Particle-in-cell simulation study of the interaction between a relativistically moving leptonic micro-cloud and ambient electrons

<p>Context. The jets of compact accreting objects are composed of electrons and a mixture of positrons and ions. These outflows impinge on the interstellar or intergalactic medium and both plasmas interact via collisionless processes. Filamentation (beam-Weibel) instabilities give rise to the growth of strong electromagnetic fields. These fields thermalize the interpenetrating plasmas.��</p><p>Aims. Hitherto, the effects imposed by a spatial non-uniformity on filamentation instabilities have remained unexplored. We examine the interaction between spatially uniform background electrons and a minuscule cloud of electrons and positrons. The cloud size is comparable to that created in recent laboratory experiments and such clouds may exist close to internal and external shocks of leptonic jets. The purpose of our study is to determine the prevalent instabilities, their ability to generate electromagnetic fields and the mechanism, by which the lepton micro-cloud transfers energy to the background plasma.��</p><p>Methods. A square micro-cloud of equally dense electrons and positrons impinges in our particle-in-cell (PIC) simulation on a spatially uniform plasma at rest. The latter consists of electrons with a temperature of 1 keV and immobile ions. The initially charge- and current neutral micro-cloud has a temperature of 100 keV and a side length of 2.5 plasma skin depths of the micro-cloud. The side length is given in the reference frame of the background plasma. The mean speed of the micro-cloud corresponds to a relativistic factor of 15, which is relevant for laboratory experiments and for relativistic astrophysical outflows. The spatial distributions of the leptons and of the electromagnetic fields are examined at several times.��</p><p>Results. A filamentation instability develops between the magnetic field carried by the micro-cloud and the background electrons. The electromagnetic fields, which grow from noise levels, redistribute the electrons and positrons within the cloud, which boosts the peak magnetic field amplitude. The current density and the moduli of the electromagnetic fields grow aperiodically in time and steadily along the direction that is anti-parallel to the cloud's velocity vector. The micro-cloud remains conjoined during the simulation. The instability induces an electrostatic wakefield in the background plasma.��</p><p>Conclusions. Relativistic clouds of leptons can generate and amplify magnetic fields even if they have a microscopic size, which implies that the underlying processes can be studied in the laboratory. The interaction of the localized magnetic field and high-energy leptons will give rise to synchrotron jitter radiation. The wakefield in the background plasma dissipates the kinetic energy of the lepton cloud. Even the fastest lepton micro-clouds can be slowed down by this collisionless mechanism. Moderately fast charge- and current neutralized lepton micro-clouds will deposit their energy close to relativistic shocks and hence they do not constitute an energy loss mechanism for the shock.</p>

Developed turbulence and nonlinear amplification of magnetic fields in laboratory and astrophysical plasmas

<p>The visible matter in the universe is turbulent and magnetized. Turbulence in galaxy clusters is produced by mergers and by jets of the central galaxies and believed responsible for the amplification of magnetic fields. We report on experiments looking at the collision of two laser-produced plasma clouds, mimicking, in the laboratory, a cluster merger event. By measuring the spectrum of the density fluctuations, we infer developed, Kolmogorov-like turbulence. From spectral line broadening, we estimate a level of turbulence consistent with turbulent heating balancing radiative cooling, as it likely does in galaxy clusters. We show that the magnetic field is amplified by turbulent motions, reaching a nonlinear regime that is a precursor to turbulent dynamo. Thus, our experiment provides a promising platform for understanding the structure of turbulence and the amplification of magnetic fields in the universe.</p>

Stability of Short-span Bridges Subject to Overtopping during Floods: Case Studies from UK and the USA

As a consequence of increased levels of flooding, largely attributable to urbanization of watersheds (and perhaps climate change, more frequent extreme rainfall events are occurring and threatening existing critical infrastructure. Many of which are short-span bridges over relatively small waterways (e.g., small rivers, streams and canals). Whilst these short-span bridges were designed, often many years ago, to pass relatively minor the then standard return-period floods, in recenttimes the failure incidence of such short-span bridges has been noticeably increasing. This is suggestive of insufficient hydraulic capacity or alternative failure mechanism not envisaged at the time of design e.g. foundation scour or undermining. This paper presen ts, and draws lessons, from bridge failures in Ireland and the USA. For example, in November 2009, the UK and Ireland were subjected to extraordinarily severe weather conditions for several days. The resulting flooding led to the collapse of three UK bridges that were generally 19th century masonry arch bridges, withrelatively shallow foundations. Parallel failure events have been observed in the USA. To date, knowledge of the combined effect of waterway erosion, bridge submergence, and geotechnical collapse has not been adequately studied. Recent research carried out considered the hydraulic analysis of short span bridges under flood conditions, but no consideration was given towards the likely damage to these structures due to erosive coupling of hydraulic and geotechnical factors. Some work has been done to predict the discharge downstream of an inundated arch, focusing onpredicting afflux, as opposed to bridge scour, under both pressurized and free-surface flows, but no ! predictive equation for scour under pressurized conditions was ever considered. The case studies this paper presents will be augmented by the initial findings from the laboratory experiments investigating the effects of surcharged flow and subsequent scour within the vicinity of single span arch bridges. Velocities profiles will be shown within the vicinity of the arch, in addition to the depth of consequent scour, for a series of flows and model spans. The data will be presented and correlated to the most recent predictive equations for submerged contraction and abutment scour. The accuracy of these equations is examined, and the findings used as a basis for developing further studies in relation to short span bridges.

Design and results of a dual-gas Quasi-Phase matching (QPM) foil target

<p>Quasi-phase matching (QPM) can be used to increase the conversion efficiency of the high harmonic generation (HHG) process. We observed QPM with an improved dual-gas foil target with a 1 kHz, 10 mJ, 30 fs laser system. Phase tuning and enhancement were possible within a spectral range from 17 nm to 30 nm. Furthermore analytical calculations and numerical simulations were carried out to distinguish QPM from other effects, such as the influence of adjacent jets on each other or the laser gas interaction. The simulations were performed with a 3 dimensional code to investigate the phase matching of the short and long trajectories individually over a large spectral range.</p>

The application of a novel fluidised photo reactor under UV-Visible and natural solar irradiation in the photocatalytic generation of hydrogen

With advancements in the development of visible light responsive catalysts for H2 production frequently being reported, photocatalytic water splitting has become an attractive method as a potential ���solar fuel generator���. The development of novel photo reactors which can enhance the potential of such catalyst, however, is rarely reported. This is particularly important as many reactor configurations are mass transport limited, which in term limits the efficiency of more effective photocatalysts in larger scale applications. This paper describes the performance of a novel fluidised photo reactor for the production of H2 over two catalysts under UV-Visible light and natural solar illumination. Catalysts Pt-C3N4 and NaTaO3.La were dispersed in the reactor and the rate of H2 was determined by GC-TCD analysis of the gas headspace. The unit was an annular reactor constructed from stainless steel 316 and quartz glass with a propeller located in the base to control fluidisation of powder catalysts. Reactor properties such as propeller rotational speed were found to enhance the photo activity of the system through the elimination of mass transport limitations and increasing light penetration. The optimum conditions for H2 evolution were found to be a propeller rotational speed of 1035 rpm and 144 W of UV-Visible irradiation, which produced a rate of 89 ��mol h-1 g-1 over Pt-C3N4. Solar irradiation was provided by the George Ellery Hale Solar Telescope, located at the California Institute of Technology.

Development of a Vehicle-Bridge-Soil Dynamic Interaction Model for Scour Damage Modelling

Damage detection in bridges using vibration-based methods is an area of growing research interest. Improved assessment<br/>methodologies combined with state-of-the-art sensor technology are rapidly making these approaches applicable for real-world<br/>structures. Applying these techniques to the detection and monitoring of scour around bridge foundations has remained<br/>challenging; however this area has gained attraction in recent years. Several authors have investigated a range of methods but<br/>there is still significant work required to achieve a rounded and widely applicable methodology to detect and monitor scour.This<br/>paper presents a novel Vehicle-Bridge-Soil Dynamic Interaction (VBSDI) model which can be used to simulate the effect of scour<br/>on an integral bridge. The model outputs dynamic signals which can be analysed to determine modal parameters and the variation<br/>of these parameters with respect to scour can be examined.The key novelty of this model is that it is the first numerical model for<br/>simulating scour that combines a realistic vehicle loadingmodel with a robust foundation soil responsemodel.This paper provides a<br/>description of the model development and explains the mathematical theory underlying themodel. Finally a case study application<br/>of the model using typical bridge, soil, and vehicle properties is provided.

Parametric scans of HB and LS-RPA regimes employing Petawatt laser

<p>By contrast to the Target Normal Sheath acceleration (TNSA) mechanism [1], Radiation Pressure Acceleration (RPA) is currently attracting a substantial amount of experimental [2,3] and theoretical [4-6] attention worldwide due to its superior scaling in terms of ion energy and laser-ion conversion efficiency. Employing Vulcan Petawatt lasers of the Rutherford Appleton Laboratory, UK, both the Hole-boring (HB) and the Light-Sail (LS) regimes of the RPA have been extensively explored. When the target thickness is of the order of hole-boring velocity times the laser pulse duration, highly collimated plasma jets of near solid density are ejected from the foil, lasting up to ns after the laser interaction. By changing the linear polarisation of the laser to circular, improved homogeneity in the jet's spatial density profile is achieved which suggests more uniform and sustained radiation pressure drive on target ions. By decreasing the target areal density or increasing irradiance on the target, the LS regime of the RPA is accessed where relatively high flux (~ 1012 particles/MeV/Sr) of ions are accelerated to ~ 10 MeV/nucleon energies in a narrow energy bandwidth. The ion energy scaling obtained from the parametric scans agrees well with theoretical estimation based on RPA mechanism and the narrow bandwidth feature in the ion spectra is studied by 2D particle-in-simulations.</p>

Determining the Presence of Scour around Bridge Foundations Using Vehicle-Induced Vibrations

Bridge scour is the number one cause of failure in bridges located over waterways. Scour leads to rapid losses in foundation stiffness and can cause sudden collapse. Previous research on bridge health monitoring has used changes in natural frequency to identify damage in bridge beams. The possibility of using a similar approach to identifying scour is investigated in this paper. To assess if this approach is feasible, it is necessary to establish how scour affects the natural frequency of a bridge, and if it is possible to measure changes in frequency using the bridge dynamic response to a passing vehicle. To address these questions, a novel vehicle���bridge���soil interaction (VBSI) model was developed. By carrying out a modal study in this model, it is shown that for a wide range of possible soil states, there is a clear reduction in the natural frequency of the first mode of the bridge with scour. Moreover, it is shown that the response signals on the bridge from vehicular loading are sufficient to allow these changes in frequency to be detected.

Hydraulics of stepped chutes: experimental-numerical-theoretical study

O estudo de escoamentos turbulentos em descarregadores em degraus tem sido um desafio para os investigadores. A macro-rugosidade do leito, a ondula����o da superf��cie livre, a intermit��ncia da localiza����o da sec����o inicial de entrada de ar e o escoamento bi-f��sico a jusante da sec����o inicial de entrada de ar fazem com que a caracteriza����o do escoamento deslizante sobre turbilh��es em descarregadores em degraus n��o seja simples. Actualmente, �� poss��vel combinar t��cnicas de medi����o fi��veis com simula����es num��ricas e an��lise te��rica. Nesta disserta����o, o estudo experimental baseia-se em resultados experimentais obtidos em duas instala����es experimentais: a instala����o A, do Laborat��rio Nacional de Engenharia Civil (LNEC), dotada de um descarregador em degraus com declive de 1V:0.75H e a instala����o B, do Instituto Superior T��cnico (IST), dotada de um descarregador em degraus com declive de 1V:2H. Uma sonda de ar, um tubo de Pitot modificado e v��rios hidr��metros permitiram o estudo do escoamento deslizante sobre turbilh��es mono- e bif��sico ao longo do descarregador em degraus da instala����o A. Na bacia de dissipa����o de energia, a observa����o visual do escoamento foi auxiliada pelas leituras efectuadas em tomadas de press��o localizadas na soleira da bacia e pelas leituras da altura do escoamento efectuadas em r��guas graduadas localizadas nas paredes da bacia. Na instala����o B foram utilizados um tubo de Pitot e v��rios hidr��metros para estudar a regi��o n��o arejada do escoamento deslizante sobre turbilh��es no descarregador em degraus. O estudo num��rico �� baseado em simula����es num��ricas da regi��o n��o arejada do escoamento deslizante sobre turbilh��es sobre descarregadores em degraus com o c��digo comercial de CFD FLOW-3D�� de modo a reproduzir as condi����es ensaiadas experimentalmente. As simula����es beneficiaram da t��cnica de blocos m��ltiplos (multi-block) num sistema de coordenadas cartesianas, da determina����o da superf��cie livre pelo m��todo TruVOF e da utiliza����o de dois modelos de turbul��ncia: os modelos k-��� e RNG k-���. Por ��ltimo, o estudo te��rico consistiu em desenvolver um modelo simplificado 1D para determinar as caracter��sticas hidr��ulicas principais do trecho n��o arejado do escoamento deslizante sobre turbilh��es em descarregadores em degraus. O modelo foi desenvolvido a partir das equa����es de Navier-Stokes, conjuntamente com resultados experimentais e num��ricos. Os resultados apresentados nesta disserta����o contribuem para o conhecimento do escoamento deslizante sobre turbilh��es em descarregadores em degraus, nomeadamente na regi��o n��o arejada, na sec����o inicial de entrada de ar e na regi��o arejada. A hidr��ulica dos dissipadores de energia a jusante de descarregadores em degraus, em particular das bacias tipo III do USBR, �� tamb��m objecto de estudo. Em rela����o �� regi��o n��o arejada do escoamento em descarregadores com declive acentuado, e com base em resultados experimentais e num��ricos, s��o propostas express��es para estimar o desenvolvimento da altura equivalente de ��gua, da espessura da camada limite, da concentra����o m��dia de ar, do coeficiente de energia cin��tica, da dissipa����o de energia, do factor de resist��ncia e do coeficiente n da f��rmula de Manning. S��o ainda propostas express��es adimensionais para a energia cin��tica turbulenta e sua dissipa����o. Para declives moderados, s��o propostas express��es para estimar o desenvolvimento da altura equivalente de ��gua, do coeficiente de energia cin��tica e da energia espec��fica residual. S��o ainda apresentados valores do expoente 1/N da express��o adimensional da distribui����o de velocidades, quer para descarregadores com declive acentuado quer com declive moderado. Em conformidade com outros estudos centrados em escoamentos de parede e com deriva����es te��ricas, para a regi��o n��o arejada do escoamento em descarregadores em degraus de acentuado declive, observa-se que o factor de resist��ncia depende da macro-rugosidade criada pelos degraus e da geometria da sec����o transversal e que o coeficiente n da f��rmula de Manning aumenta com a rugosidade. A descri����o estat��stica da turbul��ncia do escoamento �� igualmente explorada, contribuindo para o conhecimento da estrutura do escoamento. Observou-se que para n��meros de Reynolds rugoso n��o superiores a 6.8x104 a energia cin��tica turbul��ncia e a sua dissipa����o cumprem leis de semelhan��a. Estas express��es adimensionais est��o de acordo com os resultados obtidos por outros autores para escoamentos completamente desenvolvidos em canais abertos e no escoamento em rios com leito de gravilha. Em acr��scimo, a taxa de dissipa����o de energia, quer para descarregadores de declive acentuado quer de moderado declive, �� baixa. Por ��ltimo, observa-se que os valores da m��dia temporal da concentra����o de ar entre 0 e 1 medidos na regi��o n��o arejada do escoamento dizem respeito n��o s�� ao ar capturado entre ondas de ��gua, na zona de ondula����o da superf��cie livre, mas tamb��m ao ar emulsionado no escoamento, i.e., sob a forma de bolhas de ar, quando perto da sec����o m��dia inicial de entrada de ar, devido �� diferen��a entre localiza����es instant��nea e m��dia temporal. Foram revistas metodologias e f��rmulas para estimar a localiza����o da sec����o inicial de entrada de ar e apresentadas express��es para estimar a concentra����o m��dia de ar e a altura equivalente de ��gua nessa sec����o. Relativamente �� regi��o de escoamento arejado em descarregadores em degraus com declive acentuado, os resultados experimentais apresentados nesta disserta����o permitiram estimar a influ��ncia da defini����o da superf��cie livre nos par��metros hidr��ulicos da regi��o do escoamento arejado e estimar a m��xima eleva����o do escoamento nesta regi��o do escoamento. Com base nos resultados experimentais obtidos na bacia de dissipa����o de energia do tipo III do USBR localizada a jusante do descarregador em degraus da instala����o A, observou-se que os perfis da altura piezom��trica e da altura do escoamento tendem a seguir o perfil recomendado pelo USBR para bacias tipo III. A excep����o ocorre �� entrada da bacia, onde as alturas piezom��tricas apresentadas nesta disserta����o excedem largamente as apresentadas pelo USBR. �� ainda observado que, tal como entre as bacias tipo I e tipo III do USBR, o ressalto hidr��ulico estabiliza muito mais rapidamente numa bacia tipo III a jusante de um descarregador em degraus do que uma bacia tipo I a jusante do mesmo descarregador em degraus. Finalmente, observa-se que os blocos de amortecimento a colocar no descarregador n��o t��m influ��ncia vis��vel nos resultados da altura piezom��trica nem da altura do escoamento ao longo da bacia. Relativamente ��s simula����es num��ricas do escoamento n��o arejado, a proximidade entre resultados experimentais e num��ricos permite validar o modelo te��rico e a integra����o num��rica usados no FLOW-3D��. As simula����es desenvolvidas tamb��m mostraram que o modelo de turbul��ncia k-��� permite representar as caracter��sticas do escoamento n��o arejado em descarregadores em degraus, uma vez que n��o foram observadas diferen��as significativas entre as simula����es com este modelo e com o modelo RNG k-���. Finalmente, observou-se que o modelo de entrada de ar usado no FLOW-3D�� �� v��lido para estimar a localiza����o da sec����o inicial de entrada de ar. Por ��ltimo, a proximidade entre os resultados obtidos da aplica����o do modelo te��rico desenvolvido no ��mbito desta disserta����o e os resultados experimentais indica que as hip��teses e simplifica����es consideradas no desenvolvimento do modelo s��o adequadas.

Coupling anchovy eggs and hydrodynamic model as an assessement tool for estuarine ecohydrological management

Nitrate and urban waste water directives have raised the need for a better understanding of coastal systems in European Union. The incorrect application of these directives can lead to important ecological or social penalties. In the paper this problem is addressed to Ria Formosa Coastal Lagoon. Ria Formosa hosts a Natural Park, important ports of the southern Portuguese coast and significant bivalve aquaculture activity. Four major urban waste water treatment plants discharging in the lagoon are considered in this study. Its treatment level must be selected, based on detailed information from a monitoring program and on a good knowledge of the processes determining the fate of the material discharged in the lagoon. In this paper the results of a monitoring program and simulations using a coupled hydrodynamic and water quality / ecological model, MOHID, are used to characterise the system and to understand the processes in Ria Formosa. It is shown that the water residence time in most of the lagoon is quite low, of the order of days, but it can be larger in the upper parts of the channels where land generated water is discharged. The main supply of nutrients to the lagoon comes from the open sea rather than from the urban discharges. For this reason the characteristics and behaviour of the general lagoon contrasts with the behaviour of the upper reaches of the channels where the influence of the waste water treatment plants are high. In this system the bottom mineralization was found to be an important mechanism, and the inclusion of that process in the model was essential to obtain good results.

Hydrodynamic and sediment fluxes through the inlets of the Ria Formosa

Tese de dout., Ci��ncias do Mar, da Terra e do Ambiente (Ci��ncias do Mar-Oceanografia F��sica), Faculdade de Ci��ncias e Tecnologia, Univ. do Algarve, 2011

Spatial variability of macroinvertebrate assemblages and the influence of hydrological and environmental variables along the Sigi River, Tanzania-East Africa

The study of investigating the spatial and temporal variability of macroinvertebrate and their relation to hydrology, hydraulic and environmental factors was done along the Sigi River during two sampling periods in the dry (March) and wet (May) periods of 2012. The river was demarcated based on slope ranges and five river zones were identified as mountains streams (MS), upper foothills (UF), lower foothills (LF), rejuvenated foothills (REJ) and mature lower river (MR). Samples of macroinvertebrate were collected from the five river zones and measurements of hydrological (discharge), hydraulics (Depth, velocity and Froude number) and Environmental (pH, Temperature, substrate, conductivity) parameters were done in each zone. In characterizing the macroinvertebrate assemblages along the Sigi River diversity indices (number of taxa, total abundances, Margalef richness index and ShannonWiener index) were calculated and the most representative species for the spatial and temporal variation were identified. Melanoides and Afronurous showed differences in abundance in two samplings periods while Cleopatra, Potamonautes, Ephemerythus, Neoperla, Caenis, Ceratogomphus and Cheumatopsyche showed significant difference among the river zones. Spearman rank correlation and Distance Linear Model (DistLM) used to revealed physical factors governing the macroinvertebrate assemblages distribution. The study demonstrated that the variation of physical factors like discharge, temperature, conductivity and pH have an important role in the spatial distribution of macroinvertebrate assemblages along the river and the life cycle of macroinvertebrate (Afronurus) is important in determining the temporal variability.

Dynamics of production and mortality of Aurelia SP. in thau lagoon, northwerstern Mediterranean

Disserta����o de mestrado, Biologia Marinha, Faculdade de Ci��ncias e Tecnologia, Universidade do Algarve, 2015

Quantifying Submerged Fluvial Topography Using Hyperspatial Resolution UAS Imagery and Structure From Motion Photogrammetry.

Quantifying the topography of rivers and their associated bedforms has been a fundamental concern of fluvial geomorphology for decades. Such data, acquired at high temporal and spatial resolutions, are increasingly in demand for process-oriented investigations of flow hydraulics, sediment dynamics and in-stream habitat. In these riverine environments, the most challenging region for topographic measurement is the wetted, submerged channel. Generally, dry bed topography and submerged bathymetry are measured using different methods and technology. This adds to the costs, logistical challenges and data processing requirements of comprehensive river surveys. However, some technologies are capable of measuring the submerged topography. Through-water photogrammetry and bathymetric LiDAR are capable of reasonably accurate measurements of channel beds in clear water. Whilst the cost of bathymetric LiDAR remains high and its resolution relatively coarse, the recent developments in photogrammetry using Structure from Motion (SfM) algorithms promise a fundamental shift in the accessibility of topographic data for a wide range of settings. Here we present results demonstrating the potential of so called SfM-photogrammetry for quantifying both exposed and submerged fluvial topography at the mesohabitat scale. We show that imagery acquired from a rotary-winged Unmanned Aerial System (UAS) can be processed in order to produce digital elevation models (DEMs) with hyperspatial resolutions (c. 0.02���m) for two different river systems over channel lengths of 50-100���m. Errors in submerged areas range from 0.016���m to 0.089���m, which can be reduced to between 0.008���m and 0.053���m with the application of a simple refraction correction. This work therefore demonstrates the potential of UAS platforms and SfM-photogrammetry as a single technique for surveying fluvial topography at the mesoscale (defined as lengths of channel from c.10���m to a few hundred metres). This article is protected by copyright. All rights reserved.

An investigation of container train service provision and load factors in Great Britain

The continued growth in the volume of international trade poses considerable economic and sustainability challenges, particularly as transport routes become more congested and concern grows about the role of transport movements in accelerating climate change. Rail freight plays a major role in the inland transport of containers passing through the main British container ports, and potentially could play a more significant role in the future. However, there is little detailed understanding of the nature of this particular rail market, especially in terms its current operating efficiency. This paper examines container train service provision to/from the four main ports, based on analysis of a representative survey of more than 500 container trains between February and August 2007. The extent to which the existing capacity is utilised is presented, and scenarios by which the number of containers carried could be increased without requiring additional train service provision are modelled, to identify the theoretical potential for greater rail volumes. Finally, the paper identifies the challenges involved in achieving higher load factors, emphasising the importance both of wider supply chain considerations and government policy decision-making.