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Experimental Results
As the performance and the stage characteristics of the compressor under consideration have already been published by Niehuis et al. (2003), only a brief description will be given below. At operating point OP1, the aerodynamic loading is highest for the last stator, which was the design intent in order to study the effect of high loading. Consequently, further throttling (OP3) increases the loading significantly on all blades except for the last stator, which exhibits only a slight increase. It is assumed that surge of this particular compressor is triggered by the last stator. Concerning the overall unsteady behavior, Niehuis et al. (2003) also presented a detailed analysis and proposed a characteristic parameter based on the calculation of the total energy of the periodic pressure fluctuations generated by the rotor blades. Doing this for each measuring plane, the influence of the pressure fluctuations of each blade row can be recognized in terms of their upstream and downstream inflience. It was concluded that
the infhence of the rotor blades on the unsteady fbw field depends on the aerodynamic loading of the blade rows
higher loading causes an increasing potential upstream inflience as well as the downstream stability of the wakes decreases
In this paper, the detailed analysis of the ft>w field is focused on the front and the last stage of the compressor. The front stage operates at the overall highest Mach number level, and it enables the separation of different secondary ft>w phenomena, as there is a comparatively low level of unsteadiness. Besides that, the differences in aerodynamic loading increase in the last stage of the compressor. The effect on the development of secondary ft>w phenomena becomes more clear.
