8,782 research outputs found
On initial-value and self-similar solutions of the compressible Euler equations
We examine numerically the issue of convergence for initial-value solutions and similarity solutions of the compressible Euler equations in two dimensions in the presence of vortex sheets (slip lines). We consider the problem of a normal shock wave impacting an inclined density discontinuity in the presence of a solid boundary. Two solution techniques are examined: the first solves the Euler equations by a Godunov method as an initial-value problem and the second as a boundary value problem, after invoking self-similarity. Our results indicate nonconvergence of the initial-value calculation at fixed time, with increasing spatial-temporal resolution. The similarity solution appears to converge to the weak 'zero-temperature' solution of the Euler equations in the presence of the slip line. Some speculations on the geometric character of solutions of the initial-value problem are presented
A neutron star progenitor for FRBs? Insights from polarisation measurements
Fast Radio Bursts (FRBs) are intense, millisecond-duration broadband radio
transients, the emission mechanisms of which are not understood. Masui et al.
recently presented Green Bank Telescope observations of FRB 110523, which
displayed temporal variation of the linear polarisation position angle (PA).
This effect is commonly seen in radio pulsars and is attributed to a changing
projected magnetic field orientation in the emission region as the star
rotates. If a neutron star is the progenitor of this FRB, and the emission
mechanism is pulsar-like, we show that the progenitor is either rapidly
rotating, or the emission originates from a region of complex magnetic field
geometry. The observed PA variation could also be caused by propagation effects
within a neutron-star magnetosphere, or by spatially varying magnetic fields if
the progenitor lies in a dense, highly magnetised environment. Although we urge
caution in generalising results from FRB 110523 to the broader FRB population,
our analysis serves as a guide to interpreting future polarisation measurements
of FRBs, and presents another means of elucidating the origins of these
enigmatic ephemera.Comment: 7 pages, 2 figures, submitted to MNRA
CMB observations using the SKA
We examine the prospects for observations of CMB anisotropy with the SKA; we
discuss the advantages of interferometric SKA imaging, observing strategies,
calibration issues and the achievable sensitivity. Although the SKA will
probably operate at cm wavelengths, where discrete source confusion dominates
the CMB anisotropy, its extreme sensitivity to point sources will make it
possible to subtract the source contamination at these wavelengths and thereby
image the low surface brightness CMB anisotropies on small angular scales. The
SKA, operating at 10-20 GHz, may usefully make high-l observations of the CMB
anisotropy spectrum and survey the sky for Sunyaev-Zeldovich decrements.Comment: 4 pages. invited talk presented at the XXVIIth General Assembly of
the URSI, 17-24 Aug 2002, Maastricht, The Netherland
Real-time food intake classification and energy expenditure estimation on a mobile device
© 2015 IEEE.Assessment of food intake has a wide range of applications in public health and life-style related chronic disease management. In this paper, we propose a real-time food recognition platform combined with daily activity and energy expenditure estimation. In the proposed method, food recognition is based on hierarchical classification using multiple visual cues, supported by efficient software implementation suitable for realtime mobile device execution. A Fischer Vector representation together with a set of linear classifiers are used to categorize food intake. Daily energy expenditure estimation is achieved by using the built-in inertial motion sensors of the mobile device. The performance of the vision-based food recognition algorithm is compared to the current state-of-the-art, showing improved accuracy and high computational efficiency suitable for realtime feedback. Detailed user studies have also been performed to demonstrate the practical value of the software environment
Intracellular Regulatory Networks are close to Monotone Systems
Several meso-scale biological intracellular regulatory networks that have specified directionality of interactions have been recently assembled from experimental literature. Directed networks where links are characterized as positive or negative can be converted to systems of differential equations and analyzed as dynamical systems. Such analyses have shown that networks containing only sign-consistent loops, such as positive feed-forward and feedback loops function as monotone systems that display well-ordered behavior. Perturbations to monotone systems have unambiguous global effects and a predictability characteristic that confers advantages for robustness and adaptability. We find that three intracellular regulatory networks: bacterial and yeast transcriptional networks and a mammalian signaling network contain far more sign-consistent feedback and feed-forward loops than expected for shuffled networks. Inconsistent loops with negative links can be more easily removed from real regulatory networks as compared to shuffled networks. This topological feature in real networks emerges from the presence of hubs that are enriched for either negative or positive links, and is not due to a preference for double negative links in paths. These observations indicate that intracellular regulatory networks may be close to monotone systems and that this network topology contributes to the dynamic stability
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