A new computational approach has been developed to study the inception of rotating stall in axial compressors. Using this approach the flow structures within the compressor blade passages have been examined in order to determine their influence on the process of rotating stall inception. Both two and three-dimensional numerical simulations were carried out. The two-dimensional computations showed a long wave-length (or modal) type of stall inception which was found to be well described by existing compressor stability models. The numerical results were used to directly confirm the various assumptions used in the formulation of the stability models. The three-dimensional computations of rotating stall displayed a short lengthscale type of stall inception with the same character as that seen in experiments. The central feature of the flow associated with the development of the short lengthscale stall cell was the tip clearance vortex moving forward of the blade row leading edge. Vortex kinematic arguments were used to provide a physical explanation of this motion. The resulting criteria for the inception of the short length-scale stall depends upon local flow phenomena related to the tip clearance flow. Thus, unlike the modal stall situation, the flow structure within the blade passages must be addressed when describing the stability of an axial compression system to short length-scale disturbances.
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