ANALYSIS OF PRE-STALL PRESSURE DISTURBANCES IN AN AXIAL COMPRESSOR

Pre-stall rotating pressure disturbances were observed at off-design conditions in an axial compressor. The temporal and spectral characteristics of the rotating disturbances were studied at different mass flow rates along a part-speed, partially-closed inlet guide vane (IGV) compressor characteristic. Specifically, unsteady casing pressure measurements were analyzed at several steady operating points. The auto-spectral density (ASD) of the pressure measurements was characterized by a broadband local maximum with distinct peaks superimposed, each corresponding to a different circumferential wavenumber, consistent with observations in the literature of pre-stall rotating disturbances, or "rotating instabilities." Using the wavenumber-dependent ASD, each peak's amplitude, center frequency, and local width were characterized as a function of circumferential wavenumber for four distinct operating points. Additionally, temporal analysis of the pressure signals indicated that the rotating disturbances were compact in space and time. A Morlet wavelet model was developed to understand how physical parameters such as the disturbance size, frequency-of-occurrence, and circumferential propagation speed affected the wavenumber-dependent ASD. The model informed the characterization of physical flow features from statistics of the casing pressure measurements.