Numerical investigation of the mistuning influence on the vibration characteristics of a radial turbomachine rotor

Synchronous blade vibrations can have a severe impact on the service life of rotors as their occurrence leads to limit cycle oscillations, which over time increase the risk of component failure due to high cycle fatigue. A property that can substantially influ- ence the vibration characteristics of the rotor is mistuning. Mistuning appears due to structural differences between the blades, which violates the rotational periodicity of the impeller. Manufacturing tolerances, material inhomogeneities and uneven wear of the blades during operation are the main causing factors. Mistuning leads to localized dissipation of the vibration energy and significant amplitude overshoots. As a result, the amplitude of individual blades can be significantly higher than those of rotationally pe- riodic structures. Additionally, mistuning mixes vibration modes that are closely spaced in the frequency domain, leading to mistuned mode shapes. In this context, mistuned mode shapes, which are composed of compressor- and turbine-expressed modes, can cou- ple both rotor sides, causing vibrations across the entire rotor. This thesis investigates the vibrational behaviour of a mistuned rotor numerically using a mistuned FEM model. A comparison between a tuned and a mistuned rotor is contacted in regard to vibration localization and rotor coupling, while an examination of the coupled modes occurring due to mistuning is performed to identify the mechanism behind the appearance of these modes. Furthermore, the impact of the boundary conditions as well as the effect of dif- ferent mistuning patterns on the coupled modes is investigated. For the implementation of mistuning patterns, experimental data are employed.