Multibody modelling of friction based interaction between turbine blades

Turbine blades are subjected to nozzle excitation frequencies that can correspond to eigen frequencies during acceleration and deceleration phases so that it is necessary to introduce some form of damping to avoid fatigue and mechanical damage of the blade material. Friction based solutions are nonlinear and offer the interesting property to operate only when the level of vibrations reaches some threshold. But they can be difficult to tune and the availability of a model proves helpful for the designer. The system analysed in this paper is an experimental device consisting of 2 blades, which interact with each other through a so-called friction element (FE), in contact with the shrouds placed at the tip of the blades. The system is expected to reproduce the vibration response of the blades subjected to axial forces. During past experimental campaigns, one of the blades was excited out-of-plane (axially with respect to the turbine) by an electromagnet and the displacements of the blade tips and friction element had been measured. Different models of the device have already been developed: comprehensive finite element model and in house models developed under matlab . The latter possibly use different friction models, different kinematics of the friction element and can be based on time domain simulation or harmonic balance method. In this paper, we present a multibody model developed within the framework EasyDyn. Due to the nonlinear effects of friction, the equations of motion are solved in the time domain. The contact is introduced by defining, for each surface, 4 points of the friction element interacting with a plane attached to the shroud. The purpose is to analyse the influence of some modelling options: Rotation of the friction element, friction model, and normal force model and to draw rules concerning the possible simplifications. It turns out that the rotation of the friction element cannot be neglected if the normal contact stiffness is large. In this case, the deflection of the blades and the subsequent rotation of the blade shrouds make that the contact planes are no longer parallel, and that the bodies do not interact over the full expected contact surface. The contact stiffness will then have to be properly identified for reliable simulations