DEM simulations of the frictional and frictionless polydisperse packings of spheres under uniaxial compression

The uniaxial compression of polydisperse assemblies of spherical frictional and frictionless particles is modeled with the discrete element method (DEM). The normal particle size distribution with standard deviation of particle mean diameter in the range from 0% to 80% was applied. The series of numerical tests have been conducted to study the micromechanical and macromechanical properties of packings of spheres. The micro-scale analyses included distribution of contact forces and average coordination number, whereas macromechanical study included the elasticity, stress transmission and angle of internal friction in the assemblies. The linear increase in solid fraction was observed for standard deviation of particle mean diameter increasing up to 50% in assemblies of both, frictional and frictionless spheres under pressure of 100kPa. Further increase in particle size heterogeneity decreased solid fraction in systems. The increase in coefficient of interparticle friction resulted in decrease in solid fraction by above 10% in the whole range of variability of SD value due to the different space-filling properties of frictional particles. The stiffness of samples increased with compressive loads increasing, however no clear effect of particle size polydispersity on the effective elastic modulus of mixtures was found in frictional sphere packings. The effective elastic modulus increased with SD value increasing up to 50% in sample composed of smooth particles that decreased for higher SD values. Discrete element method predicted decrease in pressure ratio with standard deviation of particle mean diameter increasing up to 50%. Further increase in particle size polydispersity increased value of the parameter. Increase in coefficient of interparticle friction to 0.4 resulted in about 40% decrease in pressure ratio in sphere packings

Airflow Resistance of Wheat Bedding as Influenced by the Filling Method

A study was conducted to estimate the degree of variability of the airflow resistance in wheat caused by the filling method, compaction of the sample, and airflow direction. Two types of grain chambers were used: a cylindrical column 0.95 m high and 0.196 m in diameter, and a cubical box of 0.35 m side. All factors examined were found to influence considerably the airflow resistance. Gravitational axial filling of the grain column from three heights (0.0, 0.95 and 1.8 m) resulted in the pressure drops of 1.0, 1.3, and 1.5 kPa at the airflow velocity of 0.3 m/s. Consolidation of axially filled samples by vibration resulted in a maximum 2.2 times increase in airflow resistance. The tests with cubical sample showed that in axially filled samples the pressure drop in vertical direction was maximum 1.5 times higher than in horizontal directions. In the case of asymmetrically filled samples, the pressure drop at the airflow velocity of 0.3 m/s in vertical direction Z was found to be 1.3 of that in horizontal direction X and 1.95 times higher than with horizontal direction Y, perpendicular to X. Variations in airflow resistance in values comparable to that found in the present project may be expected in practice

Sleep improvements on days with later school starts persist after 1 year in a flexible start system

Early school times fundamentally clash with the late sleep of teenagers. This mismatch results in chronic sleep deprivation posing acute and long-term health risks and impairing students' learning. Despite immediate short-term benefits for sleep, the long-term effects of later starts remain unresolved. In a pre-post design over 1 year, we studied a unique flexible school start system, in which 10–12th grade students chose daily between an 8:00 or 8:50AM-start. Missed study time (8:00–8:50) was compensated for during gap periods or after classes. Based on 2 waves (6–9 weeks of sleep diary each), we found that students maintained their ~ 1-h-sleep gain on later days, longitudinally (n = 28) and cross-sectionally (n = 79). This gain was independent of chronotype and frequency of later starts but attenuated for boys after 1 year. Students showed persistently better sleep quality and reduced alarm-driven waking and reported psychological benefits (n = 93) like improved motivation, concentration, and study quality on later days. Nonetheless, students chose later starts only infrequently (median 2 days/week), precluding detectable sleep extensions in the flexible system overall. Reasons for not choosing late starts were the need to make up lost study time, preference for extra study time and transport issues. Whether flexible systems constitute an appealing alternative to fixed delays given possible circadian and psychological advantages warrants further investigation

PCB-Caulk Replacement Project Johnson Space Center Houston, TX

Wet method reduced exposure by minimizing overall respirable particulate release. Dry method didn't introduce delays for primer/caulk application. Removed caulks came in many forms, from dry powdery to tarry sticky. Varying textures were not sampled or packaged differently. During the course of the project, EPA modified recommended practices to include full containment for exterior caulk removal. Changes are ongoing. Initial recommendations were directed to school buildings. EPA is researching risks due to caulk. Exposure guidance lacking except for 2 of 209 PCB congeners. Work was safely completed on schedule and under budget

Kepler Input Catalog: Photometric Calibration and Stellar Classification

We describe the photometric calibration and stellar classification methods used to produce the Kepler Input Catalog (KIC). The KIC is a catalog containing photometric and physical data for sources in the Kepler Mission field of view; it is used by the mission to select optimal targets. We derived atmospheric extinction corrections from hourly observations of secondary standard fields within the Kepler field of view. Repeatability of absolute photometry for stars brighter than magnitude 15 is typically 2%. We estimated stellar parameters Teff, log(g), log (Z), E_{B-V} using Bayesian posterior probability maximization to match observed colors to Castelli stellar atmosphere models. We applied Bayesian priors describing the distribution of solar-neighborhood stars in the color-magnitude diagram (CMD), in log (Z)$, and in height above the galactic plane. Comparisons with samples of stars classified by other means indicate that in most regions of the CMD, our classifications are reliable within about +/- 200 K and +/- 0.4 dex in log (g). It is difficult to assess the reliability of our log(Z) estimates, but there is reason to suspect that it is poor, particularly at extreme Teff. Of great importance for the Kepler Mission, for Teff <= 5400 K, the distinction between main-sequence stars and giants has proved to be reliable with better than 98% confidence. The KIC is available through the MAST data archive.Comment: 77 pages, 12 figures, 1 Table. Accepted by Astronomical Journal 24 July 201