Analysis of nonlinear aeroelastic interactions during forced response of coupled turbine blades

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The flutter stability represents a dominant constraint in the design of modern low pressure turbines. An important reason for this is that the shape optimization has lead to slender, twisted blades with small chord-to-length ratio, which are known for their susceptibility to flutter. The saturation behavior of flutter vibrations is determined by structural and fluid dynamical nonlinearities, in particular the contact boundary conditions in mechanical joints and unsteady flow phenomena including shocks and stall.The primary goals of the project are(a) to develop methods for the coupled simulation of steady-state flutter and the prediction of the global aeroelastic stability, and(b) to understand the essential effects of nonlinear interactions.For the first time, the question is addressed whether there are configurations that do not flutter for sufficiently small perturbations around the equilibrium, but undergo aeroelastic self-excitation caused by nonlinear effects in the case of larger disturbances (nonlinear instability). Fundamental findings shall be gained concerning the conditions under which nonlinearities in the respective physical domain have a significant influence, and when they contribute to the onset and saturation of self-excited vibrations. Only on the basis of these findings, simplified methods for the flutter analysis can be developed and the error potential of present approaches can be estimated.The focus of the project is flutter of low pressure turbine blades. The mainly considered nonlinearities in the structural domain are the contact interactions in shroud joints, and in the fluid domain, stall and shocks under transonic conditions are considered as causes of strongly unsteady and, thus, nonlinear effects.