Efficient Modelling of Rotor-Blade Interaction Using Substructuring

Abstract

For safe rotor operation it is important to predict the torsional natural frequencies of the full rotor arrangement and not only of its components. The system's natural frequencies are typically speed-dependent if rotor and blade vibrations are coupled. In this contribution we focus on the torsional rotor-blade interaction, the coupling between torsional vibrations of the shaft and bending vibrations of blade rows attached to the shaft. During the design of a turbine shaft train, rotor blades are modelled using 3D finite elements due to its complex geometry and resulting vibration modes. This kind of model incorporates typically centrifugal loading due to the rotor rotation as well as contact modelling at the rotor-blade interface. Employing the method of substructuring enables to translate any complex blade which is modelled using 3D finite elements with thousands of physical degrees of freedom into a bunch of models with a single modal degree of freedom. Natural frequencies and modal masses are assigned to each modal degree of freedom representing the blade vibrations. These single degree of freedom models are coupled via so-called modal effective moments of inertia to the rotor shaft model. The resulting model resembles the rotor-blade interaction in all its details from the rotor point of view. The efficiency of this process is two-fold. On one hand, the resulting model size of the full rotor dynamic model becomes small and simple enough to allow elaborate parametric studies and design optimisations. On the other hand, translating the complex 3D blade model into a bunch of single degrees of freedom oscillators is extracted straightforwardly from standard output of commercial finite element software packages like Abaqus or Ansys.