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 $\Phi \propto (\mathrm{Wi} -\mathrm{Wi}_c)^{\gamma}$ with $\mathrm{Wi}_c=10$ and $\gamma = 0.45$. 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 $t^{-2/3}$ 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 $t^{-1/2}$. 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 ($\vec B$) 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 $\gamma$), thereby reining in the model's overconfidence. Further improvement is expected if $\gamma$ 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 $\gamma_t$ for different groups of samples based on $\gamma_{t-1}$ 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 $\sigma$. The model interpolates between free ballistic adsorption in the limit $\sigma\to\infty$ and a strongly correlated phase, appearing for $\sigma\to0$ 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 $\sigma_c$. 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