This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among ...the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.
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Determination of respiratory rate is a necessary task in assessing the state of health in humans. This review provides a description of modern devices used for recording and monitoring respiratory ...rate. The advantages and disadvantages of the principles of operation of these devices are discussed.
The paper is focuses on the methods that have come into practice, key functions and software instruments for collecting, analysis and visualization of network traffic statistics. The source of ...information is NetFlow telemetry data collected from network equipment. In addition to being used by network engineers and technicians, including for the purposes of network monitoring, incident handling, identification of network congestion and the main bandwidth utilizers with details on autonomous systems or IP addresses of sources and recipients, protocols, services and applications, NetFlow data is of interest in the context of monitoring and analysis of network interaction between users, service providers and consumers. The paper provides a detailed description of the developed and implemented on the basis of the new generation National Research Computer Network of Russia of the up-to-date and high-performance software solution for working with network telemetry data; specific examples of the capabilities are given in order to advanced analytics and descriptive data visualization in real time, taking into account the special needs of industry telecommunications networks in the field of research and education.
Summary
At near-grounded glacier termini, calving can lead to the capsize of kilometre-scale (i.e. gigatons) unstable icebergs. The transient contact force applied by the capsizing iceberg on the ...glacier front generates seismic waves that propagate over teleseismic distances. The inversion of this seismic signal is of great interest to get insight into actual and past capsize dynamics. However, the iceberg size, which is of interest for geophysical and climatic studies, cannot be recovered from the seismic amplitude alone. This is because the capsize is a complex process involving interactions between the iceberg, the glacier and the surrounding water. This paper presents a first step towards the construction of a complete model, and is focused on the capsize in the open ocean without glacier front nor ice-mélange. The capsize dynamics of an iceberg in the open ocean is captured by computational fluid dynamics (CFD) simulations, which allows assessing the complexity of the fluid motion around a capsizing iceberg and how far the ocean is affected by iceberg rotation. Expressing the results in terms of appropriate dimensionless variables, we show that laboratory scale and field scale capsizes can be directly compared. The capsize dynamics is found to be highly sensitive to the iceberg aspect ratio and to the water and ice densities. However, dealing at the same time with the fluid dynamics and the contact between the iceberg and the deformable glacier front requires highly complex coupling that often goes beyond actual capabilities of fluid-structure interaction softwares. Therefore, we developed a semi-analytical simplified fluid-structure model (SAFIM) that can be implemented in solid mechanics computations dealing with contact dynamics of deformable solids. This model accounts for hydrodynamic forces through calibrated drag and added-mass effects, and is calibrated against the reference CFD simulations. We show that SAFIM significantly improves the accuracy of the iceberg motion compared with existing simplified models. Various types of drag forces are discussed. The one that provides the best results is an integrated pressure-drag proportional to the square of the normal local velocity at the iceberg’s surface, with the drag coefficient depending linearly on the iceberg’s aspect ratio. A new formulation based on simplified added-masses or computed added-mass proposed in the literature, is also discussed. We study in particular the change of hydrodynamic-induced forces and moments acting on the capsizing iceberg. The error of the simulated horizontal force ranges between 5 and 25 per cent for different aspect ratios. The added-masses affect the initiation period of the capsize, the duration of the whole capsize being better simulated when added-masses are accounted for. The drag force mainly affects the amplitude of the fluid forces and this amplitude is best predicted without added-masses.
We developed a computational framework for simulating thin fluid flow in narrow interfaces between contacting solids, which is relevant for a range of engineering, biological and geophysical ...applications. The treatment of this problem requires coupling between fluid and solid mechanics equations, further complicated by contact constraints and potentially complex geometrical features of contacting surfaces. We developed a monolithic finite-element framework for handling mechanical contact, thin incompressible viscous flow and fluid-induced tractions on the surface of the solid, suitable for both one- and two-way coupling approaches. Additionally, we consider the possibility of fluid entrapment in ”pools” delimited by contact patches and its pressurization following a non-linear compressibility constitutive law. Image analysis algorithms were adapted to identify the local status of each interface element (i.e. distinguish between contact, fluid flow and trapped fluid zones) within the Newton–Raphson loop. First, an application of the proposed framework for a problem with a model geometry is given, and the robustness is demonstrated by the residual-wise and status-wise convergence. The full capability of the developed two-way coupling framework is demonstrated on a problem of a fluid flow in contact interface between a solid with representative rough surface and a rigid flat. The evolution of the contact pressure, fluid flow pattern and the morphology of trapped fluid zones under increasing external load until the complete sealing of the interface is displayed. Additionally, we demonstrated an almost mesh-independent result of a refined post-processing approach to the real contact-area computation. The developed framework permits not only to study the evolution of effective properties of contact interfaces, such as transmissivity and real contact area, but also to highlight the difference between one- and two-way coupling approaches, and, in particular, to quantify the effect of trapped fluid ”pools” on the coupled problem.
•A monolithic coupling framework was established for fluid flow in contact interface.•It accounts for trapped fluid pockets governed by a nonlinear constitutive model.•A consistent comparison between one-way and two-way couplings is carried out.•A refined post-processing provides a mesh-independent real contact area computation.
Trapped fluid in contact interface Shvarts, A.G.; Yastrebov, V.A.
Journal of the mechanics and physics of solids,
10/2018, Volume:
119
Journal Article
Peer reviewed
Open access
We study the mechanical contact between a deformable body with a wavy surface and a rigid flat taking into account pressurized fluid trapped in the interface. A finite element model is formulated for ...a general problem of trapped fluid for frictionless and frictional contact. Using this model we investigate the evolution of the real contact area, maximal frictional traction and global coefficient of friction under increasing external pressure. Elastic and elasto-plastic material models, compressible and incompressible fluid models and different geometrical characteristics of the wavy surface are used. We show that in case of incompressible fluid, due to its pressurization, the real contact area and the global coefficient of friction decrease monotonically with the increasing external pressure. Eventually, the contact opens and the fluid occupies the entire interface resulting in vanishing of static friction. An asymptotic analytical result for the critical trap-opening pressure is found and shown to be independent of the surface slope if it is small. In case of compressible fluids with pressure-dependent bulk modulus we demonstrate a non-monotonous behaviour of the global coefficient of friction due to a competition between non-linear evolution of the contact area and of the fluid pressure. However, for realistic compressibility of solids and fluids, contact-opening cannot be reached at reasonable pressures. On the other hand, in case of elastic-perfectly plastic materials, we again observe fluid permeation into the contact interface. Finally, we study the distribution of frictional tractions during the depletion of the contact area under increasing external pressure. This process leads to emergence of singularity-like peaks in tangential tractions (bounded by the Coulomb’s limit) near the contact edges. We point out the similarity between the processes of trap opening and interfacial crack propagation, and estimate the complex stress intensity factor in the framework of linear elastic fracture mechanics.
We study a regularisation of Coulomb's friction law on the propagation of local slip at an interface between a deformable and a rigid solid. This regularisation, which was proposed based on ...experimental observations, smooths the effect of a sudden jump in the contact pressure over a characteristic length scale. We apply it in numerical simulations in order to analyse its influence on the behaviour of local slip. We first show that mesh convergence in dynamic simulations is achieved without any numerical damping in the bulk and draw a convergence map with respect to the characteristic length of the friction regularisation. By varying this length scale on the example of a given slip event, we observe that there is a critical length below which the friction regularisation does not affect anymore the propagation of the interface rupture. A spectral analysis of the regularisation on a periodic variation of Coulomb's friction is conducted to confirm the existence of this critical length. The results indicate that if the characteristic length of the friction regularisation is smaller than the critical length, a slip event behaves as if it was governed by Coulomb's law. We therefore propose that there is a domain of influence of the friction regularisation depending on its characteristic length and on the frequency content of the local slip event. A byproduct of the analysis is related to the existence of a physical length scale characterising a given frictional interface. We establish that the experimental determination of this interface property may be achieved by experimentally monitoring slip pulses whose frequency content is rich enough.
The fractures of compact samples after static crack resistance testing, in which the crack on the addfracture site rotates at a right angle with respect to the plane of the initial (fatigue) crack. ...Possible reasons for the formation of this shape of fractures are analyzed. It is shown that one of the possible reasons for crack rotation consists in the presence of residual stresses in the sample body. To assess the magnitude of residual stresses, a template has been cut out from a part of a compact sample tested for static crack resistance subsequently examined by means of a double-sided layer removal technique. The presence of residual stresses in the direction parallel to the initial fatigue crack has been demonstrated, which confirms the conclusions made regarding a possible effect of residual stresses exerted on crack rotation in the course of addfracture.
A one-dimensional model based on a kinetic-type equation is proposed for studying the dynamic distribution density of virus carriers in time and space while taking into account their distribution ...from a dedicated center. This model is new and fundamentally different from known models of the diffusion–reaction type. The analytical solution is built; for obtaining a series of calculations, numerical methods are also used. The model and real data from Italy, Russia, and Chile are compared. In addition to the rate of infection, the “rate of recovery” is considered. When the wave of recovery passes through a territory with the greater part of the commonwealth, a conclusion is made about the onset of global recovery, which corresponds to real data. The predictions are proved to have been accurate also for the second wave of the pandemic in Russia. The model is expected to be able also to describe adequately subsequent epidemics instead of only the development of COVID-19.
•Cylindrical indentation on Nickel base single crystal superalloy is considered.•Primary indentation direction is 001 with two secondary directions 110, 010.•3D crystal plasticity simulations provide ...dominant slip systems on free surface.•Good agreement is found with experimentally observed slip lines.•Simulations show significant differences between core and free surface behaviour.
Crystal plasticity simulations and experiments of cylindrical indentation on Nickel base single crystal superalloy specimens are presented and discussed. The subsurface stress and strain fields presented are similar to those observed in meso-scale dovetail joints in single crystal turbine blades. Load is applied in the 001 primary orientation while the secondary orientation of the single substrate is varied. The secondary orientations 1¯10 and 010 are examined at room temperature. The plastic zone below the indent is analysed in terms of the activated slip systems. The Finite Element predictions are compared to the detailed experimental observation of slip lines on the free lateral surface of the substrate. Results presented are of particular relevance to the understanding of slip localisation in single crystal dovetail contacts and subsequent crystallographic crack nucleation and propagation induced by subsurface shear stresses.
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