Time resolved PIV measurements of the unsteady wake migration in a LPT blade passage: effect of the wake passing frequency

A time resolved Particle Image Velocimetry (TR-PIV) system has been employed to investigate the unsteady propagation of upstream wakes in a low-pressure turbine cascade. Data are obtained in the steady state condition and for two passing wake reduced frequencies. The study is focused on the identification and split of the different dynamics responsible for deterministic and random oscillations, thus loss generation. A very large data set has been collected: for each condition, about 9000 instantaneous flow fields have been acquired at up to 2kHz in order to resolve with great detail the vortex shedding phenomenon characterizing the separation at steady condition as well as the propagation of the coherent structures induced by the incoming wake. Instantaneous vector maps, phase averaged velocity fields and Proper Orthogonal Decomposition (POD) have been used for the in depth characterization of the different phenomena. The paper takes advantage of the properties of POD that reduces the data set to a low number of modes that represent the most energetic dynamics of the system. It is clearly shown that the phase averaged flow field can be represented by a few number of POD modes related to the wake passing event for the unsteady cases. POD is also able to capture flow features affecting the instantaneous flow field not directly related to the wake passage (i.e. the vortex shedding phenomenon induced by the intermittent separation developing between adjacent wakes), that are smeared out in the phase averaged results. Once recognized the POD modes most involved in the unsteady flow field, a procedure for the quantification of the different contributions to the overall amount of losses is proposed.