Wave-group propagation and hydrodynamics in the inner surf and swash zones

This thesis concerns a fundamental study of grouping waves propagating over a dissipative beach slope (1:100) and the dynamics of the associated long waves. This thesis represents an important contribution to the understanding of the group modulation influence on high and low frequency motions during shoreward propagation. Two new experimental sets of bichromatic wave-groups are presented, where an effective wave generation up to second order is successfully achieved. The IBIMS-ICL data set explores the propagation of identical wave groups. The modulation is controlled by the group frequency, fg, which affects the energy transfer to high and low frequency components. The growth of the high frequency (hf) wave skewness increases when fg decreases. This is explained by nonlinear coupling between the primary frequencies, which results in a larger growth of hf components as fg decreases, causing the hf waves to break earlier. The breaking locations are very well described by the wave-height to effective-depth ratio (gamma). Due to the grouping structure, gamma increases with fg. Therefore, a modified Iribarren number is proposed leading to an improvement in reproducing the measured gamma-values. Within the surf zone, the behaviour of the incident long wave also depends on the group modulation. For low fg conditions, the lf wave decays only slightly by transferring energy back to the hf wave components. However, for high fg wave conditions, strong dissipation of low frequency (lf) components occurs, which is explained in terms of lf wave breaking. The DIFFREP-ICL data set investigates the generation and dynamics of longer waves than the wave-group structure induced by differences in the number of wave groups (Rp) within a repetition period. Consequently, an important energy content is measured at the repetition frequency fr. The cross-shore amplitude evolution at fr is partly explained by nonlinear energy transfers from the primary frequencies, and partly by a breakpoint forcing. When Rp increases, the energy transfer to fr reduces. When Rp >= 3, the amplitude of fr suddenly grows at the breakpoint displaying a node-antinode pattern within the surf zone. The observed dominance of the breakpoint forcing over the energy transfers is justified by the combination of steep-slope regime and steep-wave conditions. A new methodology is proposed to identify the amplitude and phase cross-shore evolution of the radiated and reflected components. When energy dissipation of the higher lf components occurs, the swash is dominated by wave motions occurring at fr.Open Acces

Ciudadanía política en la red. Análisis de las prácticas políticas entre jóvenes universitarios

I nternet adquiere de manera crecien - te un papel relevante en las prácticas políticas actuales. En este estudio se analizaron las llevadas a cabo por universitarios, a través de sus pro - pios registros en diarios en línea. Los hallazgos contribuyen a la discusión sobre la apropiación de esta tecnolo - gía para el ejercicio de su ciudadanía política

Passive Microstrip Transmitarray Lens for Ku Band

The aim of this paper is to introduce a novel 12 GHz radiating design based on the idea of transmitarray lens device. In this document, an overview of the functioning of this kind of devices is given and the proposed transmitarray lens is studied, with architecture discussion and selection, as well as some ideas about the design, and manufacturing. In the document, some design, manufacturing and validation of the constituting elements of the lens (radiating elements, transmission circuits and transitions) are presented, together with a complete prototype of assembled transmitarray lens. Radiation pattern measurements in anechoic chamber, as well as gain and directivity values are offered

Electronically Reconfigurable Patches for Transmit-array Structures at 12GHz

The aim of the paper below is to develop certain active radiating elements for reconfigurable patch arrays. In literature it could be found different possibilities to obtain feasible reconfigurable antennas, whether placing the active circuitry in the transmission lines or directly over the patch. In this paper, the second option is chosen and active radiating elements are analyzed, designed, simulated and prototyped. Finally, measurements of these prototypes are shown. This work is part of a more complete and ambitious project to design and prototype reconfigurable transmit-array structures for microwave applications

Stability of thick brane configurations

Presented at the XXXI International Conference of Theoretical Physics, “Matter to the Deepest”, Ustroń, Poland, September 5–11, 2007.We study higher dimensional models with a cutoff Λ (lambda) and determine conditions under which brane configurations can be generated by dynamics at scales below Λ (lambda). Then we study the stability of these configurations

Circularly polarised broadband planar lightweight reflectarray with eligible pattern for satellite communications in Ku-Band

This study presents a lightweight planar patch-array reflectarray at Ku-band for satellite communications. The reflectarray is composed of two separate planar structures: the radiating interface formed by a planar multi-layered broadband patch array, and a phase shifting device formed by 3 dB/90° couplers. The radiating element is a multi-layered patch structure, designed to provide circular polarisation (CP). Each array cell phase is controlled by open-ended shifting lines of variable length connected to the 3 dB/90° couplers. The use of this kind of couplers maintains the same CP received/transmitted: the design imposes that if a right-hand CP (RHCP) [or left-hand CP (LHCP)] is received coming from the feeder, the same RHCP (or LHCP) configuration is re-radiated by the reflectarray. Additionally, the feeding horn can be moved in the x or y axes, so that the feeder angular position let re-define the radiation pattern and its pointing direction. Finally, measurements of a lightweight portable planar reflectarray prototype are provided and very good agreement is observed when compared with theoretical result

Broadband electronically tunable reflectionbased phase shifter for active-steering microwave reflectarray systems in Ku-band

This document provides the design of an electronically reconfigurable microwave phase shifter for reflectarray systems. The phase shifter is based on a hybrid coupler with reflective circuits in three ports. Each reflective circuit introduces a phase variation that can be modified due to the variable capacity value of a varactor inserted in it. The phase shifting process includes three different stages of phase shifting for the incoming signal in its way through the phase shifter: the signal is conducted through the device towards the reflective circuits in four different ways. The input port of the device is also the output one, providing the desired reflective phase shifting effect. This device is of great interest in reflectarray applications in order to provide more than 360° of controllable phase shifting at each array element. The document includes the complete design of the phase shifter along with its design parameters and circuital behaviour

On the MIMO Capacity for Distributed System under Composite Rayleigh/Rician Fading and Shadowing

Wireless channels are commonly affected by short-term fading and long-term fading (shadowing). The shadowing effects must be taken into account also when mobility is present in the wireless scenario. Using a composite fading model, the total channel capacity can be studied for a scenario with short-term Rayleigh fading along with shadowing. This work provides quantitative results for these kinds of scenarios with Rayleigh fading and shadowing, considering also multiple-input and multiple-output systems, which have not been previously reported. In addition, the channel capacity has been studied in depth in its relation with the shadowing level, signal to noise ratio, and the number of elements in the multiple-input and multiple-output system. Moreover, the channel performance with shadowing has been compared to the one without it. Furthermore, Rician model with shadowing is studied and its results are reported. In addition, correlated and experimental results are provided. It is identified that the distributed MIMO systems can benefit from shadowing in Rician channels. This advantage has not been reported previously. This type of fading is proposed for massive MIMO by others and our results open the door to emulate massive MIMO on a reverberation chamber.This work has been supported by “Gobierno de Extremadura” with project number IB13113, PYR-2014 GENIL project (PYR-2014-CEB09-0010/MICINN), and CEIbioTIC project (mP_TIC_11)

The influence of bandwidth on the energetics of intermediate to deep water laboratory breaking waves

An experimental investigation of two-dimensional dispersively focused laboratory breaking waves is presented. We describe the bandwidth effect on breaking wave energetics, including spectral energy evolution, characteristic group velocity, energy dissipation and its rate, and breaking strength parameter, b. To evaluate the role of bandwidth, three definitions of wave group steepness are adopted where Ss and Sn are bandwidth-dependent and Sp remains constant when bandwidth is changed. Our data show two regimes of spectral energy evolution in breaking wave groups, with both regimes bandwidth-dependent: energy dissipation and gain occur at f > 0.95fp ( fp is the peak frequency) and f < 0.95fp, respectively. The characteristic group velocity, which is used in energy dissipation calculations, increases by up to 7% after wave breaking, being larger for higher bandwidth breaking waves. An unambiguous bandwidth dependence is found between Sp and both the fractional and absolute wave energy dissipation. Wave groups of larger bandwidth break at a lower value of Sp and consequently lose relatively more energy. The energy dissipation rate depends on the breaking duration which itself is bandwidth dependent. Consequently, no clear bandwidth effect is observed in energy dissipation rate when compared with either Sp or Ss. However, there is a systematic bandwidth dependence in the variation of b when parameterised in terms of Sp, with their relationship becoming increasingly nonlinear as bandwidth increases. When parameterised with Ss, b shows a markedly reduced bandwidth dependence. Finally, the numerical breaking onset and relationship between b and Ss in the numerical study of Derakhti & Kirby (J. Fluid Mech., vol. 790, 2016, pp. 553–581) is validated experimentallyNational Science Foundation (grant number OCE-1434866)Royal Society Shooter International Fellowship and a NERC Standard Grant (grant number NE/T000309/1)NERC (grant number NE/T000309/1

Electronically reconfigurable reflective phase shifter for circularly polarized reflectarray systems

This letter presents the design of a two-port electronically reconfigurable phase shifter for circularly polarized reflectarray systems at microwave frequencies. The phase shifter is based on 3 dB/90? couplers combined with reflective circuits that introduce sequentially the phase variation. Each reflective circuit, formed by printed elements (L) and tunable varactors, produces the phase variation due to the variable capacity value of the varactor. The phase shifting process includes three different stages of phase shifting for the signal in its way from the input port towards the output port through the phase shifter. Both ports are interchangeable, acting either as input or output ports. This fulfils the reflection requirements of circularly polarized reflectarrays, being especially suitable for this purpose. The complete design, together with its circuital behavior and performance results, are depicted in this document