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Authors
Abstract(s)
O conhecimento do campo de escoamento, junto de estruturas permeĂĄveis, tem permitido
implementar medidas, recorrendo a esse tipo de estruturas, na estabilização de leitos e na
proteção das margens de rios. A adoção destas medidas, baseadas em enrocamentos, também
tĂȘm sido utilizadas na proteção contra as erosĂ”es localizadas junto das fundaçÔes de pontes.
Ainda assim, persistem dĂșvidas devido Ă complexidade dos fenĂłmenos envolvidos,
nomeadamente quando as estruturas permeĂĄveis se desenvolvem em altura.
Na presente dissertação pretende-se realizar um estudo comparativo do campo de escoamentos
que se desenvolve junto de dois elementos cilĂndricos verticais, com porosidades de 0% e Ë50%,
ensaiados no canal de Estruturas HidrĂĄulicas da UBI.
No trabalho experimental, as mediçÔes das componentes bidimensionais das velocidades
instantĂąneas, longitudinal e vertical, foram realizadas em trĂȘs planos transversais, com um
sistema 3D de Velocimetria Doppler Laser (LDV). Os contornos das componentes da velocidade
média no tempo, dos vetores velocidade, das intensidades turbulentas, das tensÔes de Reynolds
e das tensÔes tangenciais totais, são utilizados na caracterização do campo de escoamento em
torno dos obstĂĄculos.
Os resultados obtidos permitiram concluir que a permeabilidade influencia as caracterĂsticas
do campo do escoamento junto ao elemento cilĂndrico. A possibilidade do escoamento, que
chega a montante do obstĂĄculo, poder passar pela estrutura porosa, em vez de a contornar,
caso esta fosse impermeåvel, possibilita a minimização do escoamento descendente, em
quantidade e velocidade, condicionando o desenvolvimento do vĂłrtice em ferradura. JĂĄ a
jusante, por influĂȘncia do escoamento que atravessa a estrutura porosa, a zona de esteira nĂŁo
apresenta vorticidades tĂŁo bem definidas, como no caso do escoamento que contorna o
elemento cilĂndrico impermeĂĄvel. A partir desta secção, a zona inferior da esteira turbulenta,
em vez de apresentar o escoamento ascendente caracterĂstico de estruturas cilĂndricas
impermeĂĄveis, que promove o transporte de sedimentos, apresenta um escoamento mais
horizontal, com tendĂȘncia descendente e capaz de promover a deposição de sedimentos para
jusante dos obstĂĄculos.
De um modo geral, a interferĂȘncia que um elemento cilĂndrico provoca no seio de um
escoamento, Ă© menor quando este Ă© dotado de alguma permeabilidade, que mantenha o melhor
possĂvel a continuidade do escoamento longitudinal, diminuĂdo a necessidade deste ter de a
contornar, e assim reduzir o escoamento descendente que irå originar as erosÔes localizadas.
The flow field knowledge, along permeable structures, has allowed the implementation of measures, resorting to this type of structures, in beds stabilization and river banks protections. The adoption of these measures, based on riprap, have also been used to protect against local scour close to bridges foundations. Nevertheless, there are still doubts due to the complexity of the phenomena involved, particularly, when the permeable structures develop in height. The present dissertation provides a comparative study between the flow field around two circular piers, with porosities of 0% and Ë50%, tested in UBIs Hydraulic Structure channel. In experimental work, the two-dimensional components of the instantaneous velocity, longitudinal and vertical, were measured on three cross sections, using a Laser Doppler Velocimeter (LDV). The contours of time-averaged velocity components, velocity vectors, turbulence intensities, Reynolds shear stresses and tangential shear stresses are presented to characterize the flow around obstacles. The results showed that the permeability influences the characteristics of the flow field along the cylindrical element. The possibility of the flow that arrives upstream of the obstacle can pass through the porous structure, rather than around, should it be solid, allows minimizing of the downflow, in quantity and velocity, conditioning the development of the horseshoe vortex. In downstream, due to the influence of the bleed-flow, the wake region has no vorticities as well defined, as in the case of deflected flow in a solid pier. From this section, the lower of the wake region, instead of displaying the typical upflow of impermeable cylindrical structures, which promotes the transport of sediments, presents a more horizontal flow, with downflow capable of promoting the deposition of sediment to the downstream of obstacles. In a general way, the interference that a cylindrical element causes within a flow is smaller when this is endowed with some permeability, which maintains the best possible longitudinal continuity of flow, reducing the deflected flow and downflow which leads to local scour.
The flow field knowledge, along permeable structures, has allowed the implementation of measures, resorting to this type of structures, in beds stabilization and river banks protections. The adoption of these measures, based on riprap, have also been used to protect against local scour close to bridges foundations. Nevertheless, there are still doubts due to the complexity of the phenomena involved, particularly, when the permeable structures develop in height. The present dissertation provides a comparative study between the flow field around two circular piers, with porosities of 0% and Ë50%, tested in UBIs Hydraulic Structure channel. In experimental work, the two-dimensional components of the instantaneous velocity, longitudinal and vertical, were measured on three cross sections, using a Laser Doppler Velocimeter (LDV). The contours of time-averaged velocity components, velocity vectors, turbulence intensities, Reynolds shear stresses and tangential shear stresses are presented to characterize the flow around obstacles. The results showed that the permeability influences the characteristics of the flow field along the cylindrical element. The possibility of the flow that arrives upstream of the obstacle can pass through the porous structure, rather than around, should it be solid, allows minimizing of the downflow, in quantity and velocity, conditioning the development of the horseshoe vortex. In downstream, due to the influence of the bleed-flow, the wake region has no vorticities as well defined, as in the case of deflected flow in a solid pier. From this section, the lower of the wake region, instead of displaying the typical upflow of impermeable cylindrical structures, which promotes the transport of sediments, presents a more horizontal flow, with downflow capable of promoting the deposition of sediment to the downstream of obstacles. In a general way, the interference that a cylindrical element causes within a flow is smaller when this is endowed with some permeability, which maintains the best possible longitudinal continuity of flow, reducing the deflected flow and downflow which leads to local scour.
Description
Keywords
Campos de Velocidade Elemento CilĂndrico Escoamento Turbulento Ldv Permeabilidade