High-resolution PIV measurements of the shock boundary layer interaction within a highly loaded transonic compressor cascade

Kurzfassung

Both snap-shot as well as time-resolved PIV (TR-PIV) measurements are performed on a transonic compressor cascade to elucidate the underlying shock-boundary layer interaction process. In order to align both the instantaneous passage shock and the boundary layer along with its separation downstream of the shock foot, shadowgraph imagery is acquired synchronous with TR-PIV recordings. Statistics from PIV snapshots reveal that the passage shock fluctuates by up to 13% of the chord. Frequencies of shock motion lay in a range between 500 and 600 Hz with an additional specific tone near 1142 Hz. With TR-PIV, the velocity field in the interaction region is sampled at 20 kHz, which temporally resolves the motion of the shock foot and captures size variations of the separation region. The thickness of the laminar boundary layer (BL) is found to grow as the shock moves downstream. A rapid upstream motion of the shock leads to even higher BL growth rates and large scale flow separation downstream. Proper orthogonal decomposition (POD) of the snap-shot PIV data reveals that 50% of the energy is contained in the first 4 modes. The corresponding POD eigenvalue spectrum exhibits a -11/9 decay slope as predicted in literature for the inertial range of inhomogeneous turbulence.