Sensitivity Analysis of Eigenmode Variations on the Flutter Stability of a Highly Loaded Transonic Fan

This paper focuses on the flutter analysis of a scaled high - speed fan. The fan performance prediction is validated against rig data using Reynolds - Averaged Navier - Stokes (RANS) CFD simulations. All flutter stability calculations are based on the energy exchange approach which is used to predict the whole flutter map for the first bending mode, for which flutter occurs. In order to save computational time, the requirements of the influence coefficient formulation (ICM) were verified by an amplitude and blade passage number study. Afterwards, the ICM was compared against the travelling wave mode method (TWM). Only one unsteady CFD calculation has to be performed to reconstruct the whole stability curve for a specific eigenmode and operating point, which is a major benefit of the ICM. Key parameters of flutter stability such as shock - related effects and tip gap flows are identified and investigated at off design and part speed conditions. Flutter instabilities occur downstream of the suction side because of the shock structure which leads to a high pressure gradient in that region. The presence of the shock induces separated or nearly separated flows with greater pressure amplitudes and phase changes. Additionally, an eigenfrequency and mode shape variation is carried out for a blade - only model (no blade roots) to determine their influence on the fan flutter stability. For this purpose, the impact of twist/plunge ratio for the first flap mode shape is investigated. In addition to that, the first mode shape of a blade model is considered including nonlinear contacts at the blade root within a static structural analysis and compared against the blade only model. The results show that suppression of flutter onset can be achieved for specific constellations of structural parameters.