Flutter stability assessment of a low pressure turbine rotor: A comparison between cantilever and interlocked configurations

In the last decades the aero-engine design has been more and more focusing on reducing weight and polluting emissions, yielding a fewer number of highly loaded and thinner blades. This leads to undesired vibration phenomena, such as flutter and forced response. Flutter is a major concern to be taken into account, since it is a self-excited and self-sustained aeroelastic instability phenomenon. The development of accurate numerical prediction methods is thus necessary in order to determine, during the design loop, whether a blade-row will or will not experience flutter. This paper presents a numerical assessment of flutter stability on a low pressure turbine rotor row in two different configurations designed and tested in the context of the European project FUTURE. The analyses have been carried out on both the cantilever and interlocked configurations of a single-pitch row sector for the first bending mode family. The modal analysis has been performed with an Open Source FEM tool (CalculiX) able to deal with complex interlocked rotor geometries and to model contact interfaces by using dedicated contact models with friction. On the other hand, the unsteady CFD simulations with moving blades have been carried out with the TRAF code, an in-house solver developed at the University of Florence which implements a non-linear method for flutter evaluation. The comparison between the cantilever and interlocked configurations in terms of flutter behavior is provided, showing the stabilization effect of the blade tip interlock device. The results proved to be in good agreement with the evidence coming from the experimental campaign.