Characterization of experimentally observed anomalous behavior of an intentionally mistuned LPT

Authors

Salvador Rodriguez  - Universidad Politécnica de Madrid, Spain
Carlos Martel - Universidad Politécnica de Madrid, Spain

Abstract

The investigation of the combined effect of flutter and forced response in a realistic low pressure turbine is one of the objectives of the European project ARIAS. With this purpose, some experiments were conducted at the Centro de Tecnologías Aeronáuticas in a wind tunnel facility where the blades of an aerodynamically unstable turbine were additionally excited with magnetic forces. On the post-processing of these experiments, some anomalous behavior was observed when an intentional mistuning pattern was included on the system. As expected, the system presented a response synchronous with the forcing on the regions close to the resonances. The intentional mistuning pattern was implemented by adding some extra mass to some of the turbine blades, and the response was therefore expected to show two resonant peaks with similar amplitudes, corresponding to the vibration of the blades with and without added mass. However, it was found that the two peaks were clearly different, with the peak at lower frequency presenting a much higher vibration amplitude than the higher frequency peak. In order to give a correct interpretation of the experimental results, a reduced order model is derived that includes only the amplitudes of the traveling wave modes coupled by the mistuning. This model, although extremely simple, is capable of reproducing the unexpected behavior of the experiments, and gives a clean explanation of the system response. It is shown that the relative size of the mistuning with respect to the difference in frequency of the involved traveling wave modes is the key parameter for the appearance of this phenomenon.