Relações entre a capacidade de transporte por arraste e o depósito de sedimentos em um canal de fundo fixo rugoso

A determinação da capacidade máxima de transporte por arraste é um assunto de fundamental importância para entender a propagação dos sedimentos em rios, a evolução de erosão e depósito e, em geral, para estabelecer balanços sedimentológicos em bacias hidrográficas. Este trabalho tem por objetivo principal sua quantificação a partir da observação e identificação do início de depósito., As experiências foram realizadas num canal de 26 m de comprimento, 1 m de largura e 0.90 m de altura. A metodologia de ensaio procura identificar a condição hidráulica para a qual começa o depósito dos sedimentos transportados, quando a vazão de ensaio vem sendo sistematicamente diminuída (para descarga sólida constante). Dois critérios foram utilizados para definir a vazão crítica, um experimental (baseado na observação do fenômeno) e outro analítico (baseado num conceito reformulado de taxa de depósito). Os valores ensaiados abrangeram uma faixa de descarga sólida de 300 a 5000 gr/min, declividades de 0.006, 0.008 e 0.010, vazões de 3 a 35 1/s, e diâmetros de areia de 0.77, 1.22 e 1.98 mm. Foram realizadas três series de experiências com diâmetro de areia injetada igual ao diâmetro da rugosidade de fundo do canal, e uma quarta com diâmetro de areia injetada maior que a rugosidade de fundo. Os resultados mostram que das quatro alternativas testadas para caracterizar a variável hidráulica, a potência do escoamento por unidade de superfície, teve a melhor performance. Foi verificada também a boa concordância que existe entre os resultados obtidos em pesquisas anteriores e os obtidos neste trabalho. Quanto às relações entre potência do escoamento e capacidade de transporte, foi confirmada a existência das três faixas de transporte de sedimentos definidas em pesquisas anteriores, porem, os limites foram outros. Na zona de transporte de sedimentos mais intensa (transporte de massa), pode ser constatada a existência de um mecanismo adicional de dissipação de energia, que se acredita ligado aos choques entre partículas. Este mecanismo faz com que o gradiente de potência de escoamento necessário para transportar igual volume de sedimentos seja maior para os diâmetros menores. A série de experiências com diâmetro de areia injetada maior que a rugosidade de fundo, permite analisar o comportamento de uma massa de grãos transportados (com diâmetro constante) sobre fundos com diferentes rugosidades e a sua influência no início de depósito. Os casos analisados foram 1.98/0.77, 1.98/1.22 e 1.22/0.77. Os resultados mostram que um fundo com rugosidade caracterizada por um diâmetro menor que o diâmetro do sedimento transportado não dissipa necessariamente menos energia que um fundo de rugosidade igual ao sedimento transportado. A incorporação do adimensional que caracteriza a rugosidade de fundo permitiu generalizar os resultados conseguidos anteriormente com estes novos, obtendo desta forma uma formulação final única que abrange os dois casos pesquisados.Computing maximum bed load capacity of a flow is essential to understand sediment routing in rivers, erosion and deposition, and, in general, to establish sediment budgets in river basins. The main purpose of this study is to quantify bed load capacity by observing and identifying initial deposition. Experiments were performed in a 26m long, 1m wide and 0.90m high channel. The methodology used in the tests attempts to identify the hydraulic conditions under which deposition of transported sediments begin, as discharge (for constant bed load) is successively reduced. Two criteria were used to define the critical condition, one of them experimental and the other analytical (based on a reformulated concept of the rate of deposition). The values tested covered a range of sediment load from 300 a 5000 g/min, slopes of 0.006, 0.008 and 0.01, streamflows of 3 to 35 1/s, and grain diameters of 0.77, 1.22 and 1.98 mm. Three series of experiments were performed with an injected sand diameter the diameter of the channel bed roughness, and a fourth injected sand diameter greater than the bed roughness. equal with to an The results show that, of the four alternatives tested to describe the hydraulic variable, the best performance was achieved with the stream unit power (per unit of area). It was also found that the results achieved in previous studies generally agree with present findings. As to the relationship between stream unit power and transport capacity, the existence of the three types of sediment transport defined in previous studies was confirmed, but the limits were different. ln the more intense sediment transport zone (mass transport) an additional energy dissipation mechanism was found, which is probably related to collisions between sediments. Due to this mechanism, the gradient of stream unit power required to transport an equal volume of sediment must be higher for smaller diameters. The series of experiments with an injected sand diameter larger than bed roughness allow the analysis of the behavior of a mass of transported grains, (with constant diameter), in beds with different roughness, and its influence on the beginning of deposition . The results show that a bed with roughness characterized by a smal1er diameter than that of the transported sediment, does not necessarily dissipate 1ess energ than a bed with roughness equal to transported sediment. The addition of an adimensional group for the bed·roughness made it possible to generalize the results achieved previously with these new ones, thus obtaining a final single formulation which covers both cases researched

Análise por simulação numérica do escoamento ao redor de um cilindro movendo-se em trajetórias transversal e em oito

This work aims to study a two-dimensional incompressible flow around a cylinder in forced movement in order to understand the phenomena that occur in cylindrical structures under periodic oscillation using the Direct Numerical Simulation technique. The simulations were taken using the computational code named Incompact3d. It was used the Reynolds Number as 400 in all simulations. The trajectories transversal and eight shapes were simulated for a range of displacement amplitude. The Navier-Stokes and continuity equations were used to discretize the flow in a Cartesian mesh. It was used the third-order Runge-Kutta scheme with low-storage for the passtime and the body was represented by the virtual boundary method. The temporal mean of the drag and lift coefficients and vortex shedding mode were computed. The results show large variations of the mean lift coefficient with the amplitude displacement for all trajectories, and so altering the vortex shedding mode. The 2S mode was observed for simulations with transversal trajectories, except for amplitude of 0.55D, which presented the 2P mode. Simulations with eight trajectory presented 2P mode in most cases, and for certain amplitudes the periodic shedding modes were not observed.Neste trabalho foi estudado o escoamento bidimensional de um fluido incompressível ao redor de um cilindro em movimento forçado. Foram analisados os fenômenos que ocorrem nos escoamentos em torno de estruturas cilíndricas submetidas a uma oscilação periódica utilizando a técnica de Simulação Numérica Direta. As equações que descrevem o escoamento são as equações de Navier-Stokes e a equação da continuidade, que são discretizadas em uma malha cartesiana. O Método de Runge-Kutta de terceira ordem, com armazenamento reduzido, foi usado para a discretização temporal, e um esquema de diferenças finitas de sexta ordem, na discretização espacial das equações. O cilindro é representado através do Método de Fronteiras Virtuais. Foram simuladas, para um número de Reynolds igual a 400, as trajetórias, transversal e em oito, variando as amplitudes, horizontal e vertical de movimento. O cálculo das médias temporais dos coeficientes de arrasto e sustentação bem como o estudo do padrão da esteira de vórtices permitiu a análise das variações no escoamento em função das amplitudes horizontal e vertical de deslocamento. Nas trajetórias transversais observou-se o padrão 2S, com exceção da amplitude de 0,55D, que apresentou padrão 2P. As simulações em oito apresentaram o padrão 2P na maioria dos casos e para algumas amplitudes não foi observado padrão periódico de desprendimento de vórtices

Análise por Simulação Numérica de Escoamentos ao Redor de um Cilindro Movendo-se em Trajetórias Transversal e em Oito

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Análise por simulação numérica de escoamentos ao redor de um cilindro movendo-se em trajetórias transversal e em oito

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Análise por simulação numérica de escoamentos ao redor de um cilindro movendo-se em trajetórias transversal e em oito

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Diario de Valencia: Diario de Valencia - Año IV Número 1311 - 1914 octubre 28 (28/10/1914)

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Numerical analysis of the immersed boundary method applied to the flow around a forced oscillating cylinder

In present paper, Navier-Stokes and Continuity equations for incompressible flow around an oscillating cylinder were numerically solved. Sixth order compact difference schemes were used to solve the spatial derivatives, while the time advance was carried out through second order Adams Bashforth accurate scheme. In order to represent the obstacle in the flow, the Immersed Boundary Method was adopted. In this method a force term is added to the Navier-Stokes equations representing the body. The simulations present results regarding the hydrodynamic coefficients and vortex wakes in agreement to experimental and numerical previous works and the physical lock-in phenomenon was identified. Comparing different methods to impose the IBM, it can be concluded that no alterations regarding the vortex shedding mode were observed. The Immersed Boundary Method techniques used here can represent the surface of an oscillating cylinder in the flo

Secondary Kelvin-Helmholtz instability in a 3D stably stratified temporal mixing layer by Direct Numerical Simulation

The present work investigates the nature of the transition to turbulence in the stably stratified mixing layer, which is a complex process with great importance for geophysical and industrial flows. In the stably stratified mixing layer, the streamwise density gradient, which corresponds to the spanwise component of the baroclinic torque in the Boussinesq approximation, feeds the region between the Kelvin-Helmholtz (KH) vortices with vorticity and forms a thin vorticity layer, called baroclinic layer. The competition between buoyancy and inertial forces modifies the dynamics of this layer. As consequence, two different secondary instabilities are found to develop upon the baroclinic layer: one originated near the core region of the KH vortex, called near-core instability, that propagates towards the baroclinic layer and the other of Kelvin-Helmholtz type developed in the baroclinic layer itself. The development of these instabilities in the baroclinic layer depends on the Richardson number, the Reynolds number and the initial conditions. The main objective of this paper is to investigate the occurrence of secondary instabilities in the baroclinic layer of a three-dimensional stably stratified mixing layer using Direct Numerical Simulation (DNS). The development of streamwise vortices and its interactions with the secondary KH structures are focused. Typical Richardson numbers ranging from 0.07 to 0.167 are considered while the Reynolds number is kept constant ( 500 or 1000). White noise and forced perturbation are used as initial conditions. The Navier-Stokes equations, in the Boussinesq approximation, are solved numerically using a sixth-order compact finite difference scheme to compute the spatial derivatives, while the time integration is performed with a third-order low-storage Runge-Kutta method. The numerical results show the development of a jet in the baroclinic layer adjacent to vorticity layers of opposite signs. These layers are created baroclinically by convective motions inside the primary KH vortex and amplifies the near-core instability. It is shown that this instability appears due to the formation of a negative vorticity layer generated between two co-rotating positive vortices. The negative vorticity layer unstables the baroclinic layer and forms small vortices of the KH type. The intensity of the negative vorticity layer depends on the Richardson and Reynolds numbers and defines occurrence or not of secondary KH structures. Interactions between these secondary KH structures and streamwise vortices are also observed. They strongly depend on the initial conditions