Andreas Almqvist

A mixed lubrication model incorporating measured surface topography. Part 1: Theory of flow factors:

Abstract A mixed lubrication model that permits real three-dimensional surface topography as input is developed. The theory of computing flow factors within the model is presented, and with a following paper (Part 2) the method of measuring and adapting the surface roughness, and model validation through flow measurements and application to a bearing is shown. A contact mechanics model is used to calculate the elastoplastic displacement of a periodic topography signal. A method based on homogenization is used to calculate flow factors for all lubrication regimes. The flow factors are compared with the Patir and Cheng method. Results indicate that the two methods compare well for longitudinal roughness lay, but differ significantly for a cross-patterned surface roughness due to the more complete flow description of the current model.

A mixed lubrication model incorporating measured surface topography Part 2: roughness treatment, model validation, and simulation

Abstract A mixed lubrication flow factor model that permits real three-dimensional surface topography as input has been developed. Part 1 gives the theory of computing flow factors within the model. In this article, a method of adapting the measured surface topography signal to suit the numerical models is developed and presented in detail. The mixed lubrication model is validated through flow measurements for three different rough surface test specimens. Simulation of a hydrodynamic bearing was conducted and the results are presented in terms of pressure distributions and Stribeck curves covering all lubrication regimes. The results indicate that the model may be an efficient and accurate engineering design and research tool for tribological devices operating in all lubrication regimes.

Reiterated homogenization applied in hydrodynamic lubrication

This work is devoted to studying the combined effect that arises due to surface texture and surface roughness in hydrodynamic lubrication. An effective approach in tackling this problem is by using the theory of reiterated homogenization with three scales. In the numerical analysis of such problems, a very fine mesh is needed, suggesting some type of averaging. To this end, a general class of problems is studied that, e.g. includes the incompressible Reynolds problem in both Cartesian and cylindrical coordinate forms. To demonstrate the effectiveness of the method several numerical results are presented that clearly show the convergence of the deterministic solutions towards the homogenized solution. Moreover, the convergence of the friction force and the load-carrying capacity of the lubricant film is also addressed in this paper. In conclusion, reiterated homogenization is a feasible mathematical tool that facilitates the analysis of this type of problem.

A semi-deterministic texture-roughness model of the piston ring–cylinder liner contact

Many simulations already exist to model the piston ring–cylinder liner contact; however, very few models have been used to investigate the optimum surface texture. An axi-symmetric, time-dependent two-dimensional semi-deterministic texture-roughness model of the piston ring to cylinder liner contact with periodic boundary conditions and mass preserving global cavitation has been developed. The cylinder liner texture, generated by honing, was considered deterministically on the global scale, after an investigation comparing deterministic and homogenized solutions. The surface texture of a real cylinder liner was measured with white light interferometry and an algorithm developed to generate an artificial periodic texture representative of the real surface. The effect of cylinder liner plateau roughness has been incorporated on the local scale by homogenization of the Reynolds equation and calculation of flow factors from real surface topography. Using the homogenization technique to incorporate the effect of surface roughness leads to a more efficient solution than mesh refinement of the deterministic problem as the roughness does not need to be resolved on the global solution domain, allowing for significantly less degrees of freedom in the global problem. The lubricant boundary pressures have been calculated using results from a numerical ring-pack model and the lubricant viscosity has been adjusted based on the cylinder liner wall temperature. It was found from the result of a comparison between deterministic and homogenized solutions that surface texture should be modelled on the global and not on the averaged roughness scale as is the case with many previous investigations.

A comparison between computational fluid dynamic and Reynolds approaches for simulating transient EHL line contacts

When simulating elastohydrodynamic lubrication (EHL), the Reynolds equation is the predominating partial differential equation for prediction of the fluid flow. Also very few attempts have been carried out using the full momentum and continuity equations separately. The aim of this investigation is to compare two different approaches for simulation of EHL line contacts where a single ridge travels through an EHL conjunction. One of the approaches is based on the Reynolds equation, addressing the coupling between the pressure and the film thickness. The solver uses the advantages of multilevel techniques to speed up the convergence rate. The other approach is based on commercial CFD software. The software uses the momentum and continuity equations in their basic form, enabling numerical simulations outside the contact regions, as well as in the thin film region to be carried out. The numerical experiments show that, under the running conditions chosen, only small deviations between the two approaches can be observed. The results are encouraging from several viewpoints: validation of the codes, the possibilities of further developments of the CFD approach and the justification of using a Reynolds approach under the running conditions chosen.

Similarities and Differences Between the Flow Factor Method by Patir and Cheng and Homogenization

Different averaging techniques have proved to be useful for analyzing the effects of surface roughness in hydrodynamic lubrication. This paper compares two of these averaging techniques, namely the ...

A New Approach for Studying Cavitation in Lubrication

The underlying theory, in this paper, is based on clear physical arguments related to conservation of mass flow and considers both incompressible and compressible fluids. The result of the mathematical modeling is a system of equations with two unknowns, which are related to the hydrodynamic pressure and the degree of saturation of the fluid. Discretization of the system leads to a linear complementarity problem (LCP), which easily can be solved numerically with readily available standard methods and an implementation of a model problem in matlab code is made available for the reader of the paper. The model and the associated numerical solution method have significant advantages over todays most frequently used cavitation algorithms, which are based on Elrod–Adams pioneering work.

An experimental and numerical investigation of frictional losses and film thickness for four cylinder liner variants for a heavy duty diesel engine

The piston ring pack is the single greatest contributor to mechanical losses in a heavy duty diesel engine, accounting for 1.1–6.8% of the total losses. Therefore, the piston ring-cylinder liner contact is potentially the most rewarding area to study when attempting to reduce mechanical losses in a heavy duty diesel engine. In this work, four different heavy duty diesel engine cylinder liner variants have been tested to evaluate the lubricating conditions that occur when a section of top compression ring is reciprocated against them in a lubricated environment. Two of the cylinder liners were traditional grey cast iron and plateau honed with different honing angles, one had ANS Triboconditioning® applied and the last was plasma sprayed with a stainless steel and ceramic coating, then honed. An experimental test rig was used where friction and film thickness was recorded, by means of an ultrasonic technique. A numerical model was also developed to calculate the friction and film thickness. Comparisons are made between the simulation and experiment, and the four cylinder liner variants are also evaluated. It was found that both simulation and experiment could differentiate between all surfaces and the results from the model and experiment also correlated well with each other. A lower plateau average surface roughness, as exhibited by the ANS Triboconditioning® and plasma liners, led to a significant reduction in friction.

A stochastic two-scale model for pressure-driven flow between rough surfaces.

Seal surface topography typically consists of global-scale geometric features as well as local-scale roughness details and homogenization-based approaches are, therefore, readily applied. These provide for resolving the global scale (large domain) with a relatively coarse mesh, while resolving the local scale (small domain) in high detail. As the total flow decreases, however, the flow pattern becomes tortuous and this requires a larger local-scale domain to obtain a converged solution. Therefore, a classical homogenization-based approach might not be feasible for simulation of very small flows. In order to study small flows, a model allowing feasibly-sized local domains, for really small flow rates, is developed. Realization was made possible by coupling the two scales with a stochastic element. Results from numerical experiments, show that the present model is in better agreement with the direct deterministic one than the conventional homogenization type of model, both quantitatively in terms of flow rate and qualitatively in reflecting the flow pattern.

Homogenization of the Reynolds equation governing hydrodynamic flow in a rotating device

In this paper, a method facilitating the analysis of the effects of surface roughness on the lubrication of a rotating device is presented. The analysis utilizes homogenization—a suitable technique ...