Multiple states in the flow through a sluice gate

This paper addresses the problem of hydraulic hysteresis for a supercritical open channel flow approaching a sluice gate when subcritical flow can establish downstream, against the gate. The possible flow configurations across the gate are classified on the basis of the Froude number of the incoming and downstream flows, and of the ratio of gate opening to the upstream supercritical flow depth. Within the above parameter space, two regions exist in which the problem admits a dual solution, that is, two different flow configurations can establish for the same gate opening and undisturbed flow conditions. In one of these regions, both smooth flow (i.e. the fluid flows under the gate without interacting with the gate) and free outflow conditions can establish; in the other region, both smooth flow and submerged flow can establish. For flow conditions in the above regions, the configuration that actually establishes depends on the previous history of the flow, thus implying the hysteretic character of the flow

Matter-wave solitons in radially periodic potentials

We investigate two-dimensional (2D) states of Bose-Einstein condensates (BEC) with self-attraction or self-repulsion, trapped in an axially symmetric optical-lattice potential periodic along the radius. Unlike previously studied 2D models with Bessel lattices, no localized states exist in the linear limit of the present model, hence all localized states are truly nonlinear ones. We consider the states trapped in the central potential well, and in remote circular troughs. In both cases, a new species, in the form of \textit{radial gap solitons}, are found in the repulsive model (the gap soliton trapped in a circular trough may additionally support stable dark-soliton pairs). In remote troughs, stable localized states may assume a ring-like shape, or shrink into strongly localized solitons. The existence of stable annular states, both azimuthally uniform and weakly modulated ones, is corroborated by simulations of the corresponding Gross-Pitaevskii equation. Dynamics of strongly localized solitons circulating in the troughs is also studied. While the solitons with sufficiently small velocities are stable, fast solitons gradually decay, due to the leakage of matter into the adjacent trough under the action of the centrifugal force. Collisions between solitons are investigated too. Head-on collisions of in-phase solitons lead to the collapse; $\pi$-out of phase solitons bounce many times, but eventually merge into a single soliton without collapsing. The proposed setting may also be realized in terms of spatial solitons in photonic-crystal fibers with a radial structure.Comment: 16 pages, 23 figure

High-temperature optically activated GaAs power switching for aircraft digital electronic control

Gallium arsenide high-temperature devices were fabricated and assembled into an optically activated pulse-width-modulated power control for a torque motor typical of the kinds used in jet engine actuators. A bipolar heterojunction phototransistor with gallium aluminum arsenide emitter/window, a gallium arsenide junction field-effect power transistor and a gallium arsenide transient protection diode were designed and fabricated. A high-temperature fiber optic/phototransistor coupling scheme was implemented. The devices assembled into the demonstrator were successfully tested at 250 C, proving the feasibility of actuator-located switching of control power using optical signals transmitted by fibers. Assessments of the efficiency and technical merits were made for extension of this high-temperature technology to local conversion of optical power to electrical power and its control at levels useful for driving actuators. Optical power sources included in the comparisons were an infrared light-emitting diode, an injection laser diode, tungsten-halogen lamps and arc lamps. Optical-to-electrical power conversion was limited to photovoltaics located at the actuator. Impedance matching of the photovoltaic array to the load was considered over the full temperature range, -55 C to 260 C. Loss of photovoltaic efficiency at higher temperatures was taken into account. Serious losses in efficiency are: (1) in the optical source and the cooling which they may require in the assumed 125 C ambient, (2) in the decreased conversion efficiency of the gallium arsenide photovoltaic at 260 C, and (3) in impedance matching. Practical systems require improvements in these areas

On handling urban informality in southern Africa

In this article I reconsider the handling of urban informality by urban planning and management systems in southern Africa. I argue that authorities have a fetish about formality and that this is fuelled by an obsession with urban modernity. I stress that the desired city, largely inspired by Western notions of modernity, has not been and cannot be realized. Using illustrative cases of top–down interventions, I highlight and interrogate three strategies that authorities have deployed to handle informality in an effort to create or defend the modern city. I suggest that the fetish is built upon a desire for an urban modernity based on a concept of formal order that the authorities believe cannot coexist with the “disorder” and spatial “unruliness” of informality. I question the authorities' conviction that informality is an abomination that needs to be “converted”, dislocated or annihilated. I conclude that the very configuration of urban governance and socio-economic systems in the region, like the rest of sub-Saharan Africa, renders informality inevitable and its eradication impossible

Two-mode squeezed vacuum and squeezed light in correlated interferometry

We study in detail a system of two interferometers aimed to the detection of extremely faint phase-fluctuations. This system can represent a breakthrough for detecting a faint correlated signal that would remain otherwise undetectable even using the most sensitive individual interferometric devices, that are limited by the shot noise. If the two interferometers experience identical phase-fluctuations, like the ones introduced by the so called "holographic noise", this signal should emerge if their output signals are correlated, while the fluctuations due to shot noise and other independent contributions will vanish. We show how the injecting quantum light in the free ports of the interferometers can reduce the photon noise of the system beyond the shot-noise, enhancing the resolution in the phase-correlation estimation. We analyze both the use of two-mode squeezed vacuum or twin-beam state (TWB) and of two independent squeezing states. Our results basically confirms the benefit of using squeezed beams together with strong coherent beams in interferometry, even in this correlated case. However, mainly we concentrate on the possible use of TWB, discovering interesting and probably unexplored areas of application of bipartite entanglement and in particular the possibility of reaching in principle surprising uncertainty reduction

The effect of angle of attack on flow-induced vibration of low-side-ratio rectangular cylinders

This study investigates the effect of angle of attack on flow-induced vibrations (FIVs) of sharp-edged rectangular cylinders. In particular, the effect of the afterbody of a rectangular cross-section with low mass ratio and the side ratio ranging from 0.67 to 1.5 is analysed by changing the angle of attack with respect to the oncoming free-stream. As already shown for a different side ratio’s rectangle, namely a square section (see Nemes et al., 2012), the angle of attack variation can make the flow-induced amplitude response switch between vortex-induced vibration (VIV) and galloping. Some considerations with respect to the interaction between the two phenomena typical of FIV, VIV and galloping, are also given for the side ratios of 1.5 and 0.67. The amplitude and frequency responses are carefully analysed, comparing the results with those of a square section of comparable mass ratio. The results showed a marked effect of the after-body, even for slight increments of the angle of attack. This can result in different amplitude response curves, as classified by the features of the response. In addition, the influence of interacting higher harmonics components on the amplitude response is also shown and discussed

Simplified Modeling of Wind-driven Single-sided Ventilation

In the last decades there has been an expansion in the use of building mechanical cooling systems. This increase is due to several factors such as global warming, higher internal heat gains, widespread use of unshaded glazed façades and more stringent thermal comfort requirements. In mild to warm climates, buildings without operable windows require mechanical cooling during most of the year, even in moments when the outdoor conditions would allow for natural cooling by ventilation. To reverse this trend designers are encouraged to use natural cooling strategies such as natural ventilation (NV). A well-designed building NV system can reduce building energy use and contain the increasing demand for mechanical cooling systems in buildings. Unfortunately, building designers often struggle when trying to predict the performance of an NV system due, in part, to the lack of precision of the existing simple models for NV airflow in real-world conditions. This thesis investigates the wind driven NV airflow over a multitude of NV systems: night ventilation (NVC), single sided ventilation (SS1), pumping ventilation (SS2 and CR2), and corner ventilation (CR2). For these NV systems, this thesis developed simplified correlations that improve existing models for predicting these NV airflows. The results of this research show that NV systems can be modeled using simple correlations that can assist designers in the integration of these systems in modern buildings. The experimental study of NVC showed that this strategy can significantly reduce mechanical cooling load. The study of window effects on wind driven NV in SS1 systems showed that the window geometry can significantly increase the ventilation flow. Finally, the studies on pumping ventilation showed that this recently discovered ventilation mechanism is very prevalent in isolated buildings with SS rooms with two or more openings

Effective index approximations of photonic crystal slabs: a 2-to-1-D assessment

The optical properties of slab-like photonic crystals are often discussed on the basis of effective index (EI) approximations, where a 2-D effective refractive index profile replaces the actual 3-D structure. Our aim is to assess this approximation by analogous steps that reduce finite 2-D waveguide Bragg-gratings (to be seen as sections through 3-D PC slabs and membranes) to 1-D problems, which are tractable by common transfer matrix methods. Application of the EI method is disputable in particular in cases where locally no guided modes are supported, as in the holes of a PC membrane. A variational procedure permits to derive suitable effective permittivities even in these cases. Depending on the structural properties, these values can well turn out to be lower than one, or even be negative. Both the “standard” and the variational procedures are compared with reference data, generated by a rigorous 2-D Helmholtz solver, for a series of example structures.\u