Potential Erroneous Interpretations of Pressure Signals as Beats During Established Rotating Stall

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.