Coupling of tip leakage flow and blade vibration in transonic compressors

This dissertation describes the aeroelastic results of a modern 1.5-stage research compressor with blisk rotor. The heavily instrumented compressor is investigated with different tip clearances and advanced casing treatment. In particular, flutter and a self-excited rotating instability are studied. For both phenomena the aeroelastic link establishes at blade tip where a backward traveling aerodynamic wave couples with a forward traveling blade vibration. The crucial role of the blade tip clearance and the flow leaking across blade tip is clearly exhibited. Both gain additional relevance from the observation that a 10% variation in local tip clearance may cause individual blade vibration amplitudes to vary by a factor of two. The non-synchronous vibrations occur for transonic rotor relative inflow as well as in the subsonic part speed region. Circumferential wave speed and wave Mach number of the backward waves are identified as common link between the blade vibration events at different compressor speeds with seemingly arbitrary active nodal diameter. It is further shown that the vibration wave Mach number correlates with the circumferential Mach number of a rotating stall cell at identical speed with nominal VIGV setting. Finally, this dissertation proves that the aeroelastic coupling can be prevented by either a reduction in average tip clearance or by aerodynamic mistuning through advanced casing treatment. The negation of this final conclusion implies that an increase in tip clearance due to transient operation, rub-in or component wear may lead to the onset of blade vibrations not encountered with nominal clearance. The dissertation thus clearly points out that blade vibration analyses have to be robust enough to cover the compressor's entire life cycle.