Evolution of turbulence across the stages of an axial compressor under pre-stall conditions.*

Stereo-PIV measurements performed in a refractive index matched facility examine the evolution of turbulence in a series of axial planes within and between the blade rows of a multi-stage axial compressor, consisting of an inlet guide vane (IGV), followed by a rotor, and a stator. The data are recorded at below design flow conditions when the machine is about to stall. In the IGV-rotor gap, the turbulent kinetic energy (TKE), which is dominated by axial velocity fluctuations, is low in most of the passage except for the IGV wake and in the tip region, the latter owing to upstream-propagating instabilities. Within the rotor, very high turbulence is generated at the interface between the tip flow, which has high circumferential and low axial mean velocity, and the main passage flow, which has high axial velocity. This region is characterized by large-scale vortical structures aligned diagonally upstream. Both the shear and axial contraction terms are significant contributors to the turbulence production rate, and the axial velocity fluctuations can reach 80% of the mean values. The turbulence level decreases in the rotor-stator gap, and within the stator passage to a significant part by axial extension of the flow, which creates a broad region with a negative TKE production rate.