Why Do Granular Materials Stiffen with Shear Rate? : Test of Novel Stress-Based Statistics

Peer reviewedPublisher PD

Active swarms on a sphere

Here we show that coupling to curvature has profound effects on collective motion in active systems, leading to patterns not observed in flat space. Biological examples of such active motion in curved environments are numerous: curvature and tissue folding are crucial during gastrulation, epithelial and endothelial cells move on constantly growing, curved crypts and vili in the gut, and the mammalian corneal epithelium grows in a steady-state vortex pattern. On the physics side, droplets coated with actively driven microtubule bundles show active nematic patterns. We study a model of self-propelled particles with polar alignment on a sphere. Hallmarks of these motion patterns are a polar vortex and a circulating band arising due to the incompatibility between spherical topology and uniform motion - a consequence of the hairy ball theorem. We present analytical results showing that frustration due to curvature leads to stable elastic distortions storing energy in the band.Comment: 5 pages, 4 figures plus Supporting Informatio

Simulation of elongated bubbles in a channel using the two-fluid model

This paper investigates the capability of the two-fluid model to predict the bubble drift velocity of elongated bubbles in channels. The two-fluid model is widely used in the oil and gas industry for dynamic multiphase pipeline simulations. The bubble drift velocity is an important quantity in predicting pipeline flushing and slug flow. In this paper, it is shown that the two-fluid model in its standard form predicts a bubble drift velocit

Heimat in Holland:Duitse dienstmeisjes 1920-1950

Waarom kwamen duizenden jonge, ongehuwde vrouwen in Nederlandse huishoudens werken, hoe werd op hun komst en vestiging gereageerd en hoe verging het hen hier? Uit de indringende en soms pijnlijke interviews met voormalige Duitse dienstbodes wordt duidelijk hoe het was om als Duitse vrouw in Nederland te leven en werken in tij den van groeiende werkloosheid en opkomend nationaal-socialisme. Voorzover ze niet uit zichzelf naar de Heimat terugkeerden, werden velen daar na 1938 toe gedwongen door het nazi-regime. De achterblijfsters, van wie een groot deel inmiddels getrouwd was met een Nederlander, kregen te maken met de scheidslijnen tussen 'Nederlands' en 'Duits', 'goed' en 'fout'. Tot op de dag van vandaag werkt dit stereotype beeld over 'de' Duitsers door in het leven van deze vrouwen. Heimat in Holland levert een belangrijke bijdrage aan de discussie rond de betekenis van 'nationale identiteit' en de komst en vestiging van nieuwkomers in de Nederlandse samenleving

A statistical mechanics framework for static granular matter

The physical properties of granular materials have been extensively studied in recent years. So far, however, there exists no theoretical framework which can explain the observations in a unified manner beyond the phenomenological jamming diagram [1]. This work focuses on the case of static granular matter, where we have constructed a statistical ensemble [2] which mirrors equilibrium statistical mechanics. This ensemble, which is based on the conservation properties of the stress tensor, is distinct from the original Edwards ensemble and applies to packings of deformable grains. We combine it with a field theoretical analysis of the packings, where the field is the Airy stress function derived from the force and torque balance conditions. In this framework, Point J characterized by a diverging stiffness of the pressure fluctuations. Separately, we present a phenomenological mean-field theory of the jamming transition, which incorporates the mean contact number as a variable. We link both approaches in the context of the marginal rigidity picture proposed by [3, 4].Comment: 21 pages, 15 figure

Simulation of slug propagation for by-pass pigging in two-phase stratified pipe flow

The present paper is focused on the development of an accurate 1D numerical model for pig motion in two-phase flow. The focus will be on the liquid slug that is accumulated in front of the pig, the so-called pig-generated slug. Under the assumption of a stratified flow, we first discuss the academic case of liquid slug accumulation where we neglect the viscosity of the fluids. The size of the liquid slug will then effectively be determined by the speed of the hydrostatic wave which runs ahead of the pig. We also consider the more realistic case which includes the viscosity of the fluid. Finally, we discuss the effect of the presence of a by-pass in the pig on the accumulated liquid slug

Numerical simulation of roll waves in pipelines using the two-fluid model

A finite volume discretization of the incompressible two-fluid model in four-equation form is proposed for simulating roll waves appearing in multiphase pipelines. The new formulation has two important advantages compared to existing roll wave simulators: (i) it is conservative by construction, meaning that the correct shock magnitude is obtained at the hydraulic jump, and (ii) it can be more easily extended with additional physics (e.g. Compressibility, axial diffusion, surface tension), without rederiving the model equations. A simple, robust, first-order upwind discretization of the four-equation model is able to capture the roll wave profiles, although a fine grid is needed to achieve converged results. The four-equation model leads to new roll wave solutions that differ from existing analytical and numerical results. Our solutions are believed to be physically more correct because the shock relations satisfy physically conserved quantities