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The force response of squeeze film dampers and hydrodynamic bearings is significantly influenced by the occurence of cavitation inside the lubricant films. The viscosity of the occuring mixture of oil and air bubbles is vastly different than that of pure lubricant and therefore changes the pressure build‐up and thereby the stiffness and damping properties of the lubrication films. To accurately predict the force response of a squeeze film damper, the cavitation and its effect on the pressure build‐up therefore has to be taken into account. Well known cavitation algorithms for lubrication problems, like the Elrod algorithm or the two‐phase‐model, neglect the transient nature of outgassing and assume instantaneous bubble formation and collapse. In the presented paper, a transient bubble dynamics model based on the Rayleigh‐Plesset‐Scriven equation is utilised instead and its effect on the rotordynamic behaviour of a turbocharger rotor is examined based on fully transient multi body dynamics simulations. [ABSTRACT FROM AUTHOR]

A common problem in transient rotordynamic simulations is the numerical effort necessary for the computation of hydrodynamic bearing forces. Due to the nonlinear interaction between the rotordynamic and hydrodynamic systems, an adequate prediction of shaft oscillations requires a solution of the Reynolds equation at every time step. Since closed-form analytical solutions are only known for highly simplified models, numerical methods or look-up table techniques are usually employed. Numerical solutions provide excellent accuracy and allow a consideration of various physical influences that may affect the pressure generation in the bearing (e.g., cavitation or shaft tilting), but they are computationally expensive. Look-up tables are less universal because the interpolation effort and the database size increase significantly with every considered physical effect that introduces additional independent variables. In recent studies, the Reynolds equation was solved semianalytically by means of the scaled boundary finite element method (SBFEM). Compared to the finite element method (FEM), this solution is relatively fast if a small discretization error is desired or if the slenderness ratio of the bearing is large. The accuracy and efficiency of this approach, which have already been investigated for single calls of the Reynolds equation, are now examined in the context of rotordynamic simulations. For comparison of the simulation results and the computational effort, two numerical reference solutions based on the FEM and the finite volume method (FVM) are also analyzed. [ABSTRACT FROM AUTHOR]

Squeeze film dampers (SFDs) are commonly used in turbomachinery in order to introduce external damping, thereby reducing rotor vibrations and acoustic emissions. Since SFDs are of similar geometry as hydrodynamic bearings, the REYNOLDS equation of lubrication can be utilised to predict their dynamic behaviour. However, under certain operating conditions, SFDs can experience significant fluid inertia effects, which are neglected in the usual REYNOLDS analysis. An algorithm for the prediction of these effects on the pressure build up inside a finite-length SFD is therefore presented. For this purpose, the REYNOLDS equation is extended with a first-order perturbation in the fluid velocities to account for the local and convective inertia terms of the NAVIER-STOKES equations. Cavitation is taken into account by means of a mass conserving two-phase model. The resulting equation is then discretized using the finite volume method and solved with an LU factorization. The developed algorithm is capable of calculating the pressure field, and thereby the damping force, inside an SFD for arbitrary operating points in a time-efficient manner. It is therefore suited for integration into transient simulations of turbo machinery without the need for bearing force coefficient maps, which are usually restricted to circular centralized orbits. The capabilities of the method are demonstrated on a transient run-up simulation of a turbocharger rotor with two semi-floating bearings. It can be shown that the consideration of fluid inertia effects introduces a significant shift of the pressure field inside the SFDs, and therefore the resulting damper force vector, at high oil temperatures and high rotational speeds. The effect of fluid inertia on the kinematic behaviour of the whole system on the other hand is rather limited for the examined rotor. [ABSTRACT FROM AUTHOR]

The vibration behaviour of fast rotating rotors is significantly influenced by the bearing properties. Lubricant film induced excitations can cause sub‐synchronous rotor oscillations known as oil‐whirl and oil‐whip phenomena. The non‐linear bearing properties depend primarily on the lubricant properties, kinematics of bearing partners and especially on the occurrence of cavitation. Outgassing processes lead to a two‐phase flow consisting of gas and oil, which can influence the bearing stiffness and damping respectively and consequently the rotor response behaviour. In this contribution, the oscillations of a semi‐floating ring supported turbocharger rotor are investigated under the influence of lubricant film cavitation. For this purpose, run‐up simulations are carried out under the assumption of mass‐conserving cavitation according to the two‐phase model and compared with measurements. In order to illustrate the influence of outgassing processes, a comparison is made with non‐mass‐conserving cavitation theory according to the assumptions of Half‐Sommerfeld. [ABSTRACT FROM AUTHOR]

The numerical effort of rotordynamic simulations with hydrodynamic bearings is often dominated by the solution of the Reynolds equation. Because of the nonlinear fluid–structure interaction, this equation needs to be solved in every time step. Although computationally efficient analytical approximations and look-up table techniques exist, these approaches often do not provide the necessary modeling depth and accuracy. Thus, in many cases, numerical methods are preferred despite their computational cost. In recent studies, a semi-analytical solution of the Reynolds equation has been developed based on the scaled boundary finite element method (SBFEM) with the objective of improving the numerical efficiency. The developed approach assumed Gümbel conditions, which means that cavitation was handled in a highly simplified manner. In this work, the SBFEM solution is for the first time combined with a more complex, nonlinear cavitation model based on the Elrod algorithm. Because of the semi-analytical technique, this requires a simplification of the switch function that defines the locations of the pressure and cavitation zones. However, as demonstrated in this paper, the resulting bearing forces are still in good agreement with a standard numerical reference solution based on the finite volume method (FVM). Since the SBFEM model uses only a one-dimensional discretization, its high-order formulation is very straightforward, even in case of a non-equidistant grid with varying shape functions that take the smooth and non-smooth regions of the solution into account. As a result, the numerical effort is reduced significantly in comparison to the standard FVM. [Display omitted] • An SBFEM solution of the Reynolds equation is combined with an Elrod cavitation model. • The accuracy of the hydrodynamic forces computed with this approach is adequate. • The developed method is numerically efficient compared to the standard FVM. [ABSTRACT FROM AUTHOR]