Fully Coupled Analysis of Flutter Induced Limit Cycles: Frequency Versus Time Domain Methods

Abstract

To reduce emissions, it is crucial to reduce weight of aircraft engines and further increase aerodynamic efficiency of gas and steam turbines. For turbine blades these goals often lead to flutter. Thus, flutter-tolerant designs are explored, where flutter induces limit cycle oscillations (LCOs) of tolerable level. The saturation of flutter-induced vibrations can be caused by nonlinear frictional contact interactions e.g. in tip shroud interfaces. To develop flutter-tolerant designs, efficient methods are required which compute LCOs based on an appropriate mechanical modeling. We recently developed a Frequency Domain Fluid-Structure Interaction (FD-FSI) solver for flutter-induced LCOs, which relies on the harmonic balance method. It was shown that especially for long blades with friction in shroud interfaces and strong aerodynamic influence, a coupled analysis can significantly increase the accuracy of predicted LCOs compared to the current state-of-the-art methods. In the current work the FD-FSI solver is numerically validated against Time Domain (TD-FSI) simulations and we shed light on important advantages and disadvantages of both solvers. The FD-FSI solver contributes to an increased physical understanding of the coupled vibrations: We analyze why unexpected even harmonics appear in a certain LCO. On the other hand, the FD-FSI solver does not provide information on the asymptotic stability of the LCOs. Indeed, limit torus oscillations (LTOs) appear in our test case. Using the TD-FSI solver, we confirm the internal combination resonance for LTOs, for the first time, in a fully coupled analysis.