No / A computational fluid dynamics (CFD) model is used to investigate the droplet formation and deformation under the influence of different parameters. Droplet breakup phenomenon depends on several factors such as viscosity, velocity, pressure difference, and geometry. The most important parameter for droplet breakup is the Weber number (We) which is the ratio of disrupting aerodynamics forces to the surface tension forces. Volume of fluid (VOF) model is used in present work to simulate the droplet breakup. This work presents the effect of liquid velocity, viscosity, and orifice diameters on droplet formation and breakup.

No description available.

Abstract In ladle metallurgy, gas stirring and the behaviour of the slag layer are very important for alloying and the homogenization of the steel. When gas is injected through a nozzle located at the bottom of the ladle into the metal bath, the gas jet exiting the nozzle breaks up into gas bubbles. The rising bubbles break the slag layer and create an open-eye. The size of the open-eye is very important as the efficiency of the metal-slag reactions depend on the interaction between the slag and steel created during the stirring process, and information about the position and size of the open-eye is important for effective alloying practice. Moreover, the open-eye has an effect on the energy balance since it increases heat losses. In this study, experimental measurements and numerical simulations were performed to study the effect of different operating parameters on the formation of the open-eye and mixing time in a water model and industrial ladle. Experimental measurements were performed to study the effect of the gas flow rate, slag layer thickness, slag layer densities and number of porous plugs in a 1/5 scale water model and in a 150-ton steelmaking ladle. For numerical modelling, a multi-phase volume of fluid (VOF) model was used to simulate the system including the behaviour of the slag layer. The numerical simulation of the open-eye size and mixing time was found to be in good agreement with the experimental data obtained from the water model and data obtained from the industrial measurements. / Tiivistelmä Senkkametallurgiassa kaasuhuuhtelu ja kuonakerroksen käyttäytyminen ovat tärkeitä teräksen seostamisen ja homogenisoinnin näkökulmasta. Senkan pohjalla sijaitsevasta suuttimesta puhallettava kaasu hajoaa kupliksi, jotka rikkovat kuonakerroksen ja muodostavat avoimen silmäkkeen. Avoimen silmäkkeen koko on yhteydessä voimakkaampaan kuonan emulgoitumiseen, joka tehostaa metallisulan ja kuonan välisiä reaktioita. Tietoa avoimen silmäkkeen paikasta ja koosta tarvitaan myös tehokkaaseen seostuspraktiikkaan. Avoin silmäke vaikuttaa lisäksi prosessin energiataseeseen lisäten sen lämpöhäviöitä. Tässä tutkimuksessa tutkittiin kokeellisesti ja laskennallisesti erilaisten operointiparametrien vaikutusta avoimen silmäkkeen muodostumiseen vesimallissa ja terässenkassateollisessa senkassa. Kokeellisia mittauksia tehtiin kaasuhuuhtelun, kuonakerroksen paksuuden, ja suuttimien määrän vaikutuksen tutkimiseksi 1/5-mittakaavan vesimallissa ja 150 tonnin terässenkassa. Numeerisessa mallinnuksessa systeemin ja siihen lukeutuvan kuonakerroksen käyttäytymisen simuloimiseen käytettiin volume of fluid (VOF) –monifaasimenetelmää. Avoimen silmäkkeen kokoon ja sekoittumisaikaan liittyvien numeeristen simulointien havaittiin vastaavan hyvin vesimallista ja teollisista mittauksista saatua kokeellista aineistoa.

Water management is a serious issue that affects the performance and durability of PEM fuel cells. It is known, from previous experimental investigations, that surface wettability has influence on water behaviour and fuel cell performance. This finding has lead researchers to develop numerical tools for further investigation of the liquid water behaviour in gas channels. The Volume-of-Fluid (VOF) method has been used in a wide range of studies for its advantage of showing the multi-phase interface in a Computational Fluid Dynamics (CFD) simulation to understand liquid water behaviour in gas channels. In this thesis, numerical study has been carried out to examine the behaviour of liquid water in gas channels. The dynamic movement of the liquid water in the channel and the associated pressure drop, water saturation and water coverage of the GDL have been investigated. Firstly, flow diffusion into the GDL was examined to determine its effect on liquid droplet behaviour in a small section of a gas channel. Furthermore, the effects of the percentage of flow diffusion, GDL wettability, pore size, and water inlet velocity were investigated. Fluid diffusion into GDL found to have insignificant impact on liquid water behaviour so further investigations has been carried with a solid GDL surface. Secondly, gas channel geometry effect on liquid water behaviour was studied. Square, semicircle, triangle, trapezoid with a long base and trapezoid with a short base were compared to find suitable cross section geometry to carry wall wettability investigations. Among the examined geometries, the square cross section showed reasonable results for both scenarios of geometry design, fixed Reynolds number and fixed GDL interface. The effect of wall wettability was assessed by comparing nine different wall/GDL wettability combinations for straight and bend channels. Wall wettability found to have an impact on liquid water behaviour but not as much as GDL wettability. It affects liquid water saturation in the channel by a great deal by accumulating water in the channel edges affecting water behaviour. This was also proven in the last test case of a long channel where water accumulation was investigated by running the calculation until the percentage of water saturation is stabilized. It is also concluded that changing wall wettability from hydrophobic to hydrophilic doubles the percentage of channel occupied by liquid water and increases the time to reach steady state.

Thesis (PhD)--Stellenbosch University, 2012 / ENGLISH ABSTRACT: This study presents the development and extension of free-surface modelling techniques for the purpose of modelling two-fluid systems accurately and efficiently. The volume-of-fluid (VOF) method is extended in two ways: Firstly, it is extended to account for variations in the gas density through a weakly compressible formulation. Secondly, a compressive free-surface interface capturing formulation that preserves the integrity of the interface shape is detailed. These formulations were implemented and evaluated using the Elemental software. Under certain flow conditions liquid-gas systems may be subjected to large variations in pressure, making it necessary to account for changes in gas density. Modelling this effectively has received relatively little attention in the context of free-surface modelling and remains a challenge to date. To account for the variations in gas density a weakly compressible free-surface modelling formulation is developed for low Mach number flows. The latter is formally substantiated via a non-dimensional analysis. It is proposed that the new formulation advances on existing free-surface modelling formulations by effecting an accurate representation of the dominant physics in an efficient and effective manner. The proposed weakly compressible formulation is discretised using a vertexcentred edge-base finite volume approach, which provides a computationally efficient method of data structuring and memory usage. Furthermore, this implementation is applicable to unstructured spatial discretisation and parallel computing. In this light, the discretisation is formulated to ensure a stable, oscillatory free solution. Furthermore, the governing equations are solved in a fully coupled manner using a combination of dual time-stepping and a Generalised Minimum Residual solver with Lower-Upper Symmetric Gauss-Seidel preconditioning, ensuring a fast and efficient solution. The newly developed VOF interface capturing formulation is proposed to advance on the accuracy and efficiency with which the evolution of the free-surface interface is modelled. This is achieved through a novel combination of a blended higher-resolution scheme, used to interpolate the volume fraction face value, and the addition of an artificial compressive term to the VOF equation. Furthermore, the computational efficiency of the higher-resolution scheme is improved through the reformulation of the normalised variable approach and the implementation of a new higher-resolution blending function. For the purpose of evaluating the newly developed methods, several test cases are considered. It is demonstrated that the new surface capturing formulation offers a significant improvement over existing schemes, particularly at large CFL numbers. It is shown that the proposed method achieves a sharper, better defined interface for a wide range of flow conditions. With the validation of the weakly compressible formulation, it is found that the numerical results correlate well with analytical solutions. Furthermore, the importance of accounting for gas compressibility is demonstrated via an application study. The weakly compressible formulation is also found to result in negligible additional computational cost while resulting in improved convergence rates. / AFRIKAANSE OPSOMMING: Hierdie studie behels die ontwikkeling van numeriese tegnieke met die doel om twee-vloeistof vloei akkuraat en numeries effektief te modelleer. Die volume-vanvloeistof metode word op twee maniere uitgebrei: Eerstens word variasie van die gasdigtheid in ag geneem deur gebruik te maak van ’n swak samedrukbare model. Tweedens saam is ’n hoë-resolusie metode geformuleer vir die voorstelling van die vloeistof-oppervlak. Hierdie uitbreidings is met die behulp van die Elemental programmatuur geïmplementeer en met behulp van die programmatuur geëvalueer. Onder sekere toestande ervaar vloeistof-gas mengsels groot veranderinge in druk. Dit vereis dat die variasie in gasdigtheid in berekening gebring moet word. Die modellering hiervan het egter tot dusver relatief min aandag ontvang. Om hierdie rede word ’n swak samedrukbare model vir lae Mach-getalle voorgestel om die variasie in gasdigtheid in te reken. Die formulering volg uit ’n nie-dimensionele analise. Daar word geargumenteer dat die nuwe formulering die fisika meer akkuraat verteenwoordig. ’n Gesentraliseerde hoekpunt, rant gebaseerde eindige volume metode word gevolg om die differensiaalvergelykings numeries te diskretiseer. Dit bied ’n doeltreffende manier vir datastrukturering en geheuebenutting. Hierdie benadering is verder geskik vir toepassing op ongestruktureerde roosters en parallelverwerking. Die diskretisering is geformuleer om ’n stabiele oplossing sonder numeriese ossillasies te verseker. Die vloeivergelykings word op ’n gekoppelde wyse opgelos deur gebruik te maak van ’n kombinasie van ’n pseudo tyd-stap metode en ’n Veralgemene Minimum Residu berekeningsmetode met Onder-Bo Simmetriese Gauss- Seidel voorafbewerking. Die nuut ontwikkelde skema vir die modellering van die vloeistof-oppervlak is veronderstel om ’n meer akkurate voorstelling te bied en meer doeltreffend te wees vir numeriese berekeninge. Dit word bereik deur die nuwe kombinasie van ’n hoë-resolusie skema, wat gebruik word om die volumefraksie te interpoleer, met die samevoeging van ’n kunsmatige term in die volume-van-vloeistof vergelyking om die resolusie te verfyn. Verder is die doeltreffendheid van die skema verbeter deur die genormaliseerde veranderlikes benadering te herformuleer en deur die ontwikkeling van ’n nuwe hoë-resolusie vermengingsfunksie. Verskeie toetsgevalle is uitgevoer met die doel om die nuwe modelle te evalueer. Daar word aangetoon dat die nuwe skema vir die modellering van die vloeistofoppervlak ’n meetbare verbetering bied, veral by hoër Courant-Friedrichs-Lewy getalle. Die nuwe formulering bied dus hoër akkuraatheid vir ’n wye verskeidenheid van toestande. Vir die swak samedrukbare formulering is daar ’n goeie korrelasie tussen die numeriese resultate en die analitiese oplossing. In ’n toegepassingsgeval word die noodsaaklikheid om die samedrukbaarheid van die gas in ag te neem gedemonstreer. Die addisionele berekening-kostes van die nuwe formulering is weglaatbaar en in sommige gevalle verhoog die tempo waarteen die oplossing konvergeer

abstract: The goal of this paper was to do an analysis of two-dimensional unsplit mass and momentum conserving Finite Volume Methods for Advection for Volume of Fluid Fields with interfaces and validating their rates of convergence. Specifically three unsplit transport methods and one split transport method were amalgamated individually with four Piece-wise Linear Reconstruction Schemes (PLIC) i.e. Unsplit Eulerian Advection (UEA) by Owkes and Desjardins (2014), Unsplit Lagrangian Advection (ULA) by Yang et al. (2010), Split Lagrangian Advection (SLA) by Scardovelli and Zaleski (2003) and Unsplit Averaged Eulerian-Lagrangian Advection (UAELA) with two Finite Difference Methods by Parker and Youngs (1992) and two Error Minimization Methods by Pilliod Jr and Puckett (2004). The observed order of accuracy was first order in all cases except when unsplit methods and error minimization methods were used consecutively in each iteration, which resulted in second-order accuracy on the shape error convergence. The Averaged Unsplit Eulerian-Lagrangian Advection (AUELA) did produce first-order accuracy but that was due to a temporal error in the numerical setup. The main unsplit methods, Unsplit Eulerian Advection (UEA) and Unsplit Lagrangian Advection (ULA), preserve mass and momentum and require geometric clipping to solve two-phase fluid flows. The Unsplit Lagrangian Advection (ULA) can allow for small divergence in the velocity field perhaps saving time on the iterative solver of the variable coefficient Poisson System. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2019

In 1999, California Air Resources Board (CARB) implemented a regulation that required all gasoline cars sold in California be fitted with an Onboard Refueling Vapor Recovery System (ORVR). The ORVR system is designed to prevent Volatile Organic Compounds (VOCs) from escaping into the atmosphere during refuelling by storing the gas vapours in a carbon canister. Due to the complex nature of the fuel system, making design changes could have large implications on the ORVR performance of the vehicle. It is therefore desirable to develop a CFD model that can predict the effects of design changes, thereby reducing the need to perform physical tests on each design iteration. This master thesis project was performed at the Fuel Systems department at Volvo Cars in order to help reduce project lead times and product development costs by incorporating CFD as a part of the fuel system development cycle. The CFD results obtained were validated through experimental tests that were also performed as part of this project. In this master thesis project, a CFD model was developed to simulate the refuelling of gasoline for a California specification Volvo XC90 with an OPW-11B pump pistol. The model was set up in STAR-CCM+ using the Eulerian Volume of Fluid model for multiphase flow, the RANS realizable k − ε turbulence model and the two layer all y + wall treatment. The effects of the carbon canister were modelled as a porous baffle interface in the simulations where viscous and inertial resistances of the porous media were adjusted to obtain a desired pressure drop across the canister. This method proved to be a suitable simplification for this study. The effects of evaporation as well as a chemical adsorption model for the carbon canister have been excluded from the project due to time limitations. It was found that the CFD simulations were in good agreement with the experimental results, especially with respect to capturing the overall behaviour of the fuel system during refuelling. It was found that resolving the flow spatially (and temporally) in the filler pipe was a crucial part in ensuring solver stability. A pressure difference between experiment and simulation was also observed as a consequence of excluding evaporation from the CFD model. After the CFD model had been verified and validated, changes to different parts of the fuel system were investigated to observe their effects on ORVR performance. These included changing the recirculation line diameter, changing the carbon canister properties and changing the angle of how the pump pistol was inserted into the capless unit. It was found that the recirculation line diameter is a very sensitive design parameter and increasing the diameter would result in fuel vapour leaking back out into the atmosphere. Similarly, increasing the back pressure by swapping to a different carbon canister would result in the leakage of fuel vapour. On the other hand, insignificant changes in system behaviour were observed when the fuel pistol angle was changed.In 1999, California Air Resources Board (CARB) implemented a regulation that required all gasoline cars sold in California be fitted with an Onboard Refueling Vapor Recovery System (ORVR). The ORVR system is designed to prevent Volatile Organic Compounds (VOCs) from escaping into the atmosphere during refuelling by storing the gas vapours in a carbon canister. Due to the complex nature of the fuel system, making design changes could have large implications on the ORVR performance of the vehicle. It is therefore desirable to develop a CFD model that can predict the effects of design changes, thereby reducing the need to perform physical tests on each design iteration. This master thesis project was performed at the Fuel Systems department at Volvo Cars in order to help reduce project lead times and product development costs by incorporating CFD as a part of the fuel system development cycle. The CFD results obtained were validated through experimental tests that were also performed as part of this project. In this master thesis project, a CFD model was developed to simulate the refuelling of gasoline for a California specification Volvo XC90 with an OPW-11B pump pistol. The model was set up in STAR-CCM+ using the Eulerian Volume of Fluid model for multiphase flow, the RANS realizable k − ε turbulence model and the two layer all y + wall treatment. The effects of the carbon canister were modelled as a porous baffle interface in the simulations where viscous and inertial resistances of the porous media were adjusted to obtain a desired pressure drop across the canister. This method proved to be a suitable simplification for this study. The effects of evaporation as well as a chemical adsorption model for the carbon canister have been excluded from the project due to time limitations. It was found that the CFD simulations were in good agreement with the experimental results, especially with respect to capturing the overall behaviour of the fuel system during refuelling. It was found that resolving the flow spatially (and temporally) in the filler pipe was a crucial part in ensuring solver stability. A pressure difference between experiment and simulation was also observed as a consequence of excluding evaporation from the CFD model. After the CFD model had been verified and validated, changes to different parts of the fuel system were investigated to observe their effects on ORVR performance. These included changing the recirculation line diameter, changing the carbon canister properties and changing the angle of how the pump pistol was inserted into the capless unit. It was found that the recirculation line diameter is a very sensitive design parameter and increasing the diameter would result in fuel vapour leaking back out into the atmosphere. Similarly, increasing the back pressure by swapping to a different carbon canister would result in the leakage of fuel vapour. On the other hand, insignificant changes in system behaviour were observed when the fuel pistol angle was changed.

The current standard of simulating flood flow in natural river reaches is based on solving the 1-D or 2-D St. Venant equations or using hybrid 1-D/2-D models based on the same equations. These models are not always able to accurately predict floodwave propagation, especially around and downstream of regions where 3-D effects become important, or at times when the main assumptions associated with these models are violated (e.g. flow becomes pressurized due to presence of a hydraulic structure like a bridge or a culvert). A 3-D modeling approach, though computationally much more expensive, is not subject to such limitations and should be able to predict accurately predict floodwave propagation even in regions where 3-D effects are expected to be significant. This dissertation describes the development and validation of a 3-D time-accurate RANS-based model to study flood-related problems in natural environments. It also discusses how results from these 3-D simulations can be used to better calibrate lower order models. Applications are included where the flow becomes pressurized during high flow conditions and the sediment entrainment potential of the flow during the flooding event is estimated. Another important category of applications discussed in the present study are floodwave propagation induced by a sudden dam break failure. Results show that 2-D models show fairly large differences with 3-D model predictions especially in regions where 3-D effects are expected to be significant (e.g. near channel-floodplain transition, in highly curved channels, near hydraulic structures). The study also discusses the use of the validated 3-D model as an engineering design tool to identify the optimum solution for flood protection measures intended to reduce flooding in the Iowa River near Iowa City. 3-D simulation results are also used to discuss hysteresis effects in the relationship between bed shear stress and the stage/discharge. Such effects need to be taken into consideration to accurately estimate erosion associated with the passage of a floodwave.

We present a slice-based method that combined with fluid-solid interaction to render the oceans interact with the objects of the simulation. First, according to the key slices selection one can determine the initial slices simulation for reducing the computation on the number of grid and expressing the solid appearance. Second, we used 3D vector Navier-Stokes equations and combined with 3D fluid-solid coupling to comply with the laws of physics for 2D slice simulation. Third, using a volume of fluid method one can reconstruct the 2D ocean surface and further apply interpolation to extended 2D surfaces to 3D ocean surface. Finally, using the Doo-Sabin subdivision surfaces method is to be smoother for the 3D surface. From the viewpoint of ocean simulation, we can not only solve the fluid-solid coupling problem of objects floating on the sea but also achieve better result in efficiency compared with traditional ocean simulation. From the viewpoint of fluid-solid coupling, the proposed method can greatly reduce the computation in number of grid and be applied to embedded systems, games or films effectively.

Orifices are essential devices for measurement and control of flow. It is important to define the flow field and understand the flow characteristics behind an orifice for the sake of reliability measures in many hydraulic engineering applications. Since analytical and experimental solutions are restricted, a numerical solution is obtained using volume of fluid (VOF) method with the CFD solver, FLUENT, for sharp crested orifices, orifice tubes and slots. The results are compared to the available data in the literature / also a large spectrum of data collection has been achieved.