Mustapha Lahmar

Couple Stress Effects on the Dynamic Behavior of Connecting Rod Bearings in Both Gasoline and Diesel Engines

An isothermal hydrodynamic analysis of big end connecting rod bearings for both diesel and gasoline engines lubricated with couple stress fluids is undertaken. Based on the V. K. Stokes micro-continuum theory, an incompressible modified Reynolds equation is derived from the fluid motion and mass conservation equations using the assumptions of thin-film theory. The hydrodynamic performance and the crank pin center trajectories are determined numerically by means of the Booker mobility technique. Compared with the Newtonian lubricant case, the lubricants with couple stresses provide an increase of the minimum film thickness, and a drastic decrease of the power loss, peak pressure, and flow rate over one engine cycle for both engines.

Elastohydrodynamic Lubrication Analysis of a Compliant Journal Bearing Considering Static and Dynamic Deformations of the Bearing Liner

In this article, the effect of both static and dynamic deformations of the bearing liner on the dynamic performance characteristics and stability of a water-lubricated, rubber-lined journal bearing operating under small harmonic vibrations is theoretically investigated. To take into account the dynamic deformations of the bearing liner, the first-order perturbation technique is used to determine the eight dynamic coefficients for a given excitation frequency value. The static and dynamic deformation of the fluid/bearing-liner interface is assumed to be proportional to the steady-state and dynamic fluid-film pressures. It was found that the dynamic properties and stability of the compliant finite-length journal bearing are affected by surface coatings from soft materials. It was also shown that when dynamic deformations are considered in the calculations, the dynamic coefficients depend on the excitation frequency, especially for higher values of this parameter. Moreover, the two cross-damping coefficients differ from each other, while the classical elastohydrodynamic (EHD) theory predicts them to be equal, when the dynamic deformations are ignored.

The homogenization method of roughness analysis in turbulent lubrication

The hydrodynamic lubrication theory founded by Osborne Reynolds in 1886 is based on several assumptions, including that the contact surfaces are perfectly smooth, and the lubricant flow regime is laminar. However, in certain special cases these hypotheses may not be valid, and it is seen that surface roughness and turbulence affect bearing performance. The approach adopted in the present work is based on the application of the homogenization method to the turbulent Reynolds equation. In homogenization, multiscale expansion of the fluid pressure terms leads to a system of four partial differential equations governing two types of problems (local and global). The solutions are, respectively, periodic functions, and the homogenized pressure. As an illustrative example, a journal bearing with a rough surface operating in the turbulent regime is analyzed. Numerical simulations are performed by imposing periodic isotropic roughness patterns over the stationary outer cylinder surface. The homogenized solutions are compared to direct solutions of the deterministic problem. Homogenization is shown to be a powerful but practical method to attack problems of rough surfaces and is applied to turbulent lubrication for the first time. In the plain journal bearing example, the effect of roughness is to increase the load significantly in many cases. The effect of turbulence is to greatly increase the load, in both the rough and smooth cases.

Elasto-Aerodynamic Lubrication Analysis of a Self-Acting Foil Air Journal Bearing

Most of elasto-aerodynamic approaches under dynamic conditions proposed in the technical literature include only the static pressure induced deformation of foils. This paper presents a theoretical investigation on the effects of both static and dynamic deformations of the foils on the dynamic performance characteristics and stability of a self-acting air foil journal bearing operating under small harmonic vibrations. To take into account the dynamic deformations of foils, the perturbation method is used for determining the gas-film stiffness and damping coefficients for given values of excitation frequency, compressibility number, and compliance factor of the bump foil. The nonlinear stationary Reynolds’ equation is solved by means of the Galerkin’s finite element formulation while the finite differences method are used to solve the first order complex dynamic equations resulting from the perturbation of the transient compressible Reynolds’ equation. It was found that the dynamic properties and stability of the compliant finite length journal bearing are significantly affected by the compliance of foils especially when the dynamic deformation of foils is considered in addition to the static one by applying the principle of superposition.Copyright

A Study of Combined Effects of Surface Roughness and Lubricant Rheology on the Hydrodynamic Contact Behavior Using the Homogenized Method

The combined effects of surface roughness and lubricant rheology on the performance characteristics of a hydrodynamic inclined slider bearing are investigated by means of the homogenized method. The pad surface is rough and stationary while the lower surface is assumed to be smooth and moving. The V. K. Stokes couple stress fluid model is adopted to describe the rheological behavior of the lubricant flowing between the two surfaces. The numerical simulations are performed by considering three roughness patterns (transverse, longitudinal and anisotropic), and various values of the couple stress parameter. It is found that the homogenization method is rigorous and efficient for the three roughness patterns considered. It is also found that the combined effects of the surface roughness as well as the couple stress due to the presence of polymer additives on the hydrodynamic performance characteristics such as load carrying capacity, friction factor are significant.Copyright