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[ES] La creciente preocupación por los problemas medioambientales, la disponibilidad de combustibles fósiles unido a la gran demanda de vehículos, han llevado a los gobiernos a regular las emisiones emitidas a la atmósfera. Existen propuestas de adoptar fuentes de energía renovables. Sin embargo, la sustitución de los combustibles derivados del petróleo no será fácil, rápida o rentable, y el transporte propulsado por motores de combustión interna (ICE) seguirá destacando en los próximos años. La eficiencia de la combustión y el rendimiento del motor están influenciados por el complejo proceso de inyección. La inyección directa de gasolina (GDI) aumenta el ahorro de combustible y cumple los requisitos de emisiones contaminantes, aunque queda potencial por descubrir. Por ello, ha sido objeto de estudio en los últimos años y, en consecuencia, de la presente Tesis. Este trabajo tiene como motivación mejorar el entendimiento en el campo del GDI. La compleja naturaleza transitoria del proceso de inyección hace que el estudio experimental sea un desafío. La Mecánica de Fluidos Computacional (CFD) surge como una potente alternativa a los experimentos y ha sido adoptada para esta investigación. Bajo este contexto, el objetivo de la presente Tesis es desarrollar una metodología predictiva para la caracterización hidráulica del inyector, capaz de ser aplicada a las actuales y futuras generaciones de inyectores GDI, independientemente de las características del inyector y del software de estudio. Una vez validada, el objetivo posterior es utilizar los resultados para analizar el comportamiento del chorro. Este enfoque busca seguir los pasos de la comunidad científica sustituyendo la práctica experimental. La validación de la metodología se lleva a cabo mediante su aplicación en dos inyectores GDI solenoides multi-orificio diferentes. Además, se han utilizado dos códigos CFD comerciales: CONVERGE y StarCCM+. La metodología predictiva se centra en el estudio del flujo interno y el campo cercano para caracterizar hidráulicamente el inyector. El problema a tratar se define como un sistema multifásico en un marco Euleriano y considerando un único fluido. El tratamiento del flujo multifásico se realiza mediante el enfoque Volume-of-Fluid (VOF). Además, se emplea el Homogeneous Relaxation Model (HRM) para considerar el intercambio de masa entre las fases líquida y vapor debido a cavitación y flash boiling. La turbulencia se ha tratado a partir de los enfoques Reynolds-Averaged Navier-Stokes (RANS) y Large Eddy Simulations (LES). Por otro lado, en cuanto al estudio del flujo externo, se ha adoptado el Discrete Droplet Model (DDM). La atomización y el chorro están influenciados por la geometría de la tobera, por lo que la estrategia de acoplamiento del flujo interno y externo complementa los análisis. Se han adoptado enfoques de acoplamiento unidireccional y mapeado, utilizando como parámetros de entrada los datos de flujo interno de la validada metodología. Esta Tesis aporta una nueva y valiosa metodología predictiva con una elevada precisión a la hora de caracterizar el proceso de inyección en comparativa con datos experimentales. Por otro lado, es directamente trasferible a distintos códigos de cálculo así como aplicable a inyectores con características dispares sin perjudicar las exigencias del modelo. La correcta caracterización del flujo interno ha permitido emplear los datos obtenidos para analizar el comportamiento del chorro eliminando la necesidad de usar datos experimentales. Los resultados obtenidos capturan el comportamiento macroscópico del chorro con una precisión comparable a los experimentos. Aunque todavía hay muchos retos que afrontar, la presente Tesis supone un gran avance en el campo del GDI. El remarcable progreso se debe al desarrollo y uso de una metodología totalmente predictiva, que permite prescindir de la mayoría de los experimentos para contribuir a una mayor y más amplia visión de la física del proceso de inyección., [CA] La creixent preocupació pels problemes ambientals, la limitada disponibilitat de combustibles fòssils, acompanyat a la gran demanda de vehicles, ha portat el govern a regular els nivells d'emissions emesos a l'atmosfera. Existeixen propostes d'adoptar fonts d'energia renovables. Tanmateix, la substitució dels combustibles líquids derivats del petroli no es durà a terme de forma fàcil, ràpida o rentable, i el transport propulsat per motors de combustió interna (ICE) continuarà destacant en els pròxims anys. L'eficiència de la combustió i el rendiment del motor són fortament influenciats pel complex procés d'injecció. La injecció directa de gasolina (GDI) augmenta l'estalvi de combustible i complix amb els requisits d'emissions, encara que queda molt potencial per descobrir. Per això, aquest ha sigut objecte d'investigació en els últims anys i, com a conseqüència, d'aquesta Tesi. Aquest treball té com a motivació millorar l'enteniment en el camp del GDI. La complexa natura transitòria de la injecció fa que l'estudi experimental siga força complex. La Mecànica de Fluids Computacional (CFD) sorgeix com una potent alternativa als experiments, i ha sigut adoptada per aquesta investigació. Baix aquest mateix context, es proposa com a objectiu principal d'aquesta Tesi el desenvolupament d'una metodologia predictiva per a la caracterització hidràulica de l'injector, capaç de ser aplicada a les actuals i futures generacions d'injectors GDI (independentment de les característiques de l'injector i del software d'estudi). Una vegada validada, el posterior objectiu és analitzar el comportament de l'esprai. Aquest enfocament busca seguir els passos de la comunitat científica substituint la pràctica experimental. La validació de la metodologia ha sigut duta a terme mitjançant la seva aplicació en dos injectors GDI solenoides multi-orifici. A més, s'han utilitzat dos software CFD comercials: CONVERGE i StarCCM+. La metodologia predictiva se centra en l'estudi del flux intern i el camp proper per tal de caracteritzar hidràulicament l'injector. El problema a tractar es defineix en base a un sistema multi-fàsic en un marc Eulerià i considerant un únic fluid. El tractament del fluid multi-fàsic es realitza mitjançant l'aproximació Volume-of-Fluid (VOF). A més, s'utilitza el Homogeneous Relaxation Model (HRM) per tal de considerar l'intercambi de massa entre les fases líquida i vapor degut als fenòmens de cavitació i flash boiling. La turbulència s'ha tractac a través dels enfocaments Reynolds-Averaged Navier-Stokes (RANS) i Large Eddy Simulations (LES). Pel que fa a l'estudi del fluix extern, s'ha adoptat el Discrete Droplet Model (DDM). Sent conscients que el comportament l'atomització i l'esprai estan influenciats per la geometria de la tovera, l'estratègia d'acoblament del flux intern i extern complementa les anàlisis. S'han adoptat els enfocaments d'acoblament unidireccional i mapejat, utilitzant com a paràmetres d'entrada les dades del flux intern obtingudes amb la validada metodologia. Aquesta Tesi aporta una nova i valuosa metodologia predictiva amb una elevada precisió a l'hora de caracteritzar el procés d'injecció en comparativa amb dades experimentals. És directament transferible a diversos codis de càlcul així com aplicable a injectors amb característiques dispars sense perjudicar les exigències del model. La correcta caracterització del flux intern ha permès utilitzar les dades obtingudes per tal d'analitzar el comportament de l'esprai, eliminant la necessitat d'emprar dades experimentals. Els resultats obtinguts d'aquest estudi capturen el comportament macroscòpic de l'esprai amb una precisió comparable als experiments. Encara que queden molts reptes per afrontar, aquesta Tesi aporta un important avanç al camp del GDI. La ruptura prové del desenvolupament i ús d'una metodologia completament predictiva, que substitueix els experiments requerits i així contribueix a una millor i més ampla visió de la física del procés d'injecció., [EN] Concerns about climate change, availability of fuel resources and the high demand for vehicles, have led governments to regulate the level of pollution emitted by engines into the atmosphere. There is a strong desire to adopt renewable and sustainable energy sources. However, the substitution of liquid fuels derived from petroleum will not emerge easily, quickly or economically, and Internal Combustion Engines (ICE) will continue to excel for the next few years. Combustion efficiency and engine performance are strongly influenced by the complex fuel injection process. Gasoline Direct Injection (GDI) strategies increase fuel economy and meet emission requirements, although many challenges remain, which has therefore been one of the main research objectives in recent years and of this Thesis. The present research aims to provide a better understanding in the field of GDI. The transient and complex nature of the injection process makes the experimental study of GDI quite challenging. Therefore, Computational Fluid Dynamics (CFD) emerges as a powerful alternative adopted for this research. In this context, the main objective of the present Thesis is to develop a predictive methodology capable of being applied to current and future generations of GDI injectors, regardless of the injector features and the software employed, for the hydraulic characterization of the injector. Once validated, the subsequent goal is to employ the obtained results to analyze the behavior of the spray downstream of the injector. The approach attempts to follow the footsteps of the research community to avoid experimental practice. The predictive methodology has been validated through its application to two multi-hole solenoid GDI injectors with different features. In addition, the mentioned methodology has been evaluated using diverse commercial software: CONVERGE and StarCCM+. The methodology focuses on the study of the internal and near-field flow to hydraulically characterize the injector. So the problem to be addressed is a multi-phase system, performed in an Eulerian framework, modeled through a single-fluid approach. The multi-phase flow is treated by means of the Volume-of-Fluid (VOF) approach. Homogeneous Relaxation Model (HRM) is employed to consider the mass exchange between liquid and vapor fuel phases, due to cavitation and flash boiling. The turbulence treatment has been performed from both Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES) approaches. Regarding the external flow study, the Discrete Droplet Model (DDM) has been adopted. In addition, being aware that atomization and spray behavior is greatly influenced by the nozzle geometry, the coupling strategy of the internal and external flow complements the analyses. One-way coupling and mapping approaches have been adopted, using as input parameters the internal flow data obtained from the already validated methodology. Accordingly, this Thesis provides a new and valuable predictive methodology, which has demonstrated a high accuracy in characterizing the flow behavior during the injection process through comparison with experimental data. It has also proven to be directly transferable to different CFD software and applicable to injectors with dissimilar characteristics without compromising the requirements of the model. The correct internal flow characterization has made it possible to employ the obtained data to analyze the spray patterns, which eliminates the need to consider experimental data. The outcomes of this study macroscopically capture the jet behavior with an accuracy comparable to experiments under different operating conditions. Although there are still many challenges to face, the present Thesis brings a breakthrough in the field of GDI. The quantum leap arises from the development and use of a fully predictive methodology, allowing to avoid most experiments to contribute to a greater and broader vision of the injection process physics., María Martínez García has been founded through a grant from the Government of Generalitat Valenciana with reference ACIF/2018/118 and financial support from the European Union. These same institutions, Government of Generalitat Valenciana and the European Union, supported through a grant for pre-doctoral stays out of the Comunitat Valenciana with reference BEFPI/2020/057 the research carried out during the stay at Aerothermochemistry and Combustion Systems Laboratory, Swiss Federal Institute of Technology, ETH Zurich, Switzerland. Special gratitude from the author to both institutions, Government of Generalitat Valenciana and the European Union, for making this dream possible

The paper is devoted to the simulation of maritime two-phase flows of air and water. Emphasis is put on an extension of the classical Volume-of-Fluid (VoF) method by a diffusive contribution derived from a Cahn-Hilliard (CH) model and its benefits for simulating immiscible, incompressible two-phase flows. Such flows are predominantly simulated with implicit VoF schemes, which mostly employ heuristic downwind-biased approximations for the concentration transport to mimic a sharp interface. This strategy introduces a severe time step restriction and requires pseudo time-stepping of steady flows. Our overall goal is a sound description of the free-surface region that alleviates artificial time-step restrictions, facilitates an efficient and robust numerical framework and inherently includes surface tension effects when needed. The approach is verified for an analytical Couette-flow example and the bubble formation under the influence of surface tension forces. 2D Validation examples are concerned with laminar standing waves reaching from gravity to capillary scale as well as a submerged hydrofoil flow. The final application refers to the 3D flow around an experimentally investigated container vessel at fixed floatation for Re=1.4E+07 and Fn=0.26. Results are compared with data obtained from VoF approaches, supplemented by analytical solutions and measurements. The study indicates the superior efficiency, resharpening capability and wider predictive realm of the CH-based extension for free surface flows with a confined spatial range of interface Courant numbers., Submitted to Computers & Mathematics With Application

The boundaries of numerical domains for free-surface wave simulations with marine structures generate spurious wave reflection if no special measures are taken to prevent it. The common way to prevent reflection is to use dissipation zones at the cost of increased computational effort. On many occasions, the size of the dissipation area is considerably larger than the area of interest where wave interaction with the structure takes place. Our objective is to derive a local absorbing boundary condition that has equal performance to a dissipation zone with lower computational cost. The boundary condition is designed for irregular free-surface wave simulations in numerical methods that resolve the vertical dimension with multiple cells. It is for a range of phase velocities, meaning that the reflection coefficient per wave component is lower than a chosen value, say 2%, over a range of values for the dimensionless wave number kh. This is accomplished by extending the Sommerfeld boundary condition with an approximation of the linear dispersion relation in terms of kh, in combination with vertical derivatives of the solution variables. For this article, the boundary condition is extended with a non-zero right-hand side in order to prevent wave reflection, while, at the same time, at the same boundary, generating waves that propagate into the domain. Results of irregular wave simulations are shown to correspond to the analytical reflection coefficient for a range of wave numbers, and to have similar performance to a dissipation zone at a lower cost.

TEZ12631 Tez (Yüksek Lisans) -- Çukurova Üniversitesi, Adana, 2019. Kaynakça (s. 101-104) var. XXV, 105 s. :_res. (bzs. rnk.), tablo ;_29 cm. Bu çalışmada, torpido-benzeri geometri olarak adlandırılan serbest yüzeyin etkisi altında modellerin etrafındaki akış yapısı sayısal ve deneysel yöntemlerle araştırılmıştır. Sayısal çalışmada ANSYS-Fluent yazılımının sıkıştırılamaz çözücüsü kullanılmıştır. Deneysel çalışmada ise boya görselleştirmesi ve parçacık görüntülemeli hız ölçme yöntemleri uygulanmıştır. Akış yapısının geometri etrafındaki daimi olmayan akış özelliklerini hesaplamak için Büyük Girdap Simülasyonu (LES) türbülans modeli ile çözümlemeler yapılmıştır. Serbest yüzeyde hava-su ara yüzeyindeki akış özelliklerinin belirlenmesi için sıvı/gas akış oranı hacmi modelleme (VOF) yöntemi kullanılmıştır. Elde edilen sonuçlar, torpido benzeri geometrinin serbest yüzeye yakın olması nedeniyle hız alanı, akım çizgileri, girdap ve kinetik enerji gibi akış karakteristiklerin simetrik olmadığı görülmüştür. Bu nedenle, sonuçta meydana gelen sürükleme kuvvetinin (FD), daldırma oranı h/D = 0.75’e göre üniform akışa kıyasla arttığı bulunmuştur. Daldırma oranı h/D = 2.5 için akış yapısının simetrik bir formda oluştuğu, serbest yüzey etkisinin ihmal edilebilir düzeye indiği ve buna bağlı olarak üniform akış durumuna benzer akış özellikleri olduğu görüldü. Ayrıca, Reynolds sayısı 20x103’den 40x103'e yükseldiğinde ve sürükleme kuvvetinin (FD), daldırma oranı (h/D) arttıkça azaldığı, girdap uzunluğunun kısalarak modelin kuyruk kısmına yaklaştığı ve art izi bölgesi küçülmüştür. Pasif akış kontrol uygulaması ile aynı burun geometrisine sahip modelin kuyruğuna 3 ve 4 kanat eklenmesinin akış yapısı ve sürükleme kuvveti üzerindeki etkisi incelenmiştir. Nümerik sonuçlara göre kanatların art izi bölgesindeki akışı olumlu yönde etkilediği ancak sürükleme kuvveti katsayısının kısmen yükseldiği tespit edilmiştir. The present study investigates the effect of free-surface on the flow structure around a model of an underwater vehicle called torpedo-like geometry by using experimental and numerical methods. The numerical study utilizes the pressure-based solver of ANSYS-Fluent software while the experimental study employs the dye flow visualization and particle image velocimetry (PIV) methods. The Large Eddy Simulation (LES) is performed to reveal the unsteadiness of the flow structure around the torpedo-like geometries. The interface-capturing volume of the fluid fraction (VOF) method is used to model the free-surface. The results show that the flow characteristics such as velocity field, streamline topology, vorticity, and kinetic energy becomes asymmetric as the torpedo-like geometry is located close to free-surface. Therefore, the resulted drag force, FD for h/D = 0.75 was found to increase relative to uniform flow condition. It was also found that the free-surface effect becomes negligible for submergence depth ratios higher than 2.5 owing to the near-symmetrical form of the flow structures. Also, when the Reynolds number is increased from Re = 20x103 to 40x103, time-averaged vorticity formation length and drag coefficient, CD decrease for all examined torpedo-like geometries. As a passive control method, modifications were applied by adding 3 and 4 wings to the stern section of the torpedolike geometry to examine in detail the resulted flow structure and drag force. It is observed from numerical results that the stern with wings favorably organizes the wake region while increasing the drag coefficient, CD. Bu Çalışma Ç.Ü. Bilimsel Araştırma Projeleri Birimi Tarafından Desteklenmiştir. Proje no: FYL-2019-11552.

The purpose of this research is to analyse the complex phase change and the heat transfer behavior of the Ti-6Al-4 V powder particle during the Selective Laser Melting (SLM) process. In this study, the rapid melting and solidification process is presented by Computational Fluid Dynamics (CFD) approach under the framework of the volume-of-fluid (VOF) method. The interaction between the laser velocity and power to the solidification shape and defects of the metal components will be studied numerically as a guideline to improve quality and reduce costs.

Fluid flow in the real world occurs with multiple physics interacting with one another simultaneously. Accurate simulations of fluid flow will therefore require the incorporation of the various physics for realistic simulations. In this work a holistic computational fluid dynamics (CFD) model that couples the physics of multiphase flow, multicomponent flow and mass transfer physics across the multiphase interface is presented. The CFD model uses the volume-of-fluid (VOF) multiphase model as its foundation, and is applicable for microscale simulations, whilst being able to handle complex interfacial movements with mass transfer occurring simultaneously. The model development entails three main stages. Firstly the issue of spurious velocities for multiphase simulations at the micro-scale had to be addressed. A series of smoothing operations were adopted for parameters used to calculate the surface tension force, which smooths out any numerical errors across surrounding grid cells, thus minimising its impact in causing spurious velocities. Secondly, the multiphase interfacial area used to determine mass transfer requires accurate calculation, as opposed to using the standard |∇α| function which has its limitations. An algorithm was developed which approximates the interfacial area as a plane cutting the orthogonal mesh cell, with the orientation and position of the plane cut determined from the interface normal 𝐧 and the volume fraction α. Thirdly, the multicomponent physics had to be coupled to the VOF multiphase model with the incorporation of an appropriate mass transfer model. The species transport equation was employed for multicomponent tracking, and the α-factor and expulsion operations were added to ensure the species are tracked within the correct phase. Two mass transfer models found in literature were also adopted with some modifications to the original models to improve their performance. In each of the three stages of the CFD model development, test cases were conducted and their results compared to theoretical results and literature experimental data. Comparison of the results showed agreement which attests to the functionality and accuracy for each stage of the model developed. With the full coupled multiphase-multicomponent model developed, a numerical investigation was conducted into the formation and dissolution of CO2 bubbles within silicone oil in a cross junction microchannel, which obtained numerous insights into the multiphase dissolution physics occurring at the micro scale.

There is a desire to assess extreme wave loads on offshore structures like Floating Production, Storage and Offloading (FPSO) vessels, either for design, or for evaluation when circumstances near the structure change. Design formulae for extreme wave loads are scarce and have limited validity for specific structures. Simplified theory, such as linear potential theory, which is often used for motion analysis of offshore structures, does not represent the hydrodynamics involved in a wave impact with sufficient accuracy. For this reason, extreme wave loads are often assessed in physical experiments at model scale. Extreme wave loads may also be simulated by means of detailed numerical modeling. ComFLOW is the name for a numerical method specifically developed for simulating wave impact events. Simulation of wave impacts on offshore structure with ComFLOW is the subject of this thesis. Simulations should represent wave interaction with the structure as if it were out at sea. For reasons of efficiency, the computational domain can not be much larger than the offshore structure it contains. Wave simulation in computational domains of limited size requires special measures to reduce spurious reflection of waves at the boundaries of the domain. The main objective of this thesis is to find or develop means to efficiently reduce spurious reflection from the boundaries in ComFLOW. ComFLOW is based on the Navier-Stokes equations. For the derivation of the numerical method in ComFLOW, a finite volume discretization for Cartesian grids has been adopted. The discretization yields a skew-symmetric operator for the convective term and a symmetric operator for the viscous term in the momentum equation, resembling the symmetry properties of the analytical operators. Forward Euler time discretization is applied, in which the pressure term is evaluated implicitly. Substituting the discrete momentum equation in the discrete continuity equation gives a Poisson equation for the pressure which is solved iteratively by means of Successive Over-Relaxation (SOR) with an optimal choice for the relaxation coefficient in case the absorbing boundary condition derived in this thesis is not applied in the simulation. When the absorbing boundary condition is required, the Poisson equation (with additional terms resulting from the discretization of the absorbing boundary condition that do not fit the typical Poisson-stencil) is solved with a general sparse-matrix solver, in which the stabilized Bi-Conjugate Gradient (BiCGSTAB) iteration method is combined with an Incomplete Lower-Upper preconditioner (ILU(")). The free surface in ComFLOW is advected by means of an improved Volume of Fluid (iVOF) algorithm. The improvement consists of a local height function that aggregates fluid in a stencil of three cells in all spatial directions when fluxing fluid. The local height function reduces mass loss and so-called flotsam and jetsam, a numerical artifact consisting of disconnected droplets of fluid approximately one grid cell in size. In coarse-grid simulations, a specific amount of grid-size-dependent numerical viscosity is applied to obtain a velocity field without spatial instabilities, effectively resulting in a first-order upwind discretization of the convective term in the momentum equation. Upwind discretization induces spurious wave energy dissipation, which is undesirable because it reduces the forces involved in a wave impact on the structure. As a result, the grid size needs to be chosen sufficiently fine to make sure that the structure in the simulation endures the full impact for the wave condition that was specified. A better discretization than first-order upwind for the convective term can reduce spurious wave-energy dissipation. With a better discretization the grid size can be chosen less fine, resulting in more efficient simulations. In this thesis, a Lax-Wendroff discretization with a min-max flux limiter was evaluated for the convective term in the momentum equation in simulations with standing waves. Standing wave simulations are an efficient means to investigate the effect of different discretizations on the reduction of wave energy dissipation, because all the physics of wave motion are included without disturbances from incoming wave or absorbing boundary procedures. The reduction of wave-energy dissipation with Lax- Wendroff discretization was limited compared to first-order upwind discretization of the convective term in the momentum equation. It was hypothesized and found that also the free surface displacement algorithm was a source for wave-energy dissipation, because VOF with piecewise-constant interface reconstruction (Simple Line Interface Construction, SLIC) includes a first-order upwind discretization of the fluid flux. The discretization of the fluid flux can be improved when Piecewise-Linear Interface Construction (PLIC) is adopted. The reduction of wave-energy dissipation of PLIC compared to SLIC was limited as well. A considerable decrease of the wave-energy dissipation was only obtained by combining PLIC in the free surface displacement algorithm with Lax-Wendroff discretization of the convective term in the momentum equation in propagating wave simulations. In these simulations, the combination of PLIC and Lax-Wendroff gives 2% waveenergy dissipation over two wave lengths, whereas the original combination of SLIC and first-order upwind in the convective term gives 18% wave-energy dissipation. In ComFLOW, waves are commonly generated by specifying velocities as a Dirichlet boundary condition at the inflow. The velocities originate from wave theory. Wellknown wave theories for steep waves are Stokes 5th order theory and stream function theory (Rienecker-Fenton), for which analytical solutions are available. In this thesis, Rienecker-Fenton solutions are used to generate regular waves at the boundary. It was investigated how well ComFLOW represents these analytical solutions by means of a grid convergence study. We considered both the free surface elevation and the vertical profile of the horizontal velocity two wave lengths away from the inflow boundary. The free surface in wave crests was approximated reasonably well by ComFLOW for the finest grid that was considered; in wave troughs, however, there remained a considerable difference between ComFLOW and the analytical solution. The reason for the differences during wave troughs is not well understood and further research is required. Also for the velocity profile during a wave crest there were differences: not at the free surface, where we would have expected them because of the large gradients in the velocity, but the differences were mainly near the bottom. This is also part of continued research. Steep irregular waves were generated in ComFLOW by using velocities from linear wave theory with multiple frequency components, and by using velocities obtained from solutions of a Finite-Difference Finite-Element Method (termed FDFEM in this work), which is a non-linear potential flow solver. The two generation methods were compared as follows. The solution for the surface elevation from the FDFEM method at a certain location was used as a reference solution. Velocity output from the FDFEM solution was taken two wave lengths’ distance before the reference location and imposed onto the ComFLOW domain. The ComFLOW solution at the reference location was then compared to the reference solution. It was found that the ComFLOW solution for the free surface elevation agrees well with the FDFEM solution. A similar procedure was applied for wave generation with velocities obtained from linear theory, but now the surface elevation from the FDFEM solution some distance before the reference location was used. This surface elevation was decomposed into its Fourier-components. For each component, linear potential theory gives the velocities and the combination of all these velocities provides the total velocity signal that is imposed onto the ComFLOW domain as a boundary condition. Again, the ComFLOW solution for the free surface at the reference location was compared to the FDFEM solution. Now, it was found that there are differences between the two solutions that do not decrease with increasing grid resolution. It is therefore concluded that linear theory should not be used for irregular wave generation when the ComFLOW solution is to be compared with experimental results. Waves reflect from computational domain boundaries when no special measures are taken to prevent reflection. Dissipation zones are often used near domain boundaries to induce a rapid decrease of wave energy so that when waves reach the boundary and reflection occurs, the wave height has reduced to such an extent that spurious reflection does not interfere with the processes near the structure. Dissipation zones need to be several wave lengths long to be effective. In many cases, the part of the computational domain taken up by dissipation zones exceeds the size of the domain in which the wave interaction with the structure actually occurs. Dissipation zones take up a considerable amount of the total computational effort required for the simulation. Local absorbing boundary conditions are a more efficient alternative to dissipation zones. In this thesis, a local absorbing boundary condition for long-crested irregular waves is derived that is more efficient and more effective than a two-wave-lengths-long dissipation zone. The absorbing boundary condition in this study includes an approximation of the dispersion relation which is accurate within a range of wave numbers (or, equivalently, frequencies). It also includes second-order vertical derivatives of the solution variables along the boundary. The differential equation for the absorbing boundary condition is discretized implicitly, combined with the discrete momentum equation and included in the Poisson equation for the pressure. Theoretically, the reflection coefficient, i.e. the ratio of outgoing wave amplitude and reflected wave amplitude, for this absorbing boundary condition can be as low as 2% for wave components within the range 0 < kh Experiments with a schematized model of a semi-submersible in waves were performed at MARIN. Waves were generated with a pivoting wave maker at one end of the basin. At the other end, a parabolic beach was installed to reduce spurious wave reflection. The model, which was approximately 1m long, was placed in the middle of the basin and was kept restrained during the experiment. Pressure sensors and wave gauges were placed at several locations around the columns of the model. Also the deck of the model was fitted with pressure sensors. During the experiments, there was violent wave interaction with the model: wave impacts occurred on both columns and many also reached the deck. In ComFLOW, a similar setup was created. The computational domain extended over the full width of the basin. The side walls of the domain (x-z planes) were configured to represent the side walls of the basin. The incoming-wave boundary (x-y plane) was positioned approximately one typical wave length in front of the structure. Here, waves were generated by specifying the kinematics obtained from an FDFEM simulation that contained the entire basin, including wave maker and spending beach. At the outflow end of the computational domain in ComFLOW, the absorbing boundary condition derived in this study was applied. For stability reasons, the FDFEM simulation was performed with a relatively coarse grid and with a smaller water depth than the basin. For this reason, the waves in this simulation could only compare reasonably well to the waves in the experiment. And there was another issue. The position of the model relative to the wave board was not recorded with sufficient precision. At multiple output locations, the free surface elevation in de FDFEM simulation was compared to the free surface in an undisturbed test (without structure). The position where the simulation results best resemble the measurements was selected as most likely location for the model in the simulation. Keeping in mind that the waves in the simulation were slightly different from the waves in the experiment - not only as a result of model errors, but also because the true water depth in the experiment could not be represented in the FDFEM simulation the comparison between ComFLOWand the experiment turns out rather well. Pressures and surface elevations in ComFLOW show good agreement with the measurements. The 3D validation of ComFLOW was also an important test for the absorbing boundary condition that was developed in this thesis. The simulations show that the boundary condition can be applied in practical simulations with good results. These simulations could not be performed with a dissipation zone on a desktop computer, because with a dissipation zone it did not fit in memory. It would have been interesting to compare results between a 3D simulation with an absorbing boundary condition and one with a dissipation zone. On the other hand, we can conclude that the absorbing boundary condition enables us to perform simulations that could not be run on a desktop PC before the boundary condition was there.

There is a desire to assess extreme wave loads on offshore structures like Floating Production, Storage and Offloading (FPSO) vessels, either for design, or for evaluation when circumstances near the structure change. Design formulae for extreme wave loads are scarce and have limited validity for specific structures. Simplified theory, such as linear potential theory, which is often used for motion analysis of offshore structures, does not represent the hydrodynamics involved in a wave impact with sufficient accuracy. For this reason, extreme wave loads are often assessed in physical experiments at model scale. Extreme wave loads may also be simulated by means of detailed numerical modeling. ComFLOW is the name for a numerical method specifically developed for simulating wave impact events. Simulation of wave impacts on offshore structure with ComFLOW is the subject of this thesis. Simulations should represent wave interaction with the structure as if it were out at sea. For reasons of efficiency, the computational domain can not be much larger than the offshore structure it contains. Wave simulation in computational domains of limited size requires special measures to reduce spurious reflection of waves at the boundaries of the domain. The main objective of this thesis is to find or develop means to efficiently reduce spurious reflection from the boundaries in ComFLOW. ComFLOW is based on the Navier-Stokes equations. For the derivation of the numerical method in ComFLOW, a finite volume discretization for Cartesian grids has been adopted. The discretization yields a skew-symmetric operator for the convective term and a symmetric operator for the viscous term in the momentum equation, resembling the symmetry properties of the analytical operators. Forward Euler time discretization is applied, in which the pressure term is evaluated implicitly. Substituting the discrete momentum equation in the discrete continuity equation gives a Poisson equation for the pressure which is solved iteratively by means of Successive Over-Relaxation (SOR) with an optimal choice for the relaxation coefficient in case the absorbing boundary condition derived in this thesis is not applied in the simulation. When the absorbing boundary condition is required, the Poisson equation (with additional terms resulting from the discretization of the absorbing boundary condition that do not fit the typical Poisson-stencil) is solved with a general sparse-matrix solver, in which the stabilized Bi-Conjugate Gradient (BiCGSTAB) iteration method is combined with an Incomplete Lower-Upper preconditioner (ILU(")). The free surface in ComFLOW is advected by means of an improved Volume of Fluid (iVOF) algorithm. The improvement consists of a local height function that aggregates fluid in a stencil of three cells in all spatial directions when fluxing fluid. The local height function reduces mass loss and so-called flotsam and jetsam, a numerical artifact consisting of disconnected droplets of fluid approximately one grid cell in size. In coarse-grid simulations, a specific amount of grid-size-dependent numerical viscosity is applied to obtain a velocity field without spatial instabilities, effectively resulting in a first-order upwind discretization of the convective term in the momentum equation. Upwind discretization induces spurious wave energy dissipation, which is undesirable because it reduces the forces involved in a wave impact on the structure. As a result, the grid size needs to be chosen sufficiently fine to make sure that the structure in the simulation endures the full impact for the wave condition that was specified. A better discretization than first-order upwind for the convective term can reduce spurious wave-energy dissipation. With a better discretization the grid size can be chosen less fine, resulting in more efficient simulations. In this thesis, a Lax-Wendroff discretization with a min-max flux limiter was evaluated for the convective term in the momentum equation in simulations with standing waves. Standing wave simulations are an efficient means to investigate the effect of different discretizations on the reduction of wave energy dissipation, because all the physics of wave motion are included without disturbances from incoming wave or absorbing boundary procedures. The reduction of wave-energy dissipation with Lax- Wendroff discretization was limited compared to first-order upwind discretization of the convective term in the momentum equation. It was hypothesized and found that also the free surface displacement algorithm was a source for wave-energy dissipation, because VOF with piecewise-constant interface reconstruction (Simple Line Interface Construction, SLIC) includes a first-order upwind discretization of the fluid flux. The discretization of the fluid flux can be improved when Piecewise-Linear Interface Construction (PLIC) is adopted. The reduction of wave-energy dissipation of PLIC compared to SLIC was limited as well. A considerable decrease of the wave-energy dissipation was only obtained by combining PLIC in the free surface displacement algorithm with Lax-Wendroff discretization of the convective term in the momentum equation in propagating wave simulations. In these simulations, the combination of PLIC and Lax-Wendroff gives 2% waveenergy dissipation over two wave lengths, whereas the original combination of SLIC and first-order upwind in the convective term gives 18% wave-energy dissipation. In ComFLOW, waves are commonly generated by specifying velocities as a Dirichlet boundary condition at the inflow. The velocities originate from wave theory. Wellknown wave theories for steep waves are Stokes 5th order theory and stream function theory (Rienecker-Fenton), for which analytical solutions are available. In this thesis, Rienecker-Fenton solutions are used to generate regular waves at the boundary. It was investigated how well ComFLOW represents these analytical solutions by means of a grid convergence study. We considered both the free surface elevation and the vertical profile of the horizontal velocity two wave lengths away from the inflow boundary. The free surface in wave crests was approximated reasonably well by ComFLOW for the finest grid that was considered; in wave troughs, however, there remained a considerable difference between ComFLOW and the analytical solution. The reason for the differences during wave troughs is not well understood and further research is required. Also for the velocity profile during a wave crest there were differences: not at the free surface, where we would have expected them because of the large gradients in the velocity, but the differences were mainly near the bottom. This is also part of continued research. Steep irregular waves were generated in ComFLOW by using velocities from linear wave theory with multiple frequency components, and by using velocities obtained from solutions of a Finite-Difference Finite-Element Method (termed FDFEM in this work), which is a non-linear potential flow solver. The two generation methods were compared as follows. The solution for the surface elevation from the FDFEM method at a certain location was used as a reference solution. Velocity output from the FDFEM solution was taken two wave lengths’ distance before the reference location and imposed onto the ComFLOW domain. The ComFLOW solution at the reference location was then compared to the reference solution. It was found that the ComFLOW solution for the free surface elevation agrees well with the FDFEM solution. A similar procedure was applied for wave generation with velocities obtained from linear theory, but now the surface elevation from the FDFEM solution some distance before the reference location was used. This surface elevation was decomposed into its Fourier-components. For each component, linear potential theory gives the velocities and the combination of all these velocities provides the total velocity signal that is imposed onto the ComFLOW domain as a boundary condition. Again, the ComFLOW solution for the free surface at the reference location was compared to the FDFEM solution. Now, it was found that there are differences between the two solutions that do not decrease with increasing grid resolution. It is therefore concluded that linear theory should not be used for irregular wave generation when the ComFLOW solution is to be compared with experimental results. Waves reflect from computational domain boundaries when no special measures are taken to prevent reflection. Dissipation zones are often used near domain boundaries to induce a rapid decrease of wave energy so that when waves reach the boundary and reflection occurs, the wave height has reduced to such an extent that spurious reflection does not interfere with the processes near the structure. Dissipation zones need to be several wave lengths long to be effective. In many cases, the part of the computational domain taken up by dissipation zones exceeds the size of the domain in which the wave interaction with the structure actually occurs. Dissipation zones take up a considerable amount of the total computational effort required for the simulation. Local absorbing boundary conditions are a more efficient alternative to dissipation zones. In this thesis, a local absorbing boundary condition for long-crested irregular waves is derived that is more efficient and more effective than a two-wave-lengths-long dissipation zone. The absorbing boundary condition in this study includes an approximation of the dispersion relation which is accurate within a range of wave numbers (or, equivalently, frequencies). It also includes second-order vertical derivatives of the solution variables along the boundary. The differential equation for the absorbing boundary condition is discretized implicitly, combined with the discrete momentum equation and included in the Poisson equation for the pressure. Theoretically, the reflection coefficient, i.e. the ratio of outgoing wave amplitude and reflected wave amplitude, for this absorbing boundary condition can be as low as 2% for wave components within the range 0 < kh Experiments with a schematized model of a semi-submersible in waves were performed at MARIN. Waves were generated with a pivoting wave maker at one end of the basin. At the other end, a parabolic beach was installed to reduce spurious wave reflection. The model, which was approximately 1m long, was placed in the middle of the basin and was kept restrained during the experiment. Pressure sensors and wave gauges were placed at several locations around the columns of the model. Also the deck of the model was fitted with pressure sensors. During the experiments, there was violent wave interaction with the model: wave impacts occurred on both columns and many also reached the deck. In ComFLOW, a similar setup was created. The computational domain extended over the full width of the basin. The side walls of the domain (x-z planes) were configured to represent the side walls of the basin. The incoming-wave boundary (x-y plane) was positioned approximately one typical wave length in front of the structure. Here, waves were generated by specifying the kinematics obtained from an FDFEM simulation that contained the entire basin, including wave maker and spending beach. At the outflow end of the computational domain in ComFLOW, the absorbing boundary condition derived in this study was applied. For stability reasons, the FDFEM simulation was performed with a relatively coarse grid and with a smaller water depth than the basin. For this reason, the waves in this simulation could only compare reasonably well to the waves in the experiment. And there was another issue. The position of the model relative to the wave board was not recorded with sufficient precision. At multiple output locations, the free surface elevation in de FDFEM simulation was compared to the free surface in an undisturbed test (without structure). The position where the simulation results best resemble the measurements was selected as most likely location for the model in the simulation. Keeping in mind that the waves in the simulation were slightly different from the waves in the experiment - not only as a result of model errors, but also because the true water depth in the experiment could not be represented in the FDFEM simulation the comparison between ComFLOWand the experiment turns out rather well. Pressures and surface elevations in ComFLOW show good agreement with the measurements. The 3D validation of ComFLOW was also an important test for the absorbing boundary condition that was developed in this thesis. The simulations show that the boundary condition can be applied in practical simulations with good results. These simulations could not be performed with a dissipation zone on a desktop computer, because with a dissipation zone it did not fit in memory. It would have been interesting to compare results between a 3D simulation with an absorbing boundary condition and one with a dissipation zone. On the other hand, we can conclude that the absorbing boundary condition enables us to perform simulations that could not be run on a desktop PC before the boundary condition was there.

The paper describes a coupling of the geometric Volume-of-Fluid (VOF) method with a sharp-interface phase-change model for Cartesian and axisymmetric grids. Both the interfacial position and the temperature field are resolved with subgrid accuracy. Species transport with implicit diffusion is considered in the gas phase. The numerical implementation of the method developed here is described in detail, as well as its coupling with the incompressible Navier-Stokes solver PSI-BOIL, which features a hybrid, finite-volume/finite-difference approach based on a fixed, rectangular grid. Several verification cases have been undertaken to ensure correct implementation of the method in the code and to evaluate its performance. These include simulations of the Stefan problem, sucking problem and bubble growth in superheated liquid. In all cases, very good agreement with the analytical solution has been reached and grid convergence has been demonstrated. Simulations of evaporating and condensing rising bubbles, for which high-quality measured data are available, are also presented to serve as validation tests. Reasonable agreement of simulation results with experimental data has been recorded and applicability of the method to problems without inherent symmetry and featuring turbulence is shown. This work represents the first successful application of a geometric VOF method coupled with a sharp-interface phase-change model and species transport to non-trivial problems. The achieved performance of our algorithm in the verification and validation exercise represents an important step in the development of multiphase codes capable of accurately resolving complex three-dimensional multiphase flows. (C) 2021 The Author(s). Published by Elsevier Ltd.