The shape of jamming arches in two-dimensional deposits of granular materials

We present experimental results on the shape of arches that block the outlet of a two dimensional silo. For a range of outlet sizes, we measure some properties of the arches such as the number of particles involved, the span, the aspect ratio, and the angles between mutually stabilizing particles. These measurements shed light on the role of frictional tangential forces in arching. In addition, we find that arches tend to adopt an aspect ratio (the quotient between height and half the span) close to one, suggesting an isotropic load. The comparison of the experimental results with data from numerical models of the arches formed in the bulk of a granular column reveals the similarities of both, as well as some limitations in the few existing models.Comment: 8 pages; submitted to Physical Review

The measurement of lubricant-film thickness using ultrasound

Ultrasound is reflected from a liquid layer between two solid bodies. This reflection depends on the ultrasonic frequency, the acoustic properties of the liquid and solid, and the layer thickness. If the wavelength is much greater than the liquid-layer thickness, then the response is governed by the stiffness of the layer. If the wavelength and layer thickness are similar, then the interaction of ultrasound with the layer is controlled by its resonant behaviour. This stiffness governed response and resonant response can be used to determine the thickness of the liquid layer, if the other parameters are known. In this paper, ultrasound has been developed as a method to determine the thickness of lubricating films in bearing systems. An ultrasonic transducer is positioned on the outside of a bearing shell such that the wave is focused on the lubricant-film layer. The transducer is used to both emit and receive wide-band ultrasonic pulses. For a particular lubricant film, the reflected pulse is processed to give a reflection-coefficient spectrum. The lubricant-film thickness is then obtained from either the layer stiffness or the resonant frequency. The method has been validated using fluid wedges at ambient pressure between flat and curved surfaces. Experiments on the elastohydrodynamic film formed between a sliding ball and a flat surface were performed. Film-thickness values in the range 50-500 nm were recorded, which agreed well with theoretical film-formation predictions. Similar measurements have been made on the oil film between the balls and outer raceway of a deep-groove ball bearing

Transcutaneous vagus nerve stimulation via tragus or cymba conchae: Are its psychophysiological effects dependent on the stimulation area?

Efforts in optimizing transcutaneous vagus nerve stimulation (tVNS) are crucial to further develop its potential in improving cognitive and autonomic regulation. The present study focused on this topic. The aim was to compare for the first time the main stimulation areas of the ear currently used in studies with tVNS, taking cognitive as well as neurophysiological effects into account. The main areas to be compared with one another were tragus, cymba conchae, and earlobe (sham) stimulation. Post-error slowing, which has already been shown to be influenced by tVNS, was used to investigate the cognitive effects of tVNS when applied on the different auricular areas. On the neurophysiological level, we measured pupillary responses as an index of norepinephrine activity during post-error slowing, and cardiac vagal activity to investigate the activation of neural pathways involved in post-error slowing. Stimulation of different auricular areas led to no differences in post-error slowing and in pupillary responses. However, the neurological processes involved in post-error slowing could be observed, since norepinephrine activity increased after committing an error. Further, there was an increase in cardiac vagal activity over the test period that was independent of the stimulation areas. The results suggest that tVNS targeting the ear might have a non-specific effect on the processing of error commission, on pupillary responses, and on cardiac vagal activity. We conclude that it is necessary to consider alternatives for sham conditions other than electrical earlobe stimulation. [Abstract copyright: Copyright © 2021 Elsevier B.V. All rights reserved.

A supercritical series analysis for the generalized contact process with diffusion

We study a model that generalizes the CP with diffusion. An additional transition is included in the model so that at a particular point of its phase diagram a crossover from the directed percolation to the compact directed percolation class will happen. We are particularly interested in the effect of diffusion on the properties of the crossover between the universality classes. To address this point, we develop a supercritical series expansion for the ultimate survival probability and analyse this series using d-log Pad\'e and partial differential approximants. We also obtain approximate solutions in the one- and two-site dynamical mean-field approximations. We find evidences that, at variance to what happens in mean-field approximations, the crossover exponent remains close to $\phi=2$ even for quite high diffusion rates, and therefore the critical line in the neighborhood of the multicritical point apparently does not reproduce the mean-field result (which leads to $\phi=0$) as the diffusion rate grows without bound

Three-dimensional distinct element simulation of spherocylinder crystallization

Abstract.: We present a three-dimensional distinct element model (DEM) able to handle populations of spherocylinders. We report on granular crystallization occurring when vibrating mono-disperse assemblies of spherocylinders that faithfully reproduce the corresponding results of physical experiments from the literatur

Strong Approximation of Empirical Copula Processes by Gaussian Processes

We provide the strong approximation of empirical copula processes by a Gaussian process. In addition we establish a strong approximation of the smoothed empirical copula processes and a law of iterated logarithm

On the Dominance of Trivial Knots among SAPs on a Cubic Lattice

The knotting probability is defined by the probability with which an $N$-step self-avoiding polygon (SAP) with a fixed type of knot appears in the configuration space. We evaluate these probabilities for some knot types on a simple cubic lattice. For the trivial knot, we find that the knotting probability decays much slower for the SAP on the cubic lattice than for continuum models of the SAP as a function of $N$. In particular the characteristic length of the trivial knot that corresponds to a `half-life' of the knotting probability is estimated to be $2.5 \times 10^5$ on the cubic lattice.Comment: LaTeX2e, 21 pages, 8 figur