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High-speed high-pressure(HSHP)ratio compressor surge is a transient breakdown in compression accompanied by an abrupt momentary reversal of gas flow.It commonly exists in dynamic type turbo compressors,particularly the axial compressor of the modern aero-engines.By Newtons Laws of Motion,a force is needed to change the state of any motion.So what is the force that can cause such a dramatic motion as surge? What exactly triggers its happening and how to quantify the transient surge phenomenon? This paper attempts to answer these questions and discusses the dynamics from stall to surge at its transient phase.It has generally been accepted that surge is precipitated by the onset of rotating spike or stall,not only for low speed but very high speed compressor too.The state of dynamic surge modeling today is best exemplified by the Greitzer model.However,it fails to incorporate the key elements of the transient nature of a surge inception: the extreme short time duration(on millisecond scale)and the shock wave presence as observed experimentally during the flow reversal phase of a surge cycle.An indirect approach is taken in this paper to address the transient dynamics from stall to surge by using an analogy with the classic shock tube.The link is established based on observations that sudden loss of flow velocity inside cascade cells triggers stall and surge.The results from the analogy reveal that surge initiation generates simultaneously a pair of non-linear compression and expansion waves(CW & EW)and induced reverse fluid flow(IRFF).The dynamic forces for instant flow reversing are the pushing force of upstream propagating CW and the pulling force from downstream travelling EW.Results are then compared with the experimental findings by previous researchers with good agreements.Moreover,the strength of the surge shock wave and reverse flow(IRFF)can be estimated analytically or numerically by the shock tube theory from known pre-surge conditions and routes to surge.The method provided could be used as a pedagogic tool in understanding the physics of the transient phase from stall to surge and gain valuable insight to developers of more precise CFD calculations for comprehensive surge modeling in the future.