Recent rig tests and CFD predictions have shown that acoustic reflections can play an important part on flutter stability of blades in multi-stage compressors. Moreover, as the use of blisks with very low mechanical damping becomes more common in modern aero-engine designs, accurate prediction of blade aerodynamic damping in a multi-row environment becomes vital. Accurate CFD predictions in such environment require unsteady computation of multi-row full annulus models which poses high demands on both computational time and resources and thus cannot be used routinely in early development stages.
The aim of this work is to develop a simple low-fidelity approach for evaluating flutter stability of embedded rotors in multi-stage compressors. The required phase and amplitude relation between the reflected waves and the outgoing waves at leading/trailing edge of the rotor is calculated based on established theories. In the approach taken here, the phase and amplitude of the reflected waves are prescribed as boundary conditions for the blade row of interest, and flutter analysis of the rotor blade is undertaken based on a single passage single-row approach using a validated 3D unsteady RANS solver. Comparison of predicted rotor aerodynamic damping shows good agreement with that obtained using a high-fidelity full annulus multi-row model and test data. An overall reduction of computational cost of roughly 360 times is achieved by the simplified model. The proposed simplified flutter prediction model provides a much faster way of evaluating the susceptibility to acoustic driven flutter, and allows one to study the effects of parameters such as axial gap, frequency and stator vane angles at early development stages on an engine. It should be emphasised that the aim of this paper is to present a low fidelity prediction model for flutter and not noise, and hence the main emphasis is modelling low nodal diameters as a compressor blade is most likely to flutter in such nodal diameters for 1F mode.
