Over recent years engine designs have moved increasingly toward low specific thrust cycles to deliver significant specific fuel consumption (SFC) improvements. Such fan blades are more loaded than conventional fan blades and therefore can be more prone to aerodynamic and aeroelastic instabilities. The aim of this paper is to analyse the flutter stability of a low speed/low pressure ratio fan blade. By using a validated CFD model (AU3D), three dimensional unsteady simulations are performed for a modern low speed fan rig for which extensive measured data are available. The computational domain contains a complete fan assembly with an intake duct and the downstream OGVs (whole LP domain). Flutter simulations are conducted over a range of speeds to understand flutter characteristics of this blade. Only the 1F (first flap) mode is considered in this work. Measured rig data obtained by using the same fan set but with two different intakes showed a significant difference in the flutter boundary for the two intakes. AU3D computations were performed for both intakes and were used to explain this difference between the two intakes, and showed that intake reflections play an important role in flutter of this blade. In the next phase of this work, two possible modifications for increasing the flutter margin of the fan blade were explored:
1. Changing the mode shape of the blade
2. Using acoustic liners in the casing
The results show that it is possible to increase the flutter margin of the blade by either decreasing the ratio of the twisting to plunging motion in 1F mode or by introducing deep acoustic liners in the intake. The liners have to be deep enough to attenuate the flutter pressure waves and hence influence the stability.
The results indicate the importance of reflection in flutter stability of the fan blade, and clearly show that intake duct needs to be included in flutter study of any fan blade.