Principal normal stress cavitation criterion for CFD analysis of loading capacity in journal bearings

Purpose Shear stresses have a considerable influence on the characteristics of lubricants and become significant at high rotating speeds. This study aims to investigate the influences of shear cavitation (SC) on loading capacity of journal bearings. Design/methodology/approach A principal normal stress cavitation criterion based on the stress applied to flowing lubricant in journal bearings is developed and used to investigate SC in journal bearings. A computational fluid dynamic (CFD) model for calculating the loading capacity is established using this criterion. After validation with experimental results, the loading capacity is calculated under different conditions. Findings The calculation results indicate that SC intensifies when viscosity, speed and eccentricity increase. Angle of loading capacity with SC is larger than that without SC. The magnitude of loading capacity with SC is smaller than that without SC due to the decrease in the ultimate pressure. In addition, the magnitude difference between the loading capacity with and without SC increases when viscosity, speed and eccentricity increases. Originality/value Present research can provide some guidance for calculating the loading capacity when a journal bearing is operating at high speed or with a high viscosity lubricant.

Shear Cavitation

The importance of cavitation in lubrication hydrodynamics is well recognized. Cavitation can also act as a source of experimental error in rheological measurements. Therefore, the ability to understand and predict cavitation is important for tribology. Nearly all models for cavitation prediction are based on the local hydrodynamic pressure. The appropriateness of this approach when viscous stresses are of the order of the hydrodynamic pressure is questionable. One cavitation model that considers the state of stress in a flowing liquid is the principal normal stress cavitation criterion (PNSCC), which proposes that cavitation will occur when the most tensile principal normal stress exceeds some critical value. Although this hypothesis can accommodate many experimental observations, its theoretical foundations are weak. In particular, it fails to account for the tensile strength of liquids and resulting need for nucleation sites; it neglects the role of transport of dissolved gases; and it does not consider the effect of a growing bubble on the local flow, and hence local state of stress. We demonstrate cavitation in low Reynolds number Couette flow, and present a model for cavitation in shear in the limit of creeping (Stokes) flow, which corrects for the theoretical failures of the PNSCC. We use numerical simulation to analyze cavitation onset, and obtain a more general cavitation criteria from which the PNSCC is recovered under certain conditions.