La turbulence axisymétrique est un écoulement bidimensionnel trois-composantes. L’étude de ce type de turbulence est motivée par le fait que celle-ci représente la limite asymptotique des écoulements anisotropes, et qu’elle a été le sujet des investigations théoriques dans le passé. Dans ce manuscrit, la turbulence axisymétrique a étudié en géométrie fermée en utilisant des simulations numériques spectrales et pseudo-spectrales.Études antérieures concernant la génération des structures cohérentes, obtenues dans les écoulements en déclin libre, sont considérées ici dans le contexte des écoulements statistiquement stationnaires, où l’énergie est injectée soit par un forçage spectralement localisé ou par une rotation des disques en haut et en bas du cylindre. On montre que les structures observées sont conformes aux prédictions théoriques.Lorsqu’un protocole de forçage anisotrope est utilisé, une bifurcation est observée entre un état non-tourbillonnant (bidimensionnel deux-composantes, 2D2C) et un écoulement tourbillonnant turbulent (bidimensionnel trois-composante, 2D3C). Cette transition est modélisée à travers un système de deux équations différentielles ordinaires (ODE), et on montre que ce modèle retient la physique essentielle de cette transition. La transition de l’écoulement axisymétrique à un écoulement tridimensionnel (3D3C) est ensuite étudiée à l’aide d’une dimension non-entière, en introduisant de façon continue la variation azimutale dans le système. On montre que la limite 2D2C est singulière et qu’une petite variation azimutale permet une redistribution d’énergie sur les différentes composante énergétiques. Le modèle ODE est adapté pour ce système et on montre que pour l’écoulement considéré la corrélation pression-déformation est responsable d'un niveau approximativement proportionnel à la dimension non-entière. Des Simulations des Grandes Echelles sont réalisées pour évaluer la robustesse des observations à grands nombres de Reynolds. / Axisymmetric turbulence is a two-dimensional three-component flow. The investigation of this type of turbulence is motivated by the fact that it represents the asymptotic limit of anisotropic flows and since it has been the subject of theoretical investigations in the past. In the present manuscript such a flow is investigated in wall-bounded cylindrical geometry using spectral and pseudo-spectral numerical simulations.Previous results on the generation of coherent structures, obtained for freely decaying flow, are here assessed in the context of statistically steady flow, where the energy is supplied by either a spectrally localized forcing, or by moving top and bottom plates of the cylinder. It is shown that the observed structures are consistent with theoretical predictions.When an anisotropic forcing protocol is used, a bifurcation is observed from a non-swirling (two-dimensional two-component, 2D2C) flow to a swirling (two-dimensional three-component 2D3C) turbulent flow. This transition is modelled by a system of two ordinary differential equations (ODE), and it is shown that this model retains the essential physics of the transition.The transition of the axisymmetric flow to three-dimensional (3D3C) flow is then studied using non-integer dimension, by smoothly introducing azimuthal variation into the system. It is shown that the 2D2C limit is singular and that small azimuthal variation allows a redistribution of energy over the different energy components. The ODE model is adapted for this system and it is shown that for the considered flow the pressure-strain correlation is responsible for a swirl-level approximately proportional to the non-integer dimension. Large-Eddy Simulations are carried out to assess the robustness of the observations at higher Reynolds number.

River confluences - locations where rivers join one another - are fundamental components of natural drainage networks. Differences in topography, geology, soils, land use, and human activities within watersheds upstream of confluences can produce differences in thermal or chemical properties of river flows and in the materials transported by these flows. Mixing is initiated along the mixing interface (MI) that develops between the two incoming streams with different properties. Therefore, the understanding of fluvial processes at confluences is important for determining river mixing both at and downstream of individual confluences and at the scale of drainage networks. The primary goal of this thesis is to describe the main mechanisms that control mixing and transport at river confluences and the role played by the complex flow structures in the flow and how they change with planform geometry and other flow and geometrical parameters. The study is carried out using Computational Fluid Dynamics modeling based on the state of the art Detached Eddy Simulation approach and High Performance Computing. By starting with a mixing layer between parallel streams with simple geometry, the model is validated based on laboratory experiment data. Moreover, some hypotheses regarding the growth of the mixing layer are amended with the extensive data provided by the model, which is a valuable supplement to the experiment. By performing a detailed parametric study in very long and wide domains for simplified cases one can focus on the spatial development of the MI and the large scale coherent structures forming within and in the vicinity of the MI without the complications of other factors. More specifically, the influence of velocity and density difference of the two streams, flow depth, inflow conditions and angles between the two streams on the spatial development of the MI is analyzed. The data resulting from these simulations conducted in simple geometries is a unique set of data which can be used to test and improve theoretical models used to predict global parameters describing flow and mixing at natural river confluences. In particular, this research uses for the first time well resolved Large Eddy Simulation based techniques to investigate how density differences between the incoming streams affect the spatial development of the mixing interface and mixing downstream of the confluence apex. In order to investigate flow dynamics, mixing processes and effects of temperature stratifications at natural river confluences with discordant bed, a series of simulations is performed for the confluence of the Ebro and Segre Rivers in Spain, which is one of the most studied confluences in Europe. With the detailed survey data of the confluence bed and flow conditions data provided, the goal is to understand the main mechanisms responsible for mixing at a confluence with a large bed discordance and how the velocity ratio between the two incoming streams affects mixing. Besides, more insights are provided that if temperature stratification effects affect significantly flow structure and mixing based on real conditions recorded at a natural confluence. The study provides a comprehensive set of flow data in the confluence including velocity, temperature distribution etc. It serves as important supplement to the field measurements, which are generally more difficult to obtain. It also allows estimating scale effects between field conditions and conditions at which laboratory experiments of confluence flow and mixing are conducted.

A comprehensive computational and experimental analysis has been conducted to characterize the flow dynamics and periodic structures formed in the wake of complex turning vanes. The vane packs were designed by the StreamVane swirl distortion generator technology, a design system that can efficiently reproduce swirl distortion for compressor rig and full turbofan engine testing. StreamVanes consist of an array of turning vanes that commonly contain variations in turning angle along their span, a nonaxisymmetric profile about the centerline, and vane-to-vane intersections or junctions to accurately generate the desired distortion. In this study, vane packs are considered complex if they contain two out of three of these features, a combination seen in other turbomachinery components outside of StreamVane design. Similar to all stator vanes or rotor blades, StreamVane vane packs are constructed using a series of cross-sectional airfoil profiles with blunt trailing edges and finite thicknesses. This, in turn, introduces periodic vortex structures in the wake, commonly known as trailing edge vortex shedding. To fully understand how the dynamics and coherent wake formations within vortex shedding impact both the flow distortion and structural durability of StreamVanes, it is first necessary to characterize the corresponding wakes in three dimensions. The current study provides an in-depth analysis to predict and measure the trailing edge vortex development using high-fidelity computational fluid dynamics and stereoscopic time-resolved particle image velocimetry experiments. Two testcase StreamVane geometries were specifically designed with complex features to evaluate their influence on the dynamics and coherence of the respective vane wakes. Fully three-dimensional, unsteady computational fluid dynamics simulations were performed using a Reynolds-Averaged Navier-Stokes solver coupled with a standard two-equation turbulence model and a hybrid, scale-resolving turbulence model. Both models predicted large-scale wake frequencies within 1—14% of experiment, with a mean difference of less than 3.2%. These comparisons indicated that lower fidelity simulations were capable of accurately capturing such flows for complex vane packs. Additionally, structural and modal analyses were conducted using finite element models to determine the correlations between dominant structural modes and dominant wake (flow) modes. The simulations predicted that vortex shedding modes generally contained frequencies 300% larger than dominant structural modes, and therefore, vortex induced vibrations were unlikely to occur. Lastly, mode decomposition methods were applied to the experimental results to extract energy ratios and reveal dynamic content across high-order wake modes. The vortex shedding modes generated more than 80% of the total wake energy for both complex vane packs, and dynamic decomposition methods revealed unique structures within the vane junction wake. In all analyses, comparisons were made between different vane parameters, such as trailing edge thickness and turning angle, where it was found that trailing edge thickness was the dominant vortex shedding parameter. The motivation, methodology, and results of the following research is presented to better understand the wake interactions, computational predictive capabilities, and structural dynamics associated with vortex shedding from complex vane packs. Although the results directly relate to StreamVane distortion generator technology, the qualitative and quantitative comparisons between the selected methods, geometry parameters, and flow conditions can be extrapolated to modern turbomachinery components in general. Therefore, this dissertation aims to benefit distortion generator and turbomachinery designers by providing insight into the underlying physics and overall modeling techniques of the wake dynamics in highly three-dimensional, complex components. / Doctor of Philosophy / A comprehensive analysis has been completed to characterize the unsteady wake flow produced by complex turning vane systems in three dimensions. Turning vanes are a common component utilized in the field of fluid dynamics and aerospace propulsion to effectively turn and manipulate the working fluid to the desired condition. For propulsion applications, similar vanes can alleviate performance losses by improving the overall aerodynamics and mitigating flow distortions entering the compressor of a jet engine. Conversely, complex turning vanes can also be used to reproduce the distortion for engineers to evaluate jet engine components when subjected to nonuniform flow ingestion. The distinct geometry features that make these vanes complex are also present in other turbomachinery systems outside of distortion generation. In any case, the cross-sectional profiles of the turning vanes commonly contain blunt ends or trailing edges due to engineering limitations and/or restrictions. This geometric feature introduces periodic wake structures, known as vortex shedding, that can negatively effect the performance of the overall system. It is therefore a necessity to characterize both the dynamics and coherence of vortex shedding to fully understand the flow features in highly three-dimensional flows. In the presented research, this is achieved by applying computational simulations and experimental measurements to extract the corresponding wake dynamics of complex vane packs. The selected testcases where designed using the StreamVane technology, a mature system that generates tailored turning vanes to reproduce flow distortion in jet engine or fan rig ground-testing facilities. The fluid simulations captured the expected wake flow and largescale structures convecting downstream of the vane packs. A comparison between two different flow models and the experimental results revealed minimal quantitative differences in the large-scale dynamics, which gave insight into the model selection to predict such flows. Additional structural simulations were performed to estimate the forcing and response of the vane packs when subjected to the aerodynamic loading. The results showed vortex shedding was highly unlikely to cause large amplitude vibrations and structural failures. In all analyses, the primary results were correlated with common vane parameters and operating conditions to evaluate their impact on the wake dynamics. The motivation, methodology, and results of the following research is presented to better understand the wake interactions, computational predictive capabilities, and structural dynamics associated with vortex shedding from complex vane packs. Although the results directly relate to StreamVane distortion generator technology, the qualitative and quantitative comparisons between the selected methods, geometry parameters, and flow conditions can be extrapolated to modern turbomachinery components in general. Therefore, this dissertation aims to benefit distortion generator and turbomachinery designers by providing insight into the underlying physics and overall modeling techniques of the wake dynamics in highly three-dimensional, complex components.

Laser Doppler velocimetry (LDV) measurements and hydrogen-bubble flow-visualization techniques were used to examine the near-wall flow structure of 2-D and 3-D turbulent boundary layers (TBLs) over a range of low Reynolds numbers. The goals of this research were (1) an increased understanding of the flow physics in the near wall region of turbulent boundary layers, (2) to observe and quantify differences between 2-D and 3-D TBL flow structures, and (3) to document Reynolds number effects for 3-D TBLs. An ultimate application of this work would be to improve turbulence modeling for 3-D flows. The LDV data have provided results detailing the turbulence structure of the 2-D and 3-D TBLs, as well as low uncertainty skin friction estimates. These results include mean Reynolds stress distributions, flow skewing results, and U and V spectra. Effects of Reynolds number for the 3-D flow were examined when possible. Comparison to results with the same 3-D flow geometry but at a significantly higher Reynolds number provided unique insight into the structure of 3-D TBLs. While the 3-D mean and fluctuating velocities were found to be highly dependent on Reynolds number, a previously defined shear stress parameter was discovered to be invariant with Reynolds number. The hydrogen-bubble technique was used as a flow-visualization tool to examine the near-wall flow structure of 2-D and 3-D TBLs. Both the quantitative and qualitative results displayed larger turbulent fluctuations with more highly concentrated vorticity regions for the 2-D flow. The 2-D low-speed streaky structures experienced greater interaction with the outer region high-momentum fluid than observed for the 3-D flow. The near-wall 3-D flow structures were generally more quiescent. Numerical parameters quantified the observed differences, and characterized the low-speed streak and high-speed sweep events. All observations indicated a more stable near-wall flow structure with less turbulent interactions occurring between the inner and log regions for a 3-D TBL. / Ph. D.

This study aimed to understand the processes that govern free surface behaviour in depth-limited turbulent flows. Experimental data has shown that the turbulence properties at a point near the free surface relate directly to the properties of the free surface pattern. This would suggest a direct linkage between the free surface and the underlying turbulence field, but this cannot be true since the free surface pattern is strongly dynamic while the sub-surface turbulence field is relatively persistent. An oscillatory spatial correlation function was derived which explains the de-linkage, showing that the turbulence-generated surface pattern periodically inverts as it advects downstream. A model was developed, which shows that the observed free surfaces can be considered as an ensemble of overlapping but behaviourally independent oscillons. These are shown to influence a zone of fluid beneath the surface and invert at a frequency which is a function of the root-mean-square roughness height of the free surface. The spatial frequency of free surface oscillation relates strongly to the spatial frequency of turbulent structures, suggesting that the oscillon motion may form the trigger for near-bed bursting events. Given these relationships, it is proposed that measurement of the free surface behaviour may allow remote measurement of flow conditions. An acoustic wave probe was developed, which is able to remotely recover the key features of the water surface pattern. An array of such probes is proposed for the accurate measurement of temporal and spatial properties of turbulent free surfaces and hence the underlying bulk flow conditions.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19481号 / 理博第4141号 / 新制||理||1595(附属図書館) / 32517 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 藤 定義, 教授 佐々 真一, 教授 早川 尚男 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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Yes / This review paper addresses the structure of the mean flow and key turbulence quantities in free-surface flows with emergent vegetation. Emergent vegetation in open channel flow affects turbulence, flow patterns, flow resistance, sediment transport, and morphological changes. The last 15 years have witnessed significant advances in field, laboratory, and numerical investigations of turbulent flows within reaches of different types of emergent vegetation, such as rigid stems, flexible stems, with foliage or without foliage, and combinations of these. The influence of stem diameter, volume fraction, frontal area of stems, staggered and non-staggered arrangements of stems, and arrangement of stems in patches on mean flow and turbulence has been quantified in different research contexts using different instrumentation and numerical strategies. In this paper, a summary of key findings on emergent vegetation flows is offered, with particular emphasis on: (1) vertical structure of flow field, (2) velocity distribution, 2nd order moments, and distribution of turbulent kinetic energy (TKE) in horizontal plane, (3) horizontal structures which includes wake and shear flows and, (4) drag effect of emergent vegetation on the flow. It can be concluded that the drag coefficient of an emergent vegetation patch is proportional to the solid volume fraction and average drag of an individual vegetation stem is a linear function of the stem Reynolds number. The distribution of TKE in a horizontal plane demonstrates that the production of TKE is mostly associated with vortex shedding from individual stems. Production and dissipation of TKE are not in equilibrium, resulting in strong fluxes of TKE directed outward the near wake of each stem. In addition to Kelvin–Helmholtz and von Kármán vortices, the ejections and sweeps have profound influence on sediment dynamics in the emergent vegetated flows.

The effects of trailing edge flow control on the wakes of a linear cascade of idealized fan blades was investigated experiments with a view to the likely effects on broadband aircraft engine interaction noise. Single and three-component hotwire velocity measurements were made downstream of the cascade for a chord Reynolds number of 390,000 and a Mach number of 0.07. Measurements of the two-point velocity correlation were used extensively to evaluate the impact of various flow control strategies on the organization of the coherent structures of the wakes and their potential to generate noise. A baseline flow was established by measuring the wake downstream of unmodified GE-Rotor-B blades. Four sets of serrated trailing edge blades (with two different serration sizes and with two trailing edge cambers) and three sets of blades with trailing edge blowing (a simple rectangular slot, rectangular slot with Kuethe-vane vortex generators, and rectangular slot with serrated lips) were tested. The serrated trailing edges introduce corrugations into the wake, increase the wake decay and width as well as turbulence levels (possibly because of the blunt trailing edge created at the serration valley). The serrated trailing edges also increase the turbulence scales in the direction perpendicular to the plane of the wake because of the injection of streamwise vorticity. In almost all cases the serrations reduce the spanwise and streamwise turbulence scales. Serrations do not, however, affect the apparent time scale of quasi-periodic structures in the wake, and this appears to limit the potential of this trailing edge treatment to reduce broadband noise. The analysis of the characteristic eddies (obtained from proper orthogonal decomposition combined with linear estimation) revealed that the serrations do not change the qualitative form of the eddies. Trailing edge blowing was found to significantly decrease the wake deficit and width as well as the turbulence levels at all blowing rates. Blowing through the simple rectangular slot, at mass flow rates between 1.4 and 2.0% of the total passage through flow, was shown to significantly affect the size, the organization and the strength of the coherent structures. For small blowing rates the strong spanwise eddies near the trailing edge actually appear to be enhanced. For larger blowing rates, however, the turbulent scales are reduced in all directions. The addition of Kuethe vanes on the suction side of the blowing blade results in a low momentum region just downstream of the vanes that may result from flow separation there. This further enhances the shedding and increases the blowing rate needed to overcome it. The serrated blowing blades show the greatest potential to reduce broadband noise as they reduce the turbulence levels and scales without creating potentially detrimental structures. While no acoustic measurements were made, analysis of hypothetical perpendicular and parallel interactions of blades with these wakes has made possible to characterize for the first time the impact of the changes in the eddy structure of these wakes on their potential to generate broadband noise. The serrated trailing edges (especially the larger serrations) actually increase the potential of the wake to generate broadband noise (a direct consequence in the overall increase in turbulence scale and intensity). In contrast, every trailing edge blowing configuration was found to produce large reductions in the potential noise (a maximum of 6dB reduction was obtained at 2.0% blowing). The addition of Kuethe vanes on the suction side of the blowing blades significantly reduced the efficiency of the simple blowing configuration (a result of the increased coherency associated with the shedding of streamwise vorticity by the vanes). The serrated blowing configuration was found to yield reductions similar to the simple blowing configuration. / Ph. D.

Low back pain is a common medical problem around the world afflicting 80% of the population some time in their life. Low back injury can result from a loss of torso stability causing excessive strain in soft tissue. This investigation seeks to apply existing methods to new applications and to develop new methods to assess torso stability. First, the time series averaged finite time Lyapunov exponent is calculated from data obtained during seated stability experiments. The Lyapunov exponent is found to increase with increasing task difficulty. Second, a new metric for evaluating torso stability is introduced, the threshold of stability. This parameter is defined as the maximum task difficulty in which dynamic stability can be maintained for the test duration. The threshold of stability effectively differentiates torso stability at two levels of visual feedback. Third, the state space distribution of the finite time Lyapunov exponent (FTLE) field is evaluated for deterministic and stochastic systems. Two new methods are developed to generate the FTLE field from time series data. Using these methods, Lagrangian coherent structures (LCS) are found for an inverted pendulum, the Acrobot, and planar wobble chair models. The LCS are ridges in the FTLE field that separate two inherently different types of motion when applied to rigid-body dynamic systems. As a result, LCS can be used to identify the boundaries of the basin of stability. Finally, these new methods are used to find the basin of stability from time series data collected from torso stability experiments. The LCS and basins of stability provide a richer understanding into the system dynamics when compared to existing methods. By gaining a better understanding of torso stability, it is hoped this knowledge can be used to prevent low back injury and pain in the future. These new methods may also be useful in evaluating other biodynamic systems such as standing postural sway, knee stability, or hip stability as well as time series applications outside the area of biomechanics. / Ph. D.