Particle-laden pipe flows are ubiquitous in industrial applications. Examples are industries like dredging, slurry transport, and the transport of reactants and products in chemical industries. One of the most important factors in these transport processes is the friction coefficient which relates directly to the pumping power which is a significant parameter for industries when viewed from an economic standpoint. The migration of particles in dense suspensions can significantly impact the friction coefficient in pipe flow. The clustering of particles in the core can lead to a reduction in the friction factor compared to a well-mixed particle suspension. This reduction is attributed to the decreased effective viscosity near the pipe wall. While the development lengths of single-phase flows are well known, limited knowledge exists regarding the development of velocity and concentration profiles in suspension flows.

The goal of this thesis is to study the development of neutrally buoyant suspension pipe flows. The experimental setup was validated using pressure drop measurements and the entrance length for single-phase pipe flow was obtained. The experiments for suspension flows involved varying the suspension Reynolds number and volume fractions keeping the particle size constant. Ultrasound imaging technique is used to circumvent the opacity of the suspension to study the development of suspension pipe flow. The inlet conditions for concentration were characterized, obtaining a uniform distribution at the inlet. Measurements are conducted at various locations downstream of the pipe and the velocity of the dispersed phase is obtained using ultrasound imaging velocimetry. Additionally, insights into the development of the concentration profile are obtained by checking the convergence towards a fully developed intensity profile, despite the fact that the image intensity doesn’t directly correlate to concentration profiles. The velocity profiles and intensity profiles were analyzed to understand the effect of radial migration on both concentration and velocity profiles. The entrance lengths for concentration and velocity were obtained for volume fractions ranging from 0.17 to 0.25 and suspension Reynolds numbers ranging from 500-2000. The results obtained revealed that the entrance length for concentration was greater than the entrance length for velocity. Scaling of the concentration entrance length with suspension Reynolds number and volume fractions were determined, and suspension Reynolds number scaled with an exponent of -1.62 and volume fraction scaled with an exponent of -2.1. This implies that the entrance length decreases with an increase in suspension Reynolds number and volume fraction. However, no definite trend was observed for the velocity entrance length.

Nasal airway obstruction (NAO) is one of the most common symptoms in the human respiratory system and causes a considerable financial burden to both individuals and society. Currently, NAO detection is troublesome to achieve, which can influence the accuracy and effectiveness of clinical surgery. Although several objective measurement techniques are currently available, they are found to be inconsistent with patients’ sensations. In recent years, computational fluid dynamics (CFD) has become a novel technique to objectively assess nasal airflow by simulating the nasal airflow of NAO patients. Existing literature has also shown the potential to correlate relevant CFD parameters with patients’ sensations. Nevertheless, there is still debate on the numerical setup for an accurate solution and the CFD parameter to correlate with the subjective measurement. Based on these existing issues, we mainly investigated the boundary configuration (with and without the external nose), the usage of turbulence/laminar models, the usage of steady-state/transient solvers, and briefly discussed the potential of using unilateral pressure drop ratio as a parameter for NAO detection. 

To begin with, including the external nose in the nasal airflow simulation is recommended in the nasal airflow simulations. The external nose configuration can affect the flow direction through the nostrils and downstream flow distributions. However, the static (and total) pressure drop only shows a 4% difference compared to the commonly-used plane-truncated boundary configuration. Furthermore, using the laminar model is sufficient for the nasal airflow simulations concerning the static pressure drop prediction. The laminar model shows a difference lower than 15% in static pressure drop compared to the experimental values on 3D-printed nasal airway models. We stress the caution of using the 𝑘 − 𝜔 model in the nasal airflow simulations because it tends to overpredict the turbulent viscosity ratio near the inlet unphysically. Moreover, steady-state simulations can also reasonably predict nasal airflow. We observed unsteady effects when comparing the steady-state simulation with the transient simulation with a constant flow rate and the transient simulation with a sinusoidal flow-rate-versus-time profile representing the real-life breathing cycle. Nevertheless, the steady-state simulation achieves an accurate prediction in static pressure drop, with a difference lower than 6% compared to the tested transient simulations. The steady-state simulation can also perfectly match transient simulations in the velocity profile of the recirculation zones and require a much lower computational cost. Last but not least, we also tested the possibility of using the unilateral static pressure drop ratio for NAO detection using CFD. However, we note that future studies should make corrections to account for the nasal cycle effect for NAO detection. 

Overall, we conclude that including the external nose and using the laminar simulations with the steady-state solver can give an acceptable prediction for the nasal airflow, especially concerning the static pressure drop prediction. We also state that applying CFD in the nasal airflow shows the potential for NAO detection, although future studies may consider making some corrections to include the nasal cycle effect.

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.

The turbulent boundary layer development under the influence of an air cavity is studied experimentally using planar PIV, with the aim of gaining insight and building upon the flow physics typically encountered in the application of air layer drag reduction. A detection technique based on correlation values is implemented to obtain an approximate shape of the air cavity and the location of the air-water interface. The technique was successful in identifying the maximum cavity thickness with sufficient accuracy. The leading and trailing edges of the cavity however, were harder to identify, the former owing to a limitation of the developed technique and the latter due to the dynamic nature of the flow and a slightly limited FOV. The ratio of the initial boundary layer thickness to the maximum thickness of the air cavity is 6.7, and as a consequence the boundary layer did not separate at the leeward side of the air cavity. The turbulent boundary layer is observed to feel the presence of the air cavity up to 8.5-9.5 cm upstream due to an adverse pressure gradient. Alternating streamwise pressure gradients are generated due to the curvature of the air cavity: from an adverse to favourable and back to adverse. Compared to solid bump studies in literature, additional perturbations due to a free-slip boundary condition and the unstable nature of air cavity increase the complexity of the current flow. The mean velocity profile and stresses are able to capture the effects of alternating streamwise pressure gradients and air injection, with variations mostly restricted to the inner region. Effects of streamline curvature in the outer region are found to be minimal, while potential effects of the free-slip condition were much harder to identify separately and further research would be needed to appropriately assess them. The mean velocity profile is found to deviate from the classic logarithmic behaviour at the apex of the air cavity, although the flow does not seem to relaminarise. Quadrant analysis shows differences in Reynolds stress producing events compared to the baseline turbulent boundary layer case hinting at possible alteration to coherent structuring of the turbulent boundary layer developing below the air cavity.

Multiphase flows are found in abundant natural phenomena and industrial processes. In particular, pipe flows are widely popular in industries involving chemical processing, dredging, and oil transport to name a few. Particle laden flows are often observed in applications like dredging, sediment, and slurry transport through pipes, etc. Therefore, it is imperative to understand the flow phenomenon in detail. The most important parameter in pipe flow is the pressure drop across a given length. This relates directly to the pumping power which is a significant parameter for industries when viewed from an economic standpoint. This emphasizes understanding the different regimes for particle-laden flows and the impact of particles on transition in particular. Research on transition behavior for particle-laden pipe flows is scarce and the behavior is far from being completely understood.
The literature is replete with the study of transition for single-phase pipe flows. Different perturbation mechanisms were analyzed and the lifetime studies indicated that the puffs are memoryless in nature. The literature provided different formulations to accommodate for the presence of particles with regards to modifying the viscosity of the suspension. The research in multiphase flows has provided inconclusive results in determining the critical Reynolds number for transition as different criteria were provided in different works.
The goal of the current thesis is to perform experiments to understand the transition behavior of particle-laden pipe flows for different particle concentrations. The novelty of this work is the use of an active perturbation mechanism that enabled the study of transition in perturbed and unperturbed flows. The experiments involve varying the particle concentration and keeping the ratio of pipe to particle diameter constant. The study concentrates on understanding the transition behavior using Moody diagrams. The experiments rely on pressure drop measurements to record the average pressure drop across the pipe and study the intermittent structures that drive the transition behavior. The study uses glycerol to make the solution neutrally buoyant when using particles. Single-phase measurements are performed to validate the setup including the pressure sensors and the perturbation mechanism. The Moody chart indicates that the transition is sub-critical with Spatio-temporal intermittency for particle concentrations less than 1.5 %. Interestingly, the particle-induced disturbances are significant and the transition behavior is identical for perturbed and unperturbed flow. This suggests that the disturbance created by the particles is qualitatively similar to that of the perturbation mechanism. However, the transition becomes super-critical for higher particle concentrations as the transition is driven by the fluctuations generated by the particles and the additional friction created by them. The friction factor decreases monotonically for very high particle concentrations (≥ 15 %). The transition behavior is investigated further by analyzing the time series data of pressure drop for different particle concentrations at intermittency of 10 - 20 %. The transition criteria are analyzed based on deviation from the Poiseuille line in the Moody chart and spike in pressure fluctuations in the flow. The latter provides inconclusive evidence for higher particle concentrations. The former holds good for the current study, however, it needs to be revisited for other particle sizes.

Horizontal Directional Drilling permits the creation of tunnels which pass beneath rivers and canals to allow the passing of services from one side to the other. The final stage of this process involves lining the tunnel with a plastic or steel pipe. The pipe is pulled inside into the borehole using the same drilling rig that was used to bore the tunnel. For the case of a plastic High Density Polyethylene (HDPE) pipe, issues of buoyancy may arise during this pullback process since the tunnel is pre-filled with drilling mud which now primarily acts as a lubricant. This presents a problem since with this buoyancy, the pipe is lifted up to brush against the tunnel wall, creating issues with the pullback process because of the added pulling resistance. There is the possibility of cutting slots in the pipe wall at the front end of the pipe in order to allow in drilling mud so as to ballast and offset this buoyancy. The drilling fluid is a suspension of bentonite in water and is characterised as a non-Newtonian shear-thinning Herschel-Bulkley fluid, which possesses a finite yield stress. This study aimed to find whether the current slot proportions used for a given borehole diameter, pipe diameter and pipe thickness are sufficient for allowing in drilling mud to ballast the pipe. This was a case where the multiphase flow given by the interaction of air and drilling mud can be simulated using CFD. OpenFOAM is used for this purpose to first simulate the current practice. The multiphase solver interFoam together with the non-Newtonian Herschel-Bulkley and air model was validated for a series of cases before the main simulations were run. These validations included the Marsh Funnel test and the Slump test. These are two kinds of workability tests used for cement pastes and drilling muds. Cement pastes and drilling muds are characterized as three-parameter Herschel-Bulkley fluids and the physical setups of the Marsh Funnel and Slump tests were replicated in computational space. The flow time of theMarsh funnel test and the slump diameter from the CFD simulations were compared with experimental data from literature thus validating the model in OpenFOAM. The main simulation setup recreates the situation of an HDPE pipe concentric with the borehole, with the axis of the domain at an angle to the horizontal. There is drilling mud above the slot at time t = 0s. The drilling mud comes into the domain from the annulus from below the slot. The simulations showed that the flow into the slot initially came in from both the drilling mud above the slot and from the inlet. After the volume of drilling mud above the slot is almost drained completely through into the inner pipe, the mud level inside the pipe starts to become comparable that outside the pipe and both interfaces rise upwards at approximately the same pace. Subsequently, certain parameters were changed from the first benchmark case in order to see what is the effect of these individual variables. The effects of a lower drilling mud yield stress, a longer pipe slot, lower drilling mud density, a different slot aperture shape, increased flow and a steeper angle of pipe penetration were all tested in the simulation campaign. The idea is that with better and faster filling, less pipe buoyancy results. An extension of this idea is that the difference between mud levels inside and outside the pipe should be kept to a minimum. That is why the aim is to increase the flow rate through the slot in the pipe. The results show that increasing the slot length by 30% from the current practice increases the throughput of drilling mud by 10%. The results also show that an elliptical slot profile has a neutral effect and decreasing the yield stress of the drilling mud has a slightly beneficial effect.

The phenomenon of lateral migration of neutrally buoyant rigid spheres is studied experimentally for Poiseuille Flow. The study relied on the particle migration technique to capture the distribution of particles radial position at different flow parameters. In this thesis, the varying experimental parameters are flow Reynolds number and particle concentration. These two parameters have been reported to have an opposing effect on the migration. Reynolds number is varying at Re=200-1200 and the particle concentration at φ = 0.05-0.5% . The results reveal that an increase in Reynolds number and particle concentration causes the migration to develop at a longer distance from the inlet. The migration is said to be developed when the particles have migrated to the region between the tube-axis and tube-wall. An increase in particle concentration shows a similar effect with the Reynolds number on the migration which the migration develop at a longer distance. An interesting result occurred at high particle concentration, at which the significance of the Reynolds number in altering the migration is decreasing. The study is also conducted to the secondary phenomena following the migration, the generation of inner annulus and the formation of trains of particles. It is shown that the variation of Reynolds number and particle concentration are significantly affected these secondary phenomena.

Multi-phase flows are ubiquitous in nature and in everyday life surrounding us, impacting us in almost all possible ways. The presence of particles in a flow can change the flow behaviour in an unpredictable manner. The simplest example of a particle-laden flow, that one can think of, is the settling of a single sphere under gravity in a quiescent fluid. This seemingly simple problem has very high relevance in various practical applications ranging from sedimentation of particles for water treatment, process industries, transport of a dense suspension(slurry) through a pipe and even in land reclamation. The settling/ascension of a single sphere, even after having been subject to extensive study for more than a century, remains far from being understood completely. The path and the wake of a falling/rising sphere in a quiescent fluid may be subject to various instabilities depending upon two dimensionless quantities which are sufficient to characterize the motion. One being the Galileo number (Ga), which is the ratio of the net gravity force to the viscous force and the second one being the mass density ratio, which is the ratio of the density of the solid to the density of the fluid. Depending upon Ga and mass density ratio, the sphere can take up various regimes of motion such as vertical, oblique, zigzagging, helical to name a few. This is mainly due to wake instabilities that trigger such path instabilities. Based on Ga and mass density ratio, various regime maps have been proposed in literature. There have been several disagreements regarding the characterization of such paths taken by the sphere. This is due to the strong solid-fluid coupling and the inherent complexity due to triggering of the instabilities in such cases, which is far from being trivial to model numerically and also to test experimentally. The disagreements between different numerical works and different experimental works make the problem hard pressing and tempting to study. Moreover, the settling behaviour of a single sphere can also aid in understanding the collective effects displayed in the settling of dilute suspensions. The goal of the present study is to shed light on the confusion/disagreements in literature until now and characterize various path instabilities. A detailed experimental investigation is conducted to cover the parameter space (regime map) by employment of over 250 different combinations of Ga and mass density ratio to cover as many regimes of motions as possible within the given time framework. The motion of a sphere is tracked in time using high-speed cameras and corresponding path/regime of motion, higher-order statistics like velocity and physical characteristics such as the Strouhal number/ drag coefficient has been computed. The results validate well for some simple regimes of motion for which results from the previous studies perfectly agree with each other. With the confidence obtained after the validation, the current work attempts to draw points of consensus and disagreements with these earlier works for other more controversial regimes. Some regimes, which had only been observed using numerical simulations, have been observed experimentally for the first time. Also, intriguing bi-stable regimes (coexistence of two regimes) have been observed. Moreover, attempt is also made to characterize the suppression of the high-frequency oscillations with increase in the sphere inertia. An update of the regime maps is proposed with the results obtained from the experiments conducted. The results obtained will also serve as an excellent tool for validation of new numerical models, using which the Ga-mass density ratio parameter space can be covered in great detail. Recommendations for future work are given.

Optical measurement techniques are widely used in fluid mechanics to measure quantities like velocity, vorticity, and temperature. They are usually non-intrusive visualization techniques that provide a major advantage by not influencing the flow properties. Most of the developed measurement techniques estimate vorticity by computing the gradients of the measured velocity fields. In this project, the idea is to directly measure the mean vorticity using polarized emission of nanoparticles. The nanoparticles are excited using vertically and horizontally polarized laser. Accordingly, the emission from these nanoparticles is separated into vertical and horizontal polarized emissions respectively. Depending on the percentage of fluorophores in the nanoparticles whose absorption dipole moment is parallel to the excitation electric vector of the laser, the polarized emission is varied. The nanoparticles in the quiescent fluid state have a characteristic polarization (or anisotropy) depending on the chemical composition of the nanoparticle. However, in turbulent flows, depolarization of the particles can occur because of the rotation of the nanoparticles due to vorticity. Based on this idea, the vorticity can be estimated by quantifying the depolarization in turbulent flows. To verify the above phenomena experimentally, steady-state polarization measurements were conducted. An experimental setup was built specifically to measure depolarization from these nanoparticles. Important preliminary tests like lifetime measurements, spectroscopic studies, and photo-bleaching experiments were conducted. Based on these results, hybrid europium based chelated spherical particles were chosen. To further understand the properties of these particles in a quiescent flow, a consolidated set of anisotropy measurements were conducted to study their oxygen sensitivity in the surrounding medium, viscosity dependence and the effect of its concentration in the fluids. Analytical correlations which was developed in-house to translate the depolarization of the nanoparticles to vorticity was used in this thesis. Flow in a square duct was studied to estimate the vorticity along the edges and further compare it with the DNS data.
Two additional non-dimensional numbers ND1 and ND2 were introduced to interpret the underlying physics behind the problem. The variation of synthetic signals was studied for different Re, fundamental anisotropy r0 and the non-dimensional number ND2. These synthetic signals showed a drop in anisotropy near the edges of the square duct that has occurred probably due to vorticity. Emission polarized intensity signals and the anisotropy contours for Re = 0 and Re = 4434 were analyzed. Due to the measurement error, the contours were column averaged and respectively compared their variation along the width of the square duct. A drop in anisotropy was observed for Re = 4434. Using the ratio of emission intensities for each excitation polarization, the vorticity was estimated using the optimization algorithm. The estimated vorticity Ѡy shows similar trends compared to DNS data which is a promising evidence for the proof of concept. However, the results are not overlapping from the repeatability studies due to the high sensitivity of the optimization algorithm and the random error from the measurement equipment. To improve this technique in the future, transient experiments have to be conducted to estimate vorticity accurately and to capture the underlying physics.

Over recent years, there has been an increase in demands for rare and precious minerals worldwide. Mostly this is due to the rise of the world’s population and the drive towards a green energy transition and low carbon economy. Prices are rapidly increasing, and there is an identifiable risk of an increasing supply shortage of raw materials, including those identified as critical to Europe’s high technology sector. The development of surveying techniques and advances in new technologies in remotely operated vehicles (ROV) has allowed detecting that the most valuable and rare mineral resources are spread out in the sea-floor and international waters. Currently, the most significant setback towards exploitation licenses is not because of lack of technology but because of a lack of knowledge on deep-sea biodiversity and the impacts of mining on ecosystems. The ISA (International Seabed Authority) is responsible for the regulation, and the control of mineral-related activities in the international waters is currently working on drafting environmental regulations. This presents significant opportunities for research on the development of technologies that incurs in the least environmental impact possible. Currently, the main concerns are regarding the horizontal sediment-laden plume that is generated as a result of the mining process. Therefore, the industry is working towards the development of equipment that lessens the plumes spread. Work done in this research focuses on small scale sized laboratory experiments in which the sea-floor crawler’s outlet shape is varied, and its plume’s effects of the sediment waste and other effluents (SWOE) are measured. Besides gaining insight in the horizontal plume spreading under different conditions and geometries, the outcome of the research is to provide a set of measurement data that can be used for future numerical model validation towards determining an optimal outlet shape for the seabed crawler. For this purpose, a total of three diffusers were designed and tested based on a specific scaling factor and input parameters defined by IHC Mining. Experiments consisted of capturing visual imagery of the generated plume on both top and side views for further analysis and performing velocity and concentration profile measurements in different locations. The offset jet transition from jet to plume and the respective impingement point for all diffusers was compared. Velocity and concentration measurements were analyzed and compared to determine an ideal diffuser. Using diffusers reduces the plume’s initial momentum while maintaining the density differences and therefore reduces the transition from jet to plume, allowing for gentler deposition over the surface and resuspending less material. All experiments show similar behavior in which a free jet can be observed initially. Then, after substantial entrainment takes place, the jet impacts the bed due to the negative buoyancy. Once the discharge impinges the lower boundary, it forms a radially spreading layer along the boundary which transitions into a wall jet. Lowering the momentum of the jet by diffusing the outlet gave better results in terms of drawing the impingement point, and the deposition profiles nearer to the diffuser. Also, the measured velocity and concentration profiles considerably decreased, reducing the plumes spread.

Taylor-Couette (TC) flow refers to the flow in the annulus between two coaxial, independently rotating cylinders. The TC system has been subject to multiple experiments spanning over decades due to the instability phenomena that occur in the flow. When the rotation rates of the cylinders are increased beyond a critical value, instabilities appear in the system that result in the formation of different flow regimes. Single-phase flow in the TC system has been studied extensively and the various flow transitions have been catalogued for different geometrical parameters and rotational conditions. In the current experiments (Radius ratio= 0.917, Aspect ratio= 22), flow visualisation with anisotropic reflective particles has been used to obtain qualitative and quantitative information about the different flow regimes using Space-Time (S-T) plots and their spectral analyses. An aqueous glycerine solution was used as the working fluid for single-phase flows. The critical Reynolds number (calculated based on inner cylinder shear rate) for the transition from laminar Couette flow was found to be slightly higher for the current setup in comparison to other experiments found in literature, but the order of flow transitions and their spectral characterisation for all the flow regimes showed good agreement, serving as a validation for the setup. With the established single-phase flow as a base, the effect of particle loading on the flow transitions was studied. A neutrally buoyant particle-laden suspension was prepared using an aqueous glycerine solution with Poly(methyl methacrylate) (PMMA) particles of 619-micrometer diameter. The volume fraction of the particles was varied from 0.05 till 0.40 and the flow map was constructed for multi-phase flow. The primary effect of particle addition is an earlier onset of the first transition from laminar Couette flow, thus indicating a destabilisation of the flow by particles. In addition to this, several non-axisymmetric flow structures appeared in the suspension experiments which were absent in the single-phase flow experiments. The particles caused the appearance of flow regimes such as spirals (Taylor vortices that move up the cylinder axis in a helical motion) and ribbons (block-like structures that have alternating light and dark squares), which normally occur in the case of counter-rotating cylinders for single-phase flow. The transitions across all volume fractions were characterised based on the S-T plots and/or spectra to obtain a consolidated flow map for particle-laden suspensions. The results presented point towards intriguing flow behaviour that provides a large parameter space for further research in the years to come.

Growing markets for sustainable technologies such as solar panels, wind turbines, and electric cars require an enormous amount of raw materials that might not be available from sources on land. Exploitation of seafloor mineral resources could secure the supply of raw materials for the future. However, proposed deep-sea mining (DSM) systems will return wastewater containing fine sediments and other effluents back to the sea via a pipe discharging plumes close to the seabed. Concerns are that the DSM plumes will have significant environmental impacts on the mostly uncharted deep-sea ecosystem. Until today, the environmental licensing procedure for DSM operations is incomplete. Accurate numerical models to predict the behavior of returned DSM plumes are still lacking.

This research aims to support environmental impact assessments for DSM operations. Laboratory tests were carried out to provide validation data for numerical models that can be used to predict the spreading of DSM plumes in the near-field domain. Additionally, Computational Fluid Dynamics (CFD) technique was used to predict the same laboratory simulations. Wide ranges of initial parameters such are the plume release elevations relative to the bed, source initial Suspended Sediment Concentrations (SSC) and different bed slopes were considered.

Experiments and CFD results agreed well. Negatively buoyant plumes fall vertically towards the bed and impinge generating turbidity currents. Excess buoyancy gained by increasing the initial SSC at the source leads to a considerable increase in the plume dispersion rate. Plume release height is found to be an important parameter. Discharging close to the bed leads to the formation of a circular levee around the impingement point at a distance 10 times the orifice diameter. The sedimentation ring acts as a barrier against the turbidity current leading to the deposition of sediments right outside the ring. As a consequence, the plume dispersion rate reduces and DSM wastes stay closer to the disposal location. In contrast, when plumes are released at a greater height from the bed, due to long falling distance and settling of particles, generated turbidity currents accelerate and the sediments particles are transported in suspension to large distances from the disposal location.

The main goal of this research, to provide a unique set of validation data for numerical models was achieved and a Large Eddy Simulation CFD model was successfully used, tested and approved for its capability to accurately predict the near-field spreading of DSM plumes. Based on obtained results, in order to minimise the environmental impacts, the flow velocity at the bed should be minimised to prevent re-suspension of already deposited sediments. It is recommended that sediment wastes and other effluents should be released close to the seabed. CFD model setup considered only sedimentation of particles and ignored erosion. Based on observed seabed morphological changes during the laboratory experiments, it is recommended that erosion should be considered within numerical models. To fully predict the environmental impacts of DSM operations, the near-field model used in this research should be coupled to a far-field model but also considering multiple environmental pressures caused by DSM tailings.

Noise generated due to flow over corrugated pipes has extensively been studied, because of its presence in various engineering applications. Flexible risers used in the oil and gas industry are one of these applications, where noise is produced due to corrugations present at the innermost layer of the riser. Dry-gas flow through a corrugated pipe generates an unstable shear layer over each of the corrugations. Under certain conditions, this shear layer can roll up into discrete vortices, which impinge on the downstream cavity edge, producing pressure pulsations. When the frequency of impingement matches with one of the natural pipe frequencies, a ‘lock-in’ mechanism causes intensification of the associated noise and vibrations, which can have serious implications on the structural integrity of the corrugated pipe. This phenomenon is called Flow Induced Pulsations (FIP). Liquid injection in a corrugated pipe with gas flow has shown the potential to mitigate whistling caused by FIP. A number of mechanisms play a role in the whistling mitigation. It is shown in literature that the filling of cavities due to liquid film or rivulets present on the pipe wall is an important mechanism. The cavity filling results in an altered cavity geometry, which changes the shear layer dynamics. The effect of liquid and gas flow rate on the cavity filling, however, remains unknown. The present work aims to study the liquid filling behaviour of a single cavity, due to a gas flow driven liquid film.
An experimental setup is constructed which consists of a rectangular channel containing a cavity. Liquid is injected in the channel as a film at the channel wall, which is driven upward by gas flow. The liquid film thickness upstream of the cavity is measured first, followed by the cavity filling itself. Both measurements are performed using a Laser Induced Fluorescence (LIF) based technique. After several post-processing steps, the film thickness and cavity filling are quantified.
At low liquid flow rates, a partial film is created at the channel wall with a dry patch at the center. Increasing the liquid flow rate results in a full film covering the entire channel width. The film thickness varies in the transverse direction due to the presence of localized horseshoe shaped disturbance waves on the liquid film. Overall, the film cross-sectional area increases with increasing liquid flow rate, either due to an increase in film thickness of full films, or an increase in width of the partial films. The liquid film fills up the cavity, accumulating mainly at the upstream edge. The downstream edge remains relatively free from liquid. The amount of filling is found to increase with increasing local film thickness at the measurement position away from the channel side-wall. Changes in the cavity geometry due to liquid filling are estimated based on the cavity effective depth and length. The effective cavity depth does not significantly change and only decreases by 8% with a 25% increase in filling ratio. However, the effective length decreases substantially by 30% with a 25% increase in filling ratio. This could lead to a mode parameter value (ratio of cavity length to incoming gas momentum thickness), such that it does not fall in the range where whistling is observed, resulting in mitigation of whistling.

The aortic valve is a valve made of three thin flexible leaflets which open and close in order to allow the unidirectional passage of blood from the left ventricle to the aorta. Because of this repetitive motion at each cycle, the valve is constantly exposed to large variations of pressure and hemodynamic forces. Deterioration in its peculiar functioning could lead to the arising of valvular diseases such as stenosis or regurgitation. A widely spread procedure to cure severely damaged heart valves is the replacement with artificial heart valves. However, up to now, the natural aortic valve properties and functionalities have never been equalised by any manufactured prototypes in terms of efficiency and durability. This limitation has inspired many engineering researches with the purpose of gaining a complete understanding on the working principles of the valve and the evolution of the flow through it in order to enhance the performances of these medical devices.
In this thesis, numerical simulations of blood passing through the aortic valve have been performed. In particular, this study proposes an alternative investigating approach which consists on prescribing the rigid motion of the leaflets a priori as taken from experimental values and without the employment of the structural solver. To execute this strategy, the Arbitrary Lagrangian-Euler method along with remeshing techniques has been employed in Ansys Fluent. The main objectives concern the study of the resulting hemodynamics in the aortic root and the validation with other investigating techniques such as experimental work and Fluid-Structure Interaction simulations.
First, the methodology was tested in a two-dimensional geometry which embodied a simplified representation of the aortic root. Secondly, a more realistic three-dimensional case was analysed with the employment of a commercial bileaflet mechanical valve. In this case, two different methods were adopted to deal with the transition of the flow to a turbulent regime. Namely, the analysis was conducted both by the resolution of the unsteady Reynolds-Averaged Navier-Stokes equations with the k-ε model and by maintaining a laminar approach.
Validation of the method with available experimental data showed that the two-dimensional case is well reproduced. On the other hand, mainly qualitative agreement is obtained for the three-dimensional case. This suggests that the proposed approach is able to capture the main features of the flow to a certain extent and future investigations are required to obtain more accurate predictions of the flow.

An aneurysm refers to the local dilation or ballooning of a blood vessel and if left unchecked, almost every aneurysm continues to grow in size until it eventually ruptures. The abdominal aorta is the most common location for an aneurysm to form and an abdominal aortic aneurysm (AAA) can turn lethal in the event of rupture. Since the current clinical practice to classify AAAs is on the basis of their size (diameter), it was decided to investigate the impact of varying geometric parameters viz., the expansion ratio (ER) and the expansion angle (EA), on the pulsatile flow field in a hypothetical axisymmetric AAA (4A) geometry and in particular, investigate the role turbulence. All the combinations of ER and EA lead to a total of 8 cases. The simulations show that a vortex ring is shed just after peak systole in every case. The vortex ring, at inception, is similar for all cases within visual limits. As the ring travels downstream, an azimuthal instability sets in that grows with time. This is the short wave or the elliptical instability usually associated with a vortex pair perturbed by an infinitesimally small amplitude sinusoidal wave. The number of waves formed along the circumference of the ring as a result of the instability varies from case to case but in general, the number of waves decreases as the ER increases. The growth of this instability, along with interaction of the ring with the remnants of the flow field form the previous cardiac cycle, cause it to break down into smaller vortices thereby generating turbulent fluctuations. Further, subtle differences between various cases regarding the vortex ring breakdown mechanism are observed and discussed. Investigations are also carried out to identify recirculating or dead flow regions as in the context of an AAA, such a region indicates the presence an intraluminal thrombus (ILT) which is a commonly found feature in AAAs. An ILT deprives the surrounding arterial tissue of oxygen thereby weakening the aortic wall and increasing the risk of rupture. From the simulations, it could be seen that the inflection regions of the 4A geometry are most prone to the formation of an ILT which is in agreement with the physically observed locations. Finally, the oscillatory shear index (OSI) is investigated to see the influence of turbulence on a hemodynamic parameter. It was found that for the geometries tested, there is no clear co-relation between the two. From this thesis, numerous conclusions can be drawn. But perhaps the most important conclusion is that the flow field in an AAA is very complex and sensitive to various input parameters such as the ER and EA. Although flow parameters such as the Reynolds and the Womersley number are kept constant across all the cases, existing literature clearly highlights the dependence of the flow field on them. As such, the desire to be able to formulate general co-relation trends between various flow field quantities in AAAs is wishful thinking at best.