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.

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.

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.

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.

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 $Re_ au = 60$. 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 prospect of stricter national and international emission standards for the shipping industry are a driving force in the search for alternative shipping fuels, such as liquefied natural gas (LNG). However, new challenges arise with the widespread use of LNG. For example, there is a desire to use LNG cargo containment systems at lower filling levels. These filling levels are strictly limited to prevent the movement of liquid inside the containment system, which is known as sloshing. Sloshing inside a cargo containment system can result in extreme wave impact events with the potential to cause structural damage. Therefore, a fundamental understanding of these extreme wave impact events is required before studying increasingly complex phenomena.   The study of wave impacts on a wall has been an active area of research for decades. Moreover, the impact of waves upon structures is relevant for many fields such as ocean, coastal, and maritime engineering. The generation of repeatable waves in a laboratory environment is not trivial (Bagnold, 1939). Small changes in the experimental conditions, such as the water depth and the wave generation method, result in significant impact pressure variability. The impact pressure variability is even observed in carefully repeated wave impact experiments with minimal variability of the input parameters. For these measurements, the source of the impact pressure variability is thought to be the instability development on the wave crest. However, the mechanism that is responsible for the formation of these instabilities is still largely unknown.   The aim of this work is to gain insight in the sources of wave impact pressure variability. This is accomplished using direct measurements of the liquid free surface and particle image velocimetry of the surrounding air. The measurements are limited to a single wave (i.e., with a fixed steering signal and water depth) at atmospheric conditions, because of the complexity of the experimental measurements. A plunging breaking wave with a large gas pocket is generated that impacts on a vertical wall. The compression of the large gas pocket induces a significant gas flow between the wave crest and the vertical impact wall, which results in the formation of instabilities on the wave crest.   Quantitative measurements of the liquid free surface are obtained with an extension of the planar laser induced fluorescence (PLIF) method. The newly developed scanning stereo-PLIF measurement technique uses a stereo-camera set-up with a self-calibration procedure adapted for free surface flows. Thereby, the stereo-PLIF technique enables measurements of a free surface over a two-dimensional domain (e.g., y=f(x,z,t)). The system is versatile with a minimal influence on the fluid properties and the measurement domain can be scaled as needed.  A repeatable plunging breaking wave is created in the wave flume of the Hydraulic Engineering Laboratory at the Delft University of Technology. The wave encloses a gas pocket as it approaches the vertical impact wall. Initially, the plunging breaking wave is globally comparable to waves that do not impact on a vertical wall. The aspect ratio of the cross-sectional area of the gas pocket remains relatively constant at Rx/Ry = 1.6 ( ∼√3). Furthermore, the wave velocity (√gh0) and wave tip velocity (1.2√gh0) are initially similar to that of a plunging breaking wave. On the other hand, the trajectory of the wave tip is altered compared to that of a typical plunging breaking wave.   The trajectory of the wave tip is globally similar over repeated wave impact measurements. However, moments before impact the wave tip is deflected by the gas expelled from the gas pocket. The deflection of the wave tip introduces significant variation between the repeated measurements. On close inspection, the wave tip resembles a liquid sheet, that is destabilized by an initial Kelvin-Helmholtz instability (Villermaux et al., 2002). The flapping liquid sheet accelerates the wave tip, which triggers the development of a Rayleigh-Taylor instability. This results in approximately equally spaced liquid filaments (i.e., liquid fingers) over the spanwise direction of the wave. The spanwise wavelength depends on the density ratio (ρa/ρl) and surface tension, which was previously shown to be a source of wave impact pressure variability.  Additionally, particle image velocimetry measurements are performed to determine the interaction between the liquid and gas during a wave impact event. The global gas flow is similar to that of a plunging breaking wave, where a vortex develops on the leeward side of the wave. The vortex consistently separates from the breaking wave and lingers at the back of the breaking wave in the stagnant air. The development of circulation is typical for a vortex that eventually separates at a universal time scale denoted by the formation number (Gharib et al., 1998). However, a typical formation number can not be defined in this particular case, due to the simultaneous change of both the length and velocity scales.  The velocity profile between the wave tip and the vertical impact wall resembles that of a flow past a bluff body. A fit of the measured velocity profile agrees well with the velocity derived from mass conservation. Interestingly, the velocity close to the wave tip is approximately 2 times higher than the bulk velocity estimate. The high velocity close to the wave tip can thus result in an earlier onset of instability development compared to estimates based on the bulk velocity. Furthermore, the flow tends to separate close to the tip just before impact on the vertical wall. The effect of this flow separation on the impact pressure variability depends on the global wave shape prior to impact. For the case with a disturbance on the wave crest, the secondary vortex that forms close to the wave tip tends to break up. On the other hand, if the wave crest is smooth the secondary vortex remains attached. The attached secondary vortex increases the lift on the wave tip, which results in a significant deflection of the wave tip. Consequently, the development of secondary vortices close to the wave tip results in wave impact pressure variability, as the typical deflection is larger than the membrane diameter of a contemporary pressure transducer.   Local phenomena such as flow separation and the development of instabilities define the variability of the peak pressure during wave impacts. On the other hand, the global characteristics of an air-water wave impact on a vertical wall can be retrieved with pressure impulse models (Cooker et al., 1995). The maximum wave impact pressure is relevant for LNG containments systems and wave energy converters. Consequently, numerical models that aim to quantify wave impact pressure variability require accurate models of both the gas phase and the development of free surface instabilities. @en

Dense suspension flows, both in the natural environment and industrial settings, are complex phenomena with significant implications. From rivers shaping landscapes to industrial processes involving slurry transport, these flows hold a prominent position in numerous sectors. This thesis delves into a specific facet of these intricate flows: slurry transport within horizontal pipes. Slurry, a mixture of solid particles and a viscous fluid, presents a challenging arena due to its dynamic nature, encompassing multiple flowregimes and diverse phenomena that govern its behavior. This research seeks to unravel the complexities of slurry transport, presenting a comprehensive analysis using interface-resolved Direct Numerical Simulation (DNS). In the context of slurry transport (also referred to as sediment transport), a horizontal pipe is a conduit where particles suspended in a viscous fluid are transported. The dynamics of this transport are governed by several dimensionless numbers, each highlighting distinct aspects of the flow. Prominently, in this work we explore the role of the Reynolds number (Re) which encapsulates the balance between inertial and viscous forces, the Galileo number (Ga) which characterizes the competition between inertial and viscous effects in particle settling under gravity, and concentration of particles which has an influence on particle-particle and particle-fluid interactions. Key flow dynamics that determine the behaviour of the flow include turbulent mixing, gravitational settling of particles, and shear-induced particle migration due to particle-stress gradients. Practical applications of slurry transport are numerous, spanning industries such as mining, agriculture, and chemical processing. Slurry transport is of particular relevance to the dredging industry in the Netherlands to maintain its inland waterways and for land reclamation projects. However, pipeline operators grapple with issues ranging from pressure drop and the prevention of bed formation to the control of excessive pipe abrasion, silting risks, and production instability. These challenges stem from the intricate interplay of particle behavior, fluid dynamics, and pipeline geometry....@en

Flow-induced acoustics are a well-known phenomenon, occurring in a broad variety of applications, as well as in nature. In many applications, the produced acoustics are purposeful, e.g. for communication and in musical instruments. In other circumstances, however, the sound and vibrations are a nuisance, and even harmful to human beings and the environment. Pipes with internally grooved or corrugated walls can be such a source of sound production. These pipes find broad application due to the local stiffness that is combined with larger scale flexibility of the pipes. The main industrial use of corrugated pipes is as flexible connections, transporting a gas or liquid between e.g. ships and onshore storage facilities, or between subsea bore-wells and floating production facilities.@en