Effect of Fluid Compressibility on the Transient Response Characteristics of Open-Type Constant Flow Hydrostatic Bearings

The present work describes an experimental and theoretical study to demonstrate the effect of fluid compressibility caused by air in the oil film on the transient performance of an open-type constant flow hydrostatic bearing. An experimental study is conducted to investigate the transient response of the hydrostatic bearing under a step force excitation, and the results demonstrate that the damping coefficient of the bearing decreases with increasing coefficient of oil compressibility when the bearing ascends, but negligible effects of compressibility are observed when the bearing descends. To analyze the mechanism of the compressibility effects due to air in the lubrication, the flow field of oil with air is numerically obtained by solving the incompressible Navier–Stokes equations, with an analytical model describing the dependence of the fluid bulk modulus on the pressure and density. Qualitative agreement between the experimental and numerical results is obtained, which indicates that an increase in the air concentration in the oil, hence a reduction in the bulk modulus of the fluid, results in transient enlargement of the mass flow rate and increased oil film thickness. This finding explains the decrease in the damping coefficient when the bearing ascends.