Geothermal energy can be promising to help the energy transition. Warm water is pumped up from a geothermal well and after it is cooled down, the cold water is reinjected into the hot aquifer underground. The temperature change could cause chemical balances to be disturbed and result in precipitation on the porous medium of the aquifer. This process of clogging of an aquifer is not desirable since it re- duces the efficiency of the production of warm water. This research is a stepping stone for modelling these reactions and clogging of the aquifer. The reactions rates depend on the temperature and the concentration of the reactants in the aquifer. For this reason the temperature distribution of the aquifer was numerically derived with finite difference methods. The velocity field is important for modelling both the temperature and the concentrations. Therefore the pressure and velocity field were modelled with use of mass conservation and the Darcy model. The models were solved numerically with finite difference methods and some results were compared to the analytical solution of the model. It was concluded that the numerical model for the velocity field is accurate when compared to analytical solu- tions of the model equations. In the model used, an increase in porosity resulted in a delay of cooling the aquifer. This is because there was no relation between porosity and permeability implemented in the model. A lower flow rate in the aquifer delays the cooling of the aquifer as well. For further research it is recommended to implement the chemical reactions into the model and use this model to look into how to prevent the clogging in the aquifer.

The accurate approximation of the surface tension force is paramount for continuum surface models in the field of computational fluid dynamics for multiphase flow where surface tension is relevant. This involves being able to accurately calculate the curvature at the interface. This study focuses on the use of convolution in smoothing the VOF colour field in order to obtain better approximations of the curvature. Given the sudden jump in values of the VOF colour field, the calculation of its derivative for the curvature is sensitive to errors, given the large values of high order terms that determine the truncation error. To deal with this problem, convolution of this abruptly varying field can be used to create a smoother transition. The curvature approximation of a circular interface improved as the support of the convolution was increased. It was proven analytically that, for these interfaces, the original curvature is retrieved from the convoluted field. Interfaces along which the curvature varies were also considered, and it was found that there is a critical convolution support that minimizes the error in the curvature, given that the choice of the support length can modify the curvature that is estimated. An algorithm was implemented in OpenFOAM that calculates the convolution of the VOF colour field. The resulting smoothed field was then used to calculate the curvature, which is needed for the surface tension force of the system. The simulations of a two-dimensional rising bubble resulted in more accurate results for the circularity and the rising velocity, when compared to the original OpenFOAM implementation with no smoothing. With the convolution algorithm, the terminal velocity deviated only 0.01% from a well-accepted benchmark case, a great improvement when compared to the 4.2% difference when no smoothing was used. However, simulations of a static bubble in zero-gravity rapidly resulted in unphysical flow, manifested as a wavy interface, when a convolution support larger than 2 cells was chosen. An improvement of the estimation of the surface tension force direction may be needed for this behaviour to disappear.

Brain aneurysms cause almost 500,000 deaths in the world every year. A better understanding of brain aneurysm genesis and rupture may open new opportunities to prevention and treatment. In this study, a part of the cerebral vascular system, the so called circle of Willis including an aneurysm, was analyzed with Computational Fluid Dynamics (CFD). The importance of boundary conditions in an aneurysm simulation was assessed by comparing the results to 7T MRI velocity data, obtained from the Academic Medical Center (AMC) in Amsterdam. Adequate similarities were found in velocity values, together with qualitative agreement in wall shear stress (WSS) values. We found that in a patient-specific case with an aneurysm, the velocity profile was able to develop to a parabolic profile because of its location; the aneurysm existed far downstream of the circle of Willis. This implies that it is possible to use a cropped arterial system to simulate the aneurysm and to use parabolic inlet velocity profiles for patients with this aneurysm phenotype. In previous studies, it proved hard to locate the precise location of rupture in an aneurysm. A risk assessment of the rupture location in an aneurysm can be used as a more accurate tool to assess the need for surgery. The aneurysm geometry of the CFD Rupture challenge from 2013 was simulated to predict the location of a rupture site. This rupture location was predicted by combining the following hemodynamic criteria: the time-averaged wall shear stress (WSSTA), oscillatory shear index (OSI) and vortex-saddle point structure during systole with accompanying low pressure values. A sensitivity study was performed on these criteria and a critical threshold for rupture risk was proposed. Based on these criteria it was possible to predict the exact rupture site for two analyzed aneurysm geometries. It is concluded that a CFD model could be used to assess the hemodynamics in intracranial aneurysms. Future research should focus on repeating this study on more patient specific aneurysm geometries to verify this hypothesis further.

This thesis describes a study into pairwise particle interactions within a Hele-Shaw geometry, using stop-flow lithography. By exposing a photoreactive mixture to a strong UV-pulse, a hydrogel is formed. The shape of this hydrogel is controlled by masking part of the light beam. This process takes place while the Hele-Shaw channel is placed on the stage of a microscope, which allows these hydrogel particles to be viewed and tracked. Improvements were made to increase the accuracy and precision of the experimental set-up. No- tably, the initially present mismatch of intra-pair particle thickness was greatly reduced by changing the method of particle pair production. By tracking these pairs of particles, information is obtained regarding their motions and velocity relative to one another. Experiments were performed for a selection of particle shapes and compared to numerical simulations of identical geometries. Simulations predicted that attractive and repulsive velocities should be noticed depending on the separation distance and shape of the particles. Qualitatively, the experiments agree to a certain extent with the simulations. It was demonstrated that the pairwise interactions are indeed dependent on their shape. Furthermore, the magnitude of these interactions qualitatively matched with the experimental data. Quantitatively, the experimental data did not agree with the simulations, but strong evidence was presented to indicate that the UV-light hitting the sample was not uniformly distributed. This results in a discrepancy in particle thickness between the pair, which skews the experimental data. Novel insights were gained on the applicability of the current literature model on hydrogel particle propagation. In contrast to the current model, it was experimentally demonstrated that the shape and (in-plane) size of the particle affects its thickness.

Physical Vapor Deposition (PVD) is the resublimation of a substance on a cold surface coating it with a thin solid layer. PVD coatings are utilized in industry to modify surface properties and appearance. Since the industrial process requires vacuum conditions, it has been mainly conducted in a batch process. Recently, PVD is considered a promising alternative coating technology to the hot-dip galvanization in order to apply a corrosion protective coating on steel. However, a continuous process is missing to manufacture protective coatings for strip steel on an industrial scale using PVD. First approaches suggest the following process: The steel strip is pulled into a vacuum chamber through air-tight seals to ensure a non-reactive coating atmosphere and avoid impurities; then its surface is treated to obtain high adhesion during the coating process; afterwards it passes a Vapor Distribution Box (VDB) from which vapor jets (or plumes) emerge and coat the steel surface; eventually the strip leaves the vacuum chamber again via air-tight seals, is coiled and shipped. To make this process usable on a large scale - or even superior to galvanization - multiple challenges need to be overcome: ensuring the tightness of the seals, cleaning the strip, preventing stray coating of the vacuum chamber, guaranteeing a uniform coating thickness, and providing a uniform high vapor mass flow to maintain a high speed of the production line. This thesis tackles the last challenge by modeling the vapor transport both inside the VDB and inside the vacuum chamber. First the flow inside the VDB is modeled using a SIMPLE-/PISO-based algorithm for transsonic flows. To account for the evaporation at the melt surface, a boundary condition for the inlet pressure is implemented based on the Hertz-Knudsen equation. The total mass flow rate for different melt temperatures is compared to experimental values as well as an analytical, isentropic estimation. Furthermore, the sensitivity of the model to material properties and process conditions is studied. The total mass flow rate of the system is found to depend on evaporation and choking. With higher melt temperatures the total mass flow rate increases. The trend found in the simulations resembles the one from the experiment. Both yield only 33%-54% of the mass flow rate estimated by the analytical isentropic relation. This low efficiency improves with higher melt temperature. A comparison of the pressure loss across the VDB reveals that the main losses appear due to the viscous boundary layer in the nozzles connecting the VDB with the vacuum chamber. The simulation overpredicts the experimental result by a factor of 1.3. This may be due to the used assumption of an idealized value of unity for the evaporation coefficient; a value of approximately 0.3 would produce a better match between simulations and experiments. Impurities found in the experiment may cause this reduction of the evaporation coefficient. When expanding from the nozzles into the vacuum chamber, the flow accelerates to supersonic speeds and rarefies. We study the interaction of two planar sonic plumes that causes a shock next to the interaction plane. This in turn produces peaks in deposition rate and thus in the coating. Direct Simulation Monte Carlo (DSMC) method is applied for the flow which ranges from continuum at the nozzles to rarefied and free molecular flow downstream. The results are compared to the analytical effusion solution and the inviscid continuum solution from a Riemann solver. The expansion and shock regions of the DSMC simulation are visualized by the Method of Characteristics (MOC). The mass flow distribution as a function of the degree of rarefaction, the nozzle-separation-distance and the inclination of the nozzles is studied. The DSMC result of plume interaction outside the VDB closely resembles the inviscid continuum solution at low degrees of rarefaction. The flow structure with expansions and shocks coincides, deviations are apparent in the actual number density, velocity and temperature especially in the shock region. With higher rarefaction, the shock structure diminishes and the flow field approaches the free molecular flow field. However, the rarefied flow field is not within the limits of the inviscid continuum and the free molecular flow field, but may exceed them in both deposition peaks and temperature peak in the shock region. Using the MOC for visualization reveals that with higher rarefaction the shock bends away from the interaction plane which can be explained by the increased temperature in the secondary expansion. While the shock location shifts with the nozzle-separation distance, it merges to one location when scaling it with the nozzle-separation distance. Bending the nozzle outlets towards each other produces a stronger shock starting further upstream, which in turn causes a stronger secondary expansion and thus smoother deposition. In addition to studying the impact of geometry changes in the PVD setup, the effect of adding a light, inert carrier gas on the plume interaction and the resulting deposition uniformity is investigated. To this end, the carrier-gas mole fraction is varied at a given Knudsen number. Species separation focuses the heavy species along the primary axes, whereas the light inert carrier gas is scattered towards the periphery. Due to the higher mean molecular weight, the speed of sound decreases and consequently the interaction shock occurs farther downstream, is less bent and weaker producing a more uniform deposition profile. Desirable side effects of the carrier-gas are less stray deposition and a higher conductance of the coating material from the inlet nozzle. The last part of the thesis focuses on the numerical method, since DSMC is accurate but computationally costly. The substitution of the collision step in DSMC with a kinetic relaxation using the Bhatnagar-Gross-Krook (BGK) operator is implemented in order to speed up the algorithm. The choice of the target distribution for the relaxation is crucial. The Maxwellian velocity distribution produces an incorrect Prandtl number; the Ellipsoidal-Stochastical BGK (ES-BGK) corrects for the Prandtl number by taking the stress into account; the Shakov model (S-BGK) corrects by considering the heat flux vector. The implemented models are verified against literature data and evaluated for their accuracy in simulating the interacting plumes case. In addition, we evaluated a hybrid coupling of the various kinetic relaxation models in dense, near-continuum regions with DSMC for rarefied and non-continuum regions. The switching criterion for the hybrid coupling was the gradient-length local Knudsen number. The implemented kinetic models compare well to literature data for rarefied Poiseuille flow. The lower resolution criteria lower the computational cost to approximately 30% of the one of DSMC. For the planar jet interaction, the BGK model (using the Maxwellian target distribution) overestimates the shock strength, the S-BGK model overpredicts the diffusion of the shock, whereas the ES-BGK models results are in good agreement with the DSMC results. This indicates that the velocity sorting and breakdown of temperature isotropy in the expansions have a more significant influence on the flow field than the shock, which skews the velocity distribution. Coupling the kinetic models with DSMC in the highly rarefied regions improves the flow field for the BGK and S-BGK model, but not significantly. In short, this thesis examines the influence of process conditions, geometry and carrier-gas use on the mass flow rate and deposition uniformity in continuous PVD for coating steel strips with anti-corrosive coatings. It provides modeling tools for the mass transport both inside and outside the VDB which can be used for further investigation and optimization.@en

Heat transfer by the motion of fluids, referred to as convective heat transfer, is ubiquitous. Convective heat transfer in enclosures packed with solid obstacles, is of great importance in various engineering and real-life applications, such as in blast furnaces, refrigeration devices, distribution transformers, nuclear waste disposal, energy storage, etc. Both natural convection, where the heat transfer occurs due to the flow induced by density differences, and mixed convection, where the combined influence of natural convection and forced convection is of importance, play an important role in these applications.@en

Cardiovascular diseases are one of the leading causes of death worldwide, for example by causing strokes. Timely diagnosis of such diseases is pivotal for a patient’s chance of survival. Furthermore, in the present world in which medical expenses are going through the roof, we can save greatly on costs if certain diseases are detected in an earlier stage. To that end, our research is focused on improving medical measurement techniques, to give doctors a greater arsenal to combat these diseases. Ideally, a measurement technique is cheap, accurate, and all while causing minimal discomfort to the patient. Light-based techniques have proven previously to have great potential to fulfil that role. For example, that tiny device that you can put on your finger, and similarly the sensor in a sports watch, are able to measure your heart rate using light. For our research we have developed a computer model, such that we can use the power of modern computing. Our model is able to predict how light is reflected by red blood cells flowing through an artery. The computer is then able to rapidly simulate many scenarios, producing a lot of data about what the reflected light looks like for each scenario. From that data, we are able to say something about what a certain pattern in the reflected light says about the underlying system: the flowing red blood cells. As a first step, we have used our model to figure out how we can determine the heart rate from the reflected light. You could argue that that’s nothing special, as your sports watch can already do precisely that, but it’s an important step nonetheless, since our technique is different than what your sports watch is doing. Namely, the data our technique provides is more complex, but as a consequence also contains much more information and thereby yields a greater potential if we just become able to extract that information from the data. Therefore, our second step was to determine the exact velocity of the red blood cells from the reflected light, which is quite of a magical thing when you think about it: even though we cannot ‘see’ the red blood cells directly, we can still ‘see’ how fast they are moving. Although we succeeded in determining the velocity, in reality a doctor will likely need to do some tweaking to account for patient-specific factors, such as skin tone. Finally, we studied the disease atherosclerosis, in which accumulating cholesterol causes arteries to become more narrow, which ultimately could lead to a stroke. The narrowing of an artery, alters the flow behavior of the red blood cells, which we were able to pick up by studying changing patterns in the reflected light from our simulations. By extension, it should be possible to use reflected light to detect atherosclerosis, rapidly and cheaply flagging patients who are at risk. We have shown the potential of reflected light techniques for medical diagnosis purposes. Although further research and work is still required to put these techniques into practice for doctor’s to use, we have set the groundwork to enable these techniques in the not-too-distant future. @en

To mitigate climate change, carbon capture is necessary. In addition to the energy transition towards renewable sources and green house gasses emission reduction, CO2 capture from flue gas and its sinks, including air and the ocean, must be promoted. By 2030, in less than 8 years, the global carbon capture capacity must increase 100 × (from the current ca. 40 MtCO2 yr−1 to 4 GtCO2 yr−1). To meet the net zero carbon goals of 2050, sustainable, scalable, inexpensive technologies that fit in an electrified industry and have a small footprint are needed for carbon capture. Currently, such technologies do not exist. In the framework of the necessary carbon capture, and the opportunities for electrochemical (ocean) CO2 capture, five research questions are defined and addressed in this thesis...@en

Metallic droplet deposition is of interest in the industry because of the potential use in additive manufacturing. This work discusses the complex phenomena involved in droplet impingement, especially the effect of temperature dependant surface tension. The volume of fluid (VOF) method is used to solve the axis-symmetric Navier-Stokes equations, which are used to describe the droplet's behaviour. Different temperature dependencies for the surface tension are modelled, to see the effect on the interface and substrate melting. Furthermore, the effect of droplet size, initial temperature on droplet shape and substrate melting is studied. To judge the accuracy of the VOF model, a series of benchmarks are tried. The VOF model used in this work is as accurate or more accurate than previous works. The results show that droplets with a higher percentage of oxygen flatten, this is due to thermo-capillary forces. A higher temperature results in more spreading of the droplet, this is because higher temperatures result in higher surface tensions. These surface tensions keep the droplet together when it first makes contact with the substrate. This causes less air is trapped underneath the droplet, which causes the droplet to spread out more. The air also causes the droplet to cool down less fast, this results in a phenomenon where the hotter droplet is solidified faster than the colder one.

We have explored the concept of mobile computational fluid dynamics (CFD) solving. Using the computational power of a smartphone we tried to generate real time simulations of relatively simple transient natural convection problems in the laminar regime. The focus lies on finding a compromise between accuracy, speed and stability: we want to make a real timemobile CFD solver that is as accurate as possible. A JAVA application has been created that runs on Android. The SIMPLE algorithm has been implemented in order to solve for the flow and heat transfer. The SIMPLE algorithm on the application was tested for runtime performance and accuracy. For a test problem, a real time simulation on a Nexus 5X has been realized with an accuracy of 5.4% on a 21x21 grid. Running the algorithm on a desktop gave a simulation 10 times faster than real time. Comparing this algorithm to a version converted to MATLAB, gave similar solving speed. We concluded that the bottleneck in the algorithm, solving matrix equations, could not be easily improved, because MATLAB solvers perform quite optimal: a significantly faster CFD solver implementing the SIMPLE algorithm probably does not exist. Nevertheless, when trying to use the obtained results on water and air, we could not obtain any real time solutions. It is up to further research to define restictions that can guarantee real time solutions for fluids.

In this master thesis a CFD model targetting the combustion in cement kilns is developed. The model is implemented in OpenFoam

This MSc thesis reports on an experimental study performed to test the effects of packed beds of relatively large spheres on natural convection inside a cubical enclosure which is heated from the left side and cooled from the right side. With ’relatively large’, a ratio of L/D=5 is meant where L is the length between the hot and cold plate and D is the diameter of each sphere where the packed bed consists of. To gain a better understanding and enhanced knowledge of the effects of coarse-grained porous media on side-heated natural convection, heat transfer experiments were performed as well as flow and temperature distribution measurements using the optical measurement techniques Particle Imaging Velocimetry (PIV) and Liquid Crystal Thermography (LCT). In the optical measurement experiments, hydrogel balls were used which have the same refractive index as that of water.

Due to climate change, the sea temperature is rising. This temperature change has an effect on the phytoplankton population. Phytoplankton is responsible for more than 50 percent of the oxygen production on earth, and is therefore crucial for life on earth. In this report, the research question is: is the temperature increase of the water important for the development in space and time of the plankton population in a 2D model of a coastal area of an ocean? To investigate this, first a 2D model for a coastal area of an ocean is formed. As a basis for this model the 0D model of Steele and Henderson is used to describe the predator-prey model for zooplankton and phytoplankton. Due to this model a repeating pattern in time arises, which is called a limit cycle. This predator-prey model is expanded to a 2D model by applying convection (v) and diffusion (D) to it, representing the current and dispersion in the water. The chosen parameters and boundary conditions are inspired by the coastal area of the west coast of Portugal. To evaluate the effect of the water temperature rising, first, the effect of diffusion and convection are studied by answering the following questions: is the convection important for the development in space and time of the plankton population in a 2D model of a coastal area of an ocean? And: is the diffusion important for the development in space and time of the plankton population in a 2D model of a coastal area of an ocean? The diffusion is dominant in the direction perpendicular to the coast. Due to the diffusion the limit cycle of the predator-prey model is suppressed. Its range gets smaller until the plankton population is completely constant for D > 2.5 m^2/s. This critical value is included in the realistic value of D, which varies from 1 to 2000 m^2/s. Due to convection, the limit cycle corresponding to the boundary condition occurs in the direction parallel to the coast, making every point in the domain steady state. The higher the velocity v corresponding to the convection is, the less oscillations in phytoplankton density there are in the domain, thus the larger the wavelength of the oscillation is. For v > 0.06 m/s the current is so fast that the predator-prey model has little time to develop before it reaches the right boundary, making the left boundary value more important. This critical value is included in the realistic value of v, which varies from 0.03 to 0.28 m/s. The model experiences both diffusion and convection more or less equally when the Péclet number vH^2/DL≈1, resulting in D/v≈50 m. If this number is significantly larger than 50 m, diffusion dominates the solution. If the number is significantly smaller than 50 m, convection dominates the solution. Now the main research question can be answered. By increasing the temperature of the water the growth rate of phytoplankton increases. Due to this increase, the timescale of the predator-prey (τpp ) decreases. The value of D and v for which the predator-prey model and diffusion and convection all influence the solution equally depends on the timescale with the factor 1/τpp . A smaller time scale means that the predator-prey model contributes more to the model. However, this change is very small in comparison to the scale of the realistic values. In conclusion, in the formed 2D model the temperature increase of the water is not important for the development of the plankton population in space and time. The results of this report are influenced by the assumptions and approximations made. In this study, several processes, both physical and biological, are described with constants and simplifications. Improving the models of these processes is left for further research.