This numerical study presents data relevant to the flow characteristics inside of a centrifugal compressor, at design and near-surge conditions. The main objectives were to characterize the flow structures and the associated instabilities near the stall point (prior to surge) and to contrast the obtained results against data acquired for a design operation condition. Generally, the operational range of compressors is limited at low mass flow rates by development of instabilities, e.g. stall and rotating stall. Such conditions lead to breakdown of the operability of the compressor, with flow reversal in the wheel passage. This results in large mass flow variations and pressure fluctuations within the compressor, lowering the compressor efficiency and pressure ratio. Large vibratory stresses are induced in the blade under such off-design operating conditions, affecting the blade life duration. Compressor stall and rotating stall are frequently regarded as "precursors" to the more damaging surge instability. The flow fields under design and off-design operating conditions are calculated using the Large Eddy Simulation (LES) approach. The complete geometry (360 degree) of the compressor is considered during analysis. It includes the ported shroud, the compressor wheel, the vaneless diffuser, the volute, and the exit pipe. The computationally expensive transient sliding mesh technique is used in order to capture the interaction between the wheel, the flow, and the stationary components of the compressor. The LES data are validated against available experimental measurements obtained under the same operating conditions (i.e. at design and off-design). The calculated frequency spectra when the compressor operated near-by the surge line indicated the presence of the rotating stall
