Endovascular aneurysm repair (EVAR) is widely used for the treatment of abdominal aortic aneurysms, but thrombotic complications such as iliac limb occlusion and intraluminal thrombus formation remain clinically relevant. Previous computational studies have shown that post-EVAR hemodynamics are influenced by aorto-iliac geometry, yet the isolated relation between iliac coronal angulation and prothrombotic near-wall flow indicators remains insufficiently characterised. This thesis addresses this gap using a parametrically controlled idealised aorto-iliac computational fluid dynamics (CFD) model, in which the iliac coronal angle was varied from 15° to 75° while all other parameters were kept constant. Pulsatile, non-Newtonian simulations were performed in ANSYS Fluent, and the near-wall environment was assessed using time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), and endothelial cell activation potential (ECAP). The idealised sweep was complemented by statistical shape model (SSM)-derived geometries, representing native, non-stented anatomy, to test whether the same tendencies appeared in more realistic shapes.

When the angle was varied in isolation, the adverse hemodynamic area did not grow with angle: the low-TAWSS area decreased from 61.5% to 58.0% of the analysed wall between 15° and 75°, the elevated-RRT area decreased similarly, and the high-OSI and high-ECAP areas increased from a negligible base. In the SSM-derived geometries, by contrast, the low-TAWSS area increased with the measured coronal bifurcation angle, from 86.4% to 94.0%, and showed the strongest fitted trend of any metric (R² = 0.95), while OSI, RRT, and ECAP showed no consistent trend. That the low-TAWSS area moves in opposite directions when angle is varied alone versus together with realistic anatomy is the central finding: the adverse shear response attributed to angulation arises from the combined geometry that accompanies it, not from the angle by itself.

For clinical interpretation, this suggests that iliac angulation is best interpreted not as a stand-alone marker of thrombus-prone conditions, but as one geometric factor to be assessed together with the surrounding anatomy and device. Extending the analysis towards an explicit EVAR stent-graft within realistic geometries is needed before such metrics could support patient-specific risk assessment.
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In many high-tech applications, efficient thermal dissipation is necessary to maintain components within strict operational temperature limits, ensuring system reliability and performance stability. This is achieved mainly through thermal conduction, which is the dominant mechanism between contacting surfaces. Thermal contact conduction is a function of many variables, such as contact area, material properties, and surface roughness characteristics. Various studies have focused on how these variables affect the heat flow between two contacting geometries; however, the inherent variability of the surface roughness induces a significant amount of uncertainty in the measurements. This paper describes the theory and methodology followed to develop a thermo-mechanical framework that incorporates the inherent stochasticity of the surface roughness of metallic surfaces, through a probabilistic surface representation, to efficiently quantify the heat flow conductance through the calculation of the Thermal Contact Conductance (TCC). The results indicate that the proposed methodology predicts the range of thermal contact conductance with good accuracy when compared to experimental data. Lastly, the 2D implementation showed substantially higher computational efficiency compared to the 3D model while retaining comparable predictive capability. ... Read more

Fundamental research on fluid dynamics allows us to progress in our technological endeavours, ensuring safety in our daily lives. Understanding the underlying mechanics of turbulence is a challenge due to its unsteady, chaotic, three-dimensional structure. Mathematically, this is portrayed in the continuity and Navier-Stokes equations.
In incompressible flows, the velocity field must satisfy a global, instantaneous divergence-free constraint. The fractional-step method imposes this by solving a Poisson equation for the pressure field. The present work aims at formulating, implementing and evaluating an eigendecomposition-based framework for solving the discretized pressure Poisson equation in direct numerical simulation for non-uniform meshes.
The eigendecomposition-based approach was implemented into CaNS, a CPU- and GPU-based code which solves the discretized pressure Poisson equation in a massively-parallel fashion through an FFT-based approach on a finite-difference scheme. It was validated against lid-driven cavity and turbulent square duct flows, showing good agreement with the data of Gavrilakis.
The CPU-based computational performance of the framework was assessed through strong and weak scaling tests, and compared against results of SNaC, an implementation of an iterative solver for the pressure Poisson equation through geometric multigrid methods. It is shown that performance becomes communication-bound at 4096 cores, with wall-clock times higher than the FFT-based approach and lower than the multigrid approach. This bound has been hypothesized to be a consequence of the small computational domain tested (1024³) and of possible memory fragmentation. In every test performed, regardless of scale, the eigendecomposition-based implementation is shown to have lower wall-clock times than the geometric multigrid implementation, despite its higher operation count.
Finally, the environmental and financial cost of developing the eigendecomposition-based approach was assessed. 525 thousand core hours were spent, representing ≈ 3000 kWh of energy, close to 1 ton of CO2 emitted, ≈ 400€ spent and over 160 hours of elapsed compute time. Deploying this approach in a large-scale environment for solving a turbulent square duct would require 300 to 400 sand core hours.
All in all, the eigendecomposition-based approach here developed and implemented is deemed suitable for performing large-scale simulations on a many-CPU framework, with lower wall-clock times than comparable geometric multigrid methods. Unfortunately, in cases where a uniform mesh can discretize the problem, the FFT-based approach still demonstrates comparatively lower wall-clock times, meaning it should be the preferred approach where suitable. The present framework will be made available in a separate release of CaNS at github.com/CaNS-World.

https://github.com/CaNS-World ... Read more

Accurate simulation of non-linear free-surface wave interaction with structures as if in the open sea requires truncation of domains for computational feasibility. In absence of special boundary measures, truncation introduces artificial boundaries with spurious wave reflection back into the computational domain. The common way to prevent reflection is by means of dissipation zones, which are often larger than the domain of interest in order to achieve sufficient accuracy. Local absorbing boundary conditions pose a potentially more efficient alternative, but are limited in accuracy due to corners on Cartesian domains (advantageous for future implementations of Volume-of-Fluid methods). This work proposes a methodology that preserves a Cartesian structure of the interior grid, while mapping to a circular domain boundary through the use of curvilinear coordinates. On the circular boundary, higher-order absorbing conditions can be applied without corner limitations. The linearized shallow water equations are discretised using a staggered finite volume discretisation. Two second-order Engquist-Majda boundary conditions, extended with generating wave capabilities, are expressed in the coordinate system, implemented and benchmarked using a first-order Sommerfeld boundary condition, using a centred and off-centre source configuration. Results demonstrate that the methodology allows consistent implementation of higher-order absorbing boundary conditions with low spurious reflection. ... Read more

The Reducing Electric Furnace (REF) is a promising technology for the transition to hydrogen-based steelmaking, in which Direct Reduced Iron (DRI) is melted in a slag–metal bath, but feed material entry and feed pile formation remain poorly understood. This work develops a DEM-only three-phase framework for DRI–slag–metal interactions that captures drag, convective heat transfer, melting and cohesion, and proposes two alternative buoyancy formulations: an Archimedes model for a ‘wet’ pile with slag penetration and a hydrostatic model for a ‘dry’ pile without penetration. Results show that whether the pile floats or sinks in the slag is governed primarily by the assumed degree of slag penetration into the pile. In all cases, interaction with the metal phase is limited. Slag properties such as temperature, thermal conductivity, density and layer thickness dominate pile size, shape and melting, while slag viscosity and particle thermal properties play a secondary role. This work provides a foundational DEM-only model and first insights into feed pile behaviour in Electric Smelting Furnaces. ... Read more

Aligned with the European Union’s ambition to achieve net zero greenhouse gas emissions by 2050, a plethora of carbon dioxide abatement technologies have been developed and are being deployed in industries. Among these, Carbon Capture, Utilisation and Storage (CCUS) technologies play a pivotal role in decarbonising hard-to-abate sectors where carbon dioxide emissions are inevitable. Geological carbon storage is an effective option for long-term carbon dioxide sequestration, especially in regions with substantial sub-surface capacity for large scale permanent storage.

In this technology, transporting carbon dioxide in supercritical conditions enhances the transport efficiency due to its high density and low viscosity. However, the rapid depressurization of supercritical carbon dioxide across control valves prior to injection into geological storage reservoirs can trigger complex phase transition phenomena, including flashing, cavitation and shock wave formation. This study investigates the expansion dynamics of supercritical carbon dioxide using converging-diverging nozzles, as a simplified yet representative geometry for control valves, to develop modelling strategies for the depressurization process.

Building on earlier work, the simulation framework was upgraded from modelling dense liquid phase to supercritical phase which is characterised by the sharp thermodynamic gradients in the vicinity of the critical point. This study employs the Span-Wagner Equation of State to capture the real gas behaviour of carbon dioxide. The implementation in Ansys Fluent was carried out via a non-uniform lookup table, refined near the critical region and smoothed using the Savitzky-Golay filter to ensure numerical robustness of the solver while preserving the thermodynamic fidelity. Simulations were performed using the Mixture multiphase model in Ansys Fluent, incorporating interphase slip velocity to represent mechanical non-equilibrium between the liquid and vapour phases. The phase transition was modelled using the
Zwart-Gerber-Belamri (ZGB) cavitation framework tuned to simulate the non-equilibrium phase transition behaviour. Both quasi-one-dimensional and two-dimensional simulations were performed and the results were validated against experimental data from converging-diverging nozzle studies.

The results highlight the significance of metastable effects, sensitivity to cavitation model coefficients and the necessity of incorporating slip velocity between phases to accurately capture the phase transition behaviour. While the quasi-1D simulations provide qualitative trends, 2D simulations resulted in better agreement with the experiments. Overall, this study establishes a scalable and robust simulation framework for modelling the depressurization behaviour of supercritical carbon dioxide, which can be extended to complex geometries such as control valves used in CCS infrastructure. ... Read more

Shipping is one of the most cost-effective and environmentally sustainable modes of transportation. Given that approximately 60% of a typical ship’s propulsive power is used to overcome frictional drag, implementing practices to reduce this resistance stands to yield substantial economic and environmental benefits, (Larsson & Raven, 2010). A promising drag reduction technique for a ship is air lubrication. Damen Shipyards Group is currently making this technology commercially available as the Damen Air Cavity System (DACS). The system reduces the frictional resistance of a ship by creating stable air cavities on the bottom hull of a ship. The air cavities cause a reduction in friction drag by decreasing the wetted area of a ship’s bottom. During the development of the DACS system, it was observed that air cavities also change the inflow into the propeller. Both the changed inflow and the frictional drag reduction affect the propulsive efficiency and required propulsive power of the vessel. This thesis aims to provide a better understanding of how the propulsive performance of a ship is affected by air cavities. Additionally, a key application for air lubrication systems is on inland waterway vessels, which frequently operate in shallow waters. However, the impact of shallow water conditions on the performance of the air cavity system is currently unknown. The research goals of this thesis are investigated with the help of computational fluid dynamics (CFD). A literature review identified the most feasible method to model a ship with air cavities i.e. representing the cavities as surfaces with a slip boundary condition. The influence of the air cavities on propulsive performance is investigated by studying the change in nominal wake field, thrust deduction, and propeller efficiency. Three ships were investigated: a cargo ship, a cruise ship, and an inland ship. The CFD results of the cargo ship were compared to sea trial data. Model test data was available for comparison of the cruise ship results. First, a grid convergence study and a sensitivity analysis were performed to investigate the accuracy of the numerical simulations and the power predictions. The biggest source of numerical uncertainty arose from the pressure drag. From the sensitivity analysis, it was found that the uncertainty of the power prediction is mainly affected by the uncertainty of the thrust prediction, followed by the uncertain propeller geometry for the cargo ship. For the cargo ship, it was found that the air cavities cause a significant frictional drag reduction. It was also found that the air cavities caused a decrease in pressure drag because they decreased flow separation at the stern. Furthermore, a strong decrease of the nominal wake fraction was observed for this ship, because the air cavities change the boundary layer on the bottom of the ship. A change in propeller efficiency was also observed because the propeller working point changes. The magnitude of the change was larger than for the other ships because this ship has a controllable pitch propeller running at a fixed rpm. When comparing sea trial data to the CFD results, it was found that CFD underpredicts the power, especially at higher speeds. Next to this, CFD predicted a larger reduction in power than measured during the trials. The prediction of the cavity length was identified as the most likely cause for the difference. A good comparison between model tests and CFD results was found for the cruise ship. It was found that the drag reduction and change in propulsive performance could be predicted reasonably accurately by modeling the air cavities as surfaces with a slip boundary condition. Furthermore, it was observed that the air cavities caused little change in propulsive efficiency on this ship. This is because the propellers of the cruise ship are located further away from the boundary layer of the ship and the wake field is therefore only slightly affected by the air cavities. On the inland ship, it was observed that the pressure drag and flow separation at the stern were influenced due to the air cavities. However, no comprehensive conclusions could be made due to scatter in the data. This is most likely due to the uncertainty present when modeling flow separation. Furthermore also for this ship, a decrease in propulsive efficiency was found because the air cavities decreased the wake fraction of the ship. Additionally, CFD simulations were conducted for the inland ship at varying water depths to assess how shallow water conditions impact the performance of the air cavity system. Since the ship’s frictional drag increased in shallow water, the total drag reduction from the air cavities also increased. Next to this, a change between pressure and flow separation when comparing air on and air off was observed. Also here, no strong conclusion could be made due to scatter in the data. Lastly, it was found that the change in wake field caused by the air cavities is larger in shallow water than in deep water. ii Based on the results of the three ships studied it can be concluded that an air cavity system affects the propulsive performance of a ship. It was found that an air cavity system reduces the wake fraction of a ship because it limits the growth of the boundary layer on the bottom of a ship. The reduction increases for ships with a high block coefficient and a large air-covered area. For a twin screw ship, the change of the wake field is less significant. The change in propeller efficiency depends on the resistance reduction, possible change of the wake field, and the original working point of the propeller. Furthermore, it was found that the thrust deduction effect is not affected by the air cavity system. It can also be concluded from the results of the cruise ship that the flow around a ship with air cavities can modeled reasonably accurately provided that the shape of the air layer under the ship is known. More research is recommended on how air cavities change the flow separation and pressure drag of a ship. ... Read more

Supercritical CO₂ (SCO₂) is a promising alternative to traditional working fluids in heat pumps and power cycles due to its high density, thermal efficiency, and stability. These properties allow for the design of more compact and efficient equipment. However, accurately modeling supercritical heat transfer, especially near its pseudocritical point, is challenging, because of extreme variations in its thermophysical properties. As a result of these inaccuracies in modeling, large fault margins must be taken into account, leading to overdesign of equipment.

Current methods for estimating heat transfer in such systems include empirical measurements, computational fluid dynamics (CFD), and Nusselt correlations. Although measurements provide accurate data, it is not a scalable solution. CFD methods offer this scalability. However, CFD cannot be applied in complex scenarios such as modeling of SCO₂ due to the trade-off between computational cost and accuracy. Nusselt correlations have low computational demand and are a scalable solution but come with low accuracy. Therefore, Nusselt correlations are not suitable for the design of equipment.

To overcome these limitations, this research applies a new model to predict heat transfer to SCO₂ flowing upward in a heated vertical tube. Current prediction methods use artificial neural networks (ANN). This research applies a convolutional neural network (CNN) for its ability to capture spatial context from the heat transfer trajectory. This property enables CNN to capture global and local patterns important for accurate prediction in SCO₂ heat transfer. In addition, the multiple kernels per layer enables the model to extract a large range of features from the heat transfer trajectory allowing for higher accuracy. The developed CNN model has shown superior performance, achieving an R² score of 0.970 and an MSE of 1477, outperforming existing ANNs and Nusselt correlations.

Furthermore, a feature importance study is done, to identify the nondimensional parameters that are important for heat transfer prediction The feature importance study highlighted the importance of parameters such as the Reynolds number and the Froude number in improving the model predictions. In addition, the feature importance study led to the development and identification of a new nondimensional parameter, (q_wk)/(T d), as one of the most important features influencing heat transfer. Furthermore, the experiments show that normalization is vital to enhance model stability and performance, countering issues such as exploding or vanishing gradients.

The findings in this research suggest great potential for using machine learning models to design more effective and compact heat transfer equipment. Future work will focus on a feature importance study for normalized, dimensional and nondimensional parameters for improved predictions. In addition, more synthetic data should be generated in the heat transfer detoriation and heat transfer enhancement areas for better prediction of those parts. Finally, a study into the application of machine learning models for designing heat transfer equipment should be done to show the benefits of such models. ... Read more

Decarbonization is an important step to achieve the goals set by the Paris Agreement. Greenhouse gas emissions should be reduced to zero, and therefore, the reliability of fossil fuels should be reduced. This causes a shift of interest towards more renewable solutions. However, the intermittent nature
of renewable energy sources such as solar and wind energy leaves a gap in the energy supply. Currently, this gap is still filled by natural gas, but biofuels could potentially help in decarbonizing this gap. Biofuels could be prevaporized and used as an alternative fuel within existing natural gas-fired power plants. Ethanol is an interesting biofuel as it has a relatively low boiling point, meaning that it is relatively easy to evaporate. Next to that, ethanol has a similar Wobbe Index (WI) compared to natural gas, meaning that it could be potentially used with only minor adjustments to the gas turbine. This research will focus on implementing ethanol as an alternative fuel for the Killingholme power plant, a
600 MW power plant in an open cycle gas turbine (OCGT) configuration that Uniper operates in the United Kingdom.

This research focused on the effect of ethanol on the process design and combustion characteristics, where the process design was only briefly touched upon. It was found that ethanol should be heated to a temperature of 467 K to be in vapor form at the relevant gas turbine conditions. A process design was
made for the baseload operation, where the required heat for the evaporation process was provided by the flue gas flowing out of the gas turbine. Next to that, the use of ethanol requires slightly higher volume flow rates, meaning that the pipes and fittings should be adjusted to keep the desired fuel pressure. The effect of ethanol on combustion characteristics was researched by a kinetic modeling study and a CFD study focusing on fuel-air mixing, where the results will be compared to methane. A RANS study was performed for the CFD study, which showed that the use of ethanol results in a better quality of mixing. From the kinetic modeling study, it was found that ethanol has a lower autoignition
delay time than methane. This will probably not lead to the autoignition of the fuel-air mixture in the mixing section, but it could lead to periodic flashes in regions close to the recirculation zones within the burner. It was also found that ethanol has a 78% higher laminar flame speed than methane. Next to that, it was found that ethanol has an effective Lewis number of 1.56 at the relevant gas turbine conditions, whereas methane has an effective Lewis number close to unity. From the laminar flame speed and the effective Lewis number, it was concluded that the use of ethanol results in an increase in turbulent flame speed. The increased turbulent flame speed and better quality of mixing of ethanol suggest a decrease in flame length. The decrease in flame length and increase in turbulent flame speed lead to higher flashback risks, but it is expected that this will be within the flashback margin of the burner. Based on the kinetic modeling study and the fuel-air mixing study, it was concluded
that ethanol will have NOx emissions similar to methane and that the driving energy source of the combustion dynamics will probably shift to higher frequencies.

Recommendations for further research are the gas turbine’s start-up, ethanol contaminants, extended CFD study of the burner, and the blending of ethanol and natural gas. Further research could complement the already promising results from this report, and eventually, this could lead to combustion tests where ethanol will be used as fuel. ... Read more

The world is witnessing unprecedented climate changes, which have worsened over the past couple of years. Despite significant progress in renewable energy, achieving a reduction of global warming remains challenging. Renewable energy sources like solar, wind, and hydroelectric technologies offer promising solutions, but challenges persist due to their intermittency and geographical dependency. Green hydrogen, generated through electrolysis using renewable energy, has come up as a solution for storing electric energy and providing a constant energy supply.

Alkaline water electrolysis has emerged as one of the most promising electrolysis methods due to its large-scale operation, long durability and low costs. This does not come without challenges, one of them being leakage currents, which reduces the efficiency of the electrolyser. Leakage currents occur when not all current is used for hydrogen production, but some of it leaks into, for example, the produced hydrogen stream. To reduce these leakage currents, more research into the origins is needed. The first step is modelling the electrolysis while considering as much as possible of the physics happening inside the system. Current models of alkaline water electrolysers are either modelled using only mathematical equations or neglect the operating parameters, which makes the results highly variable per system. Other models do use analytical methods with experimental results, but these models only comprise one electrolysis cell rather than a full stack.

This work consists of developing two three-dimensional models, validating them using experiments, and using them to predict the effect of changes in geometry. The first model was made using COMSOL Multiphysics software and used to research the water electrolysis stack, which comprised one or eight cells using electrochemical relations and physical data. It was found that a model could be made that fitted the experiments within the error margin of the experiments (<2.5%). It lacked flexibility but overall showed good results for an electrolyser stack of one or eight cells. The second model was made using an equivalent electrical circuit (EEC) of the electrolyser in Python via the PySpice module. A steady-state model, including the leakage currents, could be developed by calculating all system resistances, namely the cell, inlet/outlet, and manifold resistances. This model overestimated the performance of the electrolyser by 10-15% for low current densities and 2-4% for high current densities. Nevertheless, it was highly adaptable for different scenarios, making it valuable for research into optimising the electrolysis stack. Both models were used to predict the effect of changes in geometry; the effect of the length of the inlets, and the number of cells. This showed that the EEC model was better suited for this research.
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Ascending thoracic aortic aneurysm is a dangerous condition which is hard to locate in patients. CFD can be used to assess someone’s risk of suffering from such an aneurysm and also to assess any possible relation of an aneurysm with relevant parameters, like the shape of the aorta or how blood flows into the aorta.

Existing research on ascending aortic CFD flow generally focuses on assessing relatively high wall shear stress, seen as the cause for aortic wall damage and aneurysms, in patients and not much on any possible relationships between aortic shape and inflow with ascending aortic aneurysm. Work that does exist often uses 4D flow MRI, shown to produce results regarding peak wall shear stresses with less accuracy compared to CFD.

This thesis aims to supply that information using an aortic geometry from a large dataset of synthetically generated aortas together with synthetically generated inlet velocity profiles, showing a possibility to work with-out the need for patient measurements. The main work in this thesis focuses on finding any possible relationship between aortic inflow angle and relatively high wall shear stress, such that the results may better explain how aortic inflow can influence the appearance of aneurysms.

For this, a workflow has been established that allows working with large aortic geometry and inflow profile datasets with relative ease. This workflow process uses a mix of OpenFoam and Ansys Fluent usage.

Results have shown that the flow jet angle in the core of the flow has a significant negative correlation with peak WSS within the ascending aorta for the chosen aortic geometry. This is in contrast with other work showing positive correlations with WSS. This may suggest aortic geometry dependence together with the need to look at other flow variables, like jet impingement angle, to explain how aortic inflow can influence high WSS within the ascending aorta.

Recommendations for future work include: The same study but with a focus on aortas of healthy young people to see if aortas at risk can be found, as existing work mainly focuses on old people with ascending thoracic aortic aneurysm, a similar study with a focus on the inclusion of various aortic geometries to find how aortic geometry may influence the effect inflow variables can have on high WSS, the creation of a tool to easily calculate the impingement angle from aortic flow data and research into if aortic jet flow disruption could reduce peak WSS in the ascending aorta for use in aortic reconstruction. ... Read more

Silicon based power semiconductors have long been used as the standard in ‘semiconductor technology in power conversion applications’. Recent developments replaces the Silicon with Silicon Carbide as it results in superior performance of the power conversion applications. However, due to the increased performance, challenges regarding heat dissipation emerge and the lifetime of the power semiconductor packaging or power module is compromised. Since this leads to an increase power density, the cooling of the power module is becoming of more importance and the heat sink becomes an interesting component to optimize. The best performance of a heat sink can be obtained when the flow through the device is turbulent. Developing turbulent flow heat sinks by using topology optimization methods can significantly improve the cooling performance compared to the current designs. This work is thus aimed towards improving methods for topology optimization of turbulent flow cooling devices. However, this work focuses on turbulent flow topology optimization only and aims to improve the accuracy of current methods. It is important that the flow physics are accurate since the thermal energy transfer is dependent on the flow field. The current state-of-the-art method based on the 𝑘−𝜔 turbulence model developed by Dilgen et al. is investigated. A design domain is subdivided into elements since the finite element method (FEM) is used, such that an optimization algorithm is able to turn every element into either fluid or solid with the goal of finding the best performing structure. This density based approach, models the solid domain as a highly impermeably porous material. To inhibit flow in the solid domain a Darcy penalization is added to the momentum equation. Moreover, in the method by Dilgen et al. boundary conditions in the other turbulent fields are also enforced using a similar penalization approach. Weaknesses and errors in the density based method are investigated by comparing solutions to ones computed on a body fitted mesh. It has been found that the largest errors in the solution, by using the state-of-the-art method, appear at the solid/fluid interface in the design. In these regions the penalizations are not applied correctly for the desired boundary conditions. Therefore, in this work it is improved on by the enforcement of the boundary condition by using the Dilation method. The Dilation method focuses on the solid/fluid region where it shifts the boundary conditions for the specific dissipation rate (𝜔) and ensures it reaches the desired value at the solid/fluid interface. Secondly, severe flow leakage is found in the “porous” solid domains using the state-of-the-art method. Flow leakage is reduced by using an improved formulation of the maximum Darcy penalization in the solid domain. Finally, the improved approach is investigated in several topology optimization cases and compared to the state-of-the-art Dilgen method. It is shown that by using the new approach, different designs with a better accuracy can be obtained. In an extreme test case, the Dilgen method resulted in an infeasible design which disconnects the flow inlets from the outlets while the new and improved method resulted in a feasible design. ... Read more

The majority of Dutch homes currently use natural gas boilers to meet their space heating demand. Changes in this significantly large sector are required to achieve the sustainability goals established by the Paris Agreement. Since most renewable energy sources produce electricity, transitioning residential heating systems might cause problems managing national electricity grids. Furthermore, the intermittent nature of renewable energy sources leads to an imbalance between supply and demand.

These challenges can be overcome by combining different storage techniques. An example of such a technique is the thermal energy buffer developed by Borg, which focuses on single-household use. This thesis looks into the feasibility of such a system by comparing different scenarios for residential heating systems.

Three scenarios were modelled using Simscape. In scenario I, a natural gas boiler provided all the heating demand of the house. In scenario II, the heating system consisted of Photovoltaic Thermal (PVT) panels and the thermal energy buffer. In scenario III, the house was heated by PVT panels, the thermal energy buffer, and a heat pump. In all scenarios, the same house was connected to the heating system.

In scenario II, the system sizes were 0, 1, 2, and 3 PVT panels, combined with buffer capacities of 0, 2, 4, and 6 m3. In scenario III, the system sizes were: 3 PVT panels with 6 m3 buffer capacity, 1 PVT panel with 4 m3 buffer capacity, and 3 PVT panels with 2 m3 buffer capacity.

The total yearly heat consumption of the modelled house was 5109 kWh. In scenario I, 378 kg of CO2 was emitted. In scenario II, CO\textsubscript{2} emissions were highly dependent on the sizing of the PVT system and the TESS and ranged from 20 to 405 kg, based on a heating demand of 9444 kWh.

Scenarios I and III maintained a comfortable temperature during the entire year. The heating system of scenario II was insufficient to heat the house throughout the year for all modelled system sizes; however, the scenario could be acceptable with larger PVT and TESS sizing. By comparing CO2 emissions and payback time, the optimal capacity of the thermal energy buffer was found to be 6 m3.

The uncertainty of the future gas price causes a challenge in comparing the cost of electrified heating systems to traditional heating systems. Additionally, it underlines the necessity of decarbonizing residential heating systems to secure comfortable, affordable housing. ... Read more

The Intergovernmental Panel on Climate Change (IPCC) believes that multiple solutions must be deployed simultaneously to reduce the emission of greenhouse gases into the atmosphere. Carbon, Capture & Storage (CCS) is an unavoidable technique within this portfolio as an intermediate solution. CCS requires transport of CO₂ through pipeline systems and into wells. There are still large uncertainties on the thermodynamics of the CO₂ in flow through valves. The CO₂ flow undergoes significant changes in pressure, temperature and phase distribution when it passes this control valve. Therefore, the behaviour of the CO₂ flow flowing through a control valve is examined in this study.

The simulation of CO₂ in a 3D valve including phase transitions is complex. Furthermore, few validation experiments are available. As a first step, more simple nozzles are simulated. In these devices, the same processes occur and validation data is available. These simulations are validated with experimental data by Nakagawa et al. to examine the accuracy. Three types of models (isenthalpic, Euler and Enhanced Mass Transfer (EMT)) are implemented in increasing levels of complexity to investigate the differences between these models and to consider when complexity is needed or simplifications are valid.

The validation cases show experimental pressure data of high-pressure CO₂ flow through converging-diverging nozzles with phase transitions. The results showed that the EMT model matched the experimental data best. A substantial similarity was achieved regarding the pressure data. The mass transfer mechanism, however, needed adjustments in its coefficients to match the experimental data. Finally, after finding the right values, the EMT shows the best technique for modelling flashing or cavitation.

In short, the overall consequences of the transition in a valve are substantial and must be considered. The behaviour of the high-pressure CO₂ flow is heavily influenced by flowing through a valve. Substantial amounts of vapour are formed, but only after the throat. This is the same for choking condition, which is achieved in the diverging section of the nozzle. The large expected drop in temperature due to pressure reduction has also been noted.

Although a high degree of similarity between the results of the model with experimental data is obtained, there is room for improvement regarding the model. A flaw was discovered in the handling of the thermodynamic properties of the fluid near critical points. Also, the surface tension has not been considered, but might have a substantial influence. Next steps in the research are 2D and 3D simulations of actual valves, but require experimental validation data. ... Read more

The transportation of high viscosity oil is an ongoing research topic for quite some years because of its importance in industrial applications and the increasing energy requirements of the world population. The water lubricated oil core with a core-annular flow configuration is a viable option for the transport of high viscosity oil. It was observed that there are significant differences between the predictions from the RANS simulations and the experimental data for turbulent oil-water core-annular flow obtained in the flow loop in the lab at TU Delft. Thus, this study is performed to understand the difference between the RANS simulations and the experimental data. 1D and 2D OpenFOAM simulations for turbulent vertical core-annular flow are performed using the Launder-Sharma low-Re k-ε turbulence model. The pressure drop and the oil holdup are imposed while the total flow rate and the water-cut is obtained as the output from the simulations. The results from the OpenFOAM simulations are compared with the DNS data available in literature to understand the impact of the turbulence characteristics and the interfacial waves. Both 1D and 2D simulations were carried out. The 1D model does not include the interface waves. The 2D axisymmetric model is able to capture the travelling interface waves. The 1D OpenFOAM simulations are carried out for five cases with different oil holdups and the results are compared with the asymptotic wall laws. It is concluded that the turbulence is sufficiently resolved by the 1D simulations. The 2D simulations are also performed for five different oil holdups. The 2D simulations are first validated by performing grid independence tests. The dependence of the simulation results on the streamwise domain length, presence of gravity and oil viscosity is also analysed. It was found that the streamwise domain length and the oil viscosity used does not have a significant influence on the prediction of the results. The comparison of the RANS simulations with the DNS data shows that the difference depends on the turbulence levels inside the simulation domain. The difference for the total flow rate between the 2D OpenFOAMsimulations and the DNS data is found to be 10% for fully turbulent core-annular flow while the difference increases to 29% for core-annular flow with no turbulence. The friction factor for the 2D Open- FOAM simulations is found to be about 17% lower for fully turbulent flow while it is 39% lower than the DNS predictions for the simulation with no turbulence. The difference for the holdup ratio (when comparing 2D OpenFOAM simulations with respect to DNS) decreases first from 15% higher for the low oil holdup cases to 8% higher for simulation with oil holdup fraction 0.71 and then increases to 14% higher for the highest oil holdup case. It is also observed from the comparison of the mean streamwise velocities and the mean shear stresses that the turbulence level in the simulation domain impact the extent of the difference. The agreement in the wave characteristics between the 2D OpenFOAM simulations and the DNS data depends on the mixture of wavelengths present in the simulation domain. The RANS simulations predict a specific dominant wavelength while the DNS data gives a mixture of wavelengths for lower oil holdups. The dominance of a specific wavelength increases with increasing oil holdup for the DNS predictions. ... Read more

The human body is the subject of several interesting phenomena and blood flow comes under that category. The main motivator for this thesis is the flow of blood in aneurysms. An aneurysm is a sudden expansion of an artery, with large expansion angles causing an adverse pressure gradient and leading to flow separation. Several studies have shown the transitional nature of the flow in aneurysms, and it has been seen that the variation of the wall shear stress from cycle-to-cycle is one of the major reasons for the growth of aneurysms, and possibly leading to their rupture at a later stage. It has also been noticed that the turbulent kinetic energy (TKE) does not decay with the mean flow kinetic energy and the periodic kinetic energy of the cardiac cycle. Therefore, it is intersting to research the turbulent decay to see how the flow in an aneurysm can be affected. However, the number of variables in an aneurysm are too high to effectively characterize this, and therefore, a simplified geometry was chosen. Pipe flow is a good choice to start with, as it can mimic the wall-bounded nature of an aneurysm. It also has a well-defined statistically steady turbulent state from where the decay of turbulence can be studied. Additionally, blood being a non-Newtonian fluid, makes it interesting to study the effects of shear-thinning.

To this extent, a Direct Numerical Simulation (DNS) study has been carried out using a higher-order spectral element method code. First, statistics for fully-developed pipe flow are compared with existing results. To a fully-developed turbulent state, a deceleration is applied to bring the flow to a steady, laminar state. The decay of the turbulent quantities is monitored during this process. Comparison studies are undertaken to study the influence of the ramp rate, the dependence of the decay on the initial Reynolds number, and the variation of the results between Newtonian and generalized Newtonian fluids. A modelling approach using RANS has also been undertaken to see if only studying the mean flow is sufficient to characterize the decay.

It is seen that two regimes of decay exist -- a power-law decay based on turbulent scaling, and an exponential viscous decay. The power-law decay is further divided into two stages -- one before the saturation of the integral length scale, and one after the saturation of the length scale -- with the maximum length scale being set by the diameter of the pipe. The exponential model has been validated using the hypothesis of Skrbek (2008). The point of divergence from the power-law to the exponential decay has been hypothesized here. It is seen that for all the cases studied, the point of divergence occurred at $Re_\tau = 60$. It is noticed that the decay is independent of the ramp rates when they are applied at time on the order of magnitude of 1 Eddy Turnover Time (ETT). The decay does show a dependence on the initial Reynolds number and the reasons for this are hypothesized. The RANS modelling used was found to be insufficient due to the inability of the RANS model to gauge the size of the domain. For generalized Newtonian fluids, it is noticed that the decay rate increases with shear-thinning. The results obtained are discussed in the context of an aneurysm. Based on the diameter, length and flow rate of the aneurysm, it can be hypothesized at which stage of decay the flow is, and based on this, it has been discussed whether using a non-Newtonian modelling approach is more beneficial than using a Newtonian approach for the decay. ... Read more

Experiments to characterise vortices in hyperbolic shaped funnels are being conducted at the Water Application Centre (WAC) in Wetsus. These have demonstrated their higher gas transfer rates in comparison to the conventional aeration systems presently in use. Depending on the imposed flow conditions, different regimes of vortices are formed, among which the Twisted vortical structure is observed to have the highest gas dissolution rates. This has probed several questions on the physical mechanisms responsible on both micro-and macroscopic scales. The present research aims to numerically analyse the flow field organisation in these vortices to reason the observed high gas transfer rates using Computational Fluid Dynamics (CFD).  Transient simulations were performed on a three-dimensional radially structured hexahedral mesh. Multi-phase modelling was done using the Euler-Euler approach-based Volume-of-Fluid (VOF) method, while the turbulent flow was modelled using Shear Stress Transport (SST) based on k - ω equations with curvature correction. The choice of boundary conditions and their location is crucial for forming a stable vortex in the hyperbolic funnels. The position of the air-water interface from experimental results was used to validate the obtained numerical results. Two regimes of the vortex, namely the Twisted and Straight vortical structures, were evaluated for their gas transfer capabilities in terms of Hydraulic Retention Time (HRT), interfacial area and mixing in the bulk.  Instabilities arise in the secondary flow field of these vortical structures analogous to the Taylor-vortices that develop in the well known Taylor-Couette flow systems. In hyperbolic funnels, these instabilities aid in advecting the bulk of liquid to the air-water interfacial region and also enhance mixing within the bulk of water. The former enhances the gas transfer rates while the latter promotes uniform mixing. The strength of these instabilities is qualitatively analysed in terms of average vorticity per unit mass of water. This is found to be higher in the Twisted regime in comparison to other regimes. This is augmented by high air-water interfacial area making this regime possess superior gas transfer rates.  Although the gas transfer rates are high, water exiting the funnel is undersaturated at the given operating conditions. In order to further enhance the amount of gas dissolved few possibilities are qualitatively discussed at the end of this study.      ... Read more

In order to reach the goals of the Paris Agreement, global emissions must decrease at a high rate. Looking at the industrial sector, the largest share of energy is used for process heating, with fossil fuels as its primary source. A way to improve the energy efficiency and reduce the emissions of process heating is by the integration of heat pump technology.

However, operational costs are still a limiting factor in the widespread uptake of this technology. A potential of industrial heat pumps that will have a positive effect on the operational costs, as well as its general applicability, is the possibility to operate the equipment in a flexible manner. At this point, the potential to respond with high temperature industrial heat pumps to heat demand, the electricity market or to grid congestion, is not yet unlocked.

The focus in this study is on gaining more insight into the dynamic characteristics of industrial heat pump operation. The approach is based on the modeling of a 2 MW high temperature industrial heat pump in the Dymola simulation environment. Based on the system dynamics, a suitable control method is selected and the controller settings are optimized. With this control system in place, the dynamic limitations are studied.

The limiting factor in the considered economizer based heat pump cycle is the ability of the control system to keep the superheat at the screw compressor injection port within acceptable limits during load level changes. The heat pump is found to be able to ramp up and down at a maximum rate of approximately 20%/min. This level of flexibility allows use of the heat pump for both heat demand and electricity price response, as well as for participation in grid load balancing pools. On a higher level, based on these results flexible heat pump operation can be considered possible. ... Read more

Throughout the last century, high energy-density fossil fuels have been the main energy source for merchant shipping. As the world tries to minimize the effects of global warming, shipowners are forced to significantly reduce their $CO_2$ emissions. Because alternative fuels are expensive, the interest in wind energy, which has been the single energy source for centuries, is renewed. There is a desire for passive and inexpensive wind-assisted ship propulsion (WASP) systems. The two-element wingsail is a promising option. This combination of a wing with a slotted flap can generate more than one and a half times more lift compared to a one-element alternative. It is used in the aviation industry as a high-lift device and has been applied on board yachts in the America's Cup and other races. Used as WASP system places new requirements as limited deck space and the ability to tack. This report describes the effects of the geometry on the performance of the wingsail and advises an ideal configuration that maximizes the overall thrust contribution. Both fixed design parameters such as the profiles and chord lengths and adjustable settings as the angle of attack and flap angle are examined. This is achieved by doing systematical geometry variations at a Reynolds number of 1.86 $\cdot 10^6$ using 2D CFD simulations. The design goal was to maximize the lift coefficient. The variations show that the lift coefficient is very sensitive to the flap angle. Increasing this angle increases the lift, but requires a well-positioned slot to ensure attachment of the flow along the suction side of the flap. The influence of the profile thickness is small. However, a minimum flap thickness was required to avoid flow separation at low angles of attack. Stall was always observed around the leading edge of the flap. The maximum lift coefficient of 2.73 at an angle of attack of 10 degrees was found for a configuration with a NACA0024 wing and NACA0012 flap profile, equal chord lengths, and a flap angle of 25 degrees. The corresponding slot had an offset of 0.5 and a side placement of 2.5 percent of the total chord length. Using the same method, the effects of adding a steering flap downstream of the wingsail were investigated. This third flap should keep the wingsail stable at a specific angle of attack. This ability is interesting for WASP purposes because it eliminates the need for fast-responding software-based control systems. The simulations show that the steering flap, which had a fixed chord length of 30 percent of the wingsail, can stabilize the system. Compared to the standard configuration the decrease of the lift was approximately 10 percent. The steering flap operates in the wake of the wingsail, which causes a decrease of drag of approximately 5 percent. This is an interesting feature because it reduces the negative effects of the steering flap. Depending on their positions, the interaction between all three elements can further enhance the performance of the whole system. ... Read more

Marine structures can sustain damage due to violent wave impacts that are characterized by complex dynamics involving both water and air. Methods that predict impacts loads for structural design often assume the water as incompressible. These predictions can be inaccurate during impacts where air is entrained in water, as that greatly increases its compressibility. Initial impact forces may decrease due to a cushioning effect. However, the total severity of the impact may increase due to temporal spreading and oscillations of impact pressure. Previous research on aerated water impacts covers breaking waves against fixed structures, flat water slamming or green water events, but not the interactive motion of rigid marine structures in free surface waves. This work aims to evaluate the effect of aerated water impacts on the dynamics of floating rigid bodies in irregular waves. As a part of this work, a state-of-the-art numerical simulation method for aerated water impacts is extended with a monolithic coupling for rigid body motion. Additionally, boundary conditions for wave generation were implemented for a validation of the method during breaking wave impact. Results of the verification of both extensions compare well to other literature. It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications.It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications.It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications. It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications. ... Read more