Design of mistuning patterns to control the vibration amplitude of unstable rotor blades

Flutter onset is one of the major causes for increased vibration levels in low pressure turbine (LPT) rotor blades. This paper describes the design process and the experimental testing of intentional mistuning patterns specifically chosen to show the possibility to control the flutter characteristics of an LPT rotor. The Asymptotic Mistuning Model (AMM) methodology is used to select the intentional mistuning patterns. The AMM formulation incorporates elastic and aerodynamic data from detailed FEM and CFD computations, and measured values of the rotor blades intrinsic mistuning. The intentional mistuning patterns are implemented in the rotor by mounting small masses at the tip-shroud of the blades, and the effect of these small masses is also introduced in the AMM description. Two intentional mistuning patterns are selected. The classical alternate mistuning pattern, designed to fully suppress flutter, and a second intentional mistuning pattern that is designed with the idea of halving the vibration amplitude of the tuned unstable rotor. This second mistuning pattern demonstrates that, through the implementation of the appropriate intentional mistuning pattern, flutter cannot only be suppressed but also modulated. The two mistuned LPT rotors were tested in a free flutter experiment at a high speed rotating wind-tunnel, and the experimental results showed a good agreement with the AMM stability predictions. This is the first time, to our knowledge, that the possibility to control flutter through intentional mistuning has been experimentally validated in a rotating rig. The AMM is also applied to evaluate the effect of the aerodynamic coupling in the stability calculations of the mistuned rotors, and the AMM results are compared with high fidelity numerical calculations. It is shown that, despite its very reduced formulation, the AMM produces quite accurate stability predictions, and that it is essential to take into account the aerodynamic coupling; if it is not considered then the instability level of the mistuned rotors can be substantially underestimated.