POD analysis of the unsteady behavior of blade wake under the influence of laminar separation vortex shedding in a compressor cascade

Abstract

When the laminar-to-turbulent transition occurs close to the blade trailing edge, the periodic passage of shear layer roll-up vortices over the trailing edge would influence the unsteady behavior of the blade wake. In this study, time-resolved Particle Image Velocimetry (PIV) measurements were performed, to investigate the unsteady behavior of blade wake taking account of upstream shear layer instability on the suction side and the associated turbulence production mechanism. The suction side of the wake shows slower velocity recovery in comparison with the pressure side of the wake. The Proper Orthogonal Decomposition (POD) eigenvectors were used to reconstruct the phase averaged flow field of the wake instability. In the deterministic process of the wake instability, the laminar separation vortex shedding mainly intensifies the momentum transport along the mainstream direction in the suction side of the wake. The wake instability, the Kelvin–Helmholtz instability, and the finer scales flow structures were isolated, and each of their contributions to the total turbulence production was quantified. The former two instabilities contribute comparable portions (each approximately accounts for 40%) to the overall flow loss. The reduced-order model with the first ten modes is able to capture the dominant flow instability.