Anomalies of ac driven solitary waves with internal modes: Nonparametric resonances induced by parametric forces

We study the dynamics of kinks in the $\phi^4$ model subjected to a parametric ac force, both with and without damping, as a paradigm of solitary waves with internal modes. By using a collective coordinate approach, we find that the parametric force has a non-parametric effect on the kink motion. Specifically, we find that the internal mode leads to a resonance for frequencies of the parametric driving close to its own frequency, in which case the energy of the system grows as well as the width of the kink. These predictions of the collective coordinate theory are verified by numerical simulations of the full partial differential equation. We finally compare this kind of resonance with that obtained for non-parametric ac forces and conclude that the effect of ac drivings on solitary waves with internal modes is exactly the opposite of their character in the partial differential equation.Comment: To appear in Phys Rev

Flow-Induced Noise in Heat Exchangers

Flow-induced tonal noise and acoustic resonance problems are found in a variety of applications that range from nuclear power plant heat exchangers to automobile air conditioning evaporators. Flow-induced noise in heat exchangers is a very complex phenomenon. The complexities are caused by several factors that affect the noise generation and attenuation mechanisms of sound sources inside ducts, and by fluid-acoustic-structural coupling effects. To predict the noise from bluff bodies inside a duct, one needs to account for the sound source???s strength and directionality, the damping and sound attenuation mechanisms inside the duct, and the effects produced by the coupling between the acoustic field and the vortex generation process. Flow-induced tonal noise generated in plate heat exchangers has unusual features that have not been previously explored. Measurements of the flow-induced noise produced by plate heat exchangers, arrays of cylinders, cylinders in tandem, side by side cylinders, single cylinders of constant diameter and ???hourglass??? shaped cylinders inside a rectangular duct have been made. The acoustic field in parts of the duct in which traveling hydrodynamic pressure fluctuations produced by vortex shedding do not contaminate the results was investigated by measuring sound pressure levels. Many of the results are believed to be unique. The measurements were then used to infer noise source strength and system damping using an acoustic model. The acoustic model was based on the inhomogeneous convected Helmholtz equation with a point dipole source term, volumetric damping and damping at the duct walls. The acoustic model was able to closely match the measured sound pressure field and the phase relationships between measurement points for sound generated by flow over short aspect ratio single cylinders inside a duct. The acoustic model based technique was also applied to investigate some of the flow-induced noise behavior and trends of side by side cylinders, cylinders in tandem and a staggered cylinder array with many cylinders

Effect of single and multi-site calibration techniques on hydrological model performance, parameter estimation and predictive uncertainty: a case study in the Logone catchment, Lake Chad basin

Understanding hydrological processes at catchment scale through the use of hydrological model parameters is essential for enhancing water resource management. Given the difficulty of using lump parameters to calibrate distributed catchment hydrological models in spatially heterogeneous catchments, a multiple calibration technique was adopted to enhance model calibration in this study. Different calibration techniques were used to calibrate the Soil and Water Assessment Tool (SWAT) model at different locations along the Logone river channel. These were: single-site calibration (SSC); sequential calibration (SC); and simultaneous multi-site calibration (SMSC). Results indicate that it is possible to reveal differences in hydrological behavior between the upstream and downstream parts of the catchment using different parameter values. Using all calibration techniques, model performance indicators were mostly above the minimum threshold of 0.60 and 0.65 for Nash Sutcliff Efficiency (NSE) and coefficient of determination (R 2 ) respectively, at both daily and monthly time-steps. Model uncertainty analysis showed that more than 60% of observed streamflow values were bracketed within the 95% prediction uncertainty (95PPU) band after calibration and validation. Furthermore, results indicated that the SC technique out-performed the other two methods (SSC and SMSC). It was also observed that although the SMSC technique uses streamflow data from all gauging stations during calibration and validation, thereby taking into account the catchment spatial variability, the choice of each calibration method will depend on the application and spatial scale of implementation of the modelling results in the catchment

Localizing gravitational wave sources with optical telescopes and combining electromagnetic and gravitational wave data

Neutron star binaries, which are among the most promising sources for the direct detection of gravitational waves (GW) by ground based detectors, are also potential electromagnetic (EM) emitters. Gravitational waves will provide a new window to observe these events and hopefully give us glimpses of new astrophysics. In this paper, we discuss how EM information of these events can considerably improve GW parameter estimation both in terms of accuracy and computational power requirement. And then in return how GW sky localization can help EM astronomers in follow-up studies of sources which did not yield any prompt emission. We discuss how both EM source information and GW source localization can be used in a framework of multi-messenger astronomy. We illustrate how the large error regions in GW sky localizations can be handled in conducting optical astronomy in the advance detector era. We show some preliminary results in the context of an array of optical telescopes called BlackGEM, dedicated for optical follow-up of GW triggers, that is being constructed in La Silla, Chile and is expected to operate concurrent to the advanced GW detectors.Comment: 8 pages, 8 figures, Proceeding for Sant Cugat Forum for Astrophysic

Nonabelian D-branes and Noncommutative Geometry

We discuss the nonabelian world-volume action which governs the dynamics of N coincident Dp-branes. In this theory, the branes' transverse displacements are described by matrix-valued scalar fields, and so this is a natural physical framework for the appearance of noncommutative geometry. One example is the dielectric effect by which Dp-branes may be polarized into a noncommutative geometry by external fields. Another example is the appearance of noncommutative geometries in the description of intersecting D-branes of differing dimensions, such as D-strings ending on a D3- or D5-brane. We also describe the related physics of giant gravitons.Comment: 21 pages, Latex, ref. adde

SCUBA Observations of the Host Galaxies of Gamma-Ray Bursts

In recent years, a population of galaxies with huge infrared luminosities and dust masses has been discovered in the submillimetre. Observations suggest that the AGN contribution to the luminosities of these submillimetre-selected galaxies is low; instead their luminosities are thought to be mainly due to strong episodes of star formation following merger events. Our current understanding of GRBs as the endpoints in the life of massive stars suggest that they will be located in such galaxies.We have observed a sample of well-located GRB host galaxies in the submillimetre. Comparing the results with the general submillimetre-selected galaxy population, we find that at low fluxes (S850 ≤ 4 mJy), the two agree well. However, there is a lack of bright GRB hosts in the submillimetre. This finding is reinforced when the results of other groups are included. Possible explanations are discussed. These results help us assess the roles of both GRB host galaxies and submillimetre-selected galaxies in the evolution of the Universe

Examining Periodic Solar Wind Density Structures Observed in the SECCHI Heliospheric Imagers

We present an analysis of small-scale, periodic, solar-wind density enhancements (length-scales as small as \approx 1000 Mm) observed in images from the Heliospheric Imager (HI) aboard STEREO A. We discuss their possible relationship to periodic fluctuations of the proton density that have been identified at 1 AU using in-situ plasma measurements. Specifically, Viall, Kepko, and Spence (2008) examined 11 years of in-situ solar-wind density measurements at 1 AU and demonstrated that not only turbulent structures, but also non-turbulent periodic density structures exist in the solar wind with scale sizes of hundreds to one thousand Mm. In a subsequent paper, Viall, Spence, and Kasper (2009) analyzed the {\alpha} to proton solar-wind abundance ratio measured during one such event of periodic density structures, demonstrating that the plasma behavior was highly suggestive that either temporally or spatially varying coronal source plasma created those density structures. Large periodic density structures observed at 1 AU, which were generated in the corona, can be observable in coronal and heliospheric white-light images if they possess sufficiently high density contrast. Indeed, we identify such periodic density structures as they enter the HI field of view and follow them as they advect with the solar wind through the images. The smaller periodic density structures that we identify in the images are comparable in size to the larger structures analyzed in-situ at 1 AU, yielding further evidence that periodic density enhancements are a consequence of coronal activity as the solar wind is formed.Comment: 15 pages, 12 figures. The final publication is available at http://www.springerlink.co

Three-Point Functions in N=4 SYM Theory at One-Loop

We analyze the one-loop correction to the three-point function coefficient of scalar primary operators in N=4 SYM theory. By applying constraints from the superconformal symmetry, we demonstrate that the type of Feynman diagrams that contribute depends on the choice of renormalization scheme. In the planar limit, explicit expressions for the correction are interpreted in terms of the hamiltonians of the associated integrable closed and open spin chains. This suggests that at least at one-loop, the planar conformal field theory is integrable with the anomalous dimensions and OPE coefficients both obtainable from integrable spin chain calculations. We also connect the planar results with similar structures found in closed string field theory.Comment: 34 pages, 9 figures, harvmac; references adde

Alter ego, state of the art on user profiling: an overview of the most relevant organisational and behavioural aspects regarding User Profiling.

This report gives an overview of the most relevant organisational and\ud behavioural aspects regarding user profiling. It discusses not only the\ud most important aims of user profiling from both an organisation’s as\ud well as a user’s perspective, it will also discuss organisational motives\ud and barriers for user profiling and the most important conditions for\ud the success of user profiling. Finally recommendations are made and\ud suggestions for further research are given