A comprehensive understanding of the hydrodynamics in vegetated open-channels and flow-vegetation interaction is of high interest to researchers and practitioners alike for instance in the content of river and coastal restoration schemes. The focus of this study was to investigate the effect of the presence of vegetation on flow resistance, turbulence statistics, and the instantaneous flow in open channels by performing three-dimensional computational-fluid-dynamics (CFD) simulations. Firstly, fully developed turbulent flow in fully-vegetated channel was analyzed by employing the method of high-resolution Large-Eddy Simulation (LES). Flow through a staggered array of rigid, emergent cylinders was simulated and the LES was validated through experiments. After validation, numerical simulations were performed at an extended parameter range of two different cylinder Reynolds numbers (ReD = 500 and 1340) and three different vegetation densities (φ = 0.016, 0.063, and 0.251). Flow structures and statistics were analyzed on the instantaneous flow and the effect of the vegetation density and cylinder Reynolds number was assessed. Moreover, drag forces exerted on the cylinders were calculated explicitly, and the effect of both ReD and φ on the drag coefficient was quantified. Secondly, two new alternative simulation strategies, a RANS based strategy with a vegetative closure model and a low-resolution Large-Eddy Simulation, were devised. They were evaluated by simulating several experimental cases with diverse conditions of the cylinder arrangement (i.e., staggered vs. random distribution), vegetation densities (φ = 0.016, 0.022, 0.063, 0.087, 0.091, 0.150, and 0.251), and cylinder Reynolds number (ReD = 170 - 1700). For the RANS based strategy, the importance of a-priori knowledge was assessed, and for the low-resolution LES, the efficiency and accuracy was demonstrated. Finally, a numerical strategy based on a porosity approach was developed and applied to open-channel flow through a natural plant. The simulated velocities were compared with experimentally acquired ones and results showed reasonable agreement. The results obtained in this research contribute to the understanding of fundamental mechanism of flow-vegetation interaction in vegetated open-channels, resolving turbulent flow-vegetation interaction explicitly. In addition, the new numerical strategies developed as part of this research are expected to allow describing the behavior of turbulent flow through artificial and natural vegetation with high efficiency and accuracy.

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Simulation of fresh concrete flow has spurged with the advent of Self-Compacting Concrete, SCC. The fresh concrete rheology must be compatible with the reinforced formwork geometry to ensure complete and reliable form filling with smooth concrete surfaces. Predicting flow behavior in the formwork and linking the required rheological parameters to flow tests performed on the site will ensure an optimization of the casting process. In this thesis, numerical simulation of concrete flow and particle behaviour is investigated, using both discrete as well as a continuous approach. Good correspondence was achieved with a Bingham material model used to simulate concrete laboratory tests (e.g. slump flow). It is known that aggregate properties such as size, shape and surface roughness as well as its grading curve affect fresh concrete properties. An increased share of non-spherical particles in concrete increases the level of yield stress, τ0, and plastic viscosity, µpl. The yield stress level may be decreased by adding superplasticizers, however, the plastic viscosity may not. An explanation for the behaviour of particles is sought after experimentally, analytically and numerically. Bingham parameter plastic viscosity is experimentally linked to particle shape. It was found that large particles orient themselves aligning their major axis with the fluid flow, whereas small particles in the colloidal range may rotate between larger particles. The rotation of crushed, non-spherical fine particles as well as particles of a few microns that agglomorate leads to an increased viscosity of the fluid. Generally, numerical simulation of large scale quantitative analyses are performed rather smoothly with the continuous approach. Smaller scale details and phenomena are better captured qualitatively with the discrete particle approach. As computer speed and capacity constantly evolves, simulation detail and sample volume will be allowed to increase. A future merging of the homogeneous fluid model with the particle approach to form particles in the fluid will feature the flow of concrete as the physical suspension that it represents. One single ellipsoidal particle in fluid was studied as a first step. / <p>QC 20150326</p>

[pt] Utiliza-se a variável de similaridade proposta por L. Crocco para transformar as equações da camada limite, hidrodinâmica e térmica no escoamento laminar bi-dimensional de um fluido com propriedades fisicas constantes em um canal convergente. As equações de conservação de massa, momentum e energia, que formam um sistema de três equações diferenciais parciais não lineares são modificadas num novo sistema composto de equações diferenciais ordinárias. Estes escoamentos, que admitem soluções similares são resolvidos e apresentam-se soluções para várias combinações dos parâmetros envolvidos. Na solução destes casos considera-se inclusive o efeito da função dissipação. Para alguns dos resultados obtidos no presente trabalho foi possível comparar com soluções existentes na literatura especializada, e uma excelente concordância foi observada. Entre os resultados apresentados, destacam-se expressões para o número de Nusselt local como função dos números de Prandtl e de Reynolds. / [en] The similarity variable proposed by L.Crocco is employed in this work to transform the boundary layer equations for the constant-property, two dimensional, laminar flow in a convergent channel. The governing equations (conservation of mass, momentum and energy) constitute a set of three non-linear partial differential equations. These equations are transformed into a simpler set of ordinary differential equations. Solutions of this transformed set are presented for several combinations of the relevant parameters. Among the effects studied in the heat generation due to viscous forces. When possible, comparisions with available results were made, and an excellent agreement was observed. Among the results presented, are of special practical interest the expressions for the local Nusselt number as a function of the Prandtl and Reynolds numbers.

L'étude fondamentale des structures turbulentes proche paroi est d'une significative importance en raison de son rôle dominant dans nombreuses applications d'ingénierie. Dans les écoulements turbulents, structures d'échelles différentes interagissent dans un phénomène complexe caractérisé par un processus autonome et continu. Bien que la présence de modèles de mouvements cohérents semble entretenir la turbulence, l'évolution et l'interaction de ces structures ne sont pas encore complètement compris. Pour analyser ces phénomènes, études d’ecoulements turbulents à proximité de paróis ont été effectuées sur une plaque plane dans une soufflerie et à l'intérieur d'un canal carré d'eau. Ce travail a employé une technique tomographique de vélocimétrie par images de particules pour mesurer de champs de vitesse tridimensionnel. Le traitement des données a été optimisé afin d'améliorer la précision des champs de vecteurs mesurés. Les mesures réalisées ont permis d’obtenir des statistiques conditionnelles des structures cohérentes, à savoir, les régions à faible et à grande vitesse, les "sweeps", les "ejections" et les tourbillons. Les résultats quantitatifs ont montré des structures tourbillonnaires distribués sur tout le flux, cependant, concentrée au voisinage des régions à faible vitesse et des "ejections". Les résultats ont indiqué que les structures tourbillonnaires étaient indirectement corrélés avec le processus de production de la turbulence, bien qu'ils ont été les principaux responsables de la dissipation d'énergie turbulente. / The fundamental study of turbulent structures near a wall is of significant importance due to its dominant role in innumerous engineering applications. In turbulent flows, eddies of different scales interact with each other in a complex phenomenon of a continuous self-sustaining process. Although the presence of patterns of coherent motion seem to be responsible for the maintenance of turbulence, the evolution and the interaction of these structures are still not completely understood. To investigate such phenomena, near-wall turbulent flows over a smooth flat plate in a wind tunnel and inside a square water channel were conducted. The present work employed a high-repetition tomographic PIV technique to measure three dimensional, time-resolved velocity fields. The tomo-PIV processing was optimized in order to improve the algorithm performance and the vector fields accuracy. Conditional statistics of coherent structures, namely, low- and high-speed regions, sweeps, ejections and vortices, were computed, providing quantitative characteristics of these structures. The statistical analyses were performed employing a data post-processing code specially developed for the present work. Results showed vortical structures spread all over the flow, nevertheless, concentrated close to low-speed regions and ejections. Results indicated that vortical structures were indirectly correlated with the process of turbulence production, although they were the main responsible for the turbulent energy dissipation.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22756号 / 工博第4755号 / 新制||工||1744(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 細田 尚, 教授 戸田 圭一, 准教授 音田 慎一郎 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

A set of laboratory experiments was carried out to distinguish flow characteristics(bed origin, shear velocity, turbulence intensity, turbulent kinetic energy) between beds of differing gravel angularity. Ten vertical profiles of velocity measurements were taken from angular and rounded fixed gravel beds with a 16 MHz micro acoustic Doppler velocimeter (ADV) sampling at 50 Hz. Both gravel beds had a bottom slope of 0.2% and were in the intermediate - roughness scale. Shear velocities were calculated using three common methods: St Venant, Reynolds stress, and Clauser. The Reynolds stress method resulted in the closest visual match to turbulence distributions proposed by others. The bed origin was found to be on average 0.24D50 and 0.21D50 for the angular and rounded gravel beds respectively. These differences, however, were not statistically significant. Turbulence intensity within 20% of the bed showed considerable scatter. The difficult measuring conditions likely prevented the ADV to detect significant differences of turbulence intensity in the longitudinal and transverse directions between the two gravel beds. But the ADV measurements in the vertical direction may well resolve turbulence even in difficult flow conditions (determined by acoustic Doppler performance curve formulation). For the vertical direction, the angular gravel bed showed an increase in TImax that extended throughout the profile. The increased turbulence intensity had a concomitant effect of increasing the turbulent kinetic energy for the angular bed.

Significant effort has been placed on the development of surfaces which reduce the amount of drag experienced by a fluid as it passes over the surface. Alterations to the fluid itself, as well as the chemical and physical composition of the surface have been investigated with varying success. Investigations into turbulent drag reduction have been mostly limited to those involving bubbles and riblets. Superhydrophobic surfaces, which combine hydrophobic surface chemistry with a regular array of microfeatures, have been shown to provide significant drag reduction in the laminar regime, with the possibility of extending these results into turbulent flows. Direct numerical simulations are used to investigate the drag reducing performance of superhydrophobic surfaces in turbulent channel flow. Slip velocities, wall shear stresses, and Reynolds stresses are considered for a variety of superhydrophobic surface microfeature geometry configurations at friction Reynolds numbers of Re = 180, Re = 395, and Re = 590. This work provides evidence that superhydrophobic surfaces are capable of reducing drag in turbulent flow situations by manipulating the laminar sublayer and turbulent energy cascade. For the largest micro-feature spacing of 90 microns an average slip velocity over 80% of the bulk velocity is obtained, and the wall shear stress reduction is found to be greater than 50%. The simulation results suggest that the mean velocity profile near the superhydrophobic wall continues to scale with the wall shear stress, but is offset by a slip velocity that increases with increasing micro-feature spacing.

Yes / This work presents new water surface elevation data including evidence of the spatial correlation of water surface waves generated in shallow water flows over a gravel bed without appreciable bed forms. Careful laboratory experiments have shown that these water surface waves are not well-known gravity or capillary waves but are caused by a different physical phenomenon. In the flow conditions studied, the shear present in shallow flows generates flow structures, which rise and impact on the water-air interface. It is shown that the spatial correlation function observed for these water surface waves can be approximated by the following analytical expression W(rho) = e(-rho 2/2 sigma w2)COS(2 pi L-0(-1)rho). The proposed approximation depends on the spatial correlation radius, sigma(w), characteristic spatial period, L-0, and spatial lag, . This approximation holds for all the hydraulic conditions examined in this study. It is shown that L-0 relates to the depth-averaged flow velocity and carries information on the shape of the vertical velocity profile and bed roughness. It is also shown that sigma(w) is related to the hydraulic roughness and the flow Reynolds number.

Yes / Bridge pier is a common feature in hydraulic structure. Its impact to the river usually occurs in group form rather than single pier, so this challenging piers-group influence towards river hydraulics and turbulence needs to be explored. In this paper, the measurements were conducted using an Acoustic doppler velocimeter (ADV) to study velocities in three dimensions (longitudinal, transversal, and vertical). Based on the experimental data, we have observed reversed depth-averaged velocity vector after each pier in the group of three-pier. The analysis has been conducted on the contribution of each bursting event to Reynolds shear stress (RSS) generation, in order to identify the critical events and turbulence structures around the piers. In the upstream near-wake flow in the bed-wall layer, strong sweep and ejection events have been observed; while at downstream, sweeps were more dominant. The pattern of burst changed in the outer layer of flow, where ejections were more dominant. Furthermore, the contribution fractional ratio to RSS variation at hole size H = 0 indicates that sweeps and ejections were significantly generated at the near wake-flow in upstream.