The ratios between the main frequency of rotating stall and rotational frequency may be considered in the form of exact ratios of small natural numbers if the pressure signals in compressors during rotating stall include the rotor rotation frequency component. During rotating stall in compressors with good rotor balancing (with absence of the rotational frequency component in the frequency characteristics of pressure signals), these ratios between the main frequency of rotating stall and rotational frequency are or are not in the form of ratios of small natural numbers. The experimentally received characteristics of power spectral density of pressure signals also show the presence of components with combinations of blade passing frequency and different harmonics of main rotating stall frequency.
Investigation of the nature and properties of dynamic processes using different methods of processing of the measured signals may lead to erroneous interpretations and conclusions. One of the reasons for erroneous interpretations is applying only one analysis method. The use of two different methods allows reducing erroneous conclusions but does not eliminate them completely. Such erroneous conclusions concerning pressure oscillations during rotating stall in axial compressors are described when two conventional methods of information processing (auto-correlation functions and frequency characteristics) were used for analyzing processes with changing frequencies and amplitudes of oscillation. These methods have been used for the analysis of aerodynamic processes during little change in frequency (a process very close to the established). This led to an erroneous estimation of the characteristics of the investigated process, namely to the interpretation of a beating effect during established rotating stall. It is shown that the use of a third method — the method of spectrograms — may allow the correct interpretation of the process, showing the absence of beats and the existence of a small change of main frequency of the rotating stall during the process, interpreted as established process. At the same time, it is shown that in the initial transient stage of pressure oscillation prior to the establishment of the rotating stall, beating or processes close to it can be observed.
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