Evolution of flow and turbulence over the entire tip region of an axial compressor rotor

Abstract

Stereo-PIV measurements performed in a refractive index-matched facility examine the mean flow and normal Reynolds stresses over the entire axial, radial, and circumferential extents of an axial compressor rotor at two operating conditions, including pre-stall. In the tip region, we follow the backward leakage jet and double leakage across the tip gap as well as the rollup, evolution, and breakdown of tip leakage vortex (TLV). With decreasing flowrate, these phenomena shift closer to the blade leading edge (LE). The TLV is surrounded by a region with elevated circumferential velocity, which expands once vortex breakdown occurs, especially at pre-stall. Conditional averaging highlights the effects of transient pre-stall features, which are ‘smeared’ by averaging, such as LE spillage, circumferential velocity exceeding the blade speed, backflow vortices (BFVs), and blade boundary layer separation. Cavitation-based flow visualization under pre-stall and stall shows the prevalence and evolution of BFVs and their role in the formation of high circumferential velocity regions. The operating conditions and transients affect the spatial distributions of normal Reynolds stresses and turbulent kinetic energy (TKE). Far from stall, the TKE peaks near the TLV center and is dominated by the radial stress. At pre-stall, the TKE increases rapidly following the TLV breakup along the periphery of high circumferential velocity regions, where the BFVs form. It is dominated by the circumferential stress within the rotor and by the axial stress downstream of the trailing edge. Many, but not all, of the observed trends can be elucidated based on the turbulence production, advection, and diffusion terms.