Three-dimensional (3D) turbulent boundary layers (TBLs) are ubiquitous in most engineering applications, but most turbulence models used to simulate these flows are built on two-dimensional turbulence theory, limiting the accuracy of simulation results. To improve the accuracy of turbulence modeling capabilities, a better understanding of 3DTBL physics is required. This dissertation outlines the experimental investigation of the attached 3D TBL flow over the Benchmark Validation Experiments for RANS/LES Investigations (BeVERLI) Hill using laser Doppler velocimetry in the Virginia Tech Stability Wind Tunnel. The mean flow and turbulence behavior of the boundary layer are studied and compared with turbulence theories to identify the validity of these assumptions in the BeVERLI Hill flow. It is shown that the pressure gradients and curvature of the hill have a significant effect on the turbulence behavior, including significant history effects at all stations due to the changing pressure gradient impact through the height of the boundary layer. Supplementing the experimental results with analysis from rapid distortion theory and simulations, it is shown that the stations lower on the hill are significantly affected by the non-linear history effects due to the varying upstream origins of the flow passing through those stations. Stations closer to the hill apex pass through a region of extremely strong favorable pressure gradient and hill constriction, resulting in behavior that matches qualitatively with the results from rapid distortion theory and provides insights into the physical mechanisms taking place in these regions of the flow. Despite the misalignment of the mean flow angle (γ_FGA) and turbulent shear stress angle (γ_SSA) throughout all of the profiles, the proposed 3D law of the wall of van den Berg (1975), which incorporates pressure gradient and inertial effects and relies on the assumption that γ_FGA=γ_SSA, is able to predict the flow behavior at more mildly non-equilibrium stations. This suggests that models that currently rely on assumptions founded on the two-dimensional law of the wall could be improved by incorporating van den Berg's model instead. The total shear stress distribution at selected stations on the BeVERLI Hill are all significantly reduced below equilibrium two-dimensional (2D) levels, indicating that turbulence models built on this assumptions will not be able to accurately simulate the 3D turbulence behavior. / Doctor of Philosophy / As an object moves through a fluid or a fluid moves past an obstacle, fluid sticks to the solid boundary of the object because of the fluid's viscosity, resulting in zero velocity on the surface (known as the "no-slip" condition). There then exists a region where the flow velocity increases from zero to the freestream velocity - this region is known as the boundary layer. The nature of the boundary layer developing around a body significantly influences how the body and fluid interact and is critical to practical items of engineering interest, such as estimating how much drag a vehicle will experience. Most bodies of engineering interest are three-dimensional (3D), like an aircraft or a car, and thus induce a three-dimensional boundary layer, but many turbulence theories used in computational fluid dynamics simulations are based on simplified two-dimensional (2D) flow behavior studied in laboratories. To further improve the accuracy of simulations, a better understanding of three-dimensional turbulent boundary layer flows is required. This dissertation outlines a study of three-dimensional turbulent flows by analyzing the three-dimensional turbulent boundary layer over the Benchmark Validation Experiments for RANS/LES Investigations (BeVERLI) Hill using laser Doppler velocimetry (LDV) in the Virginia Tech Stability Wind Tunnel. LDV uses the Doppler shift principle to measure the fluid velocity and turbulence at different points in the flow. Through analysis of the fluid velocity and turbulence in the flow, it is shown that the turbulence and flow behavior at certain stations are heavily influenced on the upstream flow history. Stations closer to the bottom of the hill are more influenced by the upstream flow history, while stations closer to the top of the hill experience such strong acceleration due to the local favorable pressure gradient and hill curvature that the upstream history has a more linear influence. In general, the turbulence on the hill is reduced due to the acceleration over the surface below 2D levels and does not match with the 2D fundamental relationships often used in turbulence theories for simulations. Thus, simulations that rely on these assumptions will not be able to accurately predict the details of the 3D flow. A proposed 3D model for the mean velocity behavior by van den Berg (1975) will perform better in simulations than the typical 2D law used in some turbulence model assumptions.

Experiments investigating complex flows in rod bundles with spacer grids that have mixing devices (such as flow mixing vanes) have mostly been performed using single-point measurements. Although these measurements allow local comparisons of experimental and numerical data they provide little insight because the discrepancies can be due to the integrated effects of many complex flow phenomena such as wake-wake, wake-vane, and vane-boundary layer interactions occurring simultaneously in a complex flow environment. In order to validate the simulations results, detailed comparison with experimental data must be done. This work describes an experimental database obtained using Time Resolved Particle Image Velocimetry (TR-PIV) measurements within a 5 x 5 rod bundle with spacer-grids. Measurements were performed using two different grid designs. One typical of Boiling Water Reactors (BWR) with swirl type mixing vanes and the other typical of Pressurized Water Reactors (PWR) with split type mixing vanes. High quality data was obtained in the vicinity of the grid using the multi-scale approach. One of the unique characteristic of this set-up is the use of the Matched Index of Refraction (MIR) technique employed in this investigation. This approach allows the use of high temporal and spatial non-intrusive dynamic measurement techniques to investigate the flow evolution below and immediately above the spacer. The experimental data presented includes explanation of the various cases tested such as test rig dimensions, measurement zones, the test equipment and the boundary conditions in order to provide appropriate data for comparison with Computational Fluid Dynamics (CFD) simulations. Turbulence parameters of the obtained data are analyzed in order to gain insight of the physical phenomena. The shape of the velocity profile at various distances from the spacer show important modifications passing the grid which delineates the significant effects of the presence of the grid spacer. Influence of the vanes wake in the global velocity was quantified to be up to a distance of 4 hydraulic diameters from the edge of the grid.Spatial and temporal correlations in the two measured dimensions were performed to quantify the time and length scales present in the flow in the vicinity of the grids and its influence in the flow modification induced by the vanes. Detection of vortex cores was performed using the vorticity, swirl strength and Galilean decomposition approach. The resulted cores were then tracked in time, in order to observe the evolution of the structures under the influence of the vanes for each grid. Vortex stretching was quantified in order to gain insight of the energy dissipation process normally associated with the phenomena. This work presents data in a single-phase flow situation and an analysis of these data for understanding complex flow structure. This data provide for the first time detailed temporal velocity full field which can be used to validate CFD codes.

The presence of gas in engine liquid cooling systems can have severe consequences for engine efficiency and life. The presence of stagnant, trapped gases will result in cooling system hotspots, causing gallery wall degradation through thermal stresses, fatigue and eventual cracking. The presence of entrained, transient gases in the coolant flow will act to reduce its bulk thermal properties and the performance of the system s coolant pump; critically the liquid flow rate, which will severely affect heat transfer throughout the engine and its ancillaries. The hold-up of gas in the pump s impeller may cause the dynamic seal to run dry, without lubrication or cooling. This poses both an immediate failure threat should the seal overheat and rubber components melt and a long term failure threat from intermittent quench cooling, which causes deposit formation on sealing faces acting to abrade and reduce seal quality. Bubbles in the coolant flow will also act as nucleation sites for cavitation growth. This will reduce the Net Positive Suction Head available (NPSHA) in the coolant flow, exacerbating cavitation and its damaging effects in locations such as the cylinder cooling liners and the pump s impeller. This thesis has analysed the occurrence of trapped gas (air) during the coolant filling process, its behaviour and break-up at engine start, the two-phase character of the coolant flow these processes generate and the effects it has on coolant pump performance. Optical and parametric data has been acquired in each of these studies, providing an understanding of the physical processes occurring, key variables and a means of validating numerical (CFD) code of integral processes. From the fundamental understanding each study has provided design rules, guidelines and validated tools have been developed, helping cooling system designers minimise the occurrence of trapped air during coolant filling, promote its breakup at engine start and to minimise its negative effects in the centrifugal coolant pump. It was concluded that whilst ideally the prevention of cooling system gases should be achieved at source, they are often unavoidable. This is due to the cost implications of finding a cylinder head gasket capable of completely sealing in-cylinder combustion pressures, the regular use of nucleate boiling regimes for engine cooling and the need to design cooling channel geometries to cool engine components and not necessarily to avoid fill entrapped air. Using the provided rules and models, it may be ensured stagnant air is minimised at source and avoided whilst an engine is running. However, to abate the effects of entrained gases in the coolant pump through redesign is undesirable due to the negative effects such changes have on a pump s efficiency and cavitation characteristics. It was concluded that the best solution to entrained gases, unavoidable at source, is to remove them from the coolant flow entirely using phase separation device(s).

Stratified two-phase flows were investigated at different test facilities with horizontal test sections in order to provide an experimental database for the development and validation of computational fluid dynamics (CFD) codes. These channels were designed with rectangular cross-sections to enable optimal observation conditions for the application of optical measurement techniques. Consequently, the local flow structure was visualised with a high-speed video camera, delivering data with high-resolution in space and time as needed for CFD code validation. Generic investigations were performed at atmospheric pressure and room temperature in two air/water channels made of acrylic glass. Divers preliminary experiments were conducted with various measuring systems in a test section mounted between two separators. The second test facility, the Horizontal Air/Water Channel (HAWAC), is dedicated to co-current flow investigations. The hydraulic jump as the quasi-stationary discontinuous transition between super- and subcritical flow was studied in this closed channel. Moreover, the instable wave growth leading to slug flow was investigated from the test section inlet. For quantitative analysis of the optical measurements, an algorithm was developed to recognise the stratified interface in the camera frames, allowing statistical treatments for comparison with CFD calculation results. The third test apparatus was installed in the pressure chamber of the TOPFLOW test facility in order to be operated at reactor typical conditions under pressure equilibrium with the vessel atmosphere. The test section representing a flat model of the hot leg of the German Konvoi pressurised water reactor (PWR) scaled at 1:3 is equipped with large glass side walls in the region of the elbow and of the steam generator inlet chamber to allow visual observations. The experiments were conducted with air and water at room temperature and maximum pressures of 3 bar as well as with steam and water at boundary conditions of up to 50 bar and 264°C. Four types of experiments were performed, including generic test cases as well as transient validation cases of typical nuclear reactor safety issues. As an example, the co-current flow experiments simulate the two-phase natural circulation in the primary circuit of a PWR. The probability distribution of the water level measured in the reactor pressure vessel simulator was used to characterise the flow in the hot leg. Moreover, the flooding behaviour in this conduit was investigated with dedicated counter-current flow limitation experiments. A comparison of the flooding characteristics with similar experimental data and correlations available in the literature shows that the channel height is the characteristic length to be used in the Wallis parameter for channels with rectangular cross-sections. Furthermore, for the analysis of steam/water experiments, condensation effects had to be taken into account. Finally, the experimental results confirm that the Wallis similarity is appropriate to scale flooding in the hot leg of a PWR over a large range of pressure and temperature conditions. Not least, different examples of comparison between experiment and simulation demonstrate the possibilities offered by the data to support the development and validation of CFD codes. Besides the comparison of qualitative aspects, it is shown exemplarily how to treat the CFD results in order to enable quantitative comparisons with the experiments.

In the continuous casting of steel, the Submerged Entry Nozzle (SEN), in particular the SEN geometry, has a primary influence on the flow pattern: the SEN controls the speed, direction and other characteristics of the jet entering the mould. The SEN is however relatively inexpensive to change (in comparison with other continuous casting equipment). Thus; there is a feasible incentive to exactly understand and predict the flow of molten steel through the SEN and into the mould, in order to maximise the quality of the steel by altering the design of the SEN. By changing the SEN geometry and SEN design, the flow pattern in the mould will also change: it is thus possible to obtain an optimum SEN design if (or when) the desired flow patterns and/or certain predetermined temperature distributions are achieved. Expensive and risky plant trials were traditionally utilised to “perfect” continuous casting processes. As opposed to the plant trials, this dissertation is concerned with the Computational Fluid Dynamics (CFD) modelling of the SEN and mould, which, when used in conjunction with the Mathematical Optimiser LS-OPT, will enable the optimisation of the SEN design to achieve desired results. The CFD models are experimentally verified and validated using 40%-scaled (designed and built in-house) and full-scale water model tests. This dissertation proves that the CFD modelling of the SEN and mould can be quite useful for optimisation and parametric studies, especially when automated model generation (geometry, mesh and solution procedures) is utilised. The importance of obtaining reliable and physically correct CFD results is also emphasised; hence the need for CFD model verification using water modelling. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2007. / Mechanical and Aeronautical Engineering / unrestricted

Gas-liquid two-phase flows occur in many industrial applications and apparatuses. The design and optimization of such apparatuses and processes requires the numerical simulation of two-phase flows. However, two-phase flow simulations are still a challenging task, especially for industrial scales. Here, the simulation of large flow domains and high Reynolds number flows require a reduction of the resolved time-scales and length-scales by a high level of modeling to decrease the computational effort. Therefore, physics-based models are needed to depict the complex transport processes between the phases. Thus, two-phase flows are the object of ongoing research. Up to now, the majority of closure models for turbulence, interfacial forces or bubble breakup and coalescence were validated against experimental data derived from experiments in simple flow channel geometries like straight pipes. Their application for the simulation of two-phase flows with three-dimensional flow structures like e.g. recirculating areas, flow separation or strong velocity gradients requires constant experimental validation and further development. Hence, improved experimental methods are required for investigations of gas-liquid two-phase flows to provide reliable data for further development and validation of numerical flow simulation models. Therefore, experiments were performed in a constricted pipe under bubbly two-phase flow conditions. Three-dimensional flow structures were created by two types of flow constrictions for a variety of gas and liquid superficial velocities up to jg = 0.1400 m⋅s-1 and jl = 1.6110 m⋅s-1. The flow fields upstream and downstream of the flow constrictions were studied using ultrafast X-ray tomography and hot-film anemometry to obtain cross-sectional phase distribution, bubble characteristics and liquid velocity. The analysis of the ultrafast X-ray tomography image data was significantly improved by development of a histogram-based gas holdup calculation. Furthermore, the spatial dependence of the axial image plane distance was studied to improve the determination of axial bubble velocities and, thus, bubble sizes. The experimental method was advanced by simultaneous application of ultrafast X-ray tomography and hot-film anemometry. Eventually, the experimental data was compared to state-of-the-art Euler/Euler two-fluid simulations. The simulations were performed in the framework of a parallel doctoral thesis in the Experimental Thermal Fluid Dynamics department at the Helmholtz-Zentrum Dresden – Rossendorf by Ms. Sibel Tas-Koehler following the baseline approach. The results were compared in terms of the phase distribution, bubble sizes and gas velocity for two operating conditions using the homogeneous multiple size group model. / Zweiphasenströmungen aus Gasen und Flüssigkeiten treten in vielen industriellen Anwendungen und Apparaten auf. Um einen sicheren, zuverlässigen und optimalen Betrieb einzelner Komponenten und gesamter Anlagen zu gewährleisten, sind die Strömungen Gegenstand zahlreicher Untersuchungen. Numerische Simulationen sind ein unverzichtbares Instrument, um Prozesse unter diesen Aspekten zu bewerten. Die Simulation von Zweiphasenströmungen, insbesondere im industriellen Maßstab, ist jedoch nach wie vor eine anspruchsvolle Aufgabe. Um den Rechenaufwand zu verringern und die Simulation von großen Strömungsgebieten und Strömungen mit hohen Reynoldszahlen zu ermöglichen, ist ein hohes Maß an Modellierung notwendig. Gleichzeitig wurden die meisten Schließungsmodelle zur Beschreibung von Turbulenz, Grenzflächenkräften oder Blasenzerfall und -koaleszenz für einfache Geometrien wie beispielsweise gerade Rohre entwickelt. Die Anwendung dieser Modelle für die Simulation von Zweiphasenströmungen mit dreidimensionalen Strömungsstrukturen, wie z.B. Rezirkulationsgebieten, Strömungsablösungen oder starken Geschwindigkeitsgradienten, erfordert eine ständige experimentelle Validierung und Weiterentwicklung. Dies wiederum erfordert eine immer höhere Auflösung der eingesetzten Messsysteme und steigende Qualität der experimentellen Daten. Um verlässliche Daten für die Weiterentwicklung und Validierung von Modellen für die numerische Strömungssimulation zu erhalten sind daher verbesserte experimentelle Methoden zur Untersuchung von Gas-Flüssig-Strömungen erforderlich. Aus diesem Grund wurden Experimente an einer Blasenströmung in einem Rohr mit einer Strömungsverengung durchgeführt. Zwei Arten von Verengungen wurden genutzt, um dreidimensionale Strömungsstrukturen für eine Vielzahl von Betriebsbedingungen zu erzeugen. Diese sind durch Gas- und Flüssigkeitsleerrohrgeschwindigkeiten bis zu jg = 0.1400 m⋅s-1 und jl = 1.6110 m⋅s-1 definiert. Um die Phasenverteilung im Querschnitt der Strömung, Blaseneigenschaften und die Flüssigphasengeschwindigkeit stromauf- und -abwärts der Verengung zu ermittelt, wurde die Strömung mit Hilfe der ultraschnellen Röntgentomographie und Heißfilm-Anemometrie untersucht. Die Datenanalyse für die Bilddaten der ultraschnellen Röntgentomographie wurde durch die Entwicklung einer Histogramm-basierten Gasgehaltsberechnung erheblich verbessert. Um die Bestimmung der axialen Blasengeschwindigkeiten und damit der Blasengrößen zu verbessern, wurde außerdem die räumliche Abhängigkeit des axialen Bildebenenabstands untersucht. Die experimentellen Methoden wurden durch die gleichzeitige Anwendung von ultraschneller Röntgentomographie und Heißfilm-Anemometrie weiterentwickelt. Die experimentellen Daten wurden mit dem Stand der Technik von Euler/Euler-Zweiphasen-Simulationen verglichen. Die Simulationen wurden im Rahmen eines parallelen Promotionsvorhabens in der Abteilung Experimentelle Thermofluiddynamik am Helmholtz-Zentrum Dresden – Rossendorf von Frau Sibel Tas-Köhler durchgeführt und folgten der Baseline-Modell Strategie. Die Ergebnisse wurden unter Verwendung des homogenen Modells mehrerer Größenklassen bezüglich der Phasenverteilung, der Blasengrößen und der Gasgeschwindigkeit für zwei Betriebsbedingungen verglichen.

Stratified two-phase flows were investigated at different test facilities with horizontal test sections in order to provide an experimental database for the development and validation of computational fluid dynamics (CFD) codes. These channels were designed with rectangular cross-sections to enable optimal observation conditions for the application of optical measurement techniques. Consequently, the local flow structure was visualised with a high-speed video camera, delivering data with high-resolution in space and time as needed for CFD code validation. Generic investigations were performed at atmospheric pressure and room temperature in two air/water channels made of acrylic glass. Divers preliminary experiments were conducted with various measuring systems in a test section mounted between two separators. The second test facility, the Horizontal Air/Water Channel (HAWAC), is dedicated to co-current flow investigations. The hydraulic jump as the quasi-stationary discontinuous transition between super- and subcritical flow was studied in this closed channel. Moreover, the instable wave growth leading to slug flow was investigated from the test section inlet. For quantitative analysis of the optical measurements, an algorithm was developed to recognise the stratified interface in the camera frames, allowing statistical treatments for comparison with CFD calculation results. The third test apparatus was installed in the pressure chamber of the TOPFLOW test facility in order to be operated at reactor typical conditions under pressure equilibrium with the vessel atmosphere. The test section representing a flat model of the hot leg of the German Konvoi pressurised water reactor (PWR) scaled at 1:3 is equipped with large glass side walls in the region of the elbow and of the steam generator inlet chamber to allow visual observations. The experiments were conducted with air and water at room temperature and maximum pressures of 3 bar as well as with steam and water at boundary conditions of up to 50 bar and 264°C. Four types of experiments were performed, including generic test cases as well as transient validation cases of typical nuclear reactor safety issues. As an example, the co-current flow experiments simulate the two-phase natural circulation in the primary circuit of a PWR. The probability distribution of the water level measured in the reactor pressure vessel simulator was used to characterise the flow in the hot leg. Moreover, the flooding behaviour in this conduit was investigated with dedicated counter-current flow limitation experiments. A comparison of the flooding characteristics with similar experimental data and correlations available in the literature shows that the channel height is the characteristic length to be used in the Wallis parameter for channels with rectangular cross-sections. Furthermore, for the analysis of steam/water experiments, condensation effects had to be taken into account. Finally, the experimental results confirm that the Wallis similarity is appropriate to scale flooding in the hot leg of a PWR over a large range of pressure and temperature conditions. Not least, different examples of comparison between experiment and simulation demonstrate the possibilities offered by the data to support the development and validation of CFD codes. Besides the comparison of qualitative aspects, it is shown exemplarily how to treat the CFD results in order to enable quantitative comparisons with the experiments.