The IDDES (Improved Delayed Detached Eddy Simulation) method was applied to simulate the highly unsteady flow in a transonic compressor at near-stall operating condition. Shock induced separated flow on the casing wall in front of the blade leading edge and the flow separation induced by the shock impinging on the blade suction surface were investigated in detail with the help of IDDES.

The 3D separation in the corner of blade suction side surface and the casing wall in the front portion of the blade was shown to be a vortex-like separation spiraling out from the blade suction surface, connecting to the blade suction wall and the casing wall. Blade tip leakage flow also contributed to the formation of the separation vortex. At near stall condition, shock wave was pushed forward out of the blade passage. The upstream propagation of the separation vortex and the related high entropy region were considered to be a characteristic phenomenon of the flow at near stall condition in this transonic compressor rotor.

The results of the present study explain the origin and the formation of the high entropy region in a transonic compressor rotor near the blade leading edge, which can be closely related to the spike type stall inception observed by other researchers in the transonic case.

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