The INFRa-Rig: Synchronous and Non-Synchronous Vibrations of an UHBR Fan at Clean and Crosswind Conditions

Abstract Due to ecological requirements, the bypass-ratio of future civil turbofan engines will be increased. To counteract the increasing weight and aerodynamic drag of the enlarged ultra-high bypass (UHBR) engines, the intake length will be shortened. This leads to a stronger coupling between intake and rotor blades that can increase the risk of the aeroelastic instabilities and a forcing due to inlet distortions. The Integrated Fan Rig Assembly (INFRa) was designed to experimentally investigate fans under variably and highly critical operating conditions. The rig is built to investigate different head and crosswind conditions. In this paper, the blade vibrations at different operating and crosswind conditions are experimentally investigated. The flow conditions are varied between low-speed subsonic and transonic flow. By varying the crosswind velocity, the inlet distortions are changed and blade vibrations are measured using strain gauges on the rotor blade, blade tip timing, and high-speed digital image correlation (DIC). Blade vibrations are observed at resonance and off-resonance excitation. At off-resonance in addition to synchronous engine order (EO) vibrations, non-synchronous vibrations (NSV) at the first two mode eigenfrequencies are observed. The responses show to be dependent on the crosswind speed. At higher cross-wind velocities, unsteady separations in the intake are present. The unsteady pressure is measured with microphones placed in the intake and compared to present NSV. Additionally, rotor induced aerodynamic disturbances are observed close to the stall line leading to NSV under clean and crosswind conditions. The results show the importance of considering crosswinds for the aeroelastic assessment of UHBR fans.