A Landing Platform Dock (LPD) facilitates amphibious operations by allowing landing vessels such as the Landing Craft Vehicle Personnel (LCVP) and Landing Craft Utility (LCU) to embark and disembark from its internal flooded well-dock. The smooth transition of these landing vessels is critical, but can be hindered by wave motions (i.e. sloshing) within the well-dock, making safe embarking and disembarking more difficult.

Existing literature mainly describes wave motions within a well-dock, but does not address how to minimise these motions. This thesis aims to develop a preliminary design to reduce wave energy entering the dock by implementing an optimised wave attenuation solution.

ComFLOW is used to observe that wave motions inside the well-dock are highly non-linear and correspond to the LPD’s oscillation period. The wave motions within the dock are caused by a drop in water level near the well-dock entrance. This causes waves to roll into the dock. In shallow water conditions like in the well-dock, the propagating waves hereby show amplitude dispersion and wave breaking.

Using a system engineering approach, a bottom-hinged pitching flap was found to be ideal for wave attenuation in shallow water conditions. The flap’s attenuation performance was examined using ComFLOW by adjusting design parameters such as: mass, center of gravity, length and mechanical damping.

It was found that high-period waves with high wave heights fully couple dynamically with the flap, resulting in several outcomes. First, the flap follows the wave frequency, resulting in sub-optimal energy dissipation with use of radiation damping. Second, tuning the flap’s natural period to match the incoming wave period, increases pitching angles but reduces angular velocity, leading to decreased wave attenuation. Third, lowering the centre of gravity increases the restoring moment and angular velocity. This results in enhanced energy dissipation of higher-order wave components. Fourth, increasing the flap’s length improves wave attenuation due to enhanced reflection and radiation damping. Fifth, adding mechanical damping decreases attenuation performance, likely because the reduction in angular velocity worsens the energy dissipation effect for higher-order wave components. This research revealed that the best performing solution, with low a centre of gravity, attenuates 28.7% of wave energy entering the well-dock. Furthermore, optimising the flaps natural period to the incoming wave period was less effective, attenuating only 19.7% of the wave energy.

With the shift to renewable energy becoming more important than ever before and offshore wind farms being one of the sustainable energy resources available, many offshore wind farms are currently being built.
One of the difficulties of these wind farms built with monopiles is that the monopiles and electricity cables must be protected from scouring and falling objects. The protection is usually performed by placing a layer of rocks around and over these objects. For deep water up to 400 meters, vertical fall pipes are used to place the rocks directly under the rock placement vessel. This is impossible when placing rocks around monopiles, as the monopile stands in the way of where the vessel would have to lie during the rock placement procedure.
The inclined fall pipe (IFP) was introduced recently to combat this issue. The IFP, which hangs diagonally next to the vessel, can place rocks a couple of meters to the vessel's side, leaving a safe distance between the vessel and the monopile. Rocks tend to form clusters inside the pipe during placement with an IFP. This means the rocks exit the pipe in bursts of clusters, resulting in uneven rock layers if the rocks are placed during the movement of the vessel. Another result is less accurate placement of the rocks as the rocks at the outer layer of a cluster, during the free fall, are pushed further out, away from the intended placement location.

This thesis investigates the processes of rocks moving inside an inclined fall pipe and simulates this process numerically. This is achieved with a Finite-Volume solver for the fluid coupled with a Discrete Element model for the particles. The results are compared to existing experimental data.

The simulation results show the same rock behavior as in the lab tests for angles up to 60 degrees. The clusters start forming from the small volume concentration differences within the rock layer. The higher densities concentrate and become clusters, while the lower densities become gaps. In the simulations, this process is caused by the differences in the flow velocity of the fluid and the resulting drag differences of the particles. This knowledge is a first step towards finding solutions to lower the formation of clusters and thus, the more accurate placement of the rocks on the sea floor.

Furthermore, multiple points of improvement for the solver have been found. For angles of 75 degrees and steeper, the rock movements of the simulations miss a certain chaoticness, which may be attributed to missing turbulent drag forces in the solver. The velocities predicted by the solver are too high; multiple possible causes are speculated. The simulation results follow the same trends as the lab results, meaning they could be used for qualitative comparisons. Various additions to enhance the solver are proposed.
The influence of a system parameter, the distance factor was noticed to be stronger in shear flows than in homogeneous flows during a system parameter variation. This distance factor relates the fluid-particle interaction radius to the particle diameter. Finding limits for this parameter in shear flow could enhance the solver's prediction capability.

With the shift to renewable energy, more monopiles will be placed. The simulations show which forces cause the cluster formations and can be used to investigate ways of reducing them. This can reduce the spillage of rocks during operation, making the entire rock placement more efficient and will reduce costs.
The solver with the enhancements for better prediction capabilities could be used to further optimize the rock placement accuracy and efficiency. This would reduce wind farm costs and help the shift to renewable energy.

Jet trenching operations in sandy seabeds have been extensively studied and are well understood, offering a preferred method for offshore cable installation due to its relatively low environmental impact. However, the transition to clay seabeds presents significant challenges, demanding a deeper comprehension of jet trenching dynamics in cohesive soils. Recognizing this need, DEME NL Offshore, a Dutch company specializing in offshore operations, seeks to develop a predictive model tailored for jet trenching in cohesive soil environments.

This thesis addresses the complexity of jet trenching in clay seabeds through a comprehensive computational fluid dynamics (CFD) modelling approach. Beginning with an extensive literature review, various methods for modelling jet trenching processes are explored, highlighting the critical parameters influencing trenching outcomes. While existing models primarily focused on jet penetration depth, this study identifies the need to integrate fluid dynamics principles and particle behavior to achieve a better understanding of jet trenching dynamics. A CFD model is developed to cover the fluid dynamics in the trench behind the jetting sword. Subsequently, a langrangian model is applied to determine the trajectory of cohesive particles of different diameter in the trench.

Key Findings:
• For the scenarios considered in this thesis, jet trenching in clay soils is not feasible by the accumulation of particles at the trench bottom before the cable touchdown point.

• Particle diameter significantly influences trenching outcomes, underscoring the importance of further investigation.

• Existing models inadequately account for clay block formation and behavior, indicating a gap in understanding.

• Empirical validation and field testing are essential to enhance model accuracy and reliability.

• Integration of field observations and experimental data can refine the model for real-world scenarios.

In response to these findings, recommendations for future research endeavors are proposed, emphasizing the importance of empirical validation and field testing to enhance model accuracy and reliability. By integrating field observations and experimental data, trenching models can be refined to better predict real-world trenching scenarios, thereby optimizing offshore cable installation processes.

In conclusion, this thesis contributes valuable insights into the challenges of jet trenching operations in cohesive seabeds and lays the groundwork for future research in the field. By addressing the recommendations outlined herein, future endeavors can advance trenching methodologies, ultimately improving the efficiency, reliability, and sustainability of offshore cable installation.

Gravel is a material with multiple purposes in construction and infrastructure. Its usage ranges from construc-tion of roads, building navigational channels to land reclaimation. Gravel can be obtained through multiple methods, such as dredging with ships. One of such dredging ship is the trailing suction hopper dredger (TSHD). However, in the seabed the gravel is thoroughly mixed with sand, thus during the collection process the unwanted sand is pumped up with the gravel. It is not uncommon to receive up to 80% sand and only 20% gravel. In order to use the storage of the ship more efficiently, the gravel is separated from the sand through an on board sieve installation. The sand is released back into the water and the gravel is collected in the hopper. This has several disadvantages, such as wasting energy by pumping unwanted sand which is returned back to the seabed. The turbidity in the seabed that occurs by releasing the fine sand which settles slowly to the seabed. And the requirement of a large sieve installation to separate the gravel from the sand, which reduces the already limited space of a ship. In order to limit the mentioned disadvantages, it would be favourable to bring the separation process at or near the seafloor. Through a literature study the best underwater separa-tor has been selected. From this it is concluded that the usage of a jet that pushes fine sediment out is the best for underwater separation. The fine sediment is pushed to the upper part of the pipe, whilst the coarse particles resist the jet and remain at the lower part of the pipe. The fine particles at the top are sucked in by a retour pipe, which guides them down to the seabed where it is released with a lowered environmental impact. Further research on this concept is done through the use of computer simulations which models the conditions using a drift-flux solver. The concept has been tested with different configurations to determine the influence of several input variables. The chosen variables are the inlet velocity, jet velocity, sediment size and sediment concentration. For these configurations the separation efficiency of the system is determined by analysing the retained gravel and the filtered sand. The containment of gravel is generally over 90% and highly satisfactory. The maximum separation of sand is 50%. These results indicate the viability of the system and thus research on improvements are recommended to further increase the systems effectiveness and efficiency.

Coastal communities around the world are facing significant challenges such as erosion, flooding, and storm surges. These issues highlight the need for nature-based coastal management solutions that can help mitigate the negative impacts of these events. Coastbusters Consortium developed such a solution in the North Sea. This solution is a system that utilizes a bivalve long line approach, where blue mussels are allowed to grow and eventually attach to the seabed, forming a reef structure that can help induce natural accretion of sand, attenuate storm waves, and reinforce the foreshore against coastal erosion. This reef structure can help to enhance coastal protection and reduce the impacts of these significant challenges on coastal communities.

To enhance the current long line system, it is imperative to develop numerical models that can accurately predict the forces acting on the slender cylinders in current and waves. One approach to this is to use the Morison equation, which accounts for both drag and inertia force. However, there is a lack of understanding regarding the drag and inertia coefficients in the literature that are used in the Morison equation. To address this issue, the present research conducts three experiments on a 3D-printed model of a dropper line, at a one-to-one scale, in a towing and wave tank. To treat the dropper line as a cylinder, a characteristic diameter is used. The 3D model is developed based on a thorough evaluation of the existing dropper lines at De Panne. These experiments aim to determine the drag and inertia coefficients for the dropper line in different steady and oscillatory flows, which can aid in the design of more efficient and effective bivalve aquaculture systems and integration into numerical models.

To determine the drag coefficient of the current towing experiments were conducted and forced oscillations and waves experiments were conducted to determine the drag and inertia coefficient in waves. The parameters used were based on the current and wave regimes at De Panne, and were expressed in terms of Reynolds and Keulegan-Carpenter numbers. The results showed that for continuous current, the drag coefficient of a dropper line containing blue mussels was determined to be $C_D$ = 1.2 for Reynolds numbers between $3.0*10^4$ and $1.0 *10^5$. In oscillatory flow, the drag coefficient varied between $C_D$ = 2.3 - 3.5, and the inertia coefficient varied between $C_M$ = 1 - 2.5 for Keulegan-Carpenter numbers between KC = 5 - 28.

The experiments conducted in this study included evaluations of the characteristic diameter and shape of the dropper line.
Results also showed that a difference existed between the coefficients obtained from the forced oscillation and wave experiments. Possible explanations for this difference were investigated, including free surface effects and flow differences. The results obtained from this study can be applied to the design of bivalve aquaculture systems and their integration into numerical models. These findings contribute to improving the efficiency and effectiveness of nature-based coastal management strategies for mitigating the effects of erosion, flooding, and storm surges on coastal communities. Further research is needed to fully understand the complex dynamics of the bivalve long line system and its interactions with the coastal environment.

The electric pulse fishing technique is no longer an option for fishermen. Therefore, they have to fall back on beam trawl fishing. This method has several disadvantages such as high fuel consumption and much soil disturbance. This system therefore deserves an investigation into improvement.
There is various literature on interaction of offshore structures with the soil. Processes in which a structure is also towed through the seabed are ploughing and cutting. Experiments with tickler chains are done and also numerical models are created.
Experiments were performed to acquire the required information that could not be retrieved from literature. In these experiments a chain element is towed through a sand layer in various configurations in a controlled way. Parameters that are varied are: velocity, chain link diameter, angle of attack, density, grain size.
A decrease of the angle of attack leads to a decrease of the tow force. The force per unit length is defined by dividing the tow force by the projected length. The projected length perpendicular to the towing direction, decreases for a decrease of angle of attack. However, the force per unit length also decreases for a decrease of the angle of attack. That means that the projected length is not the only explanation for the decrease in force.
The experiments indicate that the forces for higher penetration depths and higher chain link diameters are very high. An increase of density leads to a significant increase of the internal friction angle for low confining pressures. The increase of the internal friction angle leads to a higher passive soil pressure coefficient. The increase of this coefficient can certify the high forces.
Two dimensionless parameters are defined: the dimensionless force and the Froude number. An estimate for the increase of the passive pressure coefficient is integrated in the dimensionless force to clarify the high values for dense soil. After plotting the experimental results expressed in these dimensionless parameters, a linear trend line representing the data with reasonable accuracy of R2 = 0.611.

Green water events on ships in extreme waves pose a considerable risk to personal, operational, and structural safety. Considering these issues, novel experiments were done to investigate the interaction between focused breaking waves and a ship at forward speed. The investigated interactions are global motion, global loads, and forces on a deck structure.

For the experiments focused, breaking waves were created, which served as loads of the green water events. First, wave focusing with prescribed characteristics was investigated in numerical settings. It was shown that focusing the waves with iterative methods does not lead to a converging solution. Three breaking waves were created in a wave basin. The characteristics of the waves were retrieved from 2D numerical simulations. The parameters of the spectrum of each wave were calculated from the simulation results.

The experiments were conducted systematically, where the position of the longitudinal center of gravity (LCG) of the ship was varied with respect to the focus point of the breaking wave at the focusing time. Using the same wave input resulted in green water events and loads on deck structure with different characteristics caused by different breaking stages of the focused wave. The effects of different ship velocities and different focused breaking waves were also explored throughout the experiments. It was shown that using the same focused wave, i.e., the same energy input, the breaking stage of the wave during the impact has a large influence on both the magnitude and time development of the loads on the deck structure. Force and pressure peaks were the largest and had an initial spike-like development when the wave during impact was in an unbroken stage. Impacts with the wave in its broken stage resulted in significantly lower loads and longer-duration load development. Such changes in the loads show that the ship's position with respect to the breaking wave is essential to consider even for the same input energy, posing additional challenges for establishing the maximum load in a sea state.

When a ship reaches the shore, the water depth reduces and thus the under-keel clearance also reduces. The ship keel clearance influences the maneuverability of the sailing ship.

In this thesis project, the link between the fluid mud rheology and its density is first investigated. The exponential relation reformulated between the density and yield stress necessitates the direct measurement of rheology for navigability. It was confirmed true that the yield strength grows exponentially with the density of the fluid mud. However, at higher densities of mud, more thixotropy is observed in the flow curves. Besides, the Power-law relations were found for the relation with volumetric concentration, for yield stress in agreement with fractal dimension theory.

Second, quantification of the thixotropy effect of fluid mud is conducted to justify the importance of the time-dependency effect in the rheological modeling. We found that an unremolded fluid mud has high thixotropy even at high shear rates. However, diluted and remolded fluid mud at high shear rates found a negligible thixotropy effect. We also observed that Houska's modeling curve coincides perfectly with the flow curve of fluid mud at high shear rates, and also it integrates the thixotropy into the modeling.

Third, the total resistance of a plate moving through fluid mud is measured and compared to the frictional forces calculated using the available analytical formulas of a plate moving in Bingham fluids and Power-law fluids. The moving plate is an abstraction of a vessel's keel sailing through the fluid mud. We found that the total resistance of the plate moved in fluid mud at low velocity agrees with the frictional force of a plate moved in Bingham fluids. As the velocity increases, the stagnation pressure becomes significant, and the deviation of the total resistance of plate and frictional force of a plate increases. Thus, to estimate the total resistance of a plate moved in mud, normal forces (stagnation pressure) and vortices at edges should be investigated because these need to be subtracted.

The experimental data in this thesis are made resourceful to help researchers validate their Computational fluid dynamics(CFD) models in the application of sailing through the mud.

To meet energy demand towards a low-carbon future, the global market demand is growing for metals such as cobalt and nickel which are major elements in batteries. Polymetallic nodules, which are formed on abyssal plains at depths ranging from 4 to 6 kilometres and are distributed in high abundance on the top of the seabed, contains several times more cobalt and nickel than the entire global terrestrial reserves. This has raised the interest to exploit these resources from the deep ocean. The seafloor mining tool (SMT) can move along the soft sea bottom and can collect polymetallic nodules. While doing so, it will also entrain sediments and water. The excess of water and sediment entrained is discharged at the back of the SMT, forming a sediment plume. The sediment plume dispersion has strong adverse impacts on deep-sea environment.Thus, it is essential to study the sediment plume behavior in order to limit plume dispersion and thus to reduce its environmental impact.
Experimental research is a powerful technique to study the plume behaviour. However, experiments sometimes take a long time due to complex set-up. In comparision, numerical analysis can save time and costs when solving complex problems. Furthermore, numerical modelling can provide deeper understanding and flexibility for boundary conditions and sediment types, which is applicable on both model and prototype scale. Previous numerical studies have noted the significant role of flocculation in limiting plume dispersion, but flocculation process has not been modelled explicitly. This study aims to establish a numerical model to study flocculation process and its effect on sediment transport.
Previous flocculation-fluid dynamics modelling has applied a Euler-Euler method with additional population balance equations. The disadvantage is that many equations need to be solved. To avoid excessive computational costs, the sediment transport is described by a multiphase drift-flux model in this study. The flocculation process is modelled by a discretized form of population balance equations. The author has found that, by multiplying the particle volume, the population balance can be efficiently incorporated in the phase continuity equations in the drift-flux model. The population dynamics of particle aggregation and breakup can thus be characterized by the phase transition terms in the phase continuity equations. Hence, no additional equation needs to be introduced and solved.
Verification is carried out to check conservation relationships and iterative convergence of numerical results. Then, an initial numerical investigation has shown the results can qualitatively show the three settling stages (i.e., flocculent settling, hindered settling and compression settling) found in the experimental studies. Afterwards, the collision efficiency is calibrated using the settling column tests conducted by Enthoven (2021). The results of calibration show a good fit to the experimental data. Another advantage is that numerical simulations can provide the particle size distribution over time, which is not measured in the experiments.
The major novelty of this study is the coupling of the drift-flux model and the population balance equations, which inherits both the characteristics of population balance and the merits of drift-flux model in reducing computational costs. The flocculation modeling technique as proposed in this study can be incorporated as a module into an extended drift-flux model to predict the dispersion of deep-sea mining plumes.

In this study, the excavation process of a low pressure vertical impinging jet in cohesive soil has been investigated. Mass flow excavation is a hydraulic, low pressure, subsea excavation method whereby a large volume flow is applied to the seabed through which the seabed is eroded, and the soil is transported.
The erosion process of a cohesive soil by impinging jets depends on many variables (e.g., jet flow velocity, standoff distance, grain size, undrained shear strength). The erosion processes, especially for dynamic pressures of lower than two times the undrained shear strength, are not fully understood. It is still unknown what the influence of different soil and jet parameters are. As a result, it is still unknown what the expected scour rate will be during a mass flow excavation process of cohesive soils. The goal of the research is to quantify and be able to predict the production of a mass flow jet on cohesive soils. For this purpose, the relevant parameters of cohesive soil erosion are investigated, and fluid velocity profiles of the jet are related to clay bed failure mechanisms. Special attention is put on the effect of the consolidation coefficient on the erosion process. Based on literature and experimental research, different soil failure mechanisms have been investigated. The main result of this study is a relation between the main jet and soil parameters, and the erosion velocity development.

The research focuses on sedimentation in reservoirs. Subjects include settling distribution in a reservoir, dredging and flushing of sediment in reservoirs, impact of sedimentation on reservoir performance and financial effects of long term sedimentation in reservoirs.

Wave overtopping is responsible for many dike failures, dike breaches and severe flooding. The erosion resistance of dikes to overtopping waves is determined by the strength of the grass cover. Therefore, a thorough understanding of the erosion resistance of grass covers is desired to enable a good and effective dike management. In the existing approach to assess the erosion resistance of grass covers, the erosion resistance is empirically quantified in terms of local hydraulic loads. However, despite the cyclical nature of overtopping waves, no insights into the dependence of the erosion resistance on successive loads are available. To improve the understanding of the resistance of grass covers to wave overtopping induced loads, understanding of the development of the mechanical behaviour of grass sods is essential. In this study the mechanical behaviour of grass covers under tensile loads have been examined by means of direct pull tests executed in the field. Especially the influence of repetitive wave loading and the conditions during testing were of interest. The results of this study show that the deformation of the grass cover due to tensile load is time dependent. The failure mode of the grass cover is influenced by the degree of saturation of the grass cover. The maximum strength and mechanical behaviour of the grass cover is influenced by accumulation of damage. The findings presented in this study provide a better understanding of the processes that influence the mechanical behaviour of the grass cover. Moreover, insights in the development of the process of damage accumulation are revealed. Based on the findings of this study, a method is proposed to determine the resistance of grass covers against repetitive loads by means of direct pull tests. In this method the resistance of grass covers against repetitive loads is described by an estimation of the fatigue limit. Relating the fatigue limit of a grass cover to hydraulic loads, enables an assessment of the erosion resistance of a grass cover at a specific location.

In dredging operation, the high-pressure water jet is widely used for the excavation of soil. To study the jetting process and optimize the dredging devices, the moving vertical water jet penetrating cohesive soil experiments were carried out by Nobel (2013). However, in terms of the design optimization for the dredging devices, it is not easy to change the jet scale and soil properties during the experiment due to time and economic constraints. Some detailed physics during the jetting process, e.g. pressure on the soil surface and shear plane inside the soil during jetting, were also not monitored during the experiment. Therefore, numerical simulation is chosen to optimize the design of dredging devices and study the physics of the jetting process. A CFD (computational fluid dynamics) numerical model is built to simulate the moving jet penetrating cohesive soil. The soil is modeled as a Bingham plastic. The sediment transport is modeled by using drift-flux model. The moving jet modeling is achieved by using dynamic mesh algorithms AMI (arbitrary mesh interface) and A/R (cell layer addition removal). The CFD numerical model has been validated with the experiment of Nobel. After the validation, an analysis of the jetting process based on this CFD model is accomplished proving that the CFD model can reveal the details of the soil failure process during jetting. This thesis work reveals that it is possible to describe the hydraulic excavation of cohesive soil with reasonable accuracy using CFD numerical model. The CFD model can also reveal the details of the soil failure process that could not be retrieved from the experiment. Since the model is generic, it can be applied for a jet bar with multiple nozzles. This is helpful to improve the design of dredging equipment, optimize the operational settings and estimate the production.

Already since the start of civilisation dredging is carried out to create and maintain waterways and ports or to create new land. In recent times the scale of dredging and awareness of potential environmental impact of dredging have increased drastically. An often used dredging vessel is the trailing suction hopper dredger (TSHD). A TSHD pumps up a sediment-water mixture from the bed into a hopper. In this hopper the sediment is given time to settle and the process water is spilled overboard, often through a vertical shaft called the overflow. The spilled process water will contain some suspended sediment which has not deposited yet and this forms a turbid plume. Increased turbidity and deposition at the bed of the suspended sediment from the overflow dredging plume can have negative environmental impact. Identify the factors which create the surface plume and testing a solution to improve this is done in this thesis. Initial mixing of the overflow dredging plume under/near the TSHD is not well understood. Although the plume starts under the keel of a TSHD, it has initial downward velocity and it is denser than the ambient water, sometimes a part of the plume flows up- ward and reaches the free surface right behind the TSHD. This so called surface plume can stay suspended for long periods and is therefore important for the potential environmental impact. This thesis reports on laboratory experiments in still ambient water and numerical modelling with a crossflow. Specific attention is given to the change of overflow shape, from round to rectangular, which is tested in the laboratory experiments. Firstly, all influence factors which create a surface plume are investigated to see which factors have the highest influence. Based on the wish of a passive solution and the ability to do laboratory experiments, the change of overflow shape is chosen to investigate further. The analytical background of plume dispersion with different shapes is investigated to find the differences between them. The laboratory experiments where conducted at the dredging lab at the Technical University of Delft. In this lab, a 25m³ tank filled with water where at the top a frame is positioned which holds the overflow pipe. By adding two type of nozzles (different rectangular aspect ratio) onto the pipe, the outflow shape is changed to see if a rectangular outflow shape improves the plume path in still ambient water. This is done by measuring the concentrations at the bottom and measuring the angle of the plume. Results of the experiments show less entrainment and higher concentrations in the middle of the plume when having a rectangular outflow shape. To say something about the improvement of the plume path in real live, the plume is modelled by adding a crossflow. The information from the experiments is used to create a plume trajectory of the round outflow shape and rectangular outflow shape which are then compared with each other. Practical values are then scaled and used in the model. Results of the model show similarity from the experiments, with a deeper plume trajectory when having a rectangular outflow shape.

Understanding the erosion process of sand will help improve dredging projects. Pickup functions are used to predict the amount of sand that can be eroded by a flow with certain parameters. The most common functions that are used have been validated to data in the low velocity regime (θ < 1). However, modern day dredging processes are in the high velocity erosion regime (θ > 1).An example of such a function is the widely used pickup function of Van Rijn (1984). It predicts the pickup of grains well at low flow velocities, but overestimates pickup at high flow velocities.Recently Bisschop (2018) has performed erosion experiments at high flow velocities. In 2015 Boskalis performed experiments in the same velocity regime during a preliminary study, however, a different setup was used. It was found that there is a large (factor 2) discrepancy between the two studies.The present study has performed experiments with a setup similar to that of Van Rijn (1984), however, the flow velocity was not limited to the low velocity regime. Erosion tests were done with bed shear stresses up to 60 Pa (θ<30). The setup in question uses a sediment lift to push a prepared sand bed up into the flow.The determination of the bed shear stress was questionable in preliminary study of Boskalis. Since it is a very important parameter in erosion predictions, it was a focus of the present study to determine the bed shear stress accurately. The bed shear stress was determined with three different methods which all agree within an acceptable range (up to 8%).The erosion velocity is determined by measuring the lift velocity, which is equal to the erosion velocity when the top of the sand bed is level with the floor of the setup. Whether the top of the sand bed is level or not has been determined with the use of a high speed camera.The produced data has been compared to existing data and pickup functions of other studies. From this comparison it is clear that no existing functions match the data of the present study.In the present study an analysis is done to find out why the new data does not match existing functions. Also, based on several existing functions, a new function is presented that has a good fit with the data of the present study.

In recent years, the demand for minerals and rare-earth elements are escalating due to rapid technological advancements and developments. This condition raises the importance of Deep-Sea Mining (DSM) as an option to fulfill the global demands, for the sake of future ambitious projects. On the other hand, DSM still faces some drawbacks and obstacles in its operations, e.g. environmental impact of its tailings discharge. Thus, the presence of tools for predicting the behavior and environmental impact of DSM tailings becomes crucial for the sake of conducting sustainable DSM operations. Researches, both numerical and laboratory experiments, are then done to achieve this goal. In the study of DSM tailings behavior through numerical simulation, the challenge lies in the ability to implement the complex physics phenomena around DSM plumes to a numerical model. This research is thus aimed to observe one of the parts of the so-called physical phenomena, and numerical constraints on the simulation of DSM plumes: the effects of implementing arbitrary non-orthogonal mesh. Arbitrary non-orthogonal mesh would give users the freedom to refine the mesh based on the required resolution on a certain area in the simulation domain. In this research, an arbitrary non-orthogonal mesh is constructed forming a domain geometry of 3D tank with round pipe as a source of tailings discharge, adapting the laboratory setup of Byishimo (2018). Mesh refinement is done around the pipe discharge area, where the jet mainstream expected to occur, and near the bottom, where settling of solids fraction and high velocity-gradients are expected to occur. Top-hat approximation theory is then used for defining the inlet boundary condition of the simulation domain, and smooth solid wall is used for the bottom boundary. In observing the effect of arbitrary mesh, two parameters are varied: solids settling rate, and differentiation scheme. Following this, six simulation cases are prepared, containing three solids-settling conditions (minimum, realistic, and extreme solids fraction settling) and two differentiation schemes (Gauss Gamma and Gauss Linear). These cases are then simulated using the CFD drift-flux model in OpenFOAM for two incompressible fluids, with ambient fluid and tailings mixture as the two incompressible fluids. Turbulence is modeled using LES method, with Wall-Adapting Local Eddy-viscosity (WALE) LES model for modeling the subgrid-scale of the turbulence. Two simulated field variables are picked to be observed and compared with the laboratory measurement data: flow velocities and local Suspended Solids Concentration (SSC). Simulations show that the constructed domain able to generate stable results using not only Gauss Gamma differencing scheme but also Gauss Linear, which originally expected to give unbounded results. From simulations with various solids settling conditions, it is analyzed that the implementation of solids settling using the mentioned function leads to constant settling rate with inability to re-suspend the settled solids. Thus, simulation of cases with extreme solids settling leads to hyper-concentration of solids fraction in the cells on the bottom. The top-hat profile of the inlet boundary condition is constantly sustained throughout the simulation, resulting in uniform jet vertical velocity profile. The simulations also show that momentum-driven jet-like flow can be observed around the impingement point, while gravity current generated further from the impingement point. The simulation results show that the mesh refinement is enabling simulating and validating the flow with the required resolutions. Moreover, the constructed domain also shows that Gauss Linear can be used for simulating DSM plumes, furthermore using full tank domain. The simulated SSC also turns out not only affected by the simulation domain and mesh, but also the differencing scheme, and the presence of settling and pick-up functions.

This thesis aims at gaining insights into the operational principles of the diesel engine injection systems and reproducing the dynamic of fuel injection in the diesel engines. Two popular fuel injection systems are modeled in this thesis, including the mechanical injection system and the common-rail injection system.

Scarcity of (green) energy resources drives mankind further offshore and towards deeper waters. Protection of subsea infrastructure is of eminent importance in these environments. The key parameter to have a good control over the installation of rock is the falling time. For larger water depths up to 1200 meters, the accuracy of the falling time prediction becomes more important because a small deviation can result in a large difference between prediction and reality.

This experimental research is focused on fail mechanisms in non-cohesive soil beneath the jet with a Jet Erosion Test(JET). A vertical plain jet impinges fluid to a sand bed, which causes zero shear stress beneath the jet averaged over time. Many erosion theories would say that no pick-up happen at that point.

E_c=u/(√(H⁄d) √(ΔgD50 ))

Large range of erosion parameter was done in this research with video recordings to the activities beneath the jet. Tests varies with those parameters outflow velocities, stand off distances(SOD) and grain sizes. Comparisons are made to Rajaratnam, who has executed similar JET with varies erosion parameter too. The cavity depths and hill heights correspond well as function of the erosion parameter. Cavity widths have deviations for values above 4.0, which might cause by different grain properties or by the limitation of flume width. Secondary flow limits the width growth over time.

Weak deflected jet showed only surface erosion by micro turbulence at the soil bed till erosion parameter 1.0. One smooth bed was formed between two hill tops. Static bed did not exceed the concerning sand during and after start.
Increasing further the erosion parameter (1.0-5.0), soil deformation was also seen with ejection beneath the jet. Bearing capacity by Prandtl's theory was insufficient and failed the soil bed. Consequence, the inner cavity exceeds the internal friction angle of sand with dynamic erosion.
Erosion parameter larger than 5.0 had a wider cavity. Grains in soil translate and rotate in the start phase by insufficient bearing capacity. Vortices sweep around the jet with chaotic particle transport in the inner cavity.

Theories of Van Rijn, Meyer Peter Müller and Van Rhee underpredict for JET. The pick-ups rates are in all cases lower, which are based on the shear stress on bed. The impinging jet has different flow field and failure mechanism, which can be better related to the jet momentum.