Numerical investigation of tip clearance effects on rotating instability of a low-speed compressor

Abstract To investigate the effects of tip clearance on the rotating instability of a compressor, full-annulus numerical simulations, with and without clearance, were conducted for a low-speed compressor rotor. The analyses were focused on three spanwise positions: 99%, 50% and 10% span of the blade. The frequency, circumferential mode characteristics, and instantaneous flow structures were obtained from unsteady simulations. In the tip region, the transverse vortex in each passage and the shedding vortex at the rear of the blade suction side every two passages are associated with rotating instability (RI) when clearance exists. Without clearance, comb-like frequency distributions appear from upstream to downstream in the stationary frame at all the three radial positions, and the mode order of the RI changes from 31 to 10. The flow separations occur earlier, and the wake vortices shed periodically, similar to a Karman vortex street every two passages. According to the cross-correlation analysis of the unsteady pressure signals at different spanwise and circumferential positions, an alternative hypothesis for the RI mechanism is proposed: the instability waves generated by the blade boundary layer separation on the suction side will diffuse throughout the upstream and downstream, and a feedback mechanism will be triggered when the aerodynamic loading exceeds certain thresholds. According to this hypothesis, changing the surface feature of the suction side could have influence on the RI behaviours as well as the corresponding noise and vibration problems.