ANALYSIS OF FLUTTER MECHANISMS AND UNSTEADY AERODYNAMICS OF A TRANSONIC FAN BLADE NEAR STALL

This paper investigates the onset of aeroelastic instability of a modern low-pressure ratio transonic fan using numerical simulations, validated against experimental measurements. Different domain configurations are compared in terms of fan characteristic and radial profiles. It is shown that flutter can be predicted over the investigated speed range using a reduced computational domain. The benefit in computational speed justifies simplifications made and allows a more detailed unsteady analysis. During the unsteady analysis, it is shown that the transition from stable to aeroelastic unstable along a constant speed characteristic results from increased negative aerodynamic damping on the pressure side of the blade, accompanied by a drop of damping on the suction side near the tip. The shock and the radial migration have a stabilizing effect on the blade, while the region downstream of the shock tends to destabilize the blade. The drop in stability occurs in multiple low nodal diameters, independent of acoustic propagation conditions. While previous work tried to identify unique mechanisms that drive flutter, this research concludes that the change from stable to unstable is subtle and that it is a combination of many drivers. In addition, the tip region, especially the region downstream of the shock, tends to have a more significant impact than assumed, while the shock shows only small variations near the tip.