At the turbulent/non-turbulent interface (TNTI) of a jet flow, momentum is transferred from the turbulent jet fluid to the fluid at rest. This transfer is governed by two mechanisms. One of them is related to the large scales of the flow and the other relates to the small scales of the flow. Which of the two is dominant is still a point of discussion. To analyse the TNTI the instantaneous information of the flow and the location of the interface is of great importance. However, the interface simultaneously develops and travels downstream from the nozzle. With stationary measurement techniques this limits the number of frames the interface can be seen developing as it travels in and out of the FOV. In this thesis the TNTI is measured using a camera system that moves along with the TNTI to get high resolution instantaneous measurements of the same part of the evolving interface. The measurement techniques that are used for this experiment are Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF). The cameras for these measurements are mounted to a motorised frame to keep the same part of the interface in view of the cameras as it develops.

Three Reynolds numbers are measured with this setup and the cameras move approximately 50Dn at a velocity close to the velocity of the interface along a diagonal path to follow the evolution of the TNTI.

One measurement with a Reynolds number of approximately 1.2 × 104 has been processed to show the quality of the results that can be obtained from such a measurement. The velocities are computed by an in-house interrogation analysis program in MATLAB to overcome the wide range of particle displacement found in this experiment, due to the presence of both the centreline and the TNTI of the jet. The TNTI is detected using the LIF data and a threshold detection method from literature that determines a threshold value. The results from the PIV and LIF processing is combined to compute the average conditional vorticity over the interface.

The results show that the PIV analysis is able to compute the velocities of almost the entire jet. Only showing a lot of spurious vectors close to the nozzle in the core of the jet. The LIF edge detection algorithm, on the other hand, does not perform as expected. In multiple instances, an internal interface is detected instead of the TNTI. The TNTI is also determined by identifying a threshold value through a visual inspection of the LIF images. This manually determined TNTI is used as a point of comparison for the average conditional vorticity profiles. The average conditional vorticity profiles support the conclusion that an interface internal to the jet is detected when comparing the interface from the algorithm to the interface determined manually.

Although not quantified in this thesis, there are moving measurements that show the same section of the TNTI evolving for many frames. This gives the co-moving measurements a clear advantage in measurement time compared to stationary measurements when trying to measure the moving TNTI. Refinements to the experimental setup, the behaviour of these internal interfaces and the detection of the TNTI can be of interest for future research.

Natural gas is one of the most important global energy sources, and is commonly transported in pipelines (in gaseous state) or specially-designed LNG vessels (in liquid state). As LNG is stored inside the ship cargo containment system at atmospheric pressure and low temperatures, just below the boiling point of circa -162 ◦C (depending on the composition), small perturbations in pressure or temperature may result in phase changes between the liquid and vapor phase. During transportation overseas, the motion of the ship induces movements of the LNG inside the containment system. Sloshing may result into wave impacts that potentially cause damage to the containment system. During these wave impacts, phase changes inside the LNG fluid domain are likely to occur, which alter the fluid properties locally. Insights into these phase changes during sloshing impacts are of key importance for the accurate, efficient and safe designs of cargo containment systems inside LNG vessels. The main objectives of this dissertation are twofold : (1) the development and validation of non-intrusive measurement techniques to characterize the propagation of shock waves through multiphase fluids, and (2) quantifying the energy partitioning and emission of shock waves by collapsing vapor bubble clouds. To this aim, two novel measurements techniques are developed and validated for accurately quantifying the two-phase liquid properties and the shock wave propagation, non-intrusively. Also, the emission of shock waves by collapsing vapor bubbles is assessed non-intrusively with state-of-the-art high-speed X-ray densitometry....@en

Swimming is a sport where an incremental gain in performance can lead to either a medal or leaving the swimmer empty-handed. In the front crawl, water is driven backwards by the limbs where most propulsive forces come from the hands. Two main stroke patterns can be observed, the ”straight I pull” is commonly taught to generate forces with drag, and the ”curved S-pull” applies lift as well. This investigation uses a new industrial 6-DOF robot arm to study angle-dependent forces, stroke patterns and hand configuration variables. The forearm and hand are analysed using force measurements and PIV to obtain quantitative information, while a parameterisation of the forearm and hand is used to obtain qualitative analysis. The flow is analysed qualitatively using two analytical methods, but it is discovered that these methods can only be applied to a limited range of cases to predict the angle-dependent lift force. Numerical investigations into the parameterised arm model reveal that forces generated were similar in magnitude to those reported in the literature for a pseudo-transient simulation. Quantitative research shows that lift only surpasses drag at extreme angles of attack immediately after a rapid start. This provides clear evidence that drag-based propulsion is preferred for front-crawl swimming. We utilize PIV technology to visualise the flow field around two cases where the lift was highly prevalent. The results reveal various aspects, including the nature of the observed lift peak. Finally, the study compares three distinct strokes: one utilizing a straight drag-based approach and two utilizing a sinusoidal path, resembling a curved pull. The findings indicate that the sinusoidal path’s lift-to-drag ratio remains relatively consistent compared to steady-state, constant-angle experiments. Additionally, the results suggest that the straight stroke is optimal for propulsion, while one of the sinusoidal strokes might be more energy-efficient. An additional aim of this research is to determine the suitability of an industrial 6-DOF robot arm for stroke experiments. It has been discovered that some experiments need a longer path and should preferably be conducted in a wind or water tunnel.

A F1 car is always in a dynamic flow when racing. The most common case would be acceleration, deceleration and turning a corner. Due to the new rule by FIA since 2021, ground effect is much more significant for F1 car. Limited by the experimental set-up of F1 teams, flow under the car and accelerating flow are hard to be measured quantitatively.
This research is on an accelerating wing with ground effect. The airfoil representing the front wing is from a real F1 car, the Tyrrell 026. Prediction of downforce and flow field in steady phase was done by conformal mapping (potential flow theory), SST k−ω and 2D-DES CFD simulation.
A force and PIV measurement was done to investigate the dynamic flow behaviour.
The connection between force and flow field was discussed, considering the measured residual force and the added mass force. A conclusion is drawn with suggestion on setting the optimal clearance of an F1 car’s front wing on track, to improve the racing performance of the car.

A 1:5 scaled rowing boat had been designed by a previous research group to determine the performance of rowing blades with different sizes and blade angles. In this research modifications were done to allow more control of the rowing motion. The aim of this study is to assess whether an angled oar blade can reach faster speeds with the same input power. This is done by collecting data with force and position sensors. With the collected data, the input power and speed of the rowing boat can be compared between several modified oar blades. The results of the tests in the towing tank show that it is very likely that the rowing boat can go faster with the same input power by adjusting the oar blade angle.

In this thesis a method of combined PIV and LIF experiments on the Turbulent/Non-Turbulent interface (TNTI) of a round jet is presented.
By varying the distance from the nozzle in the measurements, the Kolmogorov scale of a large range of Reynolds number (2×10³ ≤ Re ≤ 48×10³) is kept constant, allowing for a constant resolution. The large range in Reynolds number is needed to identify a difference in scaling of the TNTI, which is difficult to capture due to the weak dependency of the separation of Kolmogorov and Taylor scales on the Reynolds number (λ/η ∼ Re^1/4). A factor of over 2.5 is achieved in the current work.
An experimental setup for these measurements is presented, as well as the design of experiments for seven Reynolds numbers based on the boundary and initial conditions of the setup. A design for measurements with a camera moving with the flow is presented as well.
Conditional statistics are used on the jet to look at the change of span-wise vorticity over the interface. For the detection of the interface scalar images from LIF are used, where the interface is detected using a threshold of dye concentration (Rhodamine B).
All Reynolds numbers show self-similarity within the measured range, comparable to literature and with a spatial resolution better than 4η. Some small differences between the low and high Reynolds number measurements can be seen. They are not expected to have significant impact on the results and can likely be solved by slight changes in the experimental setup.
The influence of the threshold on the shape and width measurements of the mean conditional vorticity profile is examined, indicating a significant dependency on chosen threshold. Current results indicate a scaling of the interface thickness with the Taylor length scale, around a value of 0.5-0.6λ, or 10-15η This is in accordance to the literature for low Reynolds numbers. Due to this significant influence of the threshold on the outcome, these results cannot be seen conclusive. For this, further investigation is needed on the dependency of the width on the threshold, and validation of the jet contour.
Universal ways to compute the threshold, jet contour and mean conditional vorticity profiles are currently missing, while they have shown to be potentially of significance on the outcome. Another method of studying TNTI scaling is via the local velocity scales, which can be found via current data, which could provide a comparison of methods. The measurements with the moving camera could help identifying the link between physical processes and the scaling of the interface.

The coalescence of two droplets suspended in a viscous fluid is the focus of this study. When two droplets that are suspended in a shearing flow come in contact with each other, it is currently uncertain if the contact would result in coalescence. To be able to make such predictions, it is necessary to study the effects of the system parameters, such as the angle of collision between the droplets and the properties of the flow and the two fluids, on the process of coalescence. This necessitates the need for a robust experimental setup in which such studies can be performed.

The characteristic of a systematic study is repeatability and control over experimental conditions. In coalescence studies, the impact angle between the droplets is a parameter that has proved difficult to control. In this study, a microfluidic device is developed that uses the concept of surface energy wells to achieve repeatability in the impact angles of droplet collisions. Using the device, droplet coalescence experiments were performed. An interesting observation was made from the experiments which guided the further course of this research work. It was seen that the droplets did not coalesce upon approach, but coalescence was driven by the separation of the droplets. In the literature, this phenomenon is referred to as 'separation-driven coalescence'. Furthermore, it was suspected that an experimental condition could be defined based on a non-dimensional parameter, namely, the Capillary number Ca, such that for Ca > Ca_cr, separation of droplets ceases to trigger coalescence.

The effect of the system parameters, namely the impact angle, θ_i, and the viscosity ratio, λ, over the Ca_cr was investigated. In this study, the presence of a Ca_cr for separation-driven coalescence is confirmed, both experimentally and through a scaling argument. The results of the study indicate that the thickness of the film on the onset of separation influences the Ca_cr. However, a dependency between Ca_cr and λ was not found experimentally. Large experimental uncertainties prevent any further conclusions to be made regarding the Ca_cr.

In this thesis, a framework is developed for the investigation of the Ca_cr for separation-driven coalescence. With more experimental data, a deeper understanding of separation-driven coalescence can be obtained.

The thesis proposes a methodology for a Surrogate Based Optimisation (SBO) study and its application in the Naval Architecture field using the open-source software DAKOTA. Surrogate Based Optimisation is a useful and powerful way of reducing time and costs in hull shape optimisation. It allows the employ of exploratory data analysis techniques and machine learning methods to get more insight, discover hidden patterns, and detect anomalies in the design space. However, an SBO consists of several interconnected steps, and each increases complexity and uncertainty to the overall process.

The main goal is the investigation of the effects of different surrogate models on ship geometry optimisation from a resistance point of view. Also, different sampling plans, infill techniques, and optimisation algorithms are analysed and compared, as elemental steps for an SBO. This is done by the use of test functions that emulate the problem of ship optimisation. Furthermore, the intent is to try and list general guidelines to follow when building up an SBO routine with CFD simulations involved, with a focus in the marine field.

This research led to the assembly of Halton sampling sequence, Kriging meta-model, Expected Improvement function, Genetic Algorithms, and Pattern Search tools to implement a working SBO routine. This routine is used to demonstrate the success of the SBO method for a Hull Vane and an aft ship optimisations. Moreover, it is used to validate the research outcomes of this thesis and to prove that this work can be safely used for every-day commercial work.

The two design applications provided a resistance reduction, with respect to a benchmark hull, of about 19% and 7%. Thus not only they were successful, but also they were excellent examples to show how a surrogate model and its correct visualisation can give the Naval Architect the right tools to critically analyse the results, gain more understanding on the problem, spot and correct anomalies, and provide creative solutions to a client.

In professional swimming, a fraction of second can determine whether a competitor will obtain a place on the podium. Swimming techniques have been analysed for decades to increase and optimize performance in order to win medals in the biggest competitions. The arms and the hands play a major role in the forward propulsion generated by a swimmer, especially in front crawl swimming. They have been widely investigated for steady motion, from the elbow position to the hand orientation and configuration. However, it is argued that the arm velocity is continuously varying during a front crawl stroke and that this unsteadiness can enhance propulsion. For this reason, investigating the effect of unsteady motion on the propulsive forces generated by the hand is of great interest. The objective of this study is to experimentally investigate the effect of acceleration on the propulsion drag of a hand in human swimming, and how this effect evolves with finger spreading. To do so, experiments on four full-scale 3D printed hands with attached forearm and different finger spreading are performed. The experiments take place in a 2 x 2 m open glass tank filled with water. The hands, mounted on a robot arm and fully submerged, are accelerated towards a constant target velocity along a linear path. The acceleration and the velocity are varied resulting in a Reynolds number of 2 x 104 ≤ 𝑅𝑒 ≤ 6 x 104 based on the hand palm. The drag force exerting on the hand is measured as a function of time and three phases are distinguished: (i) the acceleration phase during which the drag reaches a maximum, (ii) the transition phase where the drag gradually decreases to reach a constant value during (iii) the steady phase. In parallel, two-dimensional particle image velocimetry (PIV) is performed in a plan crossing the fingers. A significant effect of acceleration was found performing experiments for four different accelerations. Higher accelerations generate stronger and more concentrated vortices behind the hand, resulting in a larger amount of circulation close to the hand. Consequently, a 25% increase is found in the drag force at the end of the acceleration when the latter is multiplied by four. No significant effect of finger spreading on the drag force during the acceleration was found. However, surprisingly, an enhancement of the drag force of 5% to 10% was measured during the transition phase and the steady phase for the closed hand compared to the hands with finger spreading. PIV showed larger amount of circulation close to the hand without finger spreading.

Turbulent scalar mixing is prominent in all sorts of situations, being it in air pollution or making tea. However, this does not often lead to a homogeneously mixed fluid, but rather leads to regions of higher concentrations next to regions with significantly lower concentrations. This can be characterized by dividing the concentration field in a turbulent flow into a few regions that have almost the same concentration: Uniform Concentration Zones. How these Uniform Concentration Zones are organized is yet unclear, but a possible candidate in the velocity field is the use of Lagrangian Coherent Structures. These structures are, in principle, transfer barriers in the flow and defined as local maxima of the Finite Time Lyapunov Exponent (FTLE), which is a measure of exponential fluid-separation over time. In order to successfully apply the concept of Lagrangian Coherent Structures in an experiment, turbulent structures should remain in focus long enough to measure the exponential separation of fluid parcels. As almost all fully developed turbulence comes with a mean flow, the observation time in an Eulerian field-of-view is too short. Therefore an experiment was designed to move the observation equipment with the mean flow. With this experimental setup, the concentration field and the velocity field are measured simultaneously via respectively LIF and PIV. Comparing the Uniform Concentration Zones with both the forward and backward time FTLE-field, shows that there is not much correlation between the concentration field and the forward time FTLE. However, local maxima of the backward time FTLE seem to overlap often with boundaries between Uniform Concentration Zones.

The comfort assessment of ships is based on several criteria. There are for example criteria for motions, noise and vibrations. The criteria for motions are nowadays often defined as comfort limits on the accelerations in the ship motions. However, from other industries it followed that the rate of change of the accelerations, the jerk, can also have an influence on the sense of comfort of passengers and crew. The goal of this thesis is to investigate whether it would be useful to add jerk criteria to the comfort assessment of ships. Jerk cannot be measured directly (yet), so it has to be obtained from acceleration measurements or calculations. For this goal filter and processing procedures are defined. To be able to make comparisons different ways of quantifying the jerk are established. One is aimed at peak values, since these are often determining the limit for comfort and operability. The other is focused on quantifying the non-linearity of the response. Research is performed on what the physics behind jerk are, and what phenomena are likely to cause large jerk values. The prime cause is found in slamming. The jerk following from slamming is influenced mostly by the shape of the bow and the speed of immersion in the water. Using the measurement data of two different hulls from other research the effect of varying conditions on the jerk is investigated. From this research it follows that there can be a difference in seakeeping behaviour when looking at jerk instead of accelerations. Since jerk is the third derivative of displacement to time, the non-linearity in this response is relatively high. Therefore non-linear codes are required to calculate the jerk response correctly. The use of a Reynolds Averaged Navier Stokes (RANS) code proves to be able to predict the jerk behaviour correctly. In this thesis only model test measurements and calculations are used. The scaling of jerk to full scale, and difficulties that it might impose, are only discussed theoretically. The general conclusion is that jerk is worth investigating to compare the seakeeping behaviour of ships. Until quantitative limits are available only qualitative comparisons are possible, but these can already give additional insight.

The progress made by microfabrication technologies has escalated the interest in particle manipulation on micron scale. Particles are not only limited to solid particles but also include cells, droplets and bubbles. Activities involved in manipulation are trapping, sorting, separation and so on. The devices designed to execute such activities are aimed for a particular application. However, there lies a gap in literature to design an all-in-one device. Microfluidics research carried out at the Laboratory for Aero and Hydrodynamics is currently working on designing such devices which can carry out different manipulation tasks.
This thesis is aimed at designing and fabricating such a device where two particles can be manipulated simultaneously. The application of designing such a device would be to study cell-cell interaction, droplet coalescence and so on. To achieve this, a microfluidic device is designed as a Hele-Shaw flow cell. Since the height-averaged velocity in a Hele-Shaw flow cell is irrotational, analytical solution is possible for such a design. Although a Hele-Shaw flow is viscous flow between two closely spaced plates, it produces same streamline patterns as a 2D ideal flow. This enables the use of basic building blocks of an ideal flow field such as sources, sinks, uniform flow and so on. In this case, sources and sinks are superimposed to get the flow field inside the microfluidic device. The microfluidic device is fabricated using Polydimethylsiloxane (PDMS), a commonly used polymer to fabricate microfluidic devices. The design of the device is validated by comparing experimental flow fields, streamlines and stagnation points to the corresponding analytical solution. Single particle manipulation activities are carried out in the device. This is executed by manually controlling the flow rates or the strength of sources and sinks. Firstly, the particle is trapped at the stagnation point for a certain timespan. Finally, some manipulation activities are carried out using a single particle with the aid of streamlines produced by the sources and sinks.

The way in which mean strain affects the turbulent structures is imperative to understand various natural flows such as flow over a hill, the flow of a river in the delta, jet streams in the upper atmosphere etc. Further, it also has industrial implications viz; flow over bodies such as airfoil, turbomachinery, gas pipelines.
The strained pipe flows, in particular, have huge engineering interest due to its prevalence in industrial fittings wherein a larger pipe diameter is connected to a smaller one and vice versa. This subject also has a fundamental interest as strain highlights the interaction of various scales of turbulence. This scale interaction
essentially dictates transfer of energy in turbulence and hence is fundamental in understanding turbulence dynamics itself. The present work deals with the experimental study of the response of pipe turbulence to axisymmetric, irrotational strain using high-resolution planar Particle Image Velocimetry (PIV). A mildly rapid strain (s^*s ~ 3.2) is imposed on turbulence via a spatial contraction. It is seen that turbulence is suppressed upon straining. As a response to mean strain, transverse Reynold stress increases at the expense of streamwise Reynolds stress and anisotropy is induced in the turbulence. Despite strain being only mildly rapid, Rapid Distortion Theory (RDT) is found to predict the correct trend of normal Reynolds stress although traverse Reynolds stress is over-predicted. The effect of strain on different scales of turbulence is discerned. The large scales of turbulence are seen to get compressed in the radial direction although they do not get affected significantly in the streamwise direction. Near-wall coherent structures which were initially inclined w.r.t. the wall are seen to get aligned with the flow as they also get severely compressed in the radial direction. On the other hand, the small scales of turbulence are found to be spatially organised in the form of sheets or layers. Upon straining, these sheets are found to get aligned with the mean flow. Further, they get elongated in streamwise and compressed in the radial direction. It is observed that the small scales are more severely distorted than the large scales upon staining inside the contraction. At the Reynolds number (Re) range employed in this thesis, there is no substantial difference in which turbulence is strained inside the contraction at disparate Re. Downstream of the contraction, at the axis of the pipe, the anisotropy of Reynolds stress is found to recover slowly. Further, this relaxation is seen to be Re dependent with higher Re turbulence relaxing slightly faster.

During front crawl swimming, water is driven backwards with the limbs. Drag forces generated by the limbs are consequently used for forward propulsion. The hands are responsible for approximately 60% of the generated propulsive forces. Reaching a podium place in competitive swimming is dependent on differences in finishing times smaller than 0.5%. For this reason, investigating the effects of hand configuration on swimming performance is of interest. Configurable properties of the hand are the finger spreading and hand palm cupping. It is argued that a small finger spreading leads to a larger obstruction in the fluid flow compared to closed fingers, resulting in larger generated drag forces. Similarly, a small hand cupping is expected to increase the drag forces in analogy to the drag increase experienced by cupped disks. In this thesis, an experimental investigation is carried out to look into the effects of both finger spreading and hand cupping. Furthermore, CFD simulations are used for the abstract modelling of hands with finger spreading by use of slotted disks. 
Towing tank experiments in water are performed to investigate the effects of finger spreading for five full-scale arm models. The research showed that a small finger spreading of 5° can increase the drag coefficient of the hand with 1.7%, in comparison to closed fingers. Larger spreadings were found to influence the drag coefficient disadvantageously, where a 20° finger spreading reduced the drag with 1.5%. The found effects indicate that finishing times can be reduced with 0.3% by using 5° finger spreading instead of 20° spreading.
Wind tunnel experiments are used to look into the effects of hand cupping. Dynamic scaling based on the Reynolds number is used to account for the used air flow. Effects for five full-scale arm models with different hand cuppings were investigated, these have a 5° finger spreading which was found optimal from previous research. It appeared that small rotations around the longitudinal axis of the arm have large influences on the drag coefficient, where a maximum in drag was never found for the hand palm perpendicular to the flow, but with an abducted thumb opposing the flow. The research showed that 6% more drag is generated for a flat hand in comparison to the largest investigated hand cupping. This indicates that finishing times can be reduced with 0.8% by using a flat hand instead of a large hand cupping. 
In conclusion, the research found a hand configuration with 5° finger spreading and a flat hand palm optimal for maximizing drag forces. The found effects on finishing times indicate that using this hand configuration can play an important role in reaching podium places during front crawl swimming.

The interest in manipulating particles, droplets and bubbles have garnered significant attention in recent years, owing to the advantages offered by micro-fluidics and the advancement in micro-fabrication technologies. These manipulation activities have found its applications in myriad fields of engineering, ranging from medical diagnostics to chemical industry to drug discovery. This has increased demand for the development of devices such as 'Lab on a chip', which performs laboratory-sized experiments and analysis on a single small chip, with the same speed and accuracy as its room-sized counterpart. However, manipulation activities carried out in these devices has fixed channels, designed to serve purpose for specific manipulation tasks. This makes the device suitable for a specific application. Addressing this aspect, a device designed without having any real channels would give an opportunity to integrate multiple functionalities onto a single-chip in the long run. As a first step towards reaching this 'bigger picture', it is necessary to explore the feasibility of manipulating particles, droplets and bubbles by generating so called 'virtual channels'.
The present thesis focuses on an attempt to manipulate particles without the use of any real channels or external field. Although such manipulation is desired in the micro-scale, a top down approach is preferred and hence, the manipulation is carried out in a scaled up model. First, a Hele-Shaw flow cell is designed with sources and sinks in the millimeter scale to deviate streamlines in the same range. Thereafter, four different velocity fields are studied under different combination of sources and sinks, which are then compared to the computational ones. The property of a Hele-Shaw cell that the averaged velocity over the height of the channel is irrotational, makes it possible to compute velocity fields by the use of potential flow theory. The same velocity fields are hence, computed using a discrete source based Panel Method. A good agreement is found between computation and experiment, making PIV measurements not a necessary option for evaluation of velocity fields under these sources and sinks. Finally, individual particle is inserted into the Hele-Shaw cell and manipulated using unsteady fields. The manipulation includes tasks such as diverting particles having same initial position to different end locations; trapping particle for different instances of time and then releasing them into different directions; flipping positions of two particles; and deflecting a particle by ninety degrees. The individual particle trajectory for the above manipulation activities are tracked down using a particle tracking code and then compared with the ones generated using the Panel Method. This, however, excludes the activities where particles need to be trapped because of particle fluctuation near stagnation point. Overall, the Panel Method serves well in predicting particle path-lines and can be used as a tool for manipulating particles.

Turbulence is a commonly encountered state of fluid dynamics. Unsteady turbulent flows in pipes are present in many engineering applications and also in biological flows. However, the various processes active in such flows are not well understood. The present work employs stereo-PIV to investigate the effects of a sinusoidal pressure gradient on the various turbulence parameters, including the terms of the turbulent kinetic energy budget equation. The bulk flow rate was oscillated with a frequency of 0.5 Hz with a mean Re 26,000 and an amplitude of modulation, 0.23 times the mean value. It is seen that there is a delay in the response of turbulence to the oscillations of the bulk flow and the delay increases with increasing distance from the wall. The axial and the in-plane turbulence parameters show a difference in the delay of their responses. This delay extends into the small scales responsible for the dissipation of turbulent kinetic energy. Changes are also observed in azimuthal length scales when the flow oscillates.The effects of oscillation on the streaks of low momentum are also discussed and the structural organization in unsteady pipe flows are found to be different from that in steady pipe flows

A novel technique for measuring capillary/gravity waves travelling on air/water interfaces is investigated. The main motivation is to measure the free surface deformations of an air cavity used for air lubrication of ship hulls. Due to limitations in the available techniques, two novel methods are proposed, out of which the Stereo Particle Image Velocimetry synthetic Schlieren (Stereo-PIV-SS) technique is found to be more promising. This technique uses the input of random dot pattern images captured with and without an interface by two cameras mounted in a Stereo configuration. These images are processed to generate "pseudo" particle motion vectors using a Stereo-PIV code. An analytical study is conducted to find a simple linear relation between the pseudo vectors and interface height. Two sets of experiments are conducted to test this technique. One with defined transparent solid Plexiglas and glass profiles and another on small amplitude and slope water waves. For the experiments on water waves, the Free Surface Synthetic Schlieren (FS-SS) technique is used as validation data. In addition to experiments, a synthetic image reconstruction study is conducted to test the quality of the images generated by an in-house code.

The investigation reveals that the Stereo-PIV-SS technique is capable of capturing the flat interface height with sub millimeter precision for all interface heights tested in the range of 5 to 60 mm. Nevertheless, the technique can capture a wave profile with sub millimeter precision only for a limited range of wave parameters. The exact limits can only be defined by understanding more about the interplay of various wave parameters, which include the wave orientation, influencing the reconstruction. However, this technique can capture the profiles of waves having wavelength of the order of 1 cm and amplitude of the order of 1 mm, without requiring information about the mean interface height. Moreover, it is low cost, easy to use and apply since it requires only a standard PIV setting. The first attempt to study this new technique has been promising and it can be applied to specified wave regimes. The method can be developed further by understanding how to remove the effect of wave orientation on the reconstruction and including other wave parameters in the interface height correlation.

The research in resolving the smaller scales of turbulence has gained significant attention in recent years, owing to the advancements in the measurement techniques. High resolution experimental studies are required to investigate high Reynolds number flows where the scales of turbulence are very small. This is of particular importance in addressing the theoretical issues in studying high Reynolds number wall turbulence which finds applications in transport, energy, and aerospace industries. Hot-Wire Anemometry (HWA) is often employed to reach very high spatial resolution. Early research in Particle Image Velocimetry (PIV), a qualitative flow visualization technique, was not able to achieve a higher resolution as comparable with that of HWA. However, recent developments in PIV allow high resolution measurements, using information on the time averaged ensemble correlation.
The ensemble correlation can be used to reduce the size of the interrogation windows employed in a PIV analysis, without compromising for the particle image density in an interrogation window. This is achieved by using a large number of image pairs. In this thesis, ensemble correlation is used to study the turbulence in two specific cases: a turbulent jet and high Reynolds number pipe flow. The ensemble correlation, though being a time averaged quantity, contains information on the velocity Joint Probability Distribution Functions (JPDFs) which can be used to estimate the turbulence statistics. This information can be retrieved from the shape of the ensemble correlation. It is assumed that the ensemble correlation is the convolution of the auto-correlation and the velocity JPDFs. The velocity JPDFs are retrieved by estimating the second moments of the ensemble correlation by fitting a Gaussian profile, and further using a correction for auto-correlation. The second moments can be used to estimate the Reynolds stresses in the flow.
The proposed method is validated by implementing it to study the turbulent jet. For this purpose, 9000 image pairs are acquired. The results show that the retrieved moments predict the Reynolds stresses accurately. Further, high Reynolds number pipe flow experiments are performed in the Alpha Loop facility at Deltares. The bulk Reynolds number is varied from 0.3 to 0.6 million, acquiring 20,000 image pairs per measurement series. Also, the pressure drop is measured across the pipe along with the PIV measurements to estimate the roughness of the pipe. The results from the PIV measurements show that the mean velocity profile follows a similar trend as observed in the literature. A small bias is observed between the mean velocity obtained from the ensemble correlation and that obtained by averaging the instantaneous velocity vectors, in regions of high velocity gradients. The Reynolds shear stress estimated from the shape of the ensemble correlation is underestimated. However, the streamwise turbulent fluctuations estimated follow a trend, similar to that observed in the literature.

Fluids above the critical point are widely used in industry. Chemical, pharmaceutical, food industry and energy production are some examples. In the energy production sector they are mainly used as cooling fluids, because they allow to increase the thermal efficiency of the power plants. However, the fundamentals of their heat transfer behavior are still unknown and current heat transfer models fail to predict it. Supercritical (SC) fluids are characterized by strongly varying fluid properties, which are responsible for their particular heat transfer behavior and make them very difficult to model, simulate and experimentally investigate. In past studies, buoyancy was identified as a key cause for the heat transfer deterioration observed in SC fluids. The aim of the research described in this thesis is to investigate the possibility of performing non-intrusive local velocity measurements with the optical technique PIV and to acquire global heat transfer measurements, with strongly changing fluid properties at SC conditions. The experiments were performed in a pure buoyancydriven flow: a Rayleigh-Bénard (RB) flow. The velocity fields of RB convection with strongly varying properties, beyond the so-called Oberbeck-Boussinesq (OB) approximation, were experimentally studied at atmospheric pressure first. An increase of the time-averaged velocity close to the bottom wall of the cell with respect to the top wall of about 13% was found. This finding confirmed experimentally a top-bottom ”broken symmetry” in the velocity field, which was observed in previous numerical and theoretical studies, but it was never experimentally demonstrated before. The heat transfer with strongly variable properties at SC conditions for constant Prandtl and Rayleigh numbers, specifically defined outside the validity range of the OB approximation, was experimentally studied. The measurements were performed at the Max Planck Institute of Dynamics and Self-Organization in Göttingen (Germany), with a European EuHIT project. It was observed that the measured Nusselt number defined for non-OB conditions was different from point to point, showing that merely the Rayleigh and Prandtl numbers are not sufficient to determine the heat transfer through the cell. It was also seen that the measured Nusselt number was 16% larger with respect to the one predicted by the Grossmann-Lohse theory (2000) for the same Rayleigh and Prandtl numbers at OB conditions. A feasibility study of particle image velocimetry (PIV) at SC conditions was done by using the background oriented schlieren technique (BOS). An estimation of the PIV experimental uncertainty at SC conditions was done with the statistical correlation method proposed by Wieneke et al., (2015). PIV was successfully performed at SC conditions. Main difficulties about its applicability were due to blurring and optical distortions in the boundary layer and thermal plumes regions. PIV measurements were performed at three different magnitudes of density difference between top and bottom of the cell. Two of the three experiments were done at similar Rayleigh and Prandtl numbers, defined for non-OB conditions: one towards the liquid phase and the other one towards the gas phase. The former showed a lower large scale circulation (LSC) velocity than the latter. All cases showed the presence of one asymmetric LSC roll, which is different from a typical RB convection flow at OB conditions.
Improvements in the accuracy of PIV measurements and the acquisition of more
heat transfer data at SC conditions, would help the study of the thermal and viscous boundary layer thicknesses and turbulence modifications that are responsible for different heat transfer regimes in SC fluids.@en

The aerodynamic drag force is a crucial factor for passenger cars and trucks, where fuel economy is of utmost importance. While a lot of research has been performed to optimize the design of the upper-body of the vehicle, the flow over a rotating wheel that is enclosed by a wheelhouse is not understood to the same level. Estimates show that the addition of wheel and wheelhouse to a vehicle body, can increase its drag and lift coefficient by as much as 40 \%. With the major automobile manufacturers striving to improve the fuel economy of the vehicle, understanding the flow over a rotating wheel in a wheelhouse, offers a new dimension for the improvement of the drag coefficient of the vehicle. In the present research, the flow over a simplified car body with a wheel and a wheelhouse was studied using Large Eddy Simulation (LES). When performing an LES on a given numerical grid, it can be expected that the contributions from the subgrid viscosity model reduces on decreasing the Reynolds number of the flow. A study focusing on the dependence of the flow field on the Reynolds number was thus performed. Three Reynolds numbers based on the wheel diameter - 9 x $10^3$, 75 x $10^3$, 150 x $10^3$- were chosen for the dependence study. A comparison of the relative pressure distribution and the flow topology between the different Reynolds numbers showed that the flow over a rotating wheel in a wheelhouse was qualitatively independent of the Reynolds numbers considered in this work. Experimental results from Particle Image Velocimetry (PIV), performed on a similar vehicle model, were used to validate the numerical solution at a Reynolds number of 9 x $10^3$. The behaviour of the flow in the wheelhouse region was investigated and a new description of the flow field inside a wheelhouse, consisting of 11 vortex structures, was presented in this work. To determine the qualitative dependence between the flow field and the drag coefficient, a comparison of the flow between two geometric configurations, with different wheelhouse radius, was performed at a Reynolds number of 9 x $10^3$. It was found that the configuration with the larger wheelhouse radius had a higher wheelhouse drag coefficient and a lower wheel drag coefficient when compared to the configuration with lower wheelhouse radius. Based on a detailed flow field, four out of the eleven vortices were found to be crucial in determining differences in the drag coefficient of the wheel and wheelhouse. Based on this analysis, an alternate wheelhouse geometry, combining the advantages of the two geometric configurations considered in this work, was proposed, that could potentially lead to a reduction in the drag coefficient of the vehicle.