Influence of tip shroud modeling on the flutter stability of a low pressure turbine rotor

Since the modern design trend of low pressure turbine blades for aeronautical propulsion leads to lighter and more loaded blades, thus prone to flutter induced vibrations; flutter assessment is now an usual verification within the design loop of these components. Flutter stability assessment requires FEM and CFD tools able to predict the pressure response of fluid flow due to blade oscillation in order to compute the aerodynamic damping. Such tools are mature and validated, yet some geometrical aspects of the bladerow as contact interfaces at the blade tip shroud have to be carefully simulated to obtain accurate flutter results. The aim of this paper is to demonstrate the capability of the Open Source FEM tool (CalculiX) to deal with complex interlocked rotor geometries and to show the influence of different contact interface modeling on flutter stability. The solid mesh of a single-pitch row sector has been generated by using the Open Source suite Salome and the modal analysis has been carried out with CalculiX with cyclic symmetry conditions. The following uncoupled flutter simulations have been performed with the CFD TRAF code, an in-house solver developed at the University of Florence, which implements a non-liner method for flutter evaluation. An in-depth comparison among the FEM models with different boundary conditions in terms of modeshape frequency and aerodynamic damping curves are reported. These results show the effect of different contact interface modeling, especially on the first bending mode family, and confirm the overall row stability detected during a dedicated experimental flutter campaign.