Nonlinear dynamic response and bifurcation of asymmetric rotor system supported in axial-grooved gas-lubricated bearings

Dynamic characteristics of the asymmetric rotor system supported in axial-grooved gas-lubricated bearings are studied. In order to solve nonlinear dynamic response of rotor system effectively, a hybrid numerical model is established by coupling the motion equation of rotor with the rational function model of the gas film forces. The rational function model of the gas film forces of gas-lubricated bearing is established based on vector fitting theory. By using the hybrid numerical model, the repeated calculations of the unsteady Reynolds equation and gas film forces are avoided; the continuous rotor trajectory and the dynamic gas film forces can be calculated simultaneously; and for the rotor system supported in the same bearings, the computing cost can be saved effectively. The nonlinear dynamic responses of asymmetric rotor system supported in axial-grooved gas-lubricated bearings are investigated by trajectory diagrams, frequency spectrum, Poincaré maps, and time series. The bifurcations are analyzed by the bifurcation diagrams with different rotating speeds and mass eccentricities. The dynamic behaviors of the asymmetric rotor system appear complex nonlinear dynamic phenomenon and specific bifurcation characteristics.

Nonlinear Vibration Analysis of Turbine Bladed Disks with Mid-Span Dampers

Friction dampers are one of the most common secondary structures utilized to alleviate excessive vibration amplitudes in turbo-machinery applications. In this paper, the dynamic behavior of the turbine bladed disks coupled with one of the special damper designs, the so-called Mid-Span Dampers (MSDs) that is commonly used in steam turbines of Baker Hughes Company, is thoroughly studied. Friction between the blade and the damper is modeled through a large number of contact nodes by using 2D contact elements with a variable normal load. In the solution procedure, the coupled static/dynamic Harmonic Balance approach is utilized for the first time in the assessment of the dissipation capability of MSDs, computationally shown by predicting the forced response levels of the system at different resonances. Moreover, it is demonstrated that the nonlinear dynamic response is non-unique and it may vary considerably even if all the user-controlled inputs are kept identical. This phenomenon is a novel observation for MSDs and it is explained by an uncertainty present in the contact forces. Contact conditions corresponding to multiple responses are also investigated to unveil the different kinematics of the damper under the same nominal conditions.