeper physical insight on the mechanisms characterizing the wake-boundary layer interaction, POD has been applied at different fixed phases. Examination of POD modes has allowed to discuss in detail the evolution of different structures responsible for loss generation during the wake period. Furthermore, the inspection of the boundary layer integral parameters together with the computation of the turbulent kinetic energy has allowed a detailed loss quantification, also identifying the different contributions to the unsteady losses affecting the cascade.