14,190 research outputs found
Intermittency in two-dimensional Ekman-Navier-Stokes turbulence
We study the statistics of the vorticity field in two-dimensional
Navier-Stokes turbulence with a linear Ekman friction. We show that the
small-scale vorticity fluctuations are intermittent, as conjectured by Nam et
al. [Phys. Rev. Lett. vol.84 (2000) 5134]. The small-scale statistics of
vorticity fluctuations coincides with the one of a passive scalar with finite
lifetime transported by the velocity field itself.Comment: 4 pages, 7 figure
City-Size Distribution as a Function of Socioeconomic Conditions: An Eclectic Approach to Downscaling Global Population
http://globalchange.mit.edu/research/publications/2254In this study, we introduce a new method of downscaling global population distribution, for which
purpose conventional approaches have serious limitations in application. Our approach is âeclectic,â as
it explores the intersection between an optimization framework and the empirical regularities involved in
rank-size distributions. The novelty of our downscaling model is that it allows city-size distributions to
interact with socioeconomic variables. Our contribution to the urban studies literature is twofold. One
is our challenge to the conventional view that the proportionate growth dynamics underlies empirical
rank-size regularities. We first show that the city-size distribution of a region can deviate substantially
from a log-normal distribution with cross-regional and time variations, and then demonstrate that such
variations can be explained by certain socioeconomic conditions that each region confronts at a
particular time point. In addition to expanding academic debates on city-size distributions, our study can
pave the way for various academic and professional research projects, which need spatial distribution of
global population at fine grid cell levels as key input. Our model is applicable to the entire globe,
including regions for which reliable sub-regional population data sets are limitedly available, and can be
extended easily to function as a forecasting model.The Joint Program on the Science and Policy of Global Change is funded by the U.S.
Department of Energy, Office of Science under grants DE-FG02-94ER61937, DE-FG02-
93ER61677, DE-FG02-08ER64597, and DE-SC0003906; the U.S. Department of Energy,
National Renewable Energy Laboratory under grant XEU-0-9920-01; the U.S. Environmental
Protection Agency under grants XA-83344601-0, XA-83240101, PI-83412601-0, and RD-
83427901-0; the U.S. National Science Foundation under grants SES-0825915, EFRI-0835414,
BCS-0410344, ATM-0329759, DMS-0426845, and AGS-0944121; the U.S. National
Aeronautics and Space Administration under grants NNX07AI49G, NNX08AY59A,
NNX06AC30A, NNX09AK26G, NNX08AL73G, NNX09AI26G, NNG04GJ80G,
NNG04GP30G, and NNA06CN09A; the U.S. National Oceanic and Atmospheric
Administration under grant NA070AR4310050; the U.S. Federal Aviation Administration under
grants 06-C-NE-MIT and 09-C-NE-MIT; the U.S. Department of Transportation under grant
DTRT57-10-C-10015; the U.S. Department of Agriculture under grant 58-0111-9-001; the
Electric Power Research Institute under grant EP-P32616/C15124; and a consortium of 40
industrial and foundation sponsor
Inverse velocity statistics in two dimensional turbulence
We present a numerical study of two-dimensional turbulent flows in the
enstrophy cascade regime, with different large-scale forcings and energy sinks.
In particular, we study the statistics of more-than-differentiable velocity
fluctuations by means of two recently introduced sets of statistical
estimators, namely {\it inverse statistics} and {\it second order differences}.
We show that the 2D turbulent velocity field, , cannot be simply
characterized by its spectrum behavior, . There
exists a whole set of exponents associated to the non-trivial smooth
fluctuations of the velocity field at all scales. We also present a numerical
investigation of the temporal properties of measured in different
spatial locations.Comment: 9 pages, 12 figure
Free Energy Approach to the Formation of an Icosahedral Structure during the Freezing of Gold Nanoclusters
The freezing of metal nanoclusters such as gold, silver, and copper exhibits
a novel structural evolution. The formation of the icosahedral (Ih) structure
is dominant despite its energetic metastability. This important phenomenon,
hitherto not understood, is studied by calculating free energies of gold
nanoclusters. The structural transition barriers have been determined by using
the umbrella sampling technique combined with molecular dynamics simulations.
Our calculations show that the formation of Ih gold nanoclusters is attributed
to the lower free energy barrier from the liquid to the Ih phases compared to
the barrier from the liquid to the face-centered-cubic crystal phases
Getting Jobs, Keeping Jobs, and Earning a Living Wage: Can Welfare Reform Work?
Most discussions of welfare and work have focused on how demographic characteristics, schooling, training, and work experience limit welfare mothersâ employment and wages, but they have largely ignored factors such as inappropriate workplace behaviors, expectations of discrimination and harassment, depression, alcoholism, and domestic violence, all of which may affect welfare mothers and make employment difficult. In this paper we review the prevalence of these individual-level barriers and argue that they, in combination with an economy which does not pay low-skill workers well, are likely to impede employment and self-sufficiency for a large proportion of welfare mothers. At the end of the review, we summarize the current state of knowledge about barriers to the employment of welfare recipients and suggest several ways in which welfare-to-work programs might address these barriers.
Propagation of Exchange Bias in CoFe/FeMn/CoFe Trilayers
CoFe/FeMn, FeMn/CoFe bilayers and CoFe/FeMn/CoFe trilayers were grown in
magnetic field and at room temperature. The exchange bias field
depends strongly on the order of depositions and is much higher at CoFe/FeMn
than at FeMn/CoFe interfaces. By combining the two bilayer structures into
symmetric CoFe/FeMn()/CoFe trilayers, and
of the top and bottom CoFe layers, respectively, are both enhanced.
Reducing of the trilayers also results in enhancements of
both and . These results evidence the propagation of
exchange bias between the two CoFe/FeMn and FeMn/CoFe interfaces mediated by
the FeMn antiferromagnetic order
Pairing Reentrance Phenomenon in Heated Rotating Nuclei in the Shell Model Monte Carlo Approach
Rotational motion of heated 72-Ge is studied within the microscopic Shell
Model Monte Carlo approach. We investigate the the angular momentum alignment
and nuclear pairing correlations associated with J-pi Cooper pairs as a
function of the rotational frequency and temperature. The reentrance of pairing
correlations with temperature is predicted at high rotational frequencies. It
manifests itself through the anomalous behavior of specific heat and level
density.Comment: 4 pages; 4 figure
Prospective observational study of point-of-care creatinine in trauma.
Background:Patients with trauma are at risk for renal dysfunction from hypovolemia or urological injury. In austere environments, creatinine values are not available to guide resuscitation. A new portable device, the Stat Sensor Point-of-care (POC) Whole Blood Creatinine Analyzer, provides accurate results in <30â
s and requires minimal training. This device has not been evaluated in trauma despite the theoretical benefit it provides. The purpose of this study is to determine the clinical impact of the POC device in trauma. Methods:40 patients with trauma were enrolled in a prospective observational study. One drop of blood was used for creatinine determination on the Statsensor POC device. POC creatinine results were compared to the laboratory. Turnaround time (TAT) for POC and laboratory methods was calculated as well as time elapsed to CT scan if applicable. Results:Patients (n=40) were enrolled between December 2014 and March 2015. POC creatinine values were similar to laboratory methods with a mean bias of 0.075±0.27 (p=0.08). Mean analytical TATs for the POC measurements were significantly faster than the laboratory method (11.6±10.0â
min vs 78.1±27.9â
min, n=40, p<0.0001). Mean elapsed time before arrival at the CT scanner was 52.9±34.2â
min. Conclusions:The POC device reported similar creatinine values to the laboratory and provided significantly faster results. POC creatinine testing is a promising development for trauma practice in austere environments and workup of a subset of stable patients with trauma. Further study is warranted to determine clinical impact, both in hospital-based trauma and austere environments
Spinodal nanodecomposition in magnetically doped semiconductors
This review presents the recent progress in computational materials design,
experimental realization, and control methods of spinodal nanodecomposition
under three- and two-dimensional crystal-growth conditions in spintronic
materials, such as magnetically doped semiconductors. The computational
description of nanodecomposition, performed by combining first-principles
calculations with kinetic Monte Carlo simulations, is discussed together with
extensive electron microscopy, synchrotron radiation, scanning probe, and ion
beam methods that have been employed to visualize binodal and spinodal
nanodecomposition (chemical phase separation) as well as nanoprecipitation
(crystallographic phase separation) in a range of semiconductor compounds with
a concentration of transition metal (TM) impurities beyond the solubility
limit. The role of growth conditions, co-doping by shallow impurities, kinetic
barriers, and surface reactions in controlling the aggregation of magnetic
cations is highlighted. According to theoretical simulations and experimental
results the TM-rich regions appear either in the form of nanodots (the {\em
dairiseki} phase) or nanocolumns (the {\em konbu} phase) buried in the host
semiconductor. Particular attention is paid to Mn-doped group III arsenides and
antimonides, TM-doped group III nitrides, Mn- and Fe-doped Ge, and Cr-doped
group II chalcogenides, in which ferromagnetic features persisting up to above
room temperature correlate with the presence of nanodecomposition and account
for the application-relevant magneto-optical and magnetotransport properties of
these compounds. Finally, it is pointed out that spinodal nanodecomposition can
be viewed as a new class of bottom-up approach to nanofabrication.Comment: 72 pages, 79 figure
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