Homogeneous cooling of rough, dissipative particles: Theory and simulations

We investigate freely cooling systems of rough spheres in two and three dimensions. Simulations using an event driven algorithm are compared with results of an approximate kinetic theory, based on the assumption of a generalized homogeneous cooling state. For short times $t$, translational and rotational energy are found to change linearly with $t$. For large times both energies decay like $t^{-2}$ with a ratio independent of time, but not corresponding to equipartition. Good agreement is found between theory and simulations, as long as no clustering instability is observed. System parameters, i.e. density, particle size, and particle mass can be absorbed in a rescaled time, so that the decay of translational and rotational energy is solely determined by normal restitution and surface roughness.Comment: 10 pages, 10 eps-figure

Metaplectic Ice

Spherical Whittaker functions on the metaplectic n-fold cover of GL(r+1) over a nonarchimedean local field containing n distinct n-th roots of unity may be expressed as the partition functions of statistical mechanical systems that are variants of the six-vertex model. If n=1 then in view of the Casselman-Shalika formula this fact is related to Tokuyama's deformation of the Weyl character formula. It is shown that various properties of these Whittaker functions may be expressed in terms of the commutativity of row transfer matrices for the system. Potentially these properties (which are already proved by other methods, but very nontrivial) are amenable to proof by the Yang-Baxter equation

Propagating front in an excited granular layer

A partial monolayer of ~ 20000 uniform spherical steel beads, vibrated vertically on a flat plate, shows remarkable ordering transitions and cooperative behavior just below 1g maximum acceleration. We study the stability of a quiescent disordered or ``amorphous'' state formed when the acceleration is switched off in the excited ``gaseous'' state. The transition from the amorphous state back to the gaseous state upon increasing the plate's acceleration is generally subcritical: An external perturbation applied to one bead initiates a propagating front that produces a rapid transition. We measure the front velocity as a function of the applied acceleration. This phenomenon is explained by a model based on a single vibrated particle with multiple attractors that is perturbed by collisions. A simulation shows that a sufficiently high rate of interparticle collisions can prevent trapping in the attractor corresponding to the nonmoving ground state.Comment: 16 pages, 9 figures, revised version, to appear in Phys. Rev. E, May 199

Microscopic origin of granular ratcheting

Numerical simulations of assemblies of grains under cyclic loading exhibit ``granular ratcheting'': a small net deformation occurs with each cycle, leading to a linear accumulation of deformation with cycle number. We show that this is due to a curious property of the most frequently used models of the particle-particle interaction: namely, that the potential energy stored in contacts is path-dependent. There exist closed paths that change the stored energy, even if the particles remain in contact and do not slide. An alternative method for calculating the tangential force removes granular ratcheting.Comment: 13 pages, 18 figure

Plate bearing tests for working platforms

During piling and other construction works, a working platform is often constructed across the site. These platforms comprise aggregate material placed and compacted to a designed thickness. Satisfactory performance of the platform may be confirmed by a plate bearing test. Current guidance given on plate bearing testing of granular soils suggests that the plate be at least five times the nominal size of the coarsest material. For a working platform this may be large and the reaction load required from plant and resources to carry out the bearing test may become excessively high. The aim of the research presented in this paper was to investigate the effect of particle to plate size ratios to establish if the use of a smaller plate would still allow a reliable test to be performed on site. Plate bearing tests were carried out in a centrifuge using a large, coarse grained limestone. The limestone was graded to a scale representation of 6F2 material, a commonly specified particle size distribution for working platforms. The size of plate was varied and the load displacement response recorded. The measured bearing capacity was correlated with the ratio of particle to plate size

Responding to Sea Level Rise: Does Short-Term Risk Reduction Inhibit Successful Long-Term Adaptation?

Most existing coastal climate-adaptation planning processes, and the research supporting them, tightly focus on how to use land use planning, policy tools, and infrastructure spending to reduce risks from rising seas and changing storm conditions. While central to community response to sea level rise, we argue that the exclusive nature of this focus biases against and delays decisions to take more discontinuous, yet proactive, actions to adapt—for example, relocation and aggressive individual protection investments. Public policies should anticipate real estate market responses to risk reduction to avoid large costs—social and financial—when and if sea level rise and other climate-related factors elevate the risks to such high levels that discontinuous responses become the least bad alternative

The end-to-end testbed of the Optical Metrology System on-board LISA Pathfinder

LISA Pathfinder is a technology demonstration mission for the Laser Interferometer Space Antenna (LISA). The main experiment on-board LISA Pathfinder is the so-called LISA Technology Package (LTP) which has the aim to measure the differential acceleration between two free-falling test masses with an accuracy of 3x10^(-14) ms^(-2)/sqrt[Hz] between 1 mHz and 30 mHz. This measurement is performed interferometrically by the Optical Metrology System (OMS) on-board LISA Pathfinder. In this paper we present the development of an experimental end-to-end testbed of the entire OMS. It includes the interferometer and its sub-units, the interferometer back-end which is a phasemeter and the processing of the phasemeter output data. Furthermore, 3-axes piezo actuated mirrors are used instead of the free-falling test masses for the characterisation of the dynamic behaviour of the system and some parts of the Drag-free and Attitude Control System (DFACS) which controls the test masses and the satellite. The end-to-end testbed includes all parts of the LTP that can reasonably be tested on earth without free-falling test masses. At its present status it consists mainly of breadboard components. Some of those have already been replaced by Engineering Models of the LTP experiment. In the next steps, further Engineering Models and Flight Models will also be inserted in this testbed and tested against well characterised breadboard components. The presented testbed is an important reference for the unit tests and can also be used for validation of the on-board experiment during the mission

The mortality burden attributable to nontrauma fracture for privately insured adults with epilepsy

ObjectiveIndividuals with epilepsy have poor bone development and preservation throughout the lifespan and are vulnerable to nontrauma fracture (NTFx) and post‐NTFx complications. However, no studies have examined the contribution of NTFx to mortality among adults with epilepsy. The objective was to determine whether NTFx is a risk factor for mortality among adults with epilepsy.MethodsData from 2011 to 2016 were obtained from Optum Clinformatics Data Mart, a nationwide claims database from a single private payer in the United States. Diagnosis codes were used to identify adults (≥18 years old) with epilepsy, NTFx, and covariates (demographics and pre‐NTFx cardiovascular disease, respiratory disease, diabetes, chronic kidney disease, cancer). Crude mortality rate per 100 person‐years was estimated. Cox regression estimated hazard ratios (HRs) and 95% confidence intervals (CIs) were determined for mortality, comparing epilepsy and NTFx (EP + NTFx; n = 11 471), epilepsy without NTFx (EP without NTFx; n = 50 384), without epilepsy and with NTFx (without EP + NTFx; n = 423 041), and without epilepsy and without NTFx (without EP without NTFx; n = 6.8 million) after adjusting for covariates.ResultsThe 3‐, 6‐, and 12‐month crude mortality rates were highest among EP + NTFx (12‐month mortality rate = 8.79), followed by without EP + NTFx (12‐month mortality rate = 4.80), EP without NTFx (12‐month mortality rate = 3.06), and without EP without NTFx (12‐month mortality rate = 0.47). After adjustments, the mortality rate was elevated for EP + NTFx for all time points compared to EP without NTFx (eg, 12‐month HR = 1.70, 95% CI = 1.58‐1.85), without EP + NTFx (eg, 12‐month HR = 1.41, 95% CI = 1.32‐1.51), and without EP without NTFx (eg, 12‐month HR = 5.23, 95% CI = 4.88‐5.60). Stratified analyses showed higher adjusted HRs of 12‐month mortality for EP + NTFx for all NTFx sites (ie, vertebral column, hip, extremities), all age categories (young, middle‐aged, older), and for both women and men.SignificanceAmong adults with epilepsy and compared to adults without epilepsy, NTFx is associated with a higher 12‐month mortality rate. Findings suggest that NTFx may be a robust risk factor for mortality among adults with epilepsy.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154895/1/epi16465.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154895/2/epi16465_am.pd