Anchoring and migration of balloon in REBOA

A mechanical theory is described for a phenomenon in the surgical procedure of resuscitative endovascular balloon occlusion of the aorta (REBOA). In this procedure a balloon is pushed into the aorta by a catheter and then inflated in order to stop haemorrhage. One of the hazards of this procedure is the tendency for the balloon to migrate away from its intended position. This work examines the mechanics of balloon anchoring and migration by analysing the effects of pressure waves, the sheet flow and solid friction in the thin gap between the walls of the aorta and balloon. A viscoelastic model is adopted for the aorta wall for pressure waves between the left ventricle and the balloon. The lubrication approximation is used for blood flow in the thin gap between the walls of the balloon and aorta. Samples of quantitative predictions are discussed on how the inflation pressure and balloon characteristics affect the balloon anchoring and migration. The crucial roles of solid friction and balloon placement are pointed out, which should help in guiding the manufacturing of balloons and their usage in the field.

Effects of solid friction modifier on friction and rolling contact fatigue damage of wheel-rail surfaces

In railway network, friction is an important factor to consider in terms of the service behaviors of wheel-rail system. The objective of this study was to investigate the effect of a solid friction modifier (FM) in a railway environment. This was achieved by studying the friction, wear, and rolling contact fatigue (RCF) damage on the wheel-rail materials at different slip ratios. The results showed that when a solid FM was applied, the friction coefficient decreased. After the solid FM was separated from the wheel-rail interface, the friction coefficient gradually increased to its original level. With the application of the solid FM, the wear rates of the wheel-rail decreased. In addition, the thickness and hardness of the plastic deformation layers of the wheel-rail materials were reduced. The worn surfaces of the wheel-rail were dominated by pits and RCF cracks. Without the FM, RCF cracks ranged from 84 to 120 µm, and subsurface cracks were generated. However, with the FM, RCF cracks ranged from 17 to 97 µm and no subsurface cracks were generated. These findings indicate possible methods of improving the performance of railway rolling stock by managing friction, and reducing wear and permanent RCF damage affecting both the wheels and rails.

RTA Mathematic Simulation Principles at the Autotechnical Expertise

The features of carrying out an autotechnical expertise (ATE) are considered in case the vehicles (V) participating in the road transport accident (RTA) don’t leave skid imprints. The examples of momentum and energy conservation law application are given at developing the road accident mathematical model. Special attention is paid to the determination methods of vehicle (V) velocity, travel directions in various RTA diagrams and archeology of deformation. For this purpose it is offered to draw a momentum vector diagram. It is reasonable that for the calculation of V deformation at RTA it is necessary to determine step by step the strain-stress state in a contact area on the basis of the theories of elasticity, plasticity, solid friction and finite-element methods. The technique of constructing an RTA mathematical model is developed. It is recommended to use at ATE of RTAs at the runs-over into the fixed obstacle (a stationary V) and collisions.

Role of friction terms in two-dimensional modelling of dense snow avalanches

Voellmy–Salm friction model is one of the most extensively used theories for assessing the frictional terms of the equations that describe the motion of non-Newtonian flows such as snow avalanches. Based on the Coulomb- and turbulent-type friction, this model has been implemented in numerical tools for computation of snow avalanche dynamics based on the Shallow Water Equations (SWE). The range of the Voellmy parameters has been discussed widely, focusing mainly on the required values for achieving good results for the description of the moment and position of the avalanche when it stops. However, effects of parameters on the SWE terms, and their physical interpretation have not been investigated sufficiently. This work focuses on analysing the effects of the Voellmy–Salm parameters and cohesion on the avalanche characteristics and evolution of the new SWE-based numerical model Iber. In the numerical scheme, an upwind discretization was used for the solid friction and cohesion terms, while a centred one was used for the turbulent friction. Results show that the Voellmy–Salm model dominates the avalanche dynamics and the cohesion model allows the representation of long tails, whereas the friction and cohesion parameters may vary within a wide range.

Shear Thinning in the Prandtl Model and Its Relation to Generalized Newtonian Fluids

The Prandtl model is certainly the simplest and most generic microscopic model describing solid friction. It consists of a single, thermalized atom attached to a spring, which is dragged past a sinusoidal potential representing the surface energy corrugation of a counterface. While it was primarily introduced to rationalize how Coulomb’s friction law can arise from small-scale instabilities, Prandtl argued that his model also describes the shear thinning of liquids. Given its success regarding the interpretation of atomic-force-microscopy experiments, surprisingly little attention has been paid to the question how the Prandtl model relates to fluid rheology. Analyzing its Langevin and Brownian dynamics, we show that the Prandtl model produces friction–velocity relationships, which, converted to a dependence of effective (excess) viscosity on shear rate η ( γ ˙ ) , is strikingly similar to the Carreau–Yasuda (CY) relation, which is obeyed by many non-Newtonian liquids. The two dimensionless parameters in the CY relation are found to span a broad range of values. When thermal energy is small compared to the corrugation of the sinusoidal potential, the leading-order γ ˙ 2 corrections to the equilibrium viscosity only matter in the initial part of the cross-over from Stokes friction to the regime, where η obeys approximately a sublinear power law of 1 / γ ˙ .

Frictional anisotropy in casted seismic faults: or how to 3D print a fault to better characterize it

<p>Anisotropic phenomena have long been studied in the vicinity of seismic faults. It has for instance been shown that both in situ pore fluids and seismic mechanical waves travel at different velocities along various directions of a fault zone. Yet, while more and more complexity and disorder in seismic models are introduced to better understand earthquakes, frictional anisotropy is only rarely regarded. In many other domains than geophysics, however, such anisotropy in solid friction is believed to be crucial. For instance, the tribology of rubber tires, skis or advanced adhesives is improved when those are designed to have a preferential frictional direction. But numerous natural systems also benefit from such anisotropy: is is notably essential in the motion of numerous animal skins and in the efficient hydration of some plants. In most cases, these frictional anisotropies derive from the existence of preferential topographic orientations on, at least, one of the contact surfaces, with scales for such structural directivity that can be multiple and various. Seismic faults also exhibit such preferential directions in their topography: unique rock crystals, such as antigorite, can already display some frictional anisotropy, fault zones are  initiated by early fractures that often propagates through layered sediments, generating ramp-flat morphology in their surfaces and, finally, mature faults are marked by grooves of various wavelengths due to the slip induced erosion.</p><p> </p><p>In this work, we study how the morphology of faults affects their stability, as understood by their Coulomb static coefficient of friction. In particular we study its anisotropy with the slip direction. To do so, we make use of the 3D-printing technology and print actual fault surfaces, that were measured in the field. We perform friction experiments with gypsum casts of these 3D-printed faults, as mineral-like materials might deform differently under shear than plastic materials. With these experiments, we show that the friction coefficient along seismic faults is highly anisotropic, with slip motions that are easier in, but not limited to, the direction of the main grooves. This anisotropy could for instance be paramount to better predict the next direction of rupture along some faults under a varying stress state. In some cases, it could indeed not only be related to the orientation of the main regional stress, but also to the structural anisotropy, and  depending on stress and friction anisotropy, along which orientation a rupture criterion will first be exceeded.</p>

Changes to glacier friction law due to solid friction

<p>Theoretical laws for glacier friction over hard bedrocks rely on several assumptions. One fundamental assumption is that perfect sliding (no resistance to slip) occurs at the local scale between ice and bedrock, in which case friction only occurs at a mesoscale from ice flowing past bed irregularities - here called viscous friction. This assumption is however challenged by the numerous observations that glaciers carry debris at their basal layers, which can exert frictional resistance locally through solid-type friction between debris and rock. This is to be translated at a mesoscale as an additive frictional term to the law.<br>We study how the action of solid friction modifies the overall glacier basal friction by applying a simple effective-pressure dependant Coulomb friction law into a steady-state finite element model of a glacier over sinusoidal bedrock. We find that the viscous drag reaches the same maximum value regardless of whether there is local solid friction or not. However, we find that in the no-cavitation regime (low water pressures) the deformation-slip ratio near the bed is enhanced when solid friction occurs, although total slip is lower. As a result, the sliding parameter - ratio between viscous drag and slip - is no longer constant, as opposed to expected in a pure-sliding scenario. For high water pressures, the influence of solid friction becomes smaller and the law tends to the pure-sliding case. We propose a simple update to pure-sliding derived laws (Weertman, 1957; Fowler, 1981; Schoof, 2005; Gagliardini et al., 2007) to take into account this effect.</p>