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

Abstract

In the first part of this 3-paper-series time-resolved PIV (TR-PIV) is performed in the transonic cascade to elucidate the shock-boundary layer interaction process and to provide comparative data for numerical studies and analysis in part 2 and 3. The application of modern high-speed camera technology combined with a dual-pulse high-speed laser system enabled TR-PIV in the cascade for image areas covering 30% of the chord and 6% of the blade pitch at sampling intervals of 0.18 convective time units (CTU) for the acquisition of multiple time sequences with duration of about 2000 CTUs. The sampling rate is sufficient to resolve fluctuations of the separating region and accompanying movements of the shock system. To enable correlations between density variations of the shock system and velocity data, shadowgraphs are acquired synchronously with TR-PIV recordings at the same sampling rate. The TR-PIV and shadowgraph measurement setups, the image processing and the resulting spatial resolutions are described in this part. An analysis of power spectral densities along near-wall rows in PIV data reveals the chord-wise spatial distribution of specific peaks and bands, which are also visible in the shock buffet spectrum. A spectral proper orthogonal decomposition (SPOD) of TR-PIV data is conducted to enable inspection of spatio-temporal modes of velocity fluctuations at specific broad peaks and tones in the buffet frequency range. Results indicate, that upstream of the main shock oblique shock waves occur at higher order harmonics of the fundamental buffet frequency and at specific high-frequency tones. The upstream propagation of disturbances beyond the excursion range of the main shock is demonstrated for the dominant buffet frequency as well as for the high-frequency tone and its first harmonic using cross-correlation maps. 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 passages. The results show that with these high-frequency tones opposing vibrations of bow and lip shocks occur for neighboring blades.