464 research outputs found
Adjusting the melting point of a model system via Gibbs-Duhem integration: application to a model of Aluminum
Model interaction potentials for real materials are generally optimized with
respect to only those experimental properties that are easily evaluated as
mechanical averages (e.g., elastic constants (at T=0 K), static lattice
energies and liquid structure). For such potentials, agreement with experiment
for the non-mechanical properties, such as the melting point, is not guaranteed
and such values can deviate significantly from experiment. We present a method
for re-parameterizing any model interaction potential of a real material to
adjust its melting temperature to a value that is closer to its experimental
melting temperature. This is done without significantly affecting the
mechanical properties for which the potential was modeled. This method is an
application of Gibbs-Duhem integration [D. Kofke, Mol. Phys.78, 1331 (1993)].
As a test we apply the method to an embedded atom model of aluminum [J. Mei and
J.W. Davenport, Phys. Rev. B 46, 21 (1992)] for which the melting temperature
for the thermodynamic limit is 826.4 +/- 1.3K - somewhat below the experimental
value of 933K. After re-parameterization, the melting temperature of the
modified potential is found to be 931.5K +/- 1.5K.Comment: 9 pages, 5 figures, 4 table
Three-dimensional Aeroelastic and Aerothermoelastic Behavior in Hypersonic Flow
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76667/1/AIAA-2005-2175-915.pd
Scarred Patterns in Surface Waves
Surface wave patterns are investigated experimentally in a system geometry
that has become a paradigm of quantum chaos: the stadium billiard. Linear waves
in bounded geometries for which classical ray trajectories are chaotic are
known to give rise to scarred patterns. Here, we utilize parametrically forced
surface waves (Faraday waves), which become progressively nonlinear beyond the
wave instability threshold, to investigate the subtle interplay between
boundaries and nonlinearity. Only a subset (three main types) of the computed
linear modes of the stadium are observed in a systematic scan. These correspond
to modes in which the wave amplitudes are strongly enhanced along paths
corresponding to certain periodic ray orbits. Many other modes are found to be
suppressed, in general agreement with a prediction by Agam and Altshuler based
on boundary dissipation and the Lyapunov exponent of the associated orbit.
Spatially asymmetric or disordered (but time-independent) patterns are also
found even near onset. As the driving acceleration is increased, the
time-independent scarred patterns persist, but in some cases transitions
between modes are noted. The onset of spatiotemporal chaos at higher forcing
amplitude often involves a nonperiodic oscillation between spatially ordered
and disordered states. We characterize this phenomenon using the concept of
pattern entropy. The rate of change of the patterns is found to be reduced as
the state passes temporarily near the ordered configurations of lower entropy.
We also report complex but highly symmetric (time-independent) patterns far
above onset in the regime that is normally chaotic.Comment: 9 pages, 10 figures (low resolution gif files). Updated and added
references and text. For high resolution images:
http://physics.clarku.edu/~akudrolli/stadium.htm
Computational fluid dynamics modelling of an entire synchronous generator for improved thermal management
This study is the first in a series dedicated to investigating the airflow and thermal management of electrical machines. Owing to the temperature dependent resistive losses in the machine's windings, any improvement in cooling provides a direct reduction in losses and an increase in efficiency. This study focuses on the airflow which is intrinsically linked to the thermal behaviour of the machine as well as the windage power consumed to drive the air through the machine. A full computational fluid dynamics (CFD) model has been used to analyse the airflow around all major components of the machine. Results have been experimentally validated and investigated. At synchronous speed the experimentally tested mass flow rate and windage torque were under predicted by 4% and 7%, respectively, by the CFD. A break-down of torque by component shows that the fan consumes approximately 87% of the windage torque
Occlusion and Motion Reasoning for Long-Term Tracking
International audienceObject tracking is a reoccurring problem in computer vision. Tracking-by-detection approaches, in particular Struck (Hare et al., 2011), have shown to be competitive in recent evaluations. However, such approaches fail in the presence of long-term occlusions as well as severe viewpoint changes of the object. In this paper we propose a principled way to combine occlusion and motion reasoning with a tracking-by-detection approach. Occlusion and motion reasoning is based on state-of-the-art long-term trajectories which are labeled as object or background tracks with an energy-based formulation. The overlap between labeled tracks and detected regions allows to identify occlusions. The motion changes of the object between consecutive frames can be estimated robustly from the geometric relation between object trajectories. If this geometric change is significant, an additional detector is trained. Experimental results show that our tracker obtains state-of-the-art results and handles occlusion and viewpoints changes better than competing tracking methods
Fractal Reconnection in Solar and Stellar Environments
Recent space based observations of the Sun revealed that magnetic
reconnection is ubiquitous in the solar atmosphere, ranging from small scale
reconnection (observed as nanoflares) to large scale one (observed as long
duration flares or giant arcades). Often the magnetic reconnection events are
associated with mass ejections or jets, which seem to be closely related to
multiple plasmoid ejections from fractal current sheet. The bursty radio and
hard X-ray emissions from flares also suggest the fractal reconnection and
associated particle acceleration. We shall discuss recent observations and
theories related to the plasmoid-induced-reconnection and the fractal
reconnection in solar flares, and their implication to reconnection physics and
particle acceleration. Recent findings of many superflares on solar type stars
that has extended the applicability of the fractal reconnection model of solar
flares to much a wider parameter space suitable for stellar flares are also
discussed.Comment: Invited chapter to appear in "Magnetic Reconnection: Concepts and
Applications", Springer-Verlag, W. D. Gonzalez and E. N. Parker, eds. (2016),
33 pages, 18 figure
Optimal resource allocation for multi-queue systems with a shared server pool
We study optimal allocation of servers for a system with multiple service facilities and
with a shared pool of servers. Each service facility poses a constraint on the maximum
expected sojourn time of a job. A central decision maker can dynamically allocate servers
to each facility, where adding more servers results in faster processing speeds but against
higher utilization costs. The objective is to dynamically allocate the servers over the
different facilities such that the sojourn-time constraints are met at minimal costs. This
situation occurs frequently in practice, e.g., in Grid systems for real-time image processing
(iris scans, fingerprints). We model this problem as a Markov decision process and derive
structural properties of the relative value function. These properties, which are hard to
derive for multi-dimensional systems, give a full characterization of the optimal policy.
We demonstrate the effectiveness of these policies by extensive numerical experiments
Age-related accrual of methylomic variability is linked to fundamental ageing mechanisms
Background: Epigenetic change is a hallmark of ageing but its link to ageing mechanisms in humans remains poorly understood. While DNA methylation at many CpG sites closely tracks chronological age, DNA methylation changes relevant to biological age are expected to gradually dissociate from chronological age, mirroring the increased heterogeneity in health status at older ages. Results: Here, we report on the large-scale identification of 6366 age-related variably methylated positions (aVMPs) identified in 3295 whole blood DNA methylation profiles, 2044 of which have a matching RNA-seq gene expression profile. aVMPs are enriched at polycomb repressed regions and, accordingly, methylation at those positions is associated with the expression of genes encoding components of polycomb repressive complex 2 (PRC2) in trans. Further analysis revealed trans-associations for 1816 aVMPs with an additional 854 genes. These trans-associated aVMPs are characterized by either an age-related
The Development of Authentic Assessments to Investigate Ninth Graders? Scientific Literacy: In the Case of Scientific Cognition Concerning the Concepts of Chemistry and Physics
- …