Compressor surge is a form of aerodynamic instability in axial compressors or centrifugal compressors. The term describes violent air flow oscillating in the axial direction of a compressor, which indicates the axial component of fluid velocity varies periodically and may even become negative. In early literature, the phenomenon of compressor surge was identified by audible thumping and honking at frequencies as low as 1 Hertz, pressure pulsations throughout the machine, and severe mechanical vibration. Description Compressor surge can be classified into deep surge and mild surge. Compressor surge with negative mass flow rates is considered as deep surge while the one without reverse flows is generally termed mild surge. On a performance map, the stable operating range of a compressor is limited by the surge line. Although the line is named after a surge, technically, it is an instability boundary which denotes onsets of discernible flow instabilities, such as compressor surge or rotating stall. When the mass flow rate drops to a critical value at which discernible flow instabilities take place, nominally, the critical value should be determined as a surge mass flow rate on a constant speed line; however, in practice, the surge line on a performance map is affected by specific criteria adopted for determining discernible flow instabilities. Effects Compressor surge is catastrophic for the compressor and the whole machine. When compressor surge happens, the operating point of a compressor, which is usually denoted by the pair of the mass flow rate and pressure ratio, orbits along a surge cycle on the compressor performance map. The unstable performance caused by compressor surge is not acceptable to machines on which a compressor is mounted to ventilate or dense air. In addition to affecting performance, compressor surge is also accompanied with loud noises. Frequencies of compressor surge can range from a few to dozens Hertz depending on the configuration of a compression system. Although Helmholtz resonance frequency is often employed to characterize the unsteadiness of mild surge; it was found that Helmholtz oscillation did not trigger compressor surge in some cases. Another effect of compressor surge is on solid structure. Violent flows of compressor surge repeatedly hit blades in the compressor, resulting in blade fatigue or even mechanical failure. While fully developed compressor surge is axisymmetric, its initial phase is not necessarily axisymmetric. Actually, severe damage of compressor surge is often related to very large transverse loads on blades and