On the Effect of Single Mode Bending/Torsion Coupling on the Flutter Behavior in a Transonic Fan

Abstract

The effect of the coupling of the torsion mode blade vibration to the bending mode flutter in transonic fans has been studied by quasi-3D viscous unsteady calculations. The type of flutter in this research is that of a highly loaded blade with a tip relative Mach number just above unity, for which the mechanism is that the instability of the passage shock wave when it unstarts generates the dominant blade exciting aerodynamic work at its foot on the pressure surface of the blade. The dependence of such flutter on the blade vibration mode, i.e. the amplitude ratio and the phase difference of the bending and torsional components has been studied. The study showed that the blade exciting aerodynamic work reduced when the torsional component is added in-phase (torsional motion noses up during the upward bending motion) to the bending oscillation. The amplitude of the torsional component was shown to have an optimum amplitude. It was also shown that this tendency would switch when the shock wave is fully detached, and the blade exciting aerodynamic work would increase by adding the torsional component in-phase to the bending oscillation.