The Impact of Blade Loading and Unsteady Pressure Bifurcations on Low Pressure Turbine Flutter Boundaries

Abstract

The three major aeroelastic issues in the turbomachinery blades of jet engines and power turbines are forced response, nonsynchronous vibrations, and flutter. Flutter primarily affects highaspect ratio blades found in the fan, fore highpressure compressor stages, and aft lowpressure turbine (LPT) stages as low natural frequencies and high axial velocities create smaller reduced frequencies. Often with LPT flutter analyses, physical insights are lost in the exhaustive quest for determining whether the aerodynamic damping is positive or negative. This paper underlines some wellknown causes of the LPT flutter in addition to one novel catalyst. In particular, an emphasis is placed on revealing how local aerodynamic damping contributions change as a function of unsteady (e.g., mode shape, reduced frequency) and steady (e.g., blade torque, pressure ratio) parameters. To this end, frequency domain Reynoldsaveraged Navier–Stokes (RANS) CFD analyses are used as computational wind tunnels to investigate how aerodynamic loading variations affect flutter boundaries. Preliminary results show clear trends between the aerodynamic work influence coefficients and variations in exit Mach number and back pressure, especially for torsional mode shapes affecting the passage throat. Additionally, visualizations of qualitative bifurcations in the unsteady pressure phases around the airfoil shed light on how local damping contributions evolve with steady loading. Final results indicate a sharp drop in aeroelastic stability near specific regions of the pressure ratio, indicating a strong correlation between blade loading and flutter. Passage throat shock behavior is shown to be a controlling factor near the trailing edge, and as with critical reduced frequency, this phenomenon is shown to be highly dependent on the vibratory mode shape.