Bosonic Field Propagators on Algebraic Curves

In this paper we investigate massless scalar field theory on non-degenerate algebraic curves. The propagator is written in terms of the parameters appearing in the polynomial defining the curve. This provides an alternative to the language of theta functions. The main result is a derivation of the third kind differential normalized in such a way that its periods around the homology cycles are purely imaginary. All the physical correlation functions of the scalar fields can be expressed in terms of this object. This paper contains a detailed analysis of the techniques necessary to study field theories on algebraic curves. A simple expression of the scalar field propagator is found in a particular case in which the algebraic curves have $Z_n$ internal symmetry and one of the fields is located at a branch point.Comment: 26 pages, TeX + harvma

Self-consistent model for the saturation mechanism of the response to harmonic forcing in the backward-facing step flow

Certain flows denominated as amplifiers arc characterized by their global linear stability while showing large linear amplifications to sustained perturbations. As the forcing amplitude increases, a strong saturation of the response appears when compared to the linear prediction. However, a predictive model that describes the saturation of the response to higher amplitudes of forcing in stable laminar flows is still missing. While an asymptotic analysis based on the weakly nonlinear theory shows qualitative agreement only for very small forcing amplitudes, the linear response to harmonic forcing around the mean flow computed by direct numerical simulations presents a good prediction of the saturation also at higher forcing amplitudes. These results suggest that the saturation process is governed by the Reynolds stress and thus motivate the introduction of a simple self-consistent model. The model consists of a decomposition of the full nonlinear Navier-Stokes equations in a wan flow equation together with a linear perturbation equation around the mean flow, which arc coupled through the Reynolds stress. The full fluctuating response and the resulting Reynolds stress are approximated by the first harmonic calculated from the linear response to the forcing around the aforementioned mean flow. This closed set of coupled equations is solved in an iterative manner as partial nonlinearity is still preserved in the mean flow equation despite the assumed simplifications. The results show an accurate prediction of the response energy when compared to direct numerical simulations. The approximated coupling is strong enough to retain the main nonlinear effects of the saturation process. Hence, a simple physical picture is formalized, wherein the response modifies the mean flow through the Reynolds stress in such a way that the correct response energy is attained

Beer tapping: dynamics of bubbles after impact

Beer tapping is a well known prank where a bottle of beer is impacted from the top by a solid object, usually another bottle, leading to a sudden foam overflow. A description of the shock-driven bubble dynamics leading to foaming is presented based on an experimental and numerical study evoking the following physical picture. First, the solid impact produces a sudden downwards acceleration of the bottle creating a strong depression in the liquid bulk. The existing bubbles undergo a strong expansion and a sudden contraction ending in their collapse and fragmentation into a large amount of small bubbles. Second, the bubble clouds present a large surface area to volume ratio, enhancing the CO2 diffusion from the supersaturated liquid, hence growing rapidly and depleting the CO2. The clouds of bubbles migrate upwards in the form of plumes pulling the surrounding liquid with them and eventually resulting in the foam overflow. The sudden pressure drop that triggers the bubble dynamics with a collapse and oscillations is modelled by the Rayleigh-Plesset equation. The bubble dynamics from impact to collapse occurs over a time (t(b) similar or equal to 800 mu s) much larger than the acoustic time scale of the liquid bulk (t(ac) = 2H/c similar or equal to 80 mu s), for the experimental container of height H = 6 cm and a speed of sound around c similar or equal to 1500 m/s. This scale separation, together with the comparison of numerical and experimental results, suggests that the pressure drop is controlled by two parameters: the acceleration of the container and the distance from the bubble to the free surface

Monopoles and Holography

We present a holographic theory in AdS_4 whose zero temperature ground state develops a crystal structure, spontaneously breaking translational symmetry. The crystal is induced by a background magnetic field, but requires no chemical potential. This lattice arises from the existence of 't Hooft-Polyakov monopole solitons in the bulk which condense to form a classical object known as a monopole wall. In the infra-red, the magnetic field is screened and there is an emergent SU(2) global symmetry.Comment: 33 pages, 16 figures; v2: ref adde

Exploring the Impact of Galactic Interactions and Mergers on the Central Star Formation of APEX/EDGE-CALIFA Galaxies

Galactic interactions and subsequent mergers are a paramount channel for galaxy evolution. In this work, we use the data from 236 star forming CALIFA galaxies with integrated molecular gas observations in their central region (approximately within an effective radius) -- from the APEX millimeter telescope and the CARMA millimeter telescope array. This sample includes isolated (126 galaxies) and interacting galaxies in different merging stages (110 galaxies; from pairs, merging and post-merger galaxies). We show that the impact of interactions and mergers in the center of galaxies is revealed as an increase in the fraction of molecular gas (compared to isolated galaxies). Furthermore, our results suggest that the change in star formation efficiency is the main driver for both an enhancement and/or suppression of the central star formation -- except in merging galaxies where the enhanced star formation appears to be driven by an increase of molecular gas. We suggest that gravitational torques due to the interaction and subsequent merger transport cold molecular gas inwards, increasing the gas fraction without necessarily increasing star formation.Comment: 3 Figures, 13 Pages. Accepted for publication in Ap

Analysis of a spatial Lotka-Volterra model with a finite range predator-prey interaction

We perform an analysis of a recent spatial version of the classical Lotka-Volterra model, where a finite scale controls individuals' interaction. We study the behavior of the predator-prey dynamics in physical spaces higher than one, showing how spatial patterns can emerge for some values of the interaction range and of the diffusion parameter.Comment: 7 pages, 7 figure

(Micro)evolutionary changes and the evolutionary potential of bird migration

Seasonal migration is the yearly long-distance movement of individuals between their breeding and wintering grounds. Individuals from nearly every animal group exhibit this behavior, but probably the most iconic migration is carried out by birds, from the classic V-shape formation of geese on migration to the amazing nonstop long-distance flights undertaken by Arctic Terns Sterna paradisaea. In this chapter, we discuss how seasonal migration has shaped the field of evolution. First, this behavior is known to turn on and off quite rapidly, but controversy remains concerning where this behavior first evolved geographically and whether the ancestral state was sedentary or migratory (Fig. 7.1d, e). We review recent work using new analytical techniques to provide insight into this topic. Second, it is widely accepted that there is a large genetic basis to this trait, especially in groups like songbirds that migrate alone and at night precluding any opportunity for learning. Key hypotheses on this topic include shared genetic variation used by different populations to migrate and only few genes being involved in its control. We summarize recent work using new techniques for both phenotype and genotype characterization to evaluate and challenge these hypotheses. Finally, one topic that has received less attention is the role these differences in migratory phenotype could play in the process of speciation. Specifically, many populations breed next to one another but take drastically different routes on migration (Fig. 7.2). This difference could play an important role in reducing gene flow between populations, but our inability to track most birds on migration has so far precluded evaluations of this hypothesis. The advent of new tracking techniques means we can track many more birds with increasing accuracy on migration, and this work has provided important insight into migration's role in speciation that we will review here

ATLAS Beam Steering Mechanism (BSM) Lessons Learned

This paper describes the design, testing, and lessons learned during the development of the Advanced Topographic Laser Altimeter System (ATLAS) Beam Steering Mechanism (BSM). The BSM is a 2 degree-of-freedom tip-tilt mechanism for the purpose of pointing a flat mirror to tightly control the co-alignment of the transmitted laser and the receiver telescope of the ATLAS instrument. The high resolution needs of the mission resulted in sub-arcsecond pointing and knowledge requirements, which have been met. Development of the methodology to verify performance required significant effort. The BSM will fly as part of the Ice, Cloud, and Elevation Satellite II Mission (ICESat II), which is scheduled to be launched in 2017. The ICESat II primary mission is to map the Earth's surface topography for the determination of seasonal changes of ice sheet thickness and vegetation canopy thickness to establish long-term trends

Development of Large-Format Lithium-Ion Cells with Silicon Anode and Low Flammable Electrolyte

NASA is developing safe, high energy and high capacity lithium-ion cell designs and batteries for future missions under NASAs Advanced Space Power System (ASPS) project. Advanced cell components, such as high specific capacity silicon anodes and low-flammable electrolytes have been developed for improving the cell specific energy and enhancing safety. To advance the technology readiness level, we have developed large-format flight-type hermetically sealed battery cells by incorporating high capacity silicon anodes, commercially available lithium nickel, cobalt, aluminum oxide (NCA) cathodes, and low-flammable electrolytes. In this report, we will present the performance results of these various battery cells. In addition, we will also discuss the post-test cell analysis results as well