Multi-scale analysis of the roughness effect on lubricated rough contact

Determining friction is as equally essential as determining the film thickness in the lubricated contact, and is an important research subject. Indeed, reduction of friction in the automotive industry is important for both the minimization of fuel consumption as well as the decrease in the emissions of greenhouse gases. However, the progress in friction reduction has been limited by the difficulty in understanding the mechanism of roughness effects on friction. It was observed that micro-surface geometry or roughness was one of the major factors that affected the friction coefficient. In the present study, a new methodology coupling the multi-scale decomposition of the surface and the prediction of the friction coefficient by numerical simulation was developed to understand the influence of the scale of roughness in the friction coefficient. In particular, the real surface decomposed in different roughness scale by multi-scale decomposition, based on ridgelets transform was used as input into the model. This model predicts the effect of scale on mixed elastohydroynamic point contact. The results indicate a good influence of the fine scale of surface roughness on the friction coefficient for full-film lubrication as well as a beginning of improvement for mixed lubrication

How do liquids confined at the nanoscale influence adhesion?

Liquids play an important role in adhesion and sliding friction. They behave as lubricants in human bodies especially in the joints. However, in many biological attachment systems they acts like adhesives, e.g. facilitating insects to move on ceilings or vertical walls. Here we use molecular dynamics to study how liquids confined at the nanoscale influence the adhesion between solid bodies with smooth and rough surfaces. We show that a monolayer of liquid may strongly affect the adhesion.Comment: 5 pages, 9 color figures. Some figures are in Postscript Level 3 format. Minimal changes with respect to the previous version. Added doi and reference to the published article also inside the pape

Simulations of slip flow on nanobubble-laden surfaces

On microstructured hydrophobic surfaces, geometrical patterns may lead to the appearance of a superhydrophobic state, where gas bubbles at the surface can have a strong impact on the fluid flow along such surfaces. In particular, they can strongly influence a detected slip at the surface. We present two-phase lattice Boltzmann simulations of a flow over structured surfaces with attached gas bubbles and demonstrate how the detected slip depends on the pattern geometry, the bulk pressure, or the shear rate. Since a large slip leads to reduced friction, our results allow to assist in the optimization of microchannel flows for large throughput.Comment: 22 pages, 12 figure

Advanced interface modelling for 2D shell & 3D continuum problems

This work is motivated by the need for an efficient yet accurate approach for static and dynamic contact analysis of large-scale structures which can a) capture the optimum con- tact position with a moderate number of contact elements, and b) enable across-partition adaptive contact analysis within a parallel processing environment. In addressing these two issues, a novel adaptive node-to-surface contact approach is proposed to discretise the contact boundaries and to trace the evolution of contact locations. Contact search is a demanding process that can become quite complicated for certain types of problem. In this work, an efficient and robust contact search method is proposed, which can a) locally track the master facet of a given slave node despite the appearance of highly non-smooth contact surface, including surfaces with concave/convex regions or with distinct boundaries as well as reversible normals, and b) globally reallocate the master-slave contact pairs based on the penetration state without an expensive global search, providing an effective adaptive contact approach. A dual-interface-based domain decomposition method emphasising across-partition con- tact coupling is proposed. A pair of fully decomposed node-to-surface contact element are proposed to discretise the across-partition contact boundaries. The assumption of small incremental displacements is adopted, which a) avoids the excessive coupling between the decomposed master and slave, b) reduces significantly the communication overhead, and c) facilitates a flexible across-partition adaptive analysis. This strategy is found to provide good results for a sufficiently small time- or load-step, and it also facilitates mix-dimensional contact simulation. Another interest in current thesis is the inaccuracy in non-smooth plates modelled us- ing 2D displacement-based shell elements. In this work the dominant factor causing the inaccuracy is recognised as the incompatible tangential rotations on the two sides of the in- tersection. A 3-noded coupling element is introduced to impose a continuous constraint to couple the incompatible rotations. The significance of the discontinuity in the shell-based folded structure and the effectiveness of the coupling element is demonstrated through numerical studies comparing shell-based models to high fidelity solid-based models.Open Acces