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Projeto de investigação
Mass exchange driven by wind and temperature in lake-wetland interfaces
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Experimental Study of Density Currents within Array of Emergent Cylinders
Publication . Alves, Cátia Catarina Wang; Ricardo, Ana; Brito, Moisés
The quality of water bodies directly impacts upon density currents that promote the transport of nutrients and pollutants, and which are influenced by the presence of vegetation.
Given this interaction and its relevant impacts, density currents have appealed the research community over the last decades. While the hydrodynamics of density currents propagating over smooth and rough horizontal boundaries is considerably well understood, its interaction with vegetation or other bluff bodies is less studied. Addressing an identified research need, the present work is aimed at characterizing the generation and propagation of density currents within an array of vertical cylinders. To achieve this goal, a set of laboratory tests employing the lock-exchange technique were carried out. Two density differences were reproduced in refractive index matching conditions. Salt and alcohol mixtures were used to create the dense and ambient fluids, respectively. The currents propagation was recorded by a high-speed video camera and the density fields were obtained through image analysis techniques and a pixel-by-pixel calibration procedure. The density distributions, evolution of the currents front posi-tion and entrainment value through time were analyzed and compared with previous results of a developed current entering an array of cylinders. The analysis showed similar results for the two different reduced gravities tested and satisfactory repeatability conditions. The well-known body-head morphology of a density current over an unobstructed wall was not ob-served. The drag force created by the array did not impact the initial part of the current prop-agation as a constant front velocity similar to the Simpson solution for a slumping phase. After crossing part of the array, the front velocity reduces and was kept constant until the end of the array. Relatively to the dilution of the current, the time evolution of the entrainment showed an almost linear increase at an early stage followed by a continuously decrement towards the end of the array. When the current reached the unobstructed area after the array, it did not have enough kinetic energy to accelerate the front propagation and to incorporate more am-bient fluid. A linear behavior was observed for the front position over time with a propagating velocity smaller than the velocities registered within the array while the entrainment was kept almost constant.
Experimental characterization of liquid-liquid stratified flow interacting with vertical emergent cylinders
Publication . Ramos, João Pedro Barradas do Ó; Brito, Moisés; Ricardo, Ana
Density currents are mainly horizontal flows that are driven by density differences between contacting flows, which may result from temperature gradients, suspended solid particles or dissolved substances. These currents play an important role in nature and industry as they can have a negative environmental impact through global ocean circulation, oil spills, climate variations through water formation and the redistribution of ocean or river water. They have been widely studied since the 20th century, but despite the data acquired and the experiments carried out to date, there is still much research to be done on understanding the dynamics of density currents and their environmental impact.
The present work is part of the WinTherface project that addresses the study of wind and temperature-driven mass exchange at wetland lake interfaces and its impacts on water quality through a multidisciplinary approach integrating, field, laboratory, and numerical work. The aim of the present work is to experimentally investigate how different initial current densities and the presence of vegetation interfere with the propagation and mass exchange between the current and the ambient fluid, as well as the turbulence mechanisms around a set of cylinders present in the channel.
The laboratorial tests were carried out in a channel containing a set of cylinders intended to simulate rigid vegetation. The lock exchange technique was used to generate the density currents with a reduced gravity of 0.09 m/𝑠2 and 0.36 m/𝑠2.
During the laboratory work several tests were carried out using the Particle Image Velocimetry (PIV) measuring technique and the image analysis technique. First the PIV system was used to measure the two-dimensional instantaneous velocity fields in two planes, these being called the side view and plan view which were situated near the cylinder array. After the PIV tests, the image analysis technique was used to mainly evaluate the mass distribution of the current. This technique consists of using a concentration of dye (Rhodamine) in the flow, allowing the understanding of the current dynamics, being able to analyse the temporal evolution of the current front, and the density, its height along the channel, as well as the entrainment evolution.
It is concluded that some parameters of the current such as the height, the position of the front and the entrainment suffer a change due to the interaction with the cylinders. With the results obtained by the PIV it was observed that the interaction of the current with the set of cylinders induces a strong vertical component in the flow becoming three-dimensional. This interaction creates in the flow a set of turbulent structures dominated by a large circulation area that is evident in the approximation area of the cylinders and increases the complexity of the turbulence mechanisms in the current.
Estudo numérico do efeito do vento e da vegetação no escoamento de correntes de densidade
Publication . Sousa, António Maria Matos de; Brito, Moisés; Gil, Luís; Ricardo, Ana
Nas zonas de transição de lagos pouco profundos para zonas húmidas registam-se
frequentemente variações significativas de temperatura, diferenças na salinidade das
águas e/ou a existência de sedimentos em deposição. Estas variações induzem diferenças
na massa volúmica da água que podem originar correntes de densidade. No entanto, esta
estrutura e as propriedades físicas que caracterizam a corrente podem alterar-se caso
existam condicionantes externos como é o caso do vento e da vegetação.
Assim, o principal foco desta dissertação é definir uma metodologia de análise, utili zando o modelo de turbulência LES para estudar os efeitos do vento e da vegetação na
propagação de correntes de densidade. Os objetivos específicos são: (1) estudar a evolução
da posição e velocidade da frente da corrente; (2) analisar o impacto do vento e do seu
sentido na propagação da corrente; (3) caracterizar a influência da vegetação, quer em
termos globais como em termos locais.
Na primeira parte deste estudo, analisa-se a sensibilidade da malha utilizada na simulação numérica e verifica-se a capacidade do modelo numérico com recurso aos resultados
experimentais de Hacker (1996). Faz-se ainda uma comparação entre resultados obtidos
com e sem superfície livre, donde se conclui que a sua presença induz uma maior mistura
entre fluidos e flutuações do valor da velocidade da frente da corrente.
Em seguida, estuda-se os efeitos do vento, da sua intensidade e sentido na propagação
da corrente de densidade em 2D. Observou-se que quando o vento circula no mesmo
sentido da corrente e quanto maior a sua intensidade, menor a velocidade da corrente.
Com o vento no sentido oposto, a velocidade da corrente aumenta com o aumento da
velocidade do vento.
Na parte final, analisou-se o comportamento da corrente quando exposta a pouca densidade de vegetação, representada na simulação 3D por um único cilindro centrado com
o canal. Com uma fração volúmica de 0,2%, constatou-se que o cilindro tem influência
essencialmente a nível local (próximo do cilindro).
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Entidade financiadora
Fundação para a Ciência e a Tecnologia
Programa de financiamento
3599-PPCDT
Número da atribuição
PTDC/CTA-OHR/30561/2017