The Unsteady Shock-Boundary Layer Interaction in a Compressor Cascade – Part 1: Measurements With Time-Resolved PIV

Time-resolved PIV is performed in the transonic cascade to elucidate the shock-boundary layer interaction. Measurements are conducted in the center blade passage of the cascade at a sampling frequency of 20 kHz or 0.18 convective time units (CTU) for the acquisition of multiple time sequences with duration of approx. 2000 CTU. The sampling rate is sufficient to capture fluctuations of the separating region and accompanying movements of the shock system. To align both instantaneous density variations in the shock system and velocity data obtained in spatially separated regions, shadowgraphs are acquired synchronously with TR-PIV recordings at the same sampling rate. An analysis of power spectral densities along near-wall rows in PIV data reveals the chord-wise spatial distribution of specific peaks and bands. The upstream propagation of disturbances beyond the excursion range of the main shock was demonstrated for the dominant buffet frequency as well as for the high-frequency tone and its first harmonic using cross-correlation maps. A spectral proper orthogonal decomposition (SPOD) indicates that upstream of the main shock oblique shock waves occur at higher order harmonics of the fundamental buffet frequency as well for specific high-frequency tones. To locate possible sources of distinct tones at the entrance of the center passage, a SPOD of high-speed schlieren recordings is performed, capturing the shock systems in three neighboring blade passages. Results indicate that with these high-frequency tones opposing vibrations of bow and lip shocks occur for neighboring blades.