Numerical Investigation of Rotors in Floating Ring Bearings using Co-Simulation

In this work, the nonlinear oscillation behavior of rotors in floating ring bearings is investigated numerically by means of transient run-up simulations. Non plain bearing designs are considered, which are commonly used in turbocharger applications. Furthermore, a mass-conserving cavitation model based on two-phase theory is employed, which is well suited for transient journal motions due to subsynchronous oscillations. Special axial boundary conditions are introduced, which better reflect open-ended bearings. In contrast to classical cavitation approaches, which assume degassing of dissolved air as the main mechanism, this boundary condition leads to sucking-in of air from the surroundings during squeeze motion of the journal. The numerical model is separated into a rotor and a bearing model, which are implemented in commercial software tools. Both subsystems are joined by means of one of two coupling methods, which enables the creation of detailed, easily interchangeable and updateable subsystems. Dynamic-static solver coupling is used for rotors in single film bearings with non mass-conserving cavitation models. An explicit co-simulation approach is employed for rotors in floating ring bearings or bearings with a mass-conserving cavitation model. In the first case, only the rotor subsystem is time-dependent. In the latter case, both subsystems are time-dependent and are solved with their respective solvers. All time-dependent subsystems use stable, implicit BDF solvers. The rotor is modeled using multibody dynamics software. The partial differential equation of the bearing subsystem is discretized with the Finite Element method, which yields high flexibility concerning the gap geometry. The influences of the bearing geometry and different parameters are investigated using a symmetric Jeffcott rotor. Also, an asymmetric, heavy turbocharger is considered. A non mass-conserving penalty cavitation approach yields a smooth pressure profile. The run-up behavior and the stability threshold are almost identical to the often-used half-Sommerfeld (Gümbel) condition. The hydrodynamic pressure in pockets or grooves is negligible due to their large gap size. However, the hydrostatic pressure has to be considered, which can stabilize or destabilize the system, depending on the lubricant supply geometry. The differences between non mass-conserving and the mass-conserving cavitation model are small for purely synchronous oscillations due to small unbalance. The two-phase model yields a lower stability threshold rotor speed, which is further decreased for open-ended bearings. Non mass-conserving models implicitly assume that the bearing gap is completely filled with lubricant at all times. During squeeze motion, the pressure build-up upon load-reversal is instantaneous. The pressure build-up is delayed for the mass-conserving cavitation model, which yields reduced radial damping. Furthermore, the circumferential extend of the pressure profile during whirl motion is smaller due to cavitation for mass-conserving models. The load carrying capacity is decreased, which may often yield higher eccentricities during sub-synchronous oscillations. A two-phase model can also entail a different bifurcation behavior, especially for insufficient lubricant supply