Numerical Investigation of the Fan Flutter Mechanism Related to Acoustic Propagation Characteristics

Abstract

In this study, we numerically investigated the flutter mechanism related to acoustic propagation characteristics of a wide-chord fan rotor. We considered the first bending mode and its two and three nodal diameters traveling wave patterns. The unsteady disturbance induced by the blade vibration in the duct gradually changed from cut-off to cut-on mode by increasing the blade frequency. Flutter occurred in some specific frequency range. We observed the upstream cut-on and downstream cut-off condition with the risk of flutter. We found a new flutter risk frequency in which the blade frequency was less than the upstream cut-on frequency. The results showed that the effects of flow features and acoustic propagation characteristics on aeroelasticity were independent. To explain why flutter was more likely to occur near stall, we selected two sets of frequencies representing different acoustic propagation characteristics to carry out aeroelastic simulations for different working conditions along the same speed line. When flutter occurred, the suction side always provided positive damping, and the pressure side always provided negative damping. This study analyzed the effect of different flow features, such as shock and radial migration, in detail. The phase difference between the pressure fluctuations on the pressure side and the blade velocity played a vital role in fan flutter.

Effect of Intake Acoustic Reflection on Blade Vibration Characteristics

“…Gallardo et al [20] reported strong pressure wave interactions between blade rows even with an acoustic cut-off, revealing that multistage decay of acoustic waves may still cause aeroelastic instability of the blades. Dong et al [21] discovered that for a fan, both upstream cut-on and downstream cut-off, as well as both upstream and downstream cut-off, can induce blade flutter. When the blade vibration frequency is at the upstream acoustic resonance, the blade aeroelastic stability significantly increases.…”