Investigation of transonic buffet on a civil aero-engine fan blade

The drive towards efficient and light-weight turbofans with a very high bypass ratio leads to long and flexible fan blades which are more susceptible to aero-mechanically induced vibration. One potential problem is caused by the transonic buffet, a phenomenon in which unsteady shock-wave boundary layer interactions lead to large-scale shock movements. This results in time-varying unsteady aerodynamic loads which, when coupled with a vibration mode, can cause high vibration amplitudes and mechanical failure. This research aims to investigate transonic buffet and its interaction with blade vibration on a civil fan blade using an in-house unsteady Reynolds-averaged Navier-Stokes (URANS) solver with fluid-structure coupling capabilities. Firstly, the computational approach is validated on the fixed OAT15 aerofoil, a common benchmark for buffet. The onset of transonic buffet and buffet frequency from the numerical simulations are compared with experimental data from wind tunnel experiments and found to be in good agreement. In the next step, the same computational methodology is applied to investigate transonic buffet of a civil fan blade at a high-speed off-design operating point. The time-varying pressure fluctuations on the blade suction side are analysed to investigate the driving mechanism of transonic buffet on fan blades. It is shown that unsteady shock movement creates large forcing amplitudes, and that the third harmonic of the fundamental frequency associated with the shock movement is in resonance with the first flap vibration mode. Furthermore, blade vibration is shown to alter the frequency and amplitude of shock movement leading to a lock-in between the aerodynamic frequency and the vibration frequency of the blade. The results confirm that transonic buffet and associated vibrations (buffeting) are a potential cause of non-integral fan vibrations.