7,647 research outputs found
Local transient rheological behavior of concentrated suspensions
This paper reports experiments on the shear transient response of
concentrated non-Brownian suspensions. The shear viscosity of the suspensions
is measured using a wide-gap Couette rheometer equipped with a Particle Image
Velocimetry (PIV) device that allows measuring the velocity field. The
suspensions made of PMMA particles (31m in diameter) suspended in a
Newtonian index- and density-matched liquid are transparent enough to allow an
accurate measurement of the local velocity for particle concentrations as high
as 50%. In the wide-gap Couette cell, the shear induced particle migration is
evidenced by the measurement of the time evolution of the flow profile. A
peculiar radial zone in the gap is identified where the viscosity remains
constant. At this special location, the local particle volume fraction is taken
to be the mean particle concentration. The local shear transient response of
the suspensions when the shear flow is reversed is measured at this point where
the particle volume fraction is well defined. The local rheological
measurements presented here confirm the macroscopic measurements of
Gadala-Maria and Acrivos (1980). After shear reversal, the viscosity undergoes
a step-like reduction, decreases slower and passes through a minimum before
increasing again to reach a plateau. Upon varying the particle concentration,
we have been able to show that the minimum and the plateau viscosities do not
obey the same scaling law with respect to the particle volume fraction. These
experimental results are consistent with the scaling predicted by Mills and
Snabre (2009) and with the results of numerical simulation performed on random
suspensions [Sierou and Brady (2001)]. The minimum seems to be associated with
the viscosity of an isotropic suspension, or at least of a suspension whose
particles do not interact through non-hydrodynamic forces, while the plateau
value would correspond to the viscosity of a suspension structured by the shear
where the non-hydrodynamic forces play a crucial role
How Lyman Alpha Emission Depends On Galaxy Stellar Mass
In this work, we show how the stellar mass (M) of galaxies affects the
3<z<4.6 Ly-alpha equivalent width (EW) distribution. To this end, we design a
sample of 629 galaxies in the M range 7.6 < logM/Msun < 10.6 from the
3D-HST/CANDELS survey. We perform spectroscopic observations of this sample
using the Michigan/Magellan Fiber System, allowing us to measure Ly-alpha
fluxes and use 3D-HST/CANDELS ancillary data. In order to study the Ly-alpha EW
distribution dependence on M, we split the whole sample in three stellar mass
bins. We find that, in all bins, the distribution is best represented by an
exponential profile of the form dN(M)/dEW= A(M)exp(-EW/W0(M))/W0(M). Through a
Bayesian analysis, we confirm that lower M galaxies have higher Ly-alpha EWs.
We also find that the fraction A of galaxies featuring emission and the
e-folding scale W0 of the distribution anti- correlate with M, recovering
expressions of the forms A(M)= -0.26(.13) logM/Msun+3.01(1.2) and W0(M)=
-15.6(3.5) logM/Msun +166(34). These results are crucial for proper
interpretation of Ly-alpha emission trends reported in the literature that may
be affected by strong M selection biases.Comment: 4 pages, 5 figure
Human pulmonary responses to experimental inhalation of high concentration fine and ultrafine magnesium oxide particles.
Exposure to air polluted with particles less than 2.5 micron in size is associated epidemiologically with adverse cardiopulmonary health consequences in humans. The goal of this study was to characterize human pulmonary responses to controlled experimental high-dose exposure to fine and ultrafine magnesium oxide particles. We quantified bronchoalveolar lavage (BAL) cell and cytokine concentrations, pulmonary function, and peripheral blood neutrophil concentrations in six healthy volunteers 18 to 20 hr after inhalation of fine and ultrafine magnesium oxide particles produced from a furnace system model. We compared postexposure studies with control studies from the same six subjects. Mean +/- standard deviation (SD) cumulative magnesium dose was 4,138 +/- 2,163 min x mg/m3. By weight, 28% of fume particles were ultrafine (<0.1 micron in diameter) and over 98% of fume particles were fine (<2.5 micron in diameter). There were no significant differences in BAL inflammatory cell concentrations, BAL interleukin (IL)-1, IL-6, IL-8, tumor necrosis factor, pulmonary function, or peripheral blood neutrophil concentrations postexposure compared with control. Our findings suggest that high-dose fine and ultrafine magnesium oxide particle exposure does not produce a measurable pulmonary inflammatory response. These findings are in marked contrast with the well-described pulmonary inflammatory response following zinc oxide particle inhalation. We conclude that fine and ultrafine particle inhalation does not result in toxicity in a generic manner independent of particle composition. Our findings support the concept that particle chemical composition, in addition to particle size, is an important determinant of respiratory effects
CliCrop: a Crop Water-Stress and Irrigation Demand Model for an Integrated Global Assessment Model Approach
http://globalchange.mit.edu/research/publications/2264This paper describes the use of the CliCrop model in the context of climate change general assessment
modeling. The MIT Integrated Global System Model (IGSM) framework is a global integrated assessment
modeling framework that uses emission predictions and economic outputs from the MIT Emission Prediction
and Policy Analysis (EPPA) model and earth system modeling predictions from the IGSM to drive a
land system component, a crop model (CliCrop) and a Water Resource System (WRS) model. The global
Agriculture and Water System are dependant upon and interlinked with the global climate system. As irrigated
agriculture provides 60% of grains and 40% of all crop production on 20% of global crop lands and
accounts for 80% of global water consumption, it is crucial that the agricultural-water linkage be properly
modeled. Crop models are used to predict future yields, irrigation demand and to understand the effect of
crop and soil type on food productivity and soil fertility. In the context of an integrated global assessment, a
crop water-stress and irrigation demand model must meet certain specifications that are different for other
crop models; it needs to be global, fast and generic with a minimal set of inputs. This paper describes
how CliCrop models the physical and biological processes of crop growth and yield production and its use
within the MIT Integrated Global System Model (IGSM) framework, including the data inputs. This paper
discusses the global data bases used as input to CliCrop and provides a comparison of the accuracy of
CliCrop with the detailed biological-based crop model DSSAT as well as with measured crop yields over
the U.S. at the country level using reanalyzed weather data. In both cases CliCrop performed well and the
analysis validated its use for climate change impact assessment. We then show why correctly modeling the
soil is important for irrigation demand calculation, especially in temperate areas. Finally, we discuss a
method to estimate actual water withdrawal from modeled physical crop requirements using U.S. historical
data.The initial funding for CliCrop was provided by USAID under a program on climate change
adaptation in Niger. Further funding was provided by UN University World Institute for Development Economics
Research for the Application and Development of CliCrop in Africa, the authors would like to
particularly thank Prof. Finn Tarp, Prof. Channing Arndt and Dr. James Thurlow for their
support. The authors also would like to thank Dr. Jawoo Koo of IFPRI for his review and
contributions to the software development. The authors also gratefully acknowledge additional
financial support for this work provided by the MIT Joint Program on the Science and Policy of
Global Change through a consortium of industrial sponsors and Federal grants. Development of
the IGSM applied in this research was supported by the U.S. Department of Energy, Office of
Science (DE-FG02-94ER61937); the U.S. Environmental Protection Agency, EPRI, and other
U.S. government agencies and a consortium of 40 industrial and foundation sponsors
Sonic Booms in Atmospheric Turbulence (SonicBAT): The Influence of Turbulence on Shaped Sonic Booms
The objectives of the Sonic Booms in Atmospheric Turbulence (SonicBAT) Program were to develop and validate, via research flight experiments under a range of realistic atmospheric conditions, one numeric turbulence model research code and one classic turbulence model research code using traditional N-wave booms in the presence of atmospheric turbulence, and to apply these models to assess the effects of turbulence on the levels of shaped sonic booms predicted from low boom aircraft designs. The SonicBAT program has successfully investigated sonic boom turbulence effects through the execution of flight experiments at two NASA centers, Armstrong Flight Research Center (AFRC) and Kennedy Space Center (KSC), collecting a comprehensive set of acoustic and atmospheric turbulence data that were used to validate the numeric and classic turbulence models developed. The validated codes were incorporated into the PCBoom sonic boom prediction software and used to estimate the effect of turbulence on the levels of shaped sonic booms associated with several low boom aircraft designs. The SonicBAT program was a four year effort that consisted of turbulence model development and refinement throughout the entire period as well as extensive flight test planning that culminated with the two research flight tests being conducted in the second and third years of the program. The SonicBAT team, led by Wyle, includes partners from the Pennsylvania State University, Lockheed Martin, Gulfstream Aerospace, Boeing, Eagle Aeronautics, Technical & Business Systems, and the Laboratory of Fluid Mechanics and Acoustics (France). A number of collaborators, including the Japan Aerospace Exploration Agency, also participated by supporting the experiments with human and equipment resources at their own expense. Three NASA centers, AFRC, Langley Research Center (LaRC), and KSC were essential to the planning and conduct of the experiments. The experiments involved precision flight of either an F-18A or F-18B executing steady, level passes at supersonic airspeeds in a turbulent atmosphere to create sonic boom signatures that had been distorted by turbulence. The flights spanned a range of atmospheric turbulence conditions at NASA Armstrong and Kennedy in order to provide a variety of conditions for code validations. The SonicBAT experiments at both sites were designed to capture simultaneous F-18A or F-18B onboard flight instrumentation data, high fidelity ground based and airborne acoustic data, surface and upper air meteorological data, and additional meteorological data from ultrasonic anemometers and SODARs to determine the local atmospheric turbulence and boundary layer height
A Giant Protocluster of Galaxies at Redshift 5.7
Galaxy clusters trace the largest structures of the Universe and provide
ideal laboratories for studying galaxy evolution and cosmology. Clusters with
extended X-ray emission have been discovered at redshifts up to z ~ 2.5.
Meanwhile, there has been growing interest in hunting for protoclusters, the
progenitors of clusters, at higher redshifts. It is, however, very challenging
to find the largest protoclusters at early times when they start to assemble.
Here we report a giant protocluster of galaxies at redshift z = 5.7, when the
Universe was only one billion years old. This protocluster occupies a volume of
about 35x35x35 cubic co-moving megaparsecs. It is embedded in an even larger
overdense region with at least 41 spectroscopically confirmed, luminous
Lyman-alpha emitting galaxies (Lyman-alpha Emitters, or LAEs), including
several previously reported LAEs. Its LAE density is 6.6 times the average
density at z ~ 5.7. It is the only one of its kind in an LAE survey in four
square degrees on the sky. Such a large structure is also rarely seen in
current cosmological simulations. This protocluster will collapse into a galaxy
cluster with a mass of (3.6+/-0.9) x 10^{15} solar masses, comparable to those
of the most massive clusters or protoclusters known to date.Comment: Published in Nature Astronomy on Oct 15, 2018 (DOI:
10.1038/s41550-018-0587-9
Are Steadily Moving Crystals Unstable?
We study the dynamics of small fluctuations about the uniform state of a
crystal moving through a dissipative medium, e.g. a sedimenting colloidal
crystal or a moving flux lattice, using a set of continuum equations for the
displacement fields, and a one-dimensional driven lattice-gas model for the
coupled concentration and tilt fields. For the colloidal crystal we predict a
continuous nonequilibrium phase transition to a clumped state above a critical
Peclet number.Comment: 4 pages, revtex, 2 .eps figures, uses epsf.sty; To be published in
Phys. Rev. Lett. This version is substantially rewritten but the essential
content is the same as befor
A Magellan M2FS Spectroscopic Survey of Galaxies at 5.5<z<6.8: Program Overview and a Sample of the Brightest Lyman-alpha Emitters
We present a spectroscopic survey of high-redshift, luminous galaxies over
four square degrees on the sky, aiming to build a large and homogeneous sample
of Ly emitters (LAEs) at and 6.5, and Lyman-break
galaxies (LBGs) at . The fields that we choose to observe are
well-studied, such as SXDS and COSMOS. They have deep optical imaging data in a
series of broad and narrow bands, allowing efficient selection of galaxy
candidates. Spectroscopic observations are being carried out using the
multi-object spectrograph M2FS on the Magellan Clay telescope. M2FS is
efficient to identify high-redshift galaxies, owing to its 256 optical fibers
deployed over a circular field-of-view 30 arcmin in diameter. We have observed
square degrees. When the program is completed, we expect to identify
more than 400 bright LAEs at and 6.5, and a substantial number of
LBGs at . This unique sample will be used to study a variety of galaxy
properties and to search for large protoclusters. Furthermore, the statistical
properties of these galaxies will be used to probe cosmic reionization. We
describe the motivation, program design, target selection, and M2FS
observations. We also outline our science goals, and present a sample of the
brightest LAEs at and 6.5. This sample contains 32 LAEs with
Ly luminosities higher than 10 erg s. A few of them reach
erg s, comparable to the two most luminous LAEs
known at , `CR7' and `COLA1'. These LAEs provide ideal targets to study
extreme galaxies in the distant universe.Comment: 18 pages, 11 figures, Accepted for publication in The Astrophysical
Journa
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