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In this paper we describe a kind of unsteady phenomena in a transonic axial fan rotor at stable operating points based on the observation from numerical simulation results. Single-passage and full-annulus time-accurate simulations were implemented for several different operating points, when the unsteadiness phenomena occurred. Thanks to the recently introduced GPU acceleration technology, all these time-consuming computations were executed on a desktop computer. As is shown in frequency-domain analysis results, the frequency-domain behaviors of the calculated flow field at different operating points are different in single-passage and full-annulus results, which means that the single-passage simulation may lead to incorrect conclusions when simulating unsteady flow field near stall. It can be observed from flow fields that flow parameters in different passages are at different phases at the same moment. Spillage flows periodically appear in front of the blade leading edges at the tip region and no separation is observed at the blade leading edge in this process. The spillage flow is closely linked with a vortex structure, which is shed from the tip leakage vortex at the leading edge region. The circumferential movement of the vortex delivers a mechanism of propagation for the spillage status. Although the spillage occurs at some instants, these operating conditions are still stable because the leading edge region is occupied by the incoming main flow at other instants when the flow at this region is improved.

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In this paper, a numerical investigation into a spike-type rotating stall process is carried out considering a transonic compressor rotor (the NASA Rotor 37). Through solution of the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations, the evolution process from an initially circumferentially-symmetric near-stall flow field to a stable stall condition is simulated without adding any artificial disturbance. At the near-stall operating point, periodic fluctuations are present in the overall flow of the rotor. Moreover, the blockage region in the channel periodically shifts from middle span to the tip. This fluctuating condition does not directly lead to stall, while the full-annulus calculation eventually evolves to stall. Interestingly, a kind of "early disturbance" feature appears in the dynamic signals, which propagates forward ahead of the rotor.