Numerical investigation of flutter stability in subsonic space turbine blisk with emphasis on cut-on/cut-off modes and interblade phase angles

The so-called blisks, i.e. integrally bladed disks, are characterized by very low viscous material damping and make the flutter prediction much more critical. In that framework, a two-dimensional numerical study of a space turbine blisk featuring complex deformation of blades and high eigenfrequency (>40kHz) is performed. The simulations are based on unsteady Reynolds Averaged Navier Stokes computations linearized in the frequency domain and consist in the superposition of an unsteady linear (in time) pressure field, generated by a harmonic perturbation, upon a steady nonlinear (in space) flow. The aerodynamic damping coefficient is calculated over a range of nodal diameters, and the blades are predicted aeroelastically stable. However, violent changes occur and are rather critical since sudden and large deviations in stability appear. In that context, the nature of the waves propagating from the cascade are evaluated. Such an approach provides fundamental knowledge about the perturbations which can either propagate to the far-field (cut-on mode) or decay (cut-off mode). It is expected that the ability of the flow to damp or to amplify the blade motion is strongly affected by the way unsteady perturbations are transferred from the cascade to the far-field. The nature of the waves are first assessed from the aforementioned linearized results, then they are evaluated analytically and finally compared. A good agreement is found despite the strong assumptions of the analytical model. The results show a clear correlation between the cut-on/cut-off conditions and stability. The least stable configuration corresponds to cut-off mode at the inlet and no wave at the outlet. Without outgoing waves from the cascade, the blade is prone to be less stable: the energy from the blades vibration is necessarily dissipated or sent out by the cascade.