Forced Response of an Unstable LPT Bladed-Disk: New States With Different Blade Vibration Amplitude

Abstract In the forcing of a LPT bladed disk, the final amplitude of the vibration state that sets in is determined by the dissipation produced by the friction forces at the blade-disk contact surfaces. The friction effect is small but strongly nonlinear, and it can couple different vibration modes that would be completely independent in a purely linear description. In this work we present a vibration state where the blades exhibit different oscillation amplitude. It appears when a LPT rotor with an aeroelastically unstable first modal family is forced with a travelling wave excitation (resembling the aerodynamic forcing coming from the upstream stator). This vibration state is basically composed of the directly forced travelling wave coupled with a second unstable travelling wave through the interaction induced by the nonlinear friction. The surprising result is that the blade vibration amplitude is not uniform along the rotor, and even and odd blades show different vibration amplitudes, despite the fact that this a perfectly tuned bladed-disk. This vibration state was previously found in a reduced order model of the problem, and now we find these states using a high fidelity FEM description of a realistic unstable LPT bladed-disk with 144 blades that was developed and tested in the context of the European Project ARIAS. These states are found to be quite robust against perturbations, and show a quite extended range of existence for different forcing engine orders.