Computational Study of Intake Duct Effects on Fan Flutter Stability

Abstract

A detailed e utter analysis is presented of a civil aeroengine fan assembly using an integrated three-dimensional aeroelasticity model. Two different intake ducts are considered. The e rst one is a straight duct, the geometry of whichisrepresentativeoftestrigintakes.Thesecondductisanaxisymmetricversionofthee ightintake.Duringthe e rst phase of the study, a series of e utter analyses was conducted for the 60 ‐80% speed range. Each computation was performed at a single point along the speed characteristic by following an elevated working line that was near the expected e utter boundary. As routinely observed in rig tests, the e utter stability was predicted to drop sharply forsome very narrow speedranges, but thebehaviorwasfound to bemarkedly differentforeach individual intake. To gain further understanding of the intake effects, a large number of calculations were undertaken for thesecond intake alone. An assumed pressure perturbation, due a rotating fan assembly vibrating in a given nodal diameter mode, was imposed at the duct exit. The propagation of this perturbation was monitored at a number of stations along the duct. The cases studied were chosen to cover the combinations of speeds and nodal diameters for which theearlierfanassembly plusintakecalculationshad predictede utter.Itwasshownthat, foragiven nodaldiameter assembly mode, instabilities occurred when the perturbation frequency was sufe ciently close to the duct’ s cuton frequency in theregion closeto thefan.Such a e nding suggeststhat fan e utterand intakeductacoustics arerelated in an integral fashion. Flutter will occur when the pressure perturbation due to fan rotation and blade vibration match, both in frequency and shape, an acoustic mode of the intake.