Comparison of Perturbed Reynolds Equation and CFD Models for the Prediction of Dynamic Coefficients of Sliding Bearings

Abstract

Abstract:The accuracy and utility of rotordynamic models for machinery systems are greatly affected by the accuracy of the constituent dynamic bearing models. Primarily, the dynamic behavior of bearings is modeled as linear combination of mass, damping, and stiffness coefficients that are predicted from a perturbed Reynolds equation. In the present paper, an alternative method using Computational Fluid Dynamics (CFD) with a moving boundary is used to predict the dynamic coefficients of slider bearings and the results are compared with the more commonly employed perturbed Reynolds equation model. A linear slider bearing geometry is investigated and the results serve as precursors to similar investigations involving the more complex journal bearing geometries. Time and frequency domain methods for the estimation of dynamic coefficients are shown to give comparable results. For CFD with a moving boundary, temporal inertia is found to have a significant effect for a reduced, squeeze Reynolds number less than one. The temporal inertia effect is captured through an added mass coefficient within the dynamic model of the bearing.

A CFD-Based Frequency Response Method Applied in the Determination of Dynamic Coefficients of Hydrodynamic Bearings. Part 1: Theory

“…The present work is a follow-up to Snyder and Braun [20], and extends the authors' previous theoretical work in two main ways:…”

“…Snyder and Braun [20] previously applied a frequency domain method to CFD-predicted hydrodynamic forces to determine the dynamic coefficients of bearings. The method was shown to be a viable alternative to solving the perturbed Reynolds equation for bearing dynamic coefficients.…”

“…A notable feature of Equation 1is retention of an added mass coefficient associated with inertia of the fluid. Snyder et al [20] previously demonstrated that lubricant inertia can cause an appreciable difference between the dynamic bearing stiffness (k yy + m yy ω 2 ) predicted using CFD (lubricant inertia present) and the static stiffness (k yy ) predicted by the perturbed Reynolds equation (lubricant inertia neglected).…”

“…Alternative methods to separate the dynamic stiffness into added mass and static stiffness effects are investigated. The authors [20] previously demonstrated that temporal inertia effects as embodied in added mass coefficients are inherent within transient CFD simulations. It then becomes necessary to perturb the bearing at multiple frequencies in order to fit the static stiffness and added mass parameters.…”

A Novel Method for the Dynamic Coefficients Identification of Journal Bearings Using Kalman Filter

“…In a rotor-bearing system, the bearing dynamic coefficients affect the dynamic behaviour of the system directly, such as the critical speed, imbalance response, and stability performance. Many investigations based on the theoretical model have been carried out to calculate the dynamic coefficients [1][2][3][4]. However, the simplification in modeling inevitably leads to errors between the calculated and actual values.…”