Mechanisms and prediction methods for fan blade stall flutter

A first aim is to distinguish between two flutter regimes that are commonly referred to as stall flutter because of their occurrence between a raised working line and the stall boundary. The first one is of aeroacoustic origi n, and it arises from a match between the acoustic impedance of the intake duct and the upstream pressure perturbation due to fan vibration. The secon d type is directly related to flow separation effects and shock properties. Further objectives are to describe several numerical prediction methods, t o compare their relative computational requirements, and to establish their bounds of applicability to various flutter types. The unsteady flow is eit her a linearization about a viscous nonlinear steady-state flow for a given mode of vibration, or it is fully nonlinear. The prediction methods are cl assified according to the way they treat the unsteady flow. In all cases, t he fluid mesh was moved during the unsteady flow computations to follow the structural motion. The performance of the methods was ranked for a rig fan blade for which measurements were available. It was found that, near the s tall boundary, flutter boundary could only be captured with an adequate rep resentation of the unsteady viscous effects. It was concluded that the shoc k had a stabilizing effect whereas the separation area behind it had a dest abilizing effect.