A Parametric Study on the Influence of Acoustic Treatments in Fan Flutter

Abstract Acoustic liners in turbo engines reduce community noise but can also impact fan flutter, even though it is at lower frequencies. Without liners, reflected acoustic waves from the intake highlight can induce flutter, reducing the operability close to the stall line, a situation known as flutter bite. Liners alter this behavior by damping the acoustic waves but also shifting their phase, affecting flutter stability. A parametric study using a simplified intake model with an acoustic liner is carried out in this paper. Results show that noise liners have a significant impact in flutter stability. The length of the intake, which changes the propagation time of the acoustic waves, also has a relevant effect in fan flutter. Considering the geometry of the AneCom AeroTest (ACAT) fan, the influence of the impedance, position and length of the acoustic liner is examined. Liners covering more of the intake stabilize cut-on waves but destabilize cut-off frequencies due to reflected decaying modes. Short liners covering a quarter of the intake significantly affect flutter, with those near the inlet expanding instability, while central liners increase stability. Liners near the fan stabilize high frequencies but leave low frequencies near cut-off unstable.