Predicted Influence of Intake Acoustics Upon Part-Speed Fan Flutter

While it is known that the occurrence of flutter is dominated by aerodynamics, it also depends on such parameters as inlet distortion and acoustics, aerodynamic and mechanical mistuning, and structural damping. It is shown in this paper that recent developments in predictive methods are showing considerable promise and leading to an improved understanding of the controlling parameters. A non-linear coupled structural-fluid approach is described and applied at engine representative conditions. This involves 3D unsteady CFD calculations of the fan and intake flows. Particular emphasis is placed on the influence of intake acoustics. Earlier work on flutter prediction has focused on either the fan assembly without a proper representation of the intake, or the calculation of the acoustic properties of the intake without properly representing the interaction with the fan. The present study includes a combined fan plus intake calculation, the latter being represented via an axisymmetric approximation. With an initial prescribed velocity disturbance of the blades in a 2 nodal diameter mode, the calculations showed a strong response in a 4 nodal diameter mode. Considerable acoustic activity within the duct was also noted. This result was compared with CFD calculations for the full 3D intake geometry. It was concluded that a realistic representation of intake acoustics is required for a full description of the problem.