BYPASS TRANSITION MECHANISM INDUCED BY SECONDARY FLOW NEAR COMPRESSOR ENDWALLS

Abstract

The transverse sweeping secondary flow near the compressor endwall continuously impinges on the blade suction surface, increasing the complexity of local disturbance amplification and transition onset. In this study, large eddy simulations (LESs) were conducted to resolve the three-dimensional flow field of a compressor cascade. A new bypass transition route induced by endwall secondary flow was identified, and the associated turbulent statistics were examined. The results indicate that the multi-scale streamwise vortices within the secondary flow entrain the blade/endwall boundary layers into the mainstream, generating high- and low-speed streaks. These unsteady streaks merged intermittently to form unstable mushroom-shaped vortex structures. Instantaneous flow analysis revealed that the approaching secondary flow acted as a ‘negative jet’ above these streaks, forming a strong shear layer at the interface. The streaks subsequently undergo destabilization through an inviscid instability mechanism, leading to rapid vorticity growth and the emergence of turbulent spots. The flow in the transitional region exhibits strong turbulent non-equilibrium and anisotropy, with the turbulent kinetic energy (TKE) production term reaching up to five times the dissipation term. Among the Reynolds stress components, the streamwise normal stress contributes most significantly to the local TKE generation. This study elucidates new mechanisms of streak formation and destabilization near the compressor endwall, distinct from conventional routes under high freestream turbulence. These findings expand the understanding of bypass transition and provide direct guidance for loss control near compressor endwall.