Effect of acoustic treatment on fan flutter stability

This paper presents an investigation of the effect of acoustically treated wall on fan flutter stability. The analysis is based on a three-dimensional analytical model which captures the aerodynamic coupling between oscillating annular rotor and finite-length liner in subsonic flow. With the response function of the entire system constructed as a whole, the perturbed duct field in modal description is determined by a simultaneous solution of the rotor and liner responses. Whether or not the aeroelastic instability occurs under a given wall impedance condition is evaluated through the energy method. Numerical experiments are conducted in the parametric space of inter-blade phase angle, liner distribution and acoustic impedance. The results show that under different conditions the acoustically treated wall can take diverse effects on the fan flutter stability. In the presented cases where the rotor stability is considerably affected, it is found that the aerodynamic interaction between the rotor and the lined section modify the overall distribution of the aerodynamic loadings on blade surface rather than change their magnitudes significantly. Moreover, the aeroelastic response of the oscillating blades turns out to be very susceptive to the reactance variation when the resistance is small, further indicating the important role of the reflections on the liner surface in such aerodynamic interaction. The analysis in general implies that the actual impact of a lined wall on the fan flutter stability depends on the aerodynamic excitation condition mutually determined by duct geometry, rotor state and configurations of liner.