To explore the evolutions of inner flow structure and aerodynamic losses in a low-aspect ratio compressor rotor operating from compressor condition to highly loaded windmilling condition (turbine condition)

a simplified numerical calculation method based on energy transfer between rotor blade and working fluid was employed to capture the critical windmill points at low rotational speeds. Due attention was paid on comparisons of the evolutions of tip leakage loss and the flow separations in the blade passage at the compressor condition

critical windmill point and turbine condition on a speedline. It was found that the rotor operating mass flow for critical windmill point showned a nearly linear variation trend with the increment of rotating speed. At the same time

with the rotor operating from the compressor to turbine condition

the tip leakage flow traveled across the clearance from the suction surface to the pressure surface at turbine conditions and mixed with the low-momentum fluids on the pressure surface

resulting in flow blockages near the casing endwall. In the meanwhile

the tip flow separation switched from suction surface separation to pressure surface separation

and both the intensity and size gradually increased

leading to higher aerodynamic losses. In the hub region

the flow separation was kept on the suction surface and the size of the separation has presented a growth in the radial direction near the trailing edge.