Experimental characterization of liquid-liquid stratified flow interacting with vertical emergent cylinders

• Autor(a) principal: Ramos, João Pedro Barradas do Ó
• Data de Publicação: 2021
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10362/135861
• topic: Density currents; laboratory tests; PIV; array of cylinders; lock-exchange; Domínio/Área Científica::Engenharia e Tecnologia::Engenharia Mecânica

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/2 and 0.36 m/2. 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.