Mode shape degeneration in linear rotor dynamics for turbocharger systems

The modelling and simulation process of high-speed rotor dynamics is a rather essential task that should accompany the virtual prototype development of exhaust gas turbochargers. The system’s nonlinear nature originates primarily in the oil-film concentrated in the journal bearings, which relates to the associated gyroscopic forward and backward modes. This study focuses on linear rotor dynamics and the impact of the bearing stiffness variation on the shape alternation of gyroscopic modes, which cannot be observed in the typical critical speed maps or Campbell diagrams. The mode degeneration is investigated with the help of the modal assurance criterion according to two principles: first, quantify the shape alternation of the associated modes on the basis of the average bearing stiffness from literature, and second, compare the consecutive modes against each other, either forward or backward. The latter reveals that the aforementioned degeneration leads to a jump with respect to the mode shape into the next occurring mode although it is still being registered as a system whirl that belongs to the previous natural frequency. The investigation is extended over a wide range of turbocharger frame sizes demonstrating that both the mode degeneration and jump occur independently of the assembly dimensions, thus offering an alternative insight on the possible form of the occurring rotor-bearing oscillations arising from motor excitation, aero-loads or self-excited vibrations due to oil-whirl.