Flutter Analysis of the ECL5 Open Fan Testcase Using Harmonic Balance

Kurzfassung

This paper presents a flutter analysis of the UHBR Open Fan Testcase ECL5 for off-design points at part speed. Emphasis is put on the second eigenmode which has a strong torsional character near the blade tip. Recent studies by Pagès et al., using a time-linearised solver, showed strong negative damping for an operating point on the 80% speedline close to the maximal pressure ratio. This was identified as a phenomenon of convective resonance; for a certain nodal diameter and frequency the blade vibration is in resonance with convective disturbances that are linearly unstable. In this work, we apply a nonlinear frequency domain method (harmonic balance) to the problem of aerodynamic damping prediction for these off-design operating points. It is shown that, to obtain plausible results, it is necessary to treat the turbulence model as unsteady. While a strong negative damping is not predicted by the analysis presented here, we observe a particularly high sensitivity of the aerodynamic response with respect to the frequency for certain nodal diameters. The combination of nodal diameter and frequency of maximal sensitivities are interpreted as points near resonance. We recover from these near-resonance point convective speeds and compare them to studies of the onset of nonsynchronous vibrations of the ECL5 fan at part speed conditions.