Tail-Cone Thruster Fan Rotor Flutter Analysis

The aeroelastic stability of a tail cone thruster fan was analyzed for three structural modes and various nodal diameter patterns. Computational fluid dynamics modeling with blade vibrations was used in the analyses performed at design (peak-efficiency) and two near-stall conditions at the design rotational speed. Aerodynamic damping was used to assess flutter since this integrally bladed rotor design will have minimal structural damping. Aerodynamic damping was found to be low at the design condition. For the first mode, aerodynamic damping was seen to decrease towards stall, vanishing for a zero nodal diameter pattern. Further decrease in mass flow led to negative aerodynamic damping (flutter) for the first and second nodal diameters in the near stall condition. The flowfield at this lowest mass flow rate exhibited some additional unsteadiness at higher frequency. The sensitivity of results to time step and amplitudes of vibration was examined and found to be very satisfactory. The largest uncertainty in the results comes from the observed sensitivity of the aerodynamic damping to details of the variation of grid deformations in the blade passage mesh from the blade passage to the boundaries. For the parametric variations analyzed, the calculated aerodynamic damping is lower if the grid deformations near the blade surface are higher. Further work is needed to understand the cause of this sensitivity and find possible solutions to reduce the effects on flutter predictions.