Fluid-rigid structure interaction system with Coulomb's law

International audienceWe propose a new model in a fluid-structure system composed by a rigid body and a viscous incompress-ible fluid using a boundary condition based on Coulomb's law. This boundary condition allows the fluid to slip on the boundary if the tangential component of the stress is too large. In the opposite case, we recover the standard Dirichlet boundary condition. The governing equations are the Navier-Stokes system for the fluid and the Newton laws for the body. The corresponding coupled system can be written as a variational inequality. We prove that there exists a weak solution of this system

A history of wear and wear prevention 1700-1940

Much of the present knowledge of the processes involved in the wear of materials has been derived since the end of the Second World War. This thesis shows, however, that many of the basic concepts of wear were understood, at least empirically, prior to 1940. Factors which influenced the rate of wear of components in machines began to be investigated during the second half of the last century, and particular combinations of sliding materials were chosen to give an adequate wear life for their applications. As background, the first two chapters describe the work on sliding and rolling friction during the eighteenth and nineteenth centuries. The third chapter present evidence which shows how the empirical understanding of wear developed up to 1940. This covers wear in both sliding and rolling contact, the relationship between wear and hardness as well as wear under abrasive conditions. The next chapter shows how the concept of the real area of contact, as opposed to the apparent area, emerged from studies of the electrical resistance between two metals in contact. With this technique, measurements of the real area of contact between surfaces under various loads were made in the late nineteen thirties. The chapter also traces the development of instruments for assessing the roughness of surfaces during the same decade. Chapters 5, 6 and 7 deal with wear prevention. Chapter 5 shows how developments in plain bearings kept pace with the duties imposed on them and describes some special forms, such as the "anti-friction" pivot and the marine thrust bearing. Data is also provided on the way in which the loads and speeds of bearings increased from 1700 to 1900. Chapter 6 deals with fluid lubrication and with the pioneering work of G.A. Hirn. Hirn's experiments were the first to demonstrate convincingly the complete separation of surfaces by a film of fluid. In chapter 7 the advances in metallurgy which enabled improved bearing metals to be made are outlined. In particular, the origins of the production of high-lead bronzes is described. These alloys proved to be highly wear resistant. Some aspects of white metals (both lead and tin based) are also described. The emphasis in the thesis is on the practical steps which were taken to mitigate the detrimental aspects of wear and to develop wear resistant materials, particularly for sliding bearings. The evidence presented shows that whilst separation of surfaces by a fluid film is the ideal means of preventing wear,in many instances lubrication conditions were far from ideal

Lubricated friction at the nano and mesoscale

Friction is central to numerous natural processes and technological applications, from the motion of synovial joints to car engines and wind turbines. Despite its ubiquitous relevance, a comprehensive picture is still lacking and current models are largely semi-empirical. Experiments conducted at the atomic scale have shed light on the fundamental origins of friction, but linking findings on the single atom or molecule level to macroscale observations involving countless tribological contacts remains a considerable challenge. To bridge this gap, results are needed at the mesoscale, typically between 1 nm and 1 $\mu$m, where atomistic information is still tractable but macroscopic behaviour begins to emerge. The problem becomes even more complex when considering the presence of a fluid lubricant confined in a nanogap between the two sliding surfaces. This thesis aims at bridging the current gap between atomistic models for lubricated friction and larger scale observations. This is done mainly using atomic force microscopy (AFM) which allows investigations of both the molecular level details and the mesoscale picture within the same experiment. Wherever possible, AFM measurements have been complemented by other experimental and computational techniques. Using a variety of model systems, the thesis studies the organisation and dynamics of lubricants under nanoconfinement at the solid/liquid interface. The investigations lead to some novel insights. First, polar and non-polar lubricants are shown to experience a structural transition under nanoconfinement, with the solid-like characteristics of the boundary layer being responsible for an increase in lubricated friction. Second, the lubricant molecular ordering can be modulated by surface singularities that limit the configurational entropy of the fluid molecules. This suggests surface defects indirectly influence the lubricant properties by inducing local molecular rearrangement. External factors, such as humidity and temperature, are also investigated. Some common threads in the different model systems suggest that atomistic models can be adapted at the mesoscale to describe lubricated friction based on a thermally activated process

PFEM–DEM for particle-laden flows with free surface

The final publication is available at Springer via http://dx.doi.org/10.1007/s40571-019-00244-1This work proposes a fully Lagrangian formulation for the numerical modeling of free-surface particle-laden flows. The fluid phase is solved using the particle finite element method (PFEM), while the solid particles embedded in the fluid are modeled with the discrete element method (DEM). The coupling between the implicit PFEM and the explicit DEM is performed through a sub-stepping staggered scheme. This work only considers suspended spherical particles that are assumed not to affect the fluid motion. Several tests are presented to validate the formulation. The PFEM–DEM results show very good agreement with analytical solutions, laboratory tests and numerical results from alternative numerical methods.Peer ReviewedPostprint (author's final draft

Mechanotransduction and Ion Transport of the Endothelial Glycocalyx: A Large-Scale Molecular Dynamics Study

In our vessels, the endothelial glycocalyx is the first and foremost barrier directly exposed to the blood in the lumen. The functions of the normal endothelial glycocalyx under physiological conditions are widely accepted as a physical barrier to prevent the abnormal transportation of blood components (e.g. ions, proteins, albumin and etc.) and a mechanosensor and mechanotransducer to sense and transmit mechanical signals from the blood flow to cytoplasm. In this study, a series of large-scale molecular dynamics simulations were undertaken to study atomic events of the endothelial glycocalyx layers interacting with flow. This research is a pioneer study in which flow in the physiologically relevant range is accomplished based on an atomistic model of the glycocalyx with the to-date and detailed structural information. The coupled dynamics of flow and endothelial glycocalyx show that the glycocalyx constituents swing and swirl when the flow passes by. The active motion of the glycocalyx, as a result, disturbs the flow by modifying the velocity distributions. The glycocalyx also controls the emergence of strong shear stresses. Moreover, flow regime on complex surface was proposed based on results from a series of cases with varying surface configurations and flow velocities. Based on the dynamics of subdomains of the glycocalyx core protein, mechanism for mechanotransduction of the endothelial glycocalyx was established. The force from blood flow shear stress is transmitted via a scissor-like motion alongside the bending of the core protein with an order of magnitude of 10~ 100 pN. Finally, the mechanism of flow impact on ion transport was investigated and improved Starling principle was proposed. The flow modifies sugar chain conformations and transfers momentum to ions. The conformational changes of sugar chains then affect the Na+/sugar-chain interactions. The effects of flow velocity on the interactions are non-linear. An estimation in accordance to the improved Starling principle suggests that a physiological flow changes the osmotic part of Na+ transport by 8% compared with stationary transport

Simulating landslide-induced tsunamis in the Yangtze River at the Three Gorges in China

Landslide-induced tsunamis may cause fatalities, damages and financial losses. In the Three Gorges Reservoir Area of China, several large landslides are still unstable and persistently creeping toward the Yangtze River. In this paper, we investigate the impacts of landslide-induced tsunamis in the Three Gorges Reservoir by using a hybrid numerical approach. One of the largest unstable mass in this area, the Huangtupo landslide, is chosen as the study object. First, the landslide deformation and initiating velocities are obtained by using the finite-discrete element method. The landslide-induced tsunamis and their impacts on shipping on the Yangtze River are then investigated through smooth particle hydrodynamics modelling. Our results reveal that an approximately 80% reduction in shear strength of the tip in the landslide will lead to catastrophic failure of the landslide, with sliding velocities of up to 8 m/s. Subsequently, such a collapse may initiate a river tsunami, propagating up to 9 m on the nearby reservoir banks within 3 km. The impacts on surrounding floating objects, such as surges and sways, heaves and rolls, are up to 110 m, 8 m and 6°, respectively. The simulations indicate that although the likelihood of a catastrophic failure of the whole landslide is low, the partial sliding still poses severe threat to the nearby reservoir banks and shipping on the Yangtze River. Thus, we recommend continuous monitoring as well as landslide early warning systems at this and also other hazardous sites in this area

Dynamic explicit finite element analysis of mechanical damage in natural gas transmission pipelines

Mechanical damage is one of the largest causes of failure in gas transmission pipelines today. The damage is typically caused by external forces, unstable ground, excavation and construction equipment. The damage can lead to immediate or delayed failure of the pipelines. The failure characteristics are affected both by the geometrical parameters of the damage as well as the residual stresses around it. The most commonly used inspection technique is magnetic flux leakage (MFL) method. Although such techniques are useful for characterizing the geometrical parameters of the damage, they are generally ineffective for characterizing residual stresses. A more promising technique involves the measurement of magnetic properties, which are affected by the residual stress in the pipe wall. Measurement of the magnetic properties around the defect may allow the estimation of residual stress. Once the geometrical parameters of the defect and the residual stress are estimated, the yield pressure of the pipe can be estimated. This thesis uses the dynamic explicit finite element method to simulate two typical contact-impact processes, a denting process and a scratching process, and to predict the residual stress distribution around the defect region. The failure pressure of the pipeline is then estimated. Both the pipeline thickness and rebound effect of the pipe after removal of the indentor were considered in the analysis. Areas for pursuing additional research in the future are recommended at the end of the thesis