Uma Metodologia de Estudo de Simulação Tridimensional de Escoamento Turbulento Estratificado no Reservatório de Plantas Hidrelétricas.

Uma simulação numérica que leva em conta os efeitos de estratificação e mistura escalar (como a temperatura, salinidade ou substância solúvel em água) é necessária para estudar e prever os impactos ambientais que um reservatório de usina hidrelétrica pode produzir. Este trabalho sugere uma metodologia para o estudo de escoamentos ambientais, principalmente aqueles em que o conhecimento da interação entre a estratificação e mistura pode dar noções importantes dos fenômenos que ocorrem. Por esta razão, ferramentas de simulação numérica 3D de escoamento ambiental são desenvolvidas. Um gerador de malha de tetraedros do reservatório e o modelo de turbulência algébrico baseado no número de Richardson são as principais ferramentas desenvolvidas. A principal dificuldade na geração de uma malha de tetraedros de um reservatório é a distribuição não uniforme dos pontos relacionada com a relação desproporcional entre as escalas horizontais e verticais do reservatório. Neste tipo de distribuição de pontos, o algoritmo convencional de geração de malha de tetraedros pode tornar-se instável. Por esta razão, um gerador de malha não estruturada de tetraedros é desenvolvido e a metodologia utilizada para obter elementos conformes é descrita. A geração de malha superficial de triângulos utilizando a triangulação Delaunay e a construção do tetraedros a partir da malha triangular são os principais passos para o gerador de malha. A simulação hidrodinâmica com o modelo de turbulência fornece uma ferramenta útil e computacionalmente viável para fins de engenharia. Além disso, o modelo de turbulência baseado no número de Richardson leva em conta os efeitos da interação entre turbulência e estratificação. O modelo algébrico é o mais simples entre os diversos modelos de turbulência. Mas, fornece resultados realistas com o ajuste de uma pequena quantidade de parâmetros. São incorporados os modelos de viscosidade/difusividade turbulenta para escoamento estratificado. Na aproximação das equações médias de Reynolds e transporte de escalar é utilizando o Método dos Elementos Finitos. Os termos convectivos são aproximados utilizando o método semi-Lagrangeano, e a aproximação espacial é baseada no método de Galerkin. Os resultados computacionais são comparados com os resultados disponíveis na literatura. E, finalmente, a simulação de escoamento em um braço de reservatório é apresentada.

Horizontal and vertical movements of juvenile bluefin tuna (Thunnus thynnus) in relation to oceanographic conditions of the western North Atlantic, determined with ultrasonic telemetry

We employed ultrasonic transmitters to follow (for up to 48 h) the horizontal and vertical movements of five juvenile (6.8–18.7 kg estimated body mass) bluefin tuna (Thunnus thynnus) in the western North Atlantic (off the eastern shore of Virginia). Our objective was to document the fishes’ behavior and distribution in relation to oceanographic conditions and thus begin to address issues that currently limit population assessments based on aerial surveys. Estimation of the trends in adult and juvenile Atlantic bluefin tuna abundance by aerial surveys, and other fishery-independent measures, is considered a priority. Juvenile bluefin tuna spent the majority of their time over the continental shelf in relatively shallow water (generally less then 40 m deep). Fish used the entire water column in spite of relatively steep vertical thermal gradients (≈24°C at the surface and ≈12°C at 40 m depth), but spent the majority of their time (≈90%) above 15 m and in water warmer then 20°C. Mean swimming speeds ranged from 2.8 to 3.3 knots, and total distance covered from 152 to 289 km (82–156 nmi). Because fish generally remained within relatively con-fined areas, net displacement was only 7.7–52.7 km (4.1–28.4 nmi). Horizontal movements were not correlated with sea surface temperature. We propose that it is unlikely that juvenile bluefin tuna in this area can detect minor horizontal temperature gradients (generally less then 0.5°C/km) because of the steep vertical temperature gradients (up to ≈0.6°C/m) they experience during their regular vertical movements. In contrast, water clarity did appear to influence behavior because the fish remained in the intermediate water mass between the turbid and phytoplankton-rich plume exiting Chesapeake Bay (and similar coastal waters) and the clear oligotrophic water east of the continental shelf.

Vertical and horizontal movements of southern bluefin tuna (Thunnus maccoyii) in the Great Australian Bight observed with ultrasonic telemetry

The vertical and horizontal movements of southern bluefin tuna (SBT), Thunnus maccoyii, in the Great Australian Bight were investigated by ultrasonic telemetry. Between 1992 and 1994, sixteen tuna were tracked for up to 49 h with depth or combined temperature-depth transmitting tags. The average swimming speeds (measured over the ground) over entire tracks ranged from 0.5 to 1.4 m/s or 0.5 to 1.4 body lengths/s. The highest sustained swimming speed recorded was 2.5 m/s for 18 hours. Horizontal movements were often associated with topographical features such as lumps, reefs, islands and the shelf break. They spent long periods of time at the surface during the day (nearly 30%), which would facilitate abundance estimation by aerial survey. At night, they tended to remain just below the surface, but many remained in the upper 10 m throughout the night. SBT were often observed at the thermocline interface or at the surface while travelling. A characteristic feature of many tracks was sudden dives before dawn and after sunset during twilight, followed by a gradual return to their original depth. It is suggested that this is a behavior evolved to locate the scattering layer and its associated prey when SBT are in waters of sufficient depth. SBT maintained a difference between stomach and ambient temperature of up to 9°C.

Articulated wooden blades for a horizontal-axis wind turbine. Design, construction and testing.

A pair of blades were constructed following a Tapered Chord, Zero Twist pattern after Anderson. The construction uses the Wood Epoxy Saturation Technique, with a solid Beech main spar and leading edge joined together with laminated veneers of beech forming a D-section; the trailing edge is formed from millimetre ply skins, foam filled to resist compressive loads. This construction leads to an extremely light, flexible blade, with the centres of gravity and torsion well forward, giving good stability. Each blade has three built-in strain gauges, alowing flapwise bending to be measured. Stiffness, and natural frequencies, were measured, to input to a numerical computer model to calculate blade deformation during operation, and to determine stability boundaries of the blade. Preliminary aerodynamic performance measurements are presented and close agreement is found with theory.

Diffusion and chemical reaction in the porous structures of solid oxide fuel cells

The Rolls-Royce Integrated-Planar Solid Oxide Fuel Cell (IP-SOFC) consists of ceramic modules which have electrochemical cells printed on the outer surfaces. The cathodes are the outermost layer of each cell and are supplied with oxygen from air flowing over the outside of the module. The anodes are in direct contact with the ceramic structure and are supplied with fuel from internal gas channels. Natural gas is reformed into hydrogen for use by the fuel cells in a separate reformer module of similar design except that the fuel cells are replaced by a reforming catalyst layer. The performance of the modules is intrinsically linked to the behaviour of the gas flows within their porous structures. Because the porous layers are very thin, a one-dimensional flow model provides a good representation of the flow property variations between fuel channel and fuel cell or reforming catalyst. The multi-component convective-diffusive flows are simulated using a new theory of flow in porous material, the Cylindrical Pore Interpolation Model. The effects of the catalysed methane reforming and water-gas shift chemical reactions are also considered using appropriate kinetic models. It is found that the shift reaction, which is catalysed by the anode material, has certain beneficial effects on the fuel cell module performance. In the reformer module it was found that the flow resistance of the porous support structure makes it difficult to sustain a high methane conversion rate. Although the analysis is based on IP-SOFC geometry, the modelling approach and general conclusions are applicable to other types of SOFC.

Transient growth and triggering in the horizontal Rijke tube Matthew

This theoretical paper examines a non-normal and non-linear model of a horizontal Rijke tube. Linear and non-linear optimal initial states, which maximize acoustic energy growth over a given time from a given energy, are calculated. It is found that non-linearity and non-normality both contribute to transient growth and that, for this model, linear optimal states are only a good predictor of non-linear optimal states for low initial energies. Two types of non-linear optimal initial state are found. The first has strong energy growth during the first period of the fundamental mode but loses energy thereafter. The second has weaker energy growth during the first period but retains high energy for longer. The second type causes triggering to self-sustained oscillations from lower energy than the first and has higher energy in the fundamental mode. This suggests, for instance, that low frequency noise will be more effective at causing triggering than high frequency noise.