Comparison of Flutter Analysis Approaches in the Frequency Domain for the ECL5 Open Fan

Abstract

The topic of this paper is the flutter analysis at part speed of the ECL5/Catana open fan. The focus is on the operating point of maximal pressure ratio on the 80% speed line, where previous studies have revealed a very high sensitivity of the aerodynamic damping for the second blade eigenmode. In particular, a sudden drop of the damping near the nodal diameters of 5 was observed with time-linearised approaches, while for harmonic balance simulations with a different solver this V-shaped spike in the damping curve was observed near the nodal diameter of 6 when a fully nonlinear setup resolving the 0th and higher harmonics was used. In the previous study, this phenomenon was related to the resonance with a convective disturbance with a characteristic circumferential speed and suggested that, if non-linear effects are taken into account, then the resonance convective speed increases. In this paper, the flutter analyses based on three different frequency domain solvers are compared with each other: a time-linearised method and two harmonic balance solvers, each being based on different spatial schemes and/or turbulence models. The results in this paper show that the V-shaped damping curves can be reproduced with different solvers. If the zeroth harmonic is included in the set of harmonics which are solved for then the resonance condition is located near the nodal diameter of 6, while, for frozen mean flows, it is located at 5. This behaviour is confirmed by two harmonic balance solvers.