Analysis of Global Dynamics of Rotating Systems like Jet Engines, with Special Emphasis on Harmonic Analysis in the Presence of Bearing with Clearances

The paper presents first a description of the methods used for the analysis of global dynamics of rotating systems like jet engines but also auxiliary power units. Different methodologies are described so to model rotating parts using beam, but also Fourier multi-harmonic, three dimensional models or to take into account cyclic symmetry and multistage cyclic symmetry concepts. Advantages and disadvantages of the different model types are discussed and compared. The coupling of the rotating parts with casings and stators is then discussed both in the inertial frame and in the rotating frame. The effect on global dynamics of bearing and other linking devices is taken into account for different type of analysis from critical speed analysis, to harmonic and transient analysis.

The effect of gears and gear boxes coupling different rotors (like it is the case for auxiliary power units in a jet engine) is then discussed and appropriate methods described so to model this coupling effect.

Finally, the question of the simulation of bearings with clearances is discussed and a new formulation proposed for the harmonic analysis of such situation. Numerical examples of such a simulation are given.