Experimental Study of Density Currents within Array of Emergent Cylinders

• Autor(a) principal: Alves, Cátia Catarina Wang
• Data de Publicação: 2022
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10362/158288
• topic: Lock-exchange density currents; array of cylinders; laboratory tests image analysis technique; Domínio/Área Científica::Engenharia e Tecnologia::Engenharia Mecânica

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.