2,993 research outputs found
Elastic turbulence in two-dimensional Taylor-Couette flows
We report the onset of elastic turbulence in a two-dimensional Taylor-Couette
geometry using numerical solutions of the Oldroyd-B model, also performed at
high Weissenberg numbers with the program OpenFOAM. Beyond a critical
Weissenberg number, an elastic instability causes a supercritical transition
from the laminar Taylor-Couette to a turbulent flow. The order parameter, the
time average of secondary-flow strength, follows the scaling law with and . The power spectrum of the velocity fluctuations shows a power-law decay
with a characteristic exponent, which strongly depends on the radial position.
It is greater than two, which we relate to the dimension of the geometry
Influence of Hydrodynamic Interactions on the Kinetics of Colloidal Particle's Adsorption
The kinetics of irreversible adsorption of spherical particles onto a flat
surface is theoretically studied. Previous models, in which hydrodynamic
interactions were disregarded, predicted a power-law behavior for
the time dependence of the coverage of the surface near saturation.
Experiments, however, are in agreement with a power-law behavior of the form
. We outline that, when hydrodynamic interactions are considered, the
assymptotic behavior is found to be compatible with the experimental results in
a wide region near saturation.Comment: 4 pages, 1 figures, Phys. Rev. Lett. (in press
Thermal X-Ray Pulses Resulting From Pulsar Glitches
The non-spherically symmetric transport equations and exact thermal evolution
model are used to calculate the transient thermal response to pulsars. The
three possible ways of energy release originated from glitches, namely the
`shell', `ring' and `spot' cases are compared. The X-ray light curves resulting
from the thermal response to the glitches are calculated. Only the `spot' case
and the `ring' case are considered because the `shell' case does not produce
significant modulative X-rays. The magnetic field () effect, the
relativistic light bending effect and the rotational effect on the photons
being emitted in a finite region are considered. Various sets of parameters
result in different evolution patterns of light curves. We find that this
modulated thermal X-ray radiation resulting from glitches may provide some
useful constraints on glitch models.Comment: 48 pages, 20 figures, submitted to Ap
Adaptive end-to-end optimization of mobile video streaming using QoS negotiation
Video streaming over wireless links is a non-trivial problem due to the large and frequent changes in the quality of the underlying radio channel combined with latency constraints. We believe that every layer in a mobile system must be prepared to adapt its behavior to its environment. Thus layers must be capable of operating in multiple modes; each mode will show a different quality and resource usage. Selecting the right mode of operation requires exchange of information between interacting layers. For example, selecting the best channel coding requires information about the quality of the channel (capacity, bit-error-rate) as well as the requirements (latency, reliability) of the compressed video stream generated by the source encoder. In this paper we study the application of our generic QoS negotiation scheme to a specific configuration for mobile video transmission. We describe the results of experiments studying the overall effectiveness, stability, and dynamics of adaptation of our distributed optimization approach
Geography and art: encountering place across disciplines
This article summarises a project undertaken at the Newton Park campus of Bath Spa University over 1 week in October 2015. The project provided a space for interdisciplinary collaborations between geography and art students to explore the commonalities and differences in how they saw, interpreted and creatively re-presented the campus, using a variety of methods. This article outlines the project and reflects on the processes, outcomes, and challenges of collaboration. It highlights how this approach can enhance student learning experiences, by facilitating more interdisciplinary collaboration across the sciences, arts and humanities, and social sciences. In doing so, it explores the potential and pitfalls of collaborative cultural geography in practice across disciplines
AdaFocal: Calibration-aware Adaptive Focal Loss
Much recent work has been devoted to the problem of ensuring that a neural
network's confidence scores match the true probability of being correct, i.e.
the calibration problem. Of note, it was found that training with focal loss
leads to better calibration than cross-entropy while achieving similar level of
accuracy \cite{mukhoti2020}. This success stems from focal loss regularizing
the entropy of the model's prediction (controlled by the parameter ),
thereby reining in the model's overconfidence. Further improvement is expected
if is selected independently for each training sample
(Sample-Dependent Focal Loss (FLSD-53) \cite{mukhoti2020}). However, FLSD-53 is
based on heuristics and does not generalize well. In this paper, we propose a
calibration-aware adaptive focal loss called AdaFocal that utilizes the
calibration properties of focal (and inverse-focal) loss and adaptively
modifies for different groups of samples based on
from the previous step and the knowledge of model's under/over-confidence on
the validation set. We evaluate AdaFocal on various image recognition and one
NLP task, covering a wide variety of network architectures, to confirm the
improvement in calibration while achieving similar levels of accuracy.
Additionally, we show that models trained with AdaFocal achieve a significant
boost in out-of-distribution detection.Comment: Accepted to NeurIPS 202
Model of correlated sequential adsorption of colloidal particles
We present results of a new model of sequential adsorption in which the
adsorbing particles are correlated with the particles attached to the
substrate. The strength of the correlations is measured by a tunable parameter
. The model interpolates between free ballistic adsorption in the limit
and a strongly correlated phase, appearing for
and characterized by the emergence of highly ordered structures. The phenomenon
is manifested through the analysis of several magnitudes, as the jamming limit
and the particle-particle correlation function. The effect of correlations in
one dimension manifests in the increased tendency to particle chaining in the
substrate. In two dimensions the correlations induce a percolation transition,
in which a spanning cluster of connected particles appears at a certain
critical value . Our study could be applicable to more general
situations in which the coupling between correlations and disorder is relevant,
as for example, in the presence of strong interparticle interactions.Comment: 6 pages, 8 EPS figures. Phys. Rev. E (in press
Astrophysical Supercomputing with GPUs: Critical Decisions for Early Adopters
General purpose computing on graphics processing units (GPGPU) is
dramatically changing the landscape of high performance computing in astronomy.
In this paper, we identify and investigate several key decision areas, with a
goal of simplyfing the early adoption of GPGPU in astronomy. We consider the
merits of OpenCL as an open standard in order to reduce risks associated with
coding in a native, vendor-specific programming environment, and present a GPU
programming philosophy based on using brute force solutions. We assert that
effective use of new GPU-based supercomputing facilities will require a change
in approach from astronomers. This will likely include improved programming
training, an increased need for software development best-practice through the
use of profiling and related optimisation tools, and a greater reliance on
third-party code libraries. As with any new technology, those willing to take
the risks, and make the investment of time and effort to become early adopters
of GPGPU in astronomy, stand to reap great benefits.Comment: 13 pages, 5 figures, accepted for publication in PAS
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