Abstract
During the design process of turbomachinery, it is often not possible to use aerodynamically optimal designs due to aeroelastic constraints. The design choices are limited by possible structural failure, which can be caused by high vibration amplitudes, for example due to self-excited vibrations (flutter) or forced response. In particular, the modal damping has an important impact on these phenomena. In the absence of frictional contacts, damping is mainly created by aerodynamics. In this paper, the influence of additional damping created by the rotor bearing on the total damping and on forced vibrations will be investigated. This influence becomes relevant when blade vibrations with nodal diameters 1 and −1 couple with the shaft vibrations. The coupling is caused by a structural dynamic interaction of these two components. To investigate the blade-shaft coupling, a simulation process is set up to best represent the physical effects during operation. This simulation process is essentially based on a full structural dynamic model of the blisk-shaft assembly and a harmonic balance CFD model to account for the aeroelastic effects. In addition, mistuning identification is performed based on an experimental modal analysis at standstill. All results are incorporated into a structural reduced order model that calculates the vibrational behavior of the blading. These results are compared to damping determined during operation using an acoustic excitation system and measured forced frequency responses. The numerical results agree well with the experimental results, i.e. within the measurement uncertainty, both with respect to the damping and the mistuned frequency responses. Furthermore, the blade-shaft coupling results in significant changes of the eigen-frequencies and damping. As a consequence, damping increases by up to twelve times when taking the coupling into account. This reduces amplitudes by a factor of nine for the mistuned blade responses. Consequently, higher structural safety factors can be achieved by taking the blade-shaft coupling into account so that the remaining potentials in the aerodynamic design could be better exploited.
