An experimental investigation is performed to characterize a high-speed centrifugal compressor as it approaches instability and during subsequent surge cycles. To achieve this, data are analyzed from the inlet, the diffuser and the exit of the compressor. Analysis of the data indicates the presence of two rotating stall modes prior to and during surge cycles. A nine cell mode pattern is shown to erupt prior to the initial surge cycle. The second rotating stall mode is a single cell mode that coincides with the initiation of the surge cycles. Both stall modes are shown to be located at or near the diffuser.

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