A Conceptual Model for the Flutter Mechanism in Interlocked Turbine Blades

Many turbine rotor blades adopt the interlock configuration; among its advantages, there is a large increase in the natural frequencies with respect to the simpler cantilever configuration. Most widespread assessment methods estimate that this frequency increase should lead to a large improvement to the flutter stability of the interlocked rotor rows; some well-known approaches, such as the Panovsky-Kielb stability maps, suggest that flutter should be very unlikely for interlock configurations. Nevertheless, both the experience and detailed simulations indicate that it is possible to find flutter in interlocked blades, even with moderate aspect ratio. In this paper, the authors study the underlying physics behind flutter for interlocked blades, and present a very simplified model that is able to reproduce the main modal characteristics of interlocked blades, including the frequency increase and change in mode-shape as a function of the nodal diameter. This model also identifies a flutter mechanism, which is dependent on the aerodynamic interaction between the bending and torsion motion of the blade; even if both basic motions are individually stable, their interaction may lead to a coupled aeroelastic instability. The results from this simplified model are fully consistent with detailed three-dimensional linearized unsteady computational fluid dynamics simulations of representative turbine rotor blades, and should be of application in many realistic cases of interlock flutter. Finally, the authors present a study on the influence of the main parameters of the simplified model on the flutter stability, and derive some conclusions about the implications on flutter-free interlock design.