1,768 research outputs found
Determination of stiffness and higher gradient coefficients by means of the embedded atom method: An approach for binary alloys
For a quantitative theoretical description of phase separation and
coarsening reliable data of stiffness constants and the so called
Higher Gradient Coefficients (HGCs) are required. For that reason
pair potentials of the Lennard-Jones type were used in
[1] to provide a theoretical tool for
their quantitative determination. Following up on this work these
quantities are now calculated by means of the Embedded-Atom Method
(EAM), a recently developed approach to describe interatomic
potentials in metals. This is done, first, to achieve a better
agreement between predicted and experimentally observed stiffness
data as well as to avoid artifacts, such as the Cauchy paradox, and,
second, to increase the trustworthiness of the HGCs for which
experimental data are rarely available. After an introduction to the
fundamentals of EAM it is outlined how it can be used for
calculating stiffness constants and HGCs. In particular, Johnson's
modification of EAM for nearest neighbor interactions
[3] is applied to present explicit
numerical results for a case study alloy, Ag-Cu, which has a
``simple" face-centered-cubic crystal structure and where it is
comparatively easy to obtain all the required analysis data from the
literature and to experimentally compare the predictions of
mechanical data
Der weltwirtschaftliche Strukturwandel : Rückblick auf die sechziger Jahre und Ausblick.
Soltwedel, R.: Der Weltluftverkehr. S. 197-208
A higher gradient theory of mixtures for multi-component materials with numerical examples for binary alloys
A theory of mixture for multi-component materials is presented based on a novel, straightforward method for the exploitation of the Second Law of thermodynamics. In particular the constitutive equations for entropy, heat and diffusion flux as well as the stress tensor are formulated as a consequence of the non-negative entropy production. Furthermore we derive the established Gibbs equation as well as the Gibbs Duhem relation which also follow from the formalism. Moreover, it is illustrated, how local mechanical strains due to eigenstrains or external loadings, modify the free energy and, consequently, change the chemical potentials of the components. All consecutive steps are illustrated, first, for simple mixtures and, second, for a system containing two different phases. So-called higher gradients of the concentrations are considered, which take the nonuniform composition into account. It will also become apparent that more/other variables of modified/different physical pr oblems beyond the illustrated ones can be easily treated within the presented framework. This work ends with the specification to binary alloys and with the presentation of various numerical simulations
Migration Experience, Aspirations and the Brain Drain: Theory and Empirical Evidence
Böhme MH, Glaser T. Migration Experience, Aspirations and the Brain Drain: Theory and Empirical Evidence. Working Papers in Economics and Management. Vol 14-2014. Bielefeld: Bielefeld University, Department of Business Administration and Economics; 2014.We develop a theoretical model of human skill formation and emigration.
We extend the existing brain drain models, by partly endogenizing the heterogeneity
of individuals, by introducing aspirations. Emigration of an individual
will result in a migration experience, which increases the migrant's aspirations.
This will induce her to invest more in the education of her children back home.
We find that this aspirations effect increases the average skill level in the society
for a given migration rate. We show that the optimal migration rate
that maximizes the post-migration skill-rate of the population is higher if we
allow for the aspirations effect of migration. We use panel data from Indonesia
to demonstrate that a migration experience has an aspirations increasing
effect and calibrate our model accordingly. Our results suggest that there are
potentially more countries than previously thought which could benefit from
migration
Assessing Livestock Water Productivity in Mixed Farming Systems of Gumara Watershed, Ethiopia
A monitoring study was carried out in Gumara watershed, upper Blue Nile basin, with the objective of evaluating livestock water productivity (LWP) using a life cycle assessment method. Sixty two smallholder farmers were selected for the study implemented between November 2006 and February 2008. Data on crop and livestock production were collected to allow assessment of livestock water productivity. Study sites were situated in three different rainfed mixed crop/livestock farming systems; barley/potato based system (BPS), tef/finger-millet based system (TMS), and rice/noug based system (RNS). LWP was found to be significantly lower (p < 0.01) in RNS (0.057 USD m−3 water) than in TMS (0.066 USD m−3 water) or in BPS (0.066 USD m−3 water). Notably, water requirement per kg live weight of cattle increased towards the lower altitude area (in RNS) mainly because of increased evapo-transpiration. As a result, 20% more water was required per kg live weight of cattle in the low ground RNS compared to BPS situated in the upstream parts of the study area. Cattle herd management that involved early offtake increased LWP by 28% over the practice of late offtake. Crop water productivity expressed in monetary units (0.39 USD m−3 water) was higher than LWP (0.063 USD m−3 water) across the mixed farming systems of Gumara watershed. Strategies for improving LWP, from its present low level, could include keeping only the more productive animals, increasing pasture productivity and linking production to marketing. These strategies would also ease the imbalance between the existing high livestock population and the declining carrying capacity of natural pasture.Peer Reviewe
Face masks reduce interpersonal distance in virtual reality
During the COVID-19 pandemic several behavioral measures have been implemented to reduce viral transmission. While these measures reduce the risk of infections, they may also increase risk behavior. Here, we experimentally investigate the influence of face masks on physical distancing. Eighty-four participants with or without face masks passed virtual agents in a supermarket environment to reach a target while interpersonal distance was recorded. Agents differed in wearing face masks and age (young, elderly). In addition, situational constraints varied in whether keeping a distance of 1.5 m required an effortful detour or not. Wearing face masks (both self and other) reduced physical distancing. This reduction was most prominent when keeping the recommended distance was effortful, suggesting an influence of situational constraints. Similarly, increased distances to elderly were only observed when keeping a recommended distance was effortless. These findings highlight contextual constraints in compensation behavior and have important implications for safety policies
Desynchronization and Wave Pattern Formation in MPI-Parallel and Hybrid Memory-Bound Programs
Analytic, first-principles performance modeling of distributed-memory
parallel codes is notoriously imprecise. Even for applications with extremely
regular and homogeneous compute-communicate phases, simply adding communication
time to computation time does often not yield a satisfactory prediction of
parallel runtime due to deviations from the expected simple lockstep pattern
caused by system noise, variations in communication time, and inherent load
imbalance. In this paper, we highlight the specific cases of provoked and
spontaneous desynchronization of memory-bound, bulk-synchronous pure MPI and
hybrid MPI+OpenMP programs. Using simple microbenchmarks we observe that
although desynchronization can introduce increased waiting time per process, it
does not necessarily cause lower resource utilization but can lead to an
increase in available bandwidth per core. In case of significant communication
overhead, even natural noise can shove the system into a state of automatic
overlap of communication and computation, improving the overall time to
solution. The saturation point, i.e., the number of processes per memory domain
required to achieve full memory bandwidth, is pivotal in the dynamics of this
process and the emerging stable wave pattern. We also demonstrate how hybrid
MPI-OpenMP programming can prevent desirable desynchronization by eliminating
the bandwidth bottleneck among processes. A Chebyshev filter diagonalization
application is used to demonstrate some of the observed effects in a realistic
setting.Comment: 18 pages, 8 figure
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