Browsing by Author "Fael, Cristina"
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- Assessing equilibrium clear water scour around single cylindrical piersPublication . Lança, Rui; Fael, Cristina; Cardoso, AntónioThe objective of this research is to investigate the pertinence of existing approaches to assess the onset of the equilibrium phase of scour at single cylindrical piers in experimental studies. The results of five long-lasting experiments are reported. The discussion has profusely shown that common methods used to decide on whether a given scour experiment has reached the equilibrium phase may be erroneous. It has also shown that known predictors of time to equilibrium may imply significantly wrong predictions of equilibrium depth. Finally, it seems that, typically, 7 days-long scour depth records adjusted though a 6-parameters polynomial function and extrapolated to infinite time render robust vales of the equilibrium scour depth at single cylindrical piers.
- Clear-water scour at comparatively large cylindrical piersPublication . Lança, Rui; Fael, Cristina; Maia, Rodrigo; Pêgo, João; Cardoso, AntónioNew long-duration clear-water scour data were collected at single cylindrical piers with the objective of investigating the effect of sediment coarseness, Dp=D50 (Dp = pier diameter; D50 = median grain size) on the equilibrium scour depth and improving the scour depth time evolution modeling by making use of the exponential function suggested in the literature. Experiments were carried out for the flow intensity close to the threshold condition of initiation of sediment motion, imposing wide changes of sediment coarseness and flow shallowness, d=Dp (d = approach flow depth). The effect of sediment coarseness on the equilibrium scour depth was identified; existing predictors were modified to incorporate this effect for U=Uc ≈ 1.0; Dp=D50 > ≈60 and d=Dp ≥ 0.5; the complete characterization of a known scour depth time evolution model was achieved for U=Uc ≈ 1.0, 60 < Dp=D50 < 500 and 0.5 ≤ d=Dp ≤ 5.0.
- Clear-water scour at pile groupsPublication . Lança, Rui; Fael, Cristina; Maia, Rodrigo; Pêgo, João; Cardoso, AntónioGroups of piles are frequently used as bridge foundations. Different group configurations, characterized by different pile spacing, skew-angle, number, and arrangement of pile group columns interact differently with the flow field and lead to different scour patterns and equilibrium scour depth. There have been a number of past studies on the characterization of scouring at pile groups, but most of them report short duration scour experiments. A priori, such short durations may be postulated to inherently carry important uncertainties into existing scour predictors. In this study, 75 long-duration laboratory tests were run under steady, clear-water flow close to the threshold for initiation of sediment motion, to address the effect of time, pile spacing, skew-angle and number of pile group columns on the equilibrium scour depth. Pile groups consisted of matrical arrangements of one, two, or three columns of four rows, with spacings of 1, 2, 3, 4.5, and 6 pile diameters; the tested skew-angles were 0, 15, 30, 45, and 90°. Important contributions were achieved on (1) the impact of the duration of tests on the shape of the scour hole as well as on the precision of coefficients involved in current predictors, (2) the most unfavorable skew angle, (3) the behavior of collapsed pile groups, (4) the maximum scour depth at pile groups composed of a single alignment, and (5) the performance of two current predictors of scour depth at pile groups. Two formulations of a predictor for the calculation of an aggregated pile group factor are suggested.
- Effect of pier shape and pier alignment on the equilibrium scour depth at single piersPublication . Fael, Cristina; Lança, Cardoso; Cardoso, AntónioThe equilibrium scour depth at uniform single bridge piers depends on a large number of variables, including the pier horizontal cross-section shape and its alignment angle towards the flow direction. The influence of these variables has been studied by only a few researchers, mostly, on the basis of tests that were far from approaching equilibrium. This experimental study aims at revisiting the influence of piers׳ shape and alignment on local scouring for length–width ratios smaller than or equal to 4, by increasing the experimental evidence. Fifty five long-duration laboratory tests were run under steady, clear-water flow, close to the threshold for initiation of sediment motion. Five pier shapes were considered: circular, rectangular square-nosed, rectangular round-nosed, oblong, and zero-spacing (packed) pile-groups; the tested skew-angles were 0°, 30°, 45°, 60°, and 90°. It was concluded that i) the shape factor can be taken as 1.0, for rectangular round-nosed and oblong cross-section piers, and as 1.2, for rectangular square-nosed and packed pile-group cross-section piers, ii) the shape factor does not vary significantly with the duration of tests, this way confirming the robustness of the shape factors reported to date, iii) the effect of shape is present at skewed piers although the associated coefficients remain in the narrow range of 1.0–1.2, and iv) for length–width ratios smaller than 4, the shape factor is of the same order of magnitude as the skew angle factor and should not be neglected.
- Effect of relative sediment size on clear-water equilibrium scour depth at single cylindrical piersPublication . Lança, Rui; Fael, Cristina; Cardoso, AntónioExperiments on local scouring around cylindrical piers were performed in steady clear-water conditions to investigate the effect of sediment size, D50, on equilibrium scour depth, dse. Thirteen experiments, run for 7 to 13 days, were carried out. The dependence of the normalized equilibrium scour depth, dse/Dp, on the relative sediment size, Dp/D50, was identified, corroborating the findings of Sheppard et al. (1995; 1999; 2004). Contrarily to consecrated literature and associated practice, it was observed that the normalized equilibrium scour depth decreases with the relative sediment size, for Dp/D50 > ~100. This effect is possibly one of the causes of frequently reported over prediction of scour by formulas established on the basis of laboratory studies, where frequently Dp/D50 < 100.
- Effect of spacing and skew-angle on clear-water scour at pier alignmentsPublication . Lança, Rui; Fael, Cristina; Pêgo, João; Maia, Rodrigo; Cardoso, AntónioPier alignments, defined as single-row pier groups, displayed normal to main bridges’ axes, are frequently used to support bridge decks. Different alignments configurations, characterized by different combinations of piers’ shape and spacing and skew-angle of the alignment to the flow direction, lead to different interactions with the flow field and to different scour depths. Twenty six long lasting – 7 to 15 days – laboratory experiments were run under steady clear-water flow conditions, close to the threshold condition of beginning of sediment motion, to address the effect of pier spacing and skew-angle on the equilibrium scour depth. Alignments consisted of four cylindrical piers spaced of 1, 2, 3, 4.5 and 6 pier diameters; tested skew-angles were 0◦, 15◦, 30◦, 45◦ and 90◦. The main contribution of the paper derives from the long duration of the experiments. The functional relation of issued equilibrium scour depth with pier spacing and skew-angle is established and the dependence of the traditional group correction factor on the same variables is defined.
- Effect of viscosity on the equilibrium scour depth at single cylindrical piersPublication . Lança, Rui; Simarro, Gonzalo; Fael, Cristina; Cardoso, AntónioMost studies on scouring seem to have overlooked the effect of viscosity, the assumption being that the flow is free of viscous effects inside the scour hole because of the presence of highly turbulent flow structures such as down-flow, horseshoe vortex, and wake vortices irrespective of the approach flow regime. The present work reports the results of experiments conceived to revisit the effect of viscosity on scouring. The experiments were performed by changing the Reynolds-like numbers for two flowshallowness values and keeping constant the remaining dimensionless parameters that control the scour process. Thevalues of theReynoldsnumber corresponded to transitional approach flows. The experimental results indicate that scouring does depend on viscosity in usual laboratory conditions, the viscous effect leading to scour depths in the safety side.
- Erosões localizadas em pontes submersasPublication . Bastardo, Vânia; Lança, Rui; Almeida, Pedro; Fael, CristinaNesta comunicação analisam-se os efeitos da submersão de uma ponte na profundidade máxima da cavidade de erosão junto de um pilar cilíndrico, para escoamentos sem transporte sólido generalizado. Comparam-se os resultados obtidos com equações publicadas na literatura, tendo-se concluído que a formulação proposta por MELVILLE & COLEMAN [2000] é a mais adequada, apresentando um desvio inferior a 10%.
- Erosões localizadas junto de alinhamentos de pilares cilindricosPublication . Lança, Rui; Fael, Cristina; Maia, Rodrigo; Pêgo, João; Cardoso, AntónioÉ conhecido o facto de os pilares fundados em leitos aluvionares poderem dar origem a cavidades de erosão que, em circunstâncias excepcionais, induzem a queda desses pilares e o colapso ou a rotura parcial das pontes em que se integram. A previsão da profundidade de equilíbrio de cavidades de erosão constitui-se, por isso, num aspecto importante do projecto de pontes. Este desiderato, não sendo de concretização trivial para pilares isolados, é mais complexo no caso de alinhamentos de pilares, em face das interacções de vórtices e da interdependência de cavidades de erosão que tais alinhamentos originam. Este estudo centra-se na caracterização de cavidades de erosão junto de alinhamentos de pilares constituídos por quatro pilares cilíndricos. Analisa-se, em concreto, o efeito do espaçamento entre pilares e o efeito do ângulo de enviesamento. O estudo foi realizado para a situação mais desfavorável de profundidade e de velocidade do escoamento de aproximação, definida para a condição de início de movimento do material do fundo. Concluiu-se que, para ângulos de enviesamento de 0º e 15º, a profundidade de erosão é cerca de 15% superior à que se observa num pilar isolado e quase não varia com o espaçamento. Em contrapartida, para ângulos de 30º, 45º e 90º, as cavidades de erosão são sucessivamente menos profundas à medida que o espaçamento aumenta. Para espaçamentos (entre eixos de pilares) iguais ou superiores a seis vezes o diâmetro dos pilares, a profundidade de erosão é praticamente independente do ângulo de enviesamento. Quando o espaçamento é igual ao diâmetro do pilar, os alinhamentos podem ser tratados como um pilar de secção rectangular e topos arredondados que envolva os pilares justapostos; excluindo esta situação de espaçamento, as maiores profundidades de erosão obtêm-se para o ângulo de enviesamento de 30º. Pôde constatar-se que existem métodos de cálculo da profundidade de erosão para alinhamentos de pilares que podem conduzir a previsões largamente por defeito.
- Erosões localizadas junto de pilares de pontes sobre leitos aluvionaresPublication . Couto, Lúcia; Fael, Cristina; Lança, Rui; Cardoso, AntónioEsta comunicação reúne contribuições recentes de estudos dos autores sobre os efeitos da altura relativa do escoamento, da dimensão relativa dos sedimentos do leito, do tempo, da forma dos pilares e do seu ângulo de viés em relação à direção do escoamento na profundidade de equilíbrio de cavidades de erosão que se desenvolvem junto de pilares simples. Estas contribuições baseiam-se em campanhas experimentais de características únicas, no sentido em que foram sistematicamente conduzidas com durações superiores às da vasta maioria dos trabalhos publicados na literatura. A caracterização de parte daqueles efeitos é claramente mais completa e rigorosa por comparação com o que se encontra publicado em literatura da especialidade enquanto, noutros casos, se confirmam resultados anteriormente publicados. Sugerem-se critérios de previsão da profundidade de erosão que incorporam as novas contribuições dos autores.