Non-Synchronous Vibration: Influence of blade vibration on aerodynamic disturbances

Abstract Non-Synchronous Vibrations in compressors or fans are attributed to a coupling between blade vibration and circumferentially propagating structures called aerodynamic disturbances, appearing as machines are throttled towards stall at part speed conditions. Their ability to lock-in to structural vibration is evidenced in experiments by a sharp rise in vibration amplitudes coinciding with a shift in aerodynamic disturbance propagation speeds. Aerodynamic disturbance speeds as well as wave numbers have been identified as key parameters affecting the forcing frequencies on the rotor, and therefore the critical structural mode and nodal diameter likely to be excited. This work uses full-annulus time-accurate URANS simulations of the ECL5/CATANA research rig to investigate the fluid-structure coupling. Aerodynamic-only cases show that as massflow is reduced, disturbances increase in wavelength and pressure fluctuation amplitudes, and propagate faster and more erratically in the stationary frame. Across multiple unsteady operating points, blade vibration is included following prescribed amplitudes and frequencies. Local analysis of the disturbances shows that vibration can modulate their propagation speed based on their relative interaction phase, with a consistent sine wave relationship. Visualisations in a frame travelling at the aerodynamic propagation speed reveal that blade vibration can alter the distance between adjacent disturbances. At high enough vibration amplitudes, this can lead to disturbances merging or being created, effectively modifying their dominant circumferential wave number.