MULTI TRAVELING WAVE FLUTTER ONSET IN TUNED BLADED-DISKS

Abstract

Flutter is a major constraint on modern turbomachines; as the designs move toward more slender, thinner, and loaded blades, they become more prone to experience high cycle fatigue problems. Dry friction, present at the root attachment for cantilever configurations, is one of the main sources of energy dissipation. It saturates the flutter instability growth producing a limit cycle oscillation whose amplitude depends on the balance between the energy injected and dissipated by the system. Since both phenomena, flutter and friction, are small corrections of the elastic behavior of the structure, it takes many elastic oscillations for them to manifest and modulate the oscillation amplitude. Furthermore, even longer time scales appear when multiple traveling waves are aerodynamically unstable and exhibit similar growth rates, with a very slow energy transfer rate between them. All these slow scales make the system time integration very stiff and CPU expensive, bringing some doubts about whether the final solutions are properly converged. In order to avoid these uncertainties, a numerical continuation procedure is applied to analyze the solutions that set in, their traveling wave content, their bifurcations and their stability. The system is modeled using an asymptotic ROM and the continuation results are validated against direct time integrations. New final states with multiple traveling wave content are found and analyzed. These solutions have not been obtained before for the case of microslip friction at the blade attachment; only solutions consisting of a single traveling wave have been reported in previous works.