A study in multi-scale soft porous lubrication

In this paper, we report a comprehensive experimental and theoretical approach to examine soft porous lubrication on different scales. The problem is involved with a planing surface gliding over a soft porous layer at a tilted angle. Two sets of experimental setup were designed and constructed, one on the larger scale with porous layer thickness being 19 mm and another on the smaller scale with porous layer thickness being 3 mm. The distribution of the pore pressure and its contribution to the overall lift are examined, which clearly demonstrates the hydrodynamic similarity, governed by the Brinkman parameter, α=h₂/K_p^0.5 and compression ratio, k=h₂/h₁, exists for the flow in a soft porous media. Here h₂ and K_p are the thickness and Darcy permeability of the porous layer at the leading edge, h₁ is the thickness at the trailing edge. We have further developed a theoretical model for predicting the solid phase lifting force, which, combined with the fluid pressure, provides a comprehensive method for the prediction of the pore pressure’s contribution to the overall lift, f_air. The approach is especially meaningful for the design of soft porous bearing with appropriate porous materials. Higher f_air leads to lower friction coefficient.