Phase-Locked Investigation of Secondary Flows Perturbed by Passing Wakes in a High-Lift LPT Turbine Cascade

The present work describes the experimental investigations carried out at the Aerodynamics and Turbomachinery Laboratory of Genoa University aimed at characterizing the unsteady features of the secondary flows in a High-Lift Low Pressure Turbine cascade perturbed by incoming wakes. The investigations have been carried out at the nominal exit flow Reynolds number of 300000 in a 5-blade large-scale linear cascade. Hot-wire phase-locked ensemble-averaging technique has been applied to analyze in depth the time-dependent velocity and turbulence intensity distributions in a downstream tangential plane during a wake period. A multiple rotation technique has been used in order to measure the three velocity components as well as the Reynolds stress tensor terms. Acquired data are presented in terms of the phase-dependent mean velocity, turbulence and vorticity maps in order to distinguish between the contributions due to incoming wake velocity defect and those due to the turbulence carried by wakes on the phase-dependent secondary flow structures. Results clearly highlight a significant distortion and weakening of the passage vortex when the upstream wake passes through the measuring domain. Also an evident displacement of the passage vortex position has been observed in the wake period. This analysis allows understanding the difference in the three dimensional time mean structures of the exit flow field between the steady and unsteady operations.