Session: 22-02 Fan Aerodynamic Excitation and Inlet Distortion 2

Paper Number: 129249

129249 - Conceptual Analysis on the Impact of Acoustic Liners in Fan Flutter 

This work investigates the impact of acoustically treated intakes on fan flutter. The propagation of the fan acoustic perturbations
is studied using a simplified model, where the frequency of the waves in the intake coincides with the natural frequency of the
fan blade in the stationary frame of reference. The model consists of a constant cross-section annular duct, where the acoustic
modes are computed for hard wall and lined wall regions, assuming a uniform axial flow. The interfaces between the acoustic
liners and the hard wall are treated by mode matching. The acoustic reflections in the zero-thickness intake is computed using
the Wiener-Hopf technique following Rientra’s model. Analytical results show that the waves propagating from the fan are
reflected and scattered radially at the interfaces between the hard wall and the liner. Furthermore, the liner damps and changes
the phase velocity of acoustic modes, significantly changing the phasing. Due to the low frequency of the fan-flutter-induced
waves, the radial scattering can activate different cut-off modes, which need to be retained to make accurate predictions. It is
shown that the impact of the phasing and reflections induced by the acoustic liners is comparable to that of the intake and can alter the flutter characteristics of the fan. The likelihood of fan flutter is estimated using the phase of the reflected cut-on waves reaching the fan. This strategy is used to assess on a whole fan the impact on the stability of acoustic liners. It is shown that acoustic liners can alter the fan flutter characteristics significantly.

Presenting Author: Roque Corral Universidad Politécnica de Madrid

Presenting Author Biography: Professor of Aerospace Engineering, UPM

Authors:

Javier Gonzalez-Monge Universidad Politecnica de Madrid
Jose Miguel Perez Universidad Politecnica de Madrid
Roque Corral Universidad Politécnica de Madrid
Fanzhou Zhao Imperial College of London
Mehdi Vahdati Imperial College of London