Modal analysis of wave propagation in dispersive media

Surveys on wave propagation in dispersive media have been limited since the pioneering work of Sommerfeld [Ann. Phys. 349, 177 (1914)] by the presence of branches in the integral expression of the wave function. In this article, a method is proposed to eliminate these critical branches and hence to establish a modal expansion of the time-dependent wave function. The different components of the transient waves are physically interpreted as the contributions of distinct sets of modes and characterized accordingly. Then, the modal expansion is used to derive a modified analytical expression of the Sommerfeld precursor improving significantly the description of the amplitude and the oscillating period up to the arrival of the Brillouin precursor. The proposed method and results apply to all waves governed by the Helmholtz equations.Comment: 10 pages, 9 figure

Broadband suppression of backscattering at optical frequencies using low permittivity dielectric spheres

The exact suppression of backscattering from rotationally symmetric objects requires dual symmetric materials where ${\epsilon_r} = {\mu_r}$. This prevents their design at many frequency bands, including the optical one, because magnetic materials are not available. Electromagnetically small non-magnetic spheres of large permittivity offer an alternative. They can be tailored to exhibit balanced electric and magnetic dipole polarizabilities, which result in approximate zero backscattering. In this case, the effect is inherently narrowband. Here, we put forward a different alternative that allows broadband functionality: Electromagnetically large spheres made from low permittivity materials. The effect occurs in a parameter regime that approaches the trivial ${\epsilon_r} \to {\mu_r} =1$ case, where approximate duality is met in a weakly wavelength dependence fashion. Despite the low permittivity, the overall scattering response of the spheres is still significant. Radiation patterns from these spheres are shown to be highly directive across an octave spanning band. The effect is analytically and numerically shown using the Mie coefficients.Comment: 6 Figure

Who am I without exile? Syrian everyday life in Cairo

This thesis examines the fluidity and complexity of the everyday lives of Syrians in Egypt. While it is not meant to be comprehensive, and speaks to the very particular social in which the research was conducted, this project seeks to disrupt the processes with which categorizations and solid understandings of migration and refugeeness emerge. It argues that neat understandings of the everyday is not possible, but rather, a closer reading uncovers the undoing and reassembling of the intricate relations at play- processes that speak to the very core understandings of power, governance, and sovereignty. By methodologically employing the idea of the mess, the fragmented way in which the self navigates a contradictory everyday life becomes visible: a process that is rife with myriad encounters with various forms of powers. The thesis grapples with everyday mundane events, and not so mundane events, to trace the paths which the self configures, be it through the moment of arrival, bureaucratic navigation, modes of survival, community imagination, or the potentialities of reconfiguration. This thesis attempts to push away from the rhetoric of brush-stroking experiences assumed to be interchangeable, like “Syrian†and “refugeeâ€, and concludes instead with a note on embracing the world that is in flux

Study on bridge inspections, A: identifying barriers to new practices and providing strategies for change

2021 Summer.Includes bibliographical references.Bridge inspections are one of the key elements required for a successful bridge management process to ensure adequate bridge performance. Inspections significantly inform maintenance decisions and can help in managing maintenance activities to achieve a reliable bridge network. In the United States (U.S.) routine visual inspections are required for most bridges at a maximum interval of 24-months regardless of the bridge condition. However, limitations of current bridge inspection practices impact the quality of information provided about bridge condition and the subsequent decisions made based on that information. Accordingly, the overarching goal of this research project is to support bridge inspection practices by providing a systematic and rational framework for bridge inspection planning and identifying the factors that can facilitate innovation and research transfer in the bridge inspection field. To do so, this dissertation includes three separate yet related studies; each focusing on essential aspects of bridge inspection planning. Much research in bridge inspection has been conducted to improve the inspection planning process. The first study provides an overview of current bridge inspection practices in the U.S. and conducts a systematic literature review on innovations in the field of bridge inspection planning to identify research gaps and future needs. This study provides a background on the history of bridge inspection in the U.S., including current bridge inspection practices and their limitations, and analyzes the connections between nondestructive evaluation techniques, deterioration models and bridge inspection management. The primary emphasis of the first study is a thorough analysis of research proposing and investigating different methodologies for inspection planning. Studies were analyzed and categorized into three main types of inspection planning approaches; methods that are based on: reliability, risk analysis, and optimization approaches. This study found that one of the main barriers that may be preventing the implementation of new inspection planning frameworks in practice is that the approaches presented focus on a single bridge element or deterioration mechanism in the decision-making process. Additionally, it was concluded that approaches in the literature are either complex to apply or depend solely on expert judgement. Limitations of the uniform calendar-based approach used to schedule routine inspections have been reported in the literature. Accordingly, the objective of the second study is to provide a new systematic approach for inspection planning that integrates information from bridge condition prediction models, inspection data, and expert opinion using Bayesian analysis to enhance inspection efficiency and maintenance activities. The proposed uncertainty-based inspection framework can help bridge owners avoid unnecessary or delayed inspections and repair actions, determine the inspection method, and consider more than one deterioration process or bridge component during the inspection planning process. The inspection time and method are determined based on the uncertainty and risks associated with the bridge condition. As uncertainty in the bridge condition reaches a defined threshold, an inspection is scheduled utilizing nondestructive techniques to reduce the uncertainty level. The framework was demonstrated on a new and on an existing reinforced concrete bridge deck impacted by corrosion deterioration. The results showed that the framework can reduce the number of inspections compared to conventional scheduling methods, while also reducing the uncertainty regarding the transition in the bridge deck condition and repair time. As identified through the first study, over the last two decades many researchers have focused on providing new ideas to improve conventional bridge inspection practices, however, little guidance is provided for implementing these new research products in practice. This, along with resistance to change and complexity of the proposed ideas, resulted in a lack of consistency and success in applying new technologies in bridge inspection programs across state departments of transportation (DOTs). Accordingly, the third paper presents a qualitative study set out to identify the factors that can help improve research products and accelerate change and research transfer in bridge inspection departments. This study used semi-structured interviews, written interviews, and questionnaires for data collection and engaged with twenty-six bridge staff members from different DOTs. The findings of this study are expected to be both specific to changes in bridge inspection practice and have some generalizability to other significant changes to engineering practice at DOTs. To improve research products, this study suggested that researchers need to collaborate more with DOT staff members and provide relevant research products that are not specific to certain bridge cases and can be applied on different bridges. Also, to facilitate change in transportation organizations, change leaders should focus on showing the need for change, gaining support from the FHWA, allocating the required resources, and enhancing the capacity of DOT staff members through training and effective communication. The investigation also presented participants' opinions on some of the aspects related to conventional inspection practices such as their support of a uniform inspection interval over a variable interval, and the main barriers limiting the use of NDE methods. This study contributes to the body of knowledge in the bridge inspection field by providing a new inspection planning approach that depends on the uncertainty and the risks associated with the bridge condition and uses both computational methods and expert judgment allowing bridge owners select inspection time and method while considering more than one deterioration process or bridge element. In addition, this study presents some of the factors that can help reduce the gap between research and practice and facilitate innovation and change in transportation organizations

Magnetic micro-confinement of quantum degenerate gases

In this dissertation we explore the basic principles of the magnetic micro-confinement of the quantum degenerate gases where the approach of the so-called two-dimensional magnetic lattices has been theoretically and experimentally investigated. In this research a new generation of two-dimensional magnetic lattice has been proposed and considered as a developing phase for the previous approaches. Its advantage relies on introducing a simplified method to create single or multiple micro-traps of magnetic field local minima distributed, at a certain working distance, above the surface of a thin film of permanent magnetic material. The simplicity in creating the magnetic field local minima at the micro-scale manifests itself as a result of imprinting specific patterns through the thin film using suitable and available micro-fabrication techniques. In this approach, to create multiple micro-traps, patterned square holes of size αh X αh spaced by αs are periodically distributed across the x/y plane taking a two-dimensional grid configuration. These magnetic field local minima are recognized by their ability to trap and confine quantum single-particles and quantum degenerate gases at various levels of distribution in their phase spaces, such as ultracold atoms and virtual quantum particles. Based on the nature of the interaction between the external confining potential fields and the different types of quantum particles, this research is conducted through two separate but not different phases. We performed theoretical and/or experimental investigations, for both phases, at the vicinity of the magnetic micro-confinement and its suitability for trapping quantum particles. A special attention is paid to inspect the coherence in such systems defined in terms of providing an accessible coupling to the internal quantum states of the magnetically trapped particles. Such coherence is considered as one of the important ingredients for simulating condensed matter systems and processing quantum information