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Showing 191 to 200 of 200 (0.049 seconds)Spelling suggestions: "subject:"free surface"" "subject:"tree surface""
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Wave-Cavity Resonator: Experimental Investigation of an Alternative Energy DeviceReaume, Jonathan Daniel 21 December 2015 (has links)
A wave cavity resonator (WCR) is investigated to determine the suitability of the
device as an energy harvester in rivers or tidal flows. The WCR consists of coupling
between self-excited oscillations of turbulent flow of water in an open channel along the
opening of a rectangular cavity and the standing gravity wave in the cavity. The device
was investigated experimentally for a range of inflow velocities, cavity opening lengths,
and characteristic depths of the water. Determining appropriate models and empirical
relations for the system over a range of depths allows for accuracy when designing
prototypes and tools for determining the suitability of a particular river or tidal flow as a
potential WCR site. The performance of the system when coupled with a wave
absorber/generator is also evaluated for a range piston strokes in reference to cavity wave
height. Video recording of the oscillating free-surface inside the resonator cavity in
conjunction with free-surface elevation measurements using a capacitive wave gauge
provides representation of the resonant wave modes of the cavity as well as the degree of
the flow-wave coupling in terms of the amplitude and the quality factor of the associated
spectral peak. Moreover, application of digital particle image velocimetry (PIV) provides
insight into the evolution of the vortical structures that form across the cavity opening.
Coherent oscillations were attainable for a wide range of water depths. Variation of the
water depth affected the degree of coupling between the shear layer oscillations and the
gravity wave as well as the three-dimensionality of the flow structure. In terms of the
power investigation, conducted with the addition of a load cell and linear table-driven
piston, the device is likely limited to running low power instrumentation unless it can be
up-scaled. Up-scaling of the system, while requiring additional design considerations, is
not unreasonable; large-scale systems of resonant water waves and the generation of large
scale vortical structures due to tidal or river flows are even observed naturally. / Graduate / 0547 / 0548 / reaumejd@uvic.ca
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Análise e implementação de esquemas de convecção e modelos de turbulência para simulação de escoamentos incompressíveis envolvendo superfícies livres. / Analysis and implementation of convection schemes and turbulence models for simulation of incompressible flows involving free surfaces.Valdemir Garcia Ferreira 26 September 2001 (has links)
Uma parte significativa dos escoamentos encontrados em aplicações tecnológicas é caracterizada por envolver altos números de Reynolds, principalmente aqueles em regime turbulento e com superfície livre. Obter soluções numéricas representativas para essa classe de problemas é extremamente difícil, devido à natureza não-linear das equações diferenciais parciais envolvidas nos modelos. Conseqüentemente, o tema tem sido uma das principais preocupações da comunidade científica moderna em dinâmica de fluidos computacional. Aproximações de primeira ordem para os termos convectivos são as mais adequadas para amortecer oscilações que estão associadas às aproximações de alta ordem não-limitadas. Todavia, elas introduzem dissipação artificial nas representações discretas comprometendo os resultados numéricos. Para minimizar esse efeito não-físico e, ao mesmo tempo, conseguir aproximações incondicionalmente estáveis, é indispensável adotar uma estratégia que combine aproximações de primeira ordem com as de ordem mais alta e que leve em conta a propagação de informações físicas. Os resultados dessa composição são os esquemas "upwind" limitados de alta ordem. Em geral, espera-se que esses esquemas sejam apropriados para a representação das derivadas convectivas nos modelos de turbulência kappa-varepsilon. No contexto de diferenças finitas, a presente tese dedica-se à solução numérica das equações de Navier-Stokes no regime de números de Reynolds elevados. Em particular, ela contém uma análise de algoritmos monotônicos e antidifusivos e modelos de turbulência kappa-varepsilon para a simulação de escoamentos incompressíveis envolvendo superfícies livres. Esquemas de convecção são implementados nos códigos GENSMAC para proporcionar um tratamento robusto dos termos convectivos nas equações de transporte. Duas versões do modelo kappa-varepsilon de turbulência são implementadas nos códigos GENSMAC, para problems bidimensionais e com simetria radial, para descrever os efeitos da turbulência sobre o escoamento médio. Resultados numéricos de escoamentos com simetria radial são comparados com resultados experimentais e analíticos. Simulações numéricas de problemas tridimensionais complexos são apresentadas para avaliar o desempenho de esquemas "upwind". Finalmente, os modelos de turbulência kappa-varepsilon são utilizados para a simulação de escoamentos confinados e com superfícies livres. / A considerable part of fluid flows encountered in technological applications is characterised by involving high-Reynolds numbers, especially those in turbulent regime and with free-surface. It is extremely difficult to obtain representative numerical solutions for this class of problems, due to the non-linear nature of the partial differential equations involved in the models. Consequently, this subject has been one of main concerns in the modern computational fluid dynamics community. First-order approximation to the convective terms is one of the most appropriate to smooth out oscilations/instabilities which are associated with high-order unlimited approximation. However, it introduces numerical dissipation in the discrete representation jeopardizing the numerical results. In order to minimize this non-physical effect and, at the same time, to obtain unconditionally stable approximation, it is essential to adopt a strategy that combines first and high-order approximations and takes into account the propagation of physical information. The results of this composition are the high-order bounded upwind techniques. In general, it is expected that these algorithms are satisfactory for the representation of the convective derivatives in the kappa-varepsilon turbulence model. In the context of finite-difference, the present thesis deals with the numerical solution of the Navier-Stokes equations at high-Reynolds number regimes. In particular, it contains an analysis of monotonic and anti-difusive convection schemes and kappa-varepsilon turbulence models for the simulation of free-surface fluid flows. Upwinding methods are implemented into the GENSMAC codes to provide a robust treatment of the convective terms in the transport equations. Two versions of the K-Epsilon turbulence model are implemented into the two-dimensional and axisymmetric GENSMAC codes, in order to describe the turbulent effects on the average flow. Numerical results of axisymmetric flows are compared with experimental and analytical results. Numerical simulations of complex three-dimensional problems are presented to assess the performance of high-order bounded upwind schemes. Finally, the K-Epsilon turbulence models are employed in the simulation of confined and free-surface flows.
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Méthode SPH implicite d’ordre 2 appliquée à des fluides incompressibles munis d’une frontière libreRioux-Lavoie, Damien 05 1900 (has links)
L’objectif de ce mémoire est d’introduire une nouvelle méthode smoothed particle hydrodynamics
(SPH) implicite purement lagrangienne, pour la résolution des équations de Navier-
Stokes incompressibles bidimensionnelles en présence d’une surface libre. Notre schéma de
discrétisation est basé sur celui de Kéou Noutcheuwa et Owens [19]. Nous avons traité la
surface libre en combinant la méthode multiple boundary tangent (MBT) de Yildiz et al. [43]
et les conditions aux limites sur les champs auxiliaires de Yang et Prosperetti [42]. Ce faisant,
nous obtenons un schéma de discrétisation d’ordre $\mathcal{O}(\Delta t ^2)$ et $\mathcal{O}(\Delta x ^2)$, selon certaines
contraintes sur la longueur de lissage $h$. Dans un premier temps, nous avons testé notre
schéma avec un écoulement de Poiseuille bidimensionnel à l’aide duquel nous analysons l’erreur
de discrétisation de la méthode SPH. Ensuite, nous avons tenté de simuler un problème
d’extrusion newtonien bidimensionnel. Malheureusement, bien que le comportement de la
surface libre soit satisfaisant, nous avons rencontré des problèmes numériques sur la singularité
à la sortie du moule. / The objective of this thesis is to introduce a new implicit purely lagrangian smoothed particle
hydrodynamics (SPH) method, for the resolution of the two-dimensional incompressible
Navier-Stokes equations in the presence of a free surface. Our discretization scheme is based
on that of Kéou Noutcheuwa et Owens [19]. We have treated the free surface by combining
Yildiz et al. [43] multiple boundary tangent (MBT) method and boundary conditions on the
auxiliary fields of Yang et Prosperetti [42]. In this way, we obtain a discretization scheme of
order $\mathcal{O}(\Delta t ^2)$ and $\mathcal{O}(\Delta x ^2)$, according to certain constraints on the smoothing length $h$. First,
we tested our scheme with a two-dimensional Poiseuille flow by means of which we analyze
the discretization error of the SPH method. Then, we tried to simulate a two-dimensional
Newtonian extrusion problem. Unfortunately, although the behavior of the free surface is
satisfactory, we have encountered numerical problems on the singularity at the output of the
die.
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Desenvolvimento e teste de esquemas \"upwind\" de alta resolução e suas aplicações em escoamentos incompressíveis com superfícies livres / Development and testing of high-resolution upwind schemes and their applications in incompressible free surface flowsRafael Alves Bonfim de Queiroz 18 March 2009 (has links)
Neste trabalho são apresentados os resultados do desenvolvimento e teste de esquemas upwind de alta resolução para o controle da difusão numérica em leis de conservação gerais e problemas em dinâmica dos fluidos. Em particular, são derivados dois novos esquemas: o ALUS (Adaptive Linear Upwind Scheme) e o TOPUS (Third-Order Polynomial Upwind Scheme). Esses esquemas são testados no transporte de escalares, em equações 1D tipo convecção-difusão, em sistemas hiperbólicos 1D, nas equações de Euler 2D da dinâmica dos gases e nas equações de Navier-Stokes incompressíveis 2D/3D. Os esquemas são então associados a uma modelagem algébrica não linear para a simulação de problemas de escoamentos incompressíveis turbulentos 2D com/sem superfícies livres / In this work, results of the development and testing of high-resolution upwind schemes for controlling of the numerical diffusion for general conservation laws and fluid dynamics problems are presented. In particular, two new high-resolution upwind schemes are derived, namely, the ALUS (Adaptive Linear Upwind Scheme) and the TOPUS (Third-Order Polynomial Upwind Scheme). These schemes are tested in scalar transport, 1D convection-diffusion equations, 1D hyperbolic systems, 2D Euler equations of the gas dynamics, and in 2D/3D incompressible Navier-Stokes equations. The schemes are then combined with a nonlinear Reynolds stress algebraic equation model for the simulation of 2D incompressible turbulent flows with/without free surfaces
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Nouvelle technique de grilles imbriquées pour les équations de Saint-Venant 2D / New nested grids technique for 2D shallow water equationsAltaie, Huda 17 December 2018 (has links)
Les écoulements en eau peu profonde se rencontrent dans de nombreuses situations d’intérêts : écoulements de rivières et dans les lacs, mais aussi dans les mers et océans (courants de marée, tsunami, etc.). Ils sont modélisés par un système d’équations aux dérivées partielles, où les inconnues sont la vitesse de l’écoulement et la hauteur d’eau. On peut supposer que la composante verticale de la vitesse est petite devant les composantes horizontales et que ces dernières sont indépendantes de la profondeur. Le modèle est alors donné par les équations de shallow water (SWEs). Cette thèse se concentre sur la conception d’une nouvelle technique d’interaction de plusieurs grilles imbriquées pour modèle en eau peu profonde en utilisant des méthodes numériques. La première partie de cette thèse comprend, La dérivation complète de ces équations à partir des équations de Navier- Stokes est expliquée. Etudier le développement et l’évaluation des méthodes numériques en utilisant des méthodes de différences finies et plusieurs exemples numériques sont appliqués utilisant la condition initiale du niveau gaussien pour 2DSWEs. Dans la deuxième partie de la thèse, nous sommes intéressés à proposer une nouvelle technique d’interaction de plusieurs grilles imbriquées pour résoudre les modèles océaniques en utilisant quatre choix des opérateurs de restriction avec des résultats de haute précision. Notre travail s’est concentré sur la résolution numérique de SWE par grilles imbriquées. A chaque niveau de résolution, nous avons utilisé une méthode classique de différences finies sur une grille C d’Arakawa, avec un schéma de leapfrog complété par un filtre d’Asselin. Afin de pouvoir affiner les calculs dans les régions perturbées et de les alléger dans les zones calmes, nous avons considéré plusieurs niveaux de résolution en utilisant des grilles imbriquées. Ceci permet d’augmenter considérablement le rapport performance de la méthode, à condition de régler efficacement les interactions (spatiales et temporelles) entre les grilles. Dans la troisième partie de cette thèse, plusieurs exemples numéériques sont testés pour 2DSWE avec imbriqués 3:1 et 5:1. Finalement, la quatrième partie de ce travail, certaines applications de grilles imbriquées pour le modèle tsunami sont présentées. / Most flows in the rivers, seas, and ocean are shallow water flow in which the horizontal length andvelocity scales are much larger than the vertical ones. The mathematical formulation of these flows, so called shallow water equations (SWEs). These equations are a system of hyperbolic partial differentialequations and they are effective for many physical phenomena in the oceans, coastal regions, riversand canals. This thesis focuses on the design of a new two-way interaction technique for multiple nested grids 2DSWEs using the numerical methods. The first part of this thesis includes, proposing several ways to develop the derivation of shallow water model. The complete derivation of this system from Navier-Stokes equations is explained. Studying the development and evaluation of numerical methods by suggesting new spatial and temporal discretization techniques in a standard C-grid using an explicit finite difference method in space and leapfrog with Robert-Asselin filter in time which are effective for modeling in oceanic and atmospheric flows. Several numerical examples for this model using Gaussian level initial condition are implemented in order to validate the efficiency of the proposed method. In the second part of our work, we are interested to propose a new two-way interaction technique for multiple nested grids to solve ocean models using four choices of higher restriction operators (update schemes) for the free surface elevation and velocities with high accuracy results. Our work focused on the numerical resolution of SWEs by nested grids. At each level of resolution, we used explicit finite differences methods on Arakawa C-grid. In order to be able to refine the calculations in troubled regions and move them into quiet areas, we have considered several levels of resolution using nested grids. This makes it possible to considerably increase the performance ratio of the method, provided that the interactions (spatial and temporal) between the grids are effectively controlled. In the third part of this thesis, several numerical examples are tested to show and verify twoway interaction technique for multiple nested grids of shallow water models can works efficiently over different periods of time with nesting 3:1 and 5:1 at multiple levels. Some examples for multiple nested grids of the tsunami model with nesting 5:1 using moving boundary conditions are tested in the fourth part of this work.
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A Partitioned FSI Approach to Study the Interaction between Flexible Membranes and FluidsMakaremi Masouleh, Mahtab 27 April 2022 (has links)
The interaction between fluids and structures, which is an interdisciplinary problem, has gained importance in a wide range of scientific and engineering applications. Thanks to new advances in computer technology, the numerical analysis of multiphysics phenomena has aroused growing interest.
Fluid-structure interactions have been numerically and experimentally studied by many researchers and published by several books, papers, and review papers. Hou et al. (2012) [3] have also published a review paper entitled “Numerical methods for fluid-structure interaction”, which provides useful knowledge about different approaches for FSI analysis.
The key challenge encountered in any numerical FSI analysis is the coupling between the two independent domains with clear distinctions. For example, a structure domain requires discretizing by a Lagrangian mesh where the mesh is fixed to the mass and follows the mass motion. In fact, the Lagrangian mesh is able to deform and follows an individual structural mass as it moves through space and time. Nonetheless, the fluid mesh remains intact within the space, where the fluid flows as time passes.
The numerical approaches with regard to FSI phenomena can be divided into two main categories, namely the monolithic approach and the partitioned approach. In the former, a single system equation for the whole problem is solved simultaneously by a unified algorithm; however, in the latter, the fluid and the structure are discretized with their proper mesh and solved separately by different numerical algorithms.
When a fluid flow interacts with a structure, the pressure load arising from the fluid flow is exerted on the structure, followed by deformations, stresses, and strains of the structure. Depending on the resulting deformation and the rate of the variations, a one-way or two-way coupling analysis can be conducted. Fluid-structure interaction (FSI) is characterized by the interaction of some movable or deformable structure with an internal or surrounding fluid flow.
In a fluid-structure interaction (FSI), the laws that describe fluid dynamics and structural mechanics are coupled. There is also another classification for FSI problems on the basis of mesh methods: conforming methods and non-conforming methods. In the first method, the interface condition is regarded as a physical boundary (interface boundary) moving during the solution time, which imposes the mesh for the fluid domain to be updated in conformity with the new position for the interface.
In contrast, the implementation of the second method eliminates a need for the fluid mesh update on the account of the fact that the interface requirement is enforced by constraints on the system equations instead of the physical boundary motion.
In this work, we study numerically and experimentally the fluid-structure interaction comprising a flexible slender shaped structure, free surface flow and potentially interacting rigid structures, categorized in flood protection applications, whereas more emphasis is given to numerical analysis. Objectives of this study are defined in detail as follows:
The initial aim is the numerical analysis of the behavior of a down-scale membrane loaded by hydrostatic pressures, where the numerical results have to be validated against available experimental data.
A further case which has to be investigated is how the full scale flexible flood barrier behaves when approached and impacted by an accelerated massive flotsam. The numerical model has to be built so as to replicate the same physical phenomenon investigated experimentally. It enables a comparison between the numerical and experimental analyses to be drawn.
A more complicated case where the flexible down-scale membrane interacts with a propagated water wave is a further target area to study. Moreover, an experimental investigation is required to validate the numerical results by way of comparison.
The ultimate goal is to perform a similitude analysis upon which a correlation between the full-scale prototype and the down-scale model can be formed. The implementation of the similarity laws enables the behavior of the full scale prototype to be quantitatively assessed on the basis of the available data for the down-scale model. In addition, in order to validate the accuracy of the similitude analysis, numerical analyses have to be carried out.:Contents
Zusammenfassung I
ABSTRACT IV
Nomenclature X
1 Introduction 1
1.1 Work overview 2
1.2 Literature review 3
1.2.1 The non-conforming methods 6
1.2.2 The conforming (partitioned) approaches 11
1.2.2.1 Interface data transfer 16
1.2.2.2 Accuracy, stability and efficiency 16
1.2.2.3 Modification of interface conditions: Robin transmission conditions 18
1.3 Concluding remarks 19
2 Methodology-numerical methods for fluid-structure interaction analysis (FSI) 20
2.1 Single FV framework 21
2.1.1 The prism layer mesher 24
2.1.2 Turbulence modeling 24
2.2 Preparation of the standalone Abaqus model 27
2.2.1 Damping by bulk viscosity 28
2.2.2 Coulomb friction damping 29
2.2.3 Rayleigh damping 29
2.2.4 Determination of the Rayleigh damping parameters based on the Chowdhury procedure 29
2.2.5 The frequency response function (FRF) measurement 30
2.2.6 The half-power bandwidth method 31
2.3 Explicit partitioned coupling 33
2.4 Implicit partitioned coupling 39
2.5 Overset mesh 40
2.6 Concluding remarks 42
3 Verification and validation of the structural model 44
3.1 Numerical model setup of the down-scale membrane 44
3.2 Comparing similarity between numerical and experimental results 46
3.2.1 Hypothesis test terminology 46
3.2.2 Curve fitting 47
3.2.3 Similarity measures between two curves 48
3.3 Results (down-scale membrane) 52
3.3.1 Similarity tests for the contact length 54
3.3.2 Similarity tests for the slope 58
3.3.3 Similarity tests for the displacement in Y direction 60
3.4 Concluding remarks 63
4 Numerical model setup of the original membrane for impact analysis 66
4.1 Structure domain 67
4.2 Fluid domain 72
4.2.1 Standard mesh and results 74
4.2.2 Overset mesh 80
4.3 Co-simulation model setup and results 88
4.4 Concluding remarks 96
5 Numerical wave generation 100
5.1 Theoretical estimation of the waves 107
5.2 Numerical wave tank setup 110
5.3 Results 114
5.4 Concluding remarks 119
6 Validity of the model with dynamic pressure 121
6.1 Wave tank 123
6.2 Structure domain 127
6.3 Fluid domain 130
6.4 Co-simulation model setup 136
6.5 Experimental approach 137
6.6 Results 141
6.6.1 Similarity tests for the displacement of the membrane in X direction 156
6.6.2 Similarity tests for the displacement of the membrane in Y direction 160
6.6.3 Similarity tests for the displacement of the membrane in Z direction 164
6.7 Concluding remarks 168
7 Similarity 171
7.1 Motivation 171
7.2 Governing equations 174
7.3 Buckingham Pi theorem 175
7.4 Dimensionless numbers 175
Similitude requirement 177
7.5 Simulation setup 178
7.6 Results 179
7.7 Concluding remarks 191
8 Summary, conclusions and outlook 192
List of figures 199
List of tables 209
References 210
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Posouzení vlivu nestandardních přítokových proudových poměrů na Q/h charakteristiku ostrohranného přelivu s obdélníkovým výřezem / Assessment effects substandard tributaries flowing a proportion on Q/h characteristics thin-plate weir with a rectangular cutoutŠmidrkalová, Nina January 2012 (has links)
This master´s thesis deals with the problem of measuring flow, especially using the method of Q/H characteristics. It describes the thin-plate weir and mentions specific channels. The master´s thesis suggests options of monitoring flow capacity in fixed proffiles with free surface. The master´s thesis also includes an experiment, which is focused on simulation of substandard tributary flowing proportions before weirs for determining their effect on Q/H characteristcs. The experiment is carried out on a thin-plate weir with a rectangular notch at the Department of Water Structures at the Faculty of Civil Engeneering in Brno. Another part of the master´s thesis is data processing to get an idea about the impact of the phenomenon in real applications. Finally, options are recommended for follow-up research.
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FILAMENT GENERATED DROPLETS DURING DROP BREAKUP, SHEET RUPTURE, AND DROP IMPACTXiao Liu (15339289) 24 April 2023 (has links)
<p>Free surface flows, characterized by a deformable interface between two immiscible fluids or between a liquid and a gas, play a pivotal role in numerous natural phenomena and industrial processes. The fluid-fluid interface dynamics, governed by the complex interplay of forces such as inertia, capillary force, viscous force, and possibly elastic force, significantly influence the behavior of the fluids involved. Examples of free surface flows can be observed in everyday situations, such as droplet formation from a faucet, propagation and breaking of ocean waves, and tear films that coat the eye. An in-depth understanding of free surface flows and fluid-fluid interface dynamics has extensive implications for optimizing applications like inkjet printing, coating, spraying, and droplet formation while providing insights into the intricate behavior of natural fluid systems. Most of these applications, except for coating, involve abrupt and catastrophic topological changes of interfaces present in processes such as drop breakup, sheet rupture, and drop impact, where small droplets form from liquid sheets or filaments.</p>
<p>This thesis examines the dynamics of contracting liquid filaments through computational means. Previous computational simulations have assumed that initially the fluid within the filament is quiescent which, however, may not typically be the case in practical applications. Here, the effect of a realistic, non-zero initial velocity profile is considered with the hypothesis that the fact that the fluid is already in motion when it starts to contract may result in significant alterations in the filament’s final fate vis-a-vis whether it breaks up into multiple small droplets or contracts into a sphere as its ends retract toward each other. The transient system of governing equations, the three-dimensional but axisymmetric (3DA) Navier-Stokes and continuity equations subjected to interfacial boundary conditions, are solved using rigorous and robust numerical algorithms in both fully 3DA and one-dimensional (1D) settings using the Galerkin finite element (GFEM) method. The simulation results are then used to construct comprehensive phase diagrams to delineate regions where filaments break up into smaller droplets from those where filaments contract to spheres without breakup.</p>
<p>Polymer additives are often present in practical applications involving filament contraction and breakup. The presence of polymer molecules in an otherwise Newtonian solvent gives rise to non-Newtonian rheology. In this thesis, the dynamics of filaments containing polymer additives are analyzed using a 1D algorithm that is developed specifically for simulating viscoelastic free surface flows where the fluid’s rheology is described by the oft-used Oldroyd-B model. In real-world applications, filaments produced from nozzles are expected to be prestressed at the instant when they are created and begin to contract. It is demonstrated that the retraction velocity of tips of highly viscous, prestressed filaments is significantly increased compared to filaments in which the polymer molecules are initially relaxed and Newtonian filaments. This enhancement is explained by examining the value of f σ: D (σ: Elastic stress; D: Rate-of-strain tensor), which can be positive or negative. This quantity is positive when the flow does work on the polymer molecules but negative when the molecules do work on the flow, i.e., when elastic recoiling or unloading takes place. In prestressed filaments, elastic unloading takes place because σ: D < 0. The elastic stresses work by pulling the fluid in axially and pushing it out radially, thereby drastically increasing the tip velocity. However, this enhancement in contraction velocity is not observed in low to intermediate viscosity prestressed filaments and whose Newtonian counterparts typically experience end-pinching. It has been established by others that end-pinching can be precluded in either filaments of intermediate viscosity or surfactant-laden filaments of low viscosity through a process known as escape from end-pinching. In this study, we demonstrate that a similar escape can also occur in prestressed viscoelastic filaments of low-to-intermediate viscosity, as revealed by one-dimensional numerical simulations and rationalized by examining when and where the elastic recoil takes place.</p>
<p>Beyond cylindrical filaments, thin liquid films or planar liquid sheets are also prevalent in atomization, curtain coating, and other processes where liquid sheet stability has been a subject of extensive research. Numerous authors have examined wave formation and growth leading to sheet breakup. Free liquid films or sheets without edges or caps at their two ends, which typically have two free surfaces and are surrounded by air or sometimes another liquid, can destabilize and rupture due to intermolecular van der Waals attractive forces, despite the stabilizing influence of surface tension. In this thesis, the dynamics of contracting free films or sheets with caps---two-dimensional (2D) drops---of Newtonian fluids is examined without considering van der Waals forces to confirm or refute the hypothesis that such systems can rupture due to finite-amplitude perturbations even in the absence of intermolecular forces. In particular, both two-dimensional and one-dimensional high-accuracy simulations are employed to demonstrate that unlike inviscid 2D drops that can rupture in the absence of van der Waals forces, 2D drops or sheets can escape from pinch-off due to the action of viscous forces which are present in real systems no matter how small their viscosity. The reopening of the interface and escape from pinch-off in 2D drops and sheets are explained by demonstrating the key role played by vorticity. New power-law relations or scaling laws are obtained as a function of Ohnesorge number (ratio of viscous to the square root of the product of inertial and capillary forces) for the value of the minimum film thickness for which 2D drops or sheets stop thinning and after which the interface begins to reopen. Simple yet powerful arguments are presented rationalizing these scaling laws. It is expected that these power-law relations should be of great interest to experimentalists who study such phenomena by high-speed visualization experiments.</p>
<p>Some of the motivation for this thesis research comes from crop spraying applications in which achieving zero or negligible drift is highly desirable. To further the understanding of fluid mechanics underpinning current and future drift reduction technologies, a simplified experimental setup is adopted to generate liquid sheets and analyze their disintegration into droplets. This new setup is both simpler and more universal than commonly utilized experimental systems that use single or multiple nozzles to generate liquid sheets and spray droplets from the disintegration of free liquid films. In the current experiments, droplets of test fluids are made to collide with or impact the top planar surface of a solid cylinder or rod. A series of MATLAB codes are developed and employed to extract droplet size distributions from images that are obtained from high-speed visualization experiments. The experimental setup and the means of data analysis are then used to probe the effect of fluid properties on the dynamics of sheet disintegration and droplet size distributions. It is hoped that future researchers will be able to combine what has been done in this thesis by simulations and in this chapter via experimental observations to develop an improved mechanistic understanding of spray formation.</p>
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Methodenentwicklung zur Simulation von Strömungen mit freier Oberfläche unter dem Einfluss elektromagnetischer WechselfelderBeckstein, Pascal 16 February 2018 (has links) (PDF)
Im Bereich der industriellen Metallurgie und Kristallzüchtung treten bei zahlreichen Anwendungen, wo magnetische Wechselfelder zur induktiven Beeinflussung von leitfähigen Werkstoffen eingesetzt werden, auch Strömungen mit freier Oberfläche auf. Das Anwendungsspektrum reicht dabei vom einfachen Aufschmelzen eines Metalls in einem offenen Tiegel bis hin zur vollständigen Levitation. Auch der sogenannte RGS-Prozess, ein substratbasiertes Kristallisationsverfahren zur Herstellung siliziumbasierter Dünnschichtmaterialien, ist dafür ein Beispiel. Um bei solchen Prozessen die Interaktion von Magnetfeld und Strömung zu untersuchen, ist die numerische Simulationen ein wertvolles Hilfsmittel. Für beliebige dreidimensionale Probleme werden entsprechende Berechnungen bisher durch eine externe Kopplung kommerzieller Programme realisiert, die für Magnetfeld und Strömung jeweils unterschiedliche numerische Techniken nutzen. Diese Vorgehensweise ist jedoch im Allgemeinen mit unnötigem Rechenaufwand verbunden. In dieser Arbeit wird ein neu entwickelter Methodenapparat auf Basis der FVM vorgestellt, mit welchem sich diese Art von Berechnungen effizient durchführen lassen. Mit der Implementierung dieser Methoden in foam-extend, einer erweiterten Version der quelloffenen Software OpenFOAM, ist daraus ein leistungsfähiges Werkzeug in Form einer freien Simulationsplattform entstanden, welches sich durch einen modularen Aufbau leicht erweitern lässt. Mit dieser Plattform wurden in foam-extend auch erstmalig dreidimensionale Induktionsprozesse im Frequenzraum gelöst.
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Methodenentwicklung zur Simulation von Strömungen mit freier Oberfläche unter dem Einfluss elektromagnetischer WechselfelderBeckstein, Pascal 08 January 2018 (has links)
Im Bereich der industriellen Metallurgie und Kristallzüchtung treten bei zahlreichen Anwendungen, wo magnetische Wechselfelder zur induktiven Beeinflussung von leitfähigen Werkstoffen eingesetzt werden, auch Strömungen mit freier Oberfläche auf. Das Anwendungsspektrum reicht dabei vom einfachen Aufschmelzen eines Metalls in einem offenen Tiegel bis hin zur vollständigen Levitation. Auch der sogenannte RGS-Prozess, ein substratbasiertes Kristallisationsverfahren zur Herstellung siliziumbasierter Dünnschichtmaterialien, ist dafür ein Beispiel. Um bei solchen Prozessen die Interaktion von Magnetfeld und Strömung zu untersuchen, ist die numerische Simulationen ein wertvolles Hilfsmittel. Für beliebige dreidimensionale Probleme werden entsprechende Berechnungen bisher durch eine externe Kopplung kommerzieller Programme realisiert, die für Magnetfeld und Strömung jeweils unterschiedliche numerische Techniken nutzen. Diese Vorgehensweise ist jedoch im Allgemeinen mit unnötigem Rechenaufwand verbunden. In dieser Arbeit wird ein neu entwickelter Methodenapparat auf Basis der FVM vorgestellt, mit welchem sich diese Art von Berechnungen effizient durchführen lassen. Mit der Implementierung dieser Methoden in foam-extend, einer erweiterten Version der quelloffenen Software OpenFOAM, ist daraus ein leistungsfähiges Werkzeug in Form einer freien Simulationsplattform entstanden, welches sich durch einen modularen Aufbau leicht erweitern lässt. Mit dieser Plattform wurden in foam-extend auch erstmalig dreidimensionale Induktionsprozesse im Frequenzraum gelöst.
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