Identification of annular gas seal coefficients, rotorstator contact, and nonlinear analysis of a rotordynamic system

Abstract

This thesis deals with theoretical and experimental investigations of a test rig used for identification of annular gas seal coefficients. The work is divided in three parts: (i) the identification of coefficients from an annular seal; (ii) the simulation of the motion from a rotor-stator contact model; (iii) the analysis of rotordynamic system with anisotropic bearings and shaft, and a seal with cubic stiffness and damping. In the first part, the coefficients are computed by exciting the rotor with magnetic actuators and measuring the rotor-stator displacement with proximity sensors. The Complex Dynamic Stiffness (CDS) matrix is obtained as the transfer function between forces and displacements. The coefficients are obtained from the real and imaginary parts of the CDS matrix. For the rotor-stator contact, simplified models of the rotor and stator are used. The contact is modeled as tangential and normal forces with Coulomb friction. Three simulations are performed and different motion patterns are observed: forward, backward, subsynchronous, supersynchronous and chaotic vibration. The types of vibration are analyzed by orbits plots, Poincar´e maps, full spectra and full spectrograms. The last part of thesis considers a simplified anisotropic rotor with anisotropic bearings and an annular seal with cubic damping and stiffness coefficients. First, the system is analyzed by the Floquet theory, showing the behavior of the combination resonances with the degree of anisotropy from the bearings and rotor. Then, the fixed points of the system are computed by a semi-analytical method known as the Normal Forms method. The results are compared with a simple numerical integration of the equations.