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Flutter stability is a dominant design constraint of modern gas and steamturbines. To further increase the feasible design space, flutter-tolerantdesigns are currently explored, which may undergo Limit Cycle Oscillations(LCOs) of acceptable, yet not vanishing, level. Bounded self-excitedoscillations are a priori a nonlinear phenomenon, and can thus only beexplained by nonlinear interactions such as dry stick-slip friction inmechanical joints. The currently available simulation methods for blade flutteraccount for nonlinear interactions, at most, in only one domain, the structureor the fluid, and assume the behavior in the other domain as linear. In thiswork, we develop a fully-coupled nonlinear frequency domain method which iscapable of resolving nonlinear flow and structural effects. We demonstrate thecomputational performance of this method for a state-of-the-art aeroelasticmodel of a shrouded turbine blade row. Besides simulating limit cycles, wepredict, for the first time, the phenomenon of nonlinear instability, i.e., asituation where the equilibrium point is locally stable, but for sufficientlystrong perturbation (caused e.g. by an impact), the dry frictional dissipationcannot bound the flutter vibrations. This implies that linearized theory doesnot necessary lead to a conservative design of turbine blades. We show thatthis phenomenon is due to the nonlinear contact interactions at the tipshrouds, which cause a change of the vibrational deflection shape andfrequency, which in turn leads to a loss of aeroelastic stability. Finally, weextend the well-known energy method to capture these effects, and conclude thatit provides a good approximation and is useful for initializing thefully-coupled solver.

Development of a Fully-Coupled Harmonic Balance Method and a Refined  Energy Method for the Computation of Flutter-Induced Limit Cycle Oscillations  of Bladed Disks with Nonlinear Friction Contacts