The turbulent boundary layer is an aspect of fluid flow which dominates the performance of many engineering systems - yet the analytic solution of such flows is intractable for most applications. Our understanding of boundary layers is therefore limited by our ability to simulate and measure them. Tomographic Particle Image Velocimetry (TPIV) is a recently developed technique for direct measurement of fluid velocity within a 3D region. This allows new insight into the topological structure of turbulent boundary layers. Increasing Reynolds Number increases the range of scales at which turbulence exists; a measurement technique must have a larger 'dynamic range' to fully resolve the flow. Tomographic PIV is currently limited in spatial dynamic range (which is also linked to the spatial and temporal resolution) due to a high degree of noise. Results also contain significant bias error. This work proposes a modification of the technique to use more than two exposures in the PIV process, which (for four exposures) is shown to improve random error by a factor of 2 to 7 depending on experimental setup parameters. The dynamic range increases correspondingly and can be doubled again in highly turbulent flows. Bias error is reduced by up to 40%. An alternative reconstruction approach is also presented, based on application of a reduction strategy (elimination of coefficients based on a first guess) to the tomographic weightings matrix Wij. This facilitates a potentially significant increase in computational efficiency. Despite the achieved reduction in error, measurements contain non-zero divergence due to noise and sampling errors. The same problem affects visualisation of topology and coherent fluid structures. Using Projection Onto Convex Sets, a framework for post-processing operators is implemented which includes a divergence minimisation procedure and a scale-limited denoising strategy which is resilient to 'false' vectors contained in the data. Finally, developed techniques are showcased by visualisation of topological information in the inner region of a high Reynolds Number boundary layer (δ+ = 1890, Reθ = 3650). Comments are made on the visible flow structures and tentative conclusions are drawn.

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Thermoacoustic devices are a group of systems that make use of the thermoacoustic effect to achieve an energy conversion between thermal and acoustic energy. The thermoacoustic effect occurs when a solid boundary is introduced into an acoustic field, and a non-zero net heat transportation takes place while the net mass transfer remains null. Thermoacoustic technologies are gaining an increasing research interest because of their potential applications for building alternative prime movers or heat pumps which do not use working fluids causing environmental damage and require very little maintenance due to their lack of moving part. However, the operation of this type of system is yet to be fully understood: fluid flow and heat transfer processes within the system components such as thermoacoustic stacks and heat exchangers still require a lot of attention. The performance of the system working with relatively low amplitude acoustic wave can be predicted by the linear thermoacoustic theory, which is already well developed. However, a high amplitude acoustic wave is usually required in order to achieve high power density or high power output. Unfortunately, the performance of such systems can be seriously degraded due to nonlinear effects, such as turbulence, minor loss or high proportion of harmonics. The lack of understanding of these effects impedes the design and construction of high efficiency systems. The work described in this thesis is focused on the study of flow phenomena taking place around parallel plate stack placed in a standing wave thermoacoustic resonator, by using advanced flow diagnostics techniques such as particle image velocimetry (PIV) and hot wire anemometry (HWA). In order to carry out the experimental study, a standing wave thermoacoustic device working at relatively low frequency of 13.1Hz was designed, commissioned and tested. The frequency response of this device was carefully investigated and compared with the analytical results using linear acoustic equations and a linear model of the loudspeaker. A further comparison with the analytical results obtained with the modelling tool DeltaEC (Design Environment for Low-amplitude Thermoacoustic Energy Conversion) was also presented. The resonator was driven from low to large pressure amplitudes with drive ratios up to 10%. A good agreement is obtained for small amplitudes, but the discrepancies become larger when the driving amplitude is increased. The analysis reveals that the large discrepancy at high amplitude can be attributed to minor losses. Following the above preliminary work, a more comprehensive study of the flow field around parallel-plate stacks was conducted by means of PIV and HWA. It was shown that the flow around the two studied parallel-plate stacks exhibits rather complicated flow features when the amplitude of the acoustic oscillation varies. Symmetrical and asymmetrical vortex shedding phenomena are observed and two distinct modes of generating 'vortex streets' are identified. It shown that a velocity related parameter such as the Reynolds number, defined on the plate thickness and the velocity amplitude at the entrance to the stack, and a geometrical parameter are not sufficient to define the flow characteristics in this type of flow problem. It is also proposed to introduce an extra frequency related parameter such as the Keulegan-Carpenter number (KC) and to carry out a similarity analysis in order to understand better the physics behind the flow phenomena and their controlling parameters. Typical ensemble-averaged velocity fields are used in the analysis above. However, the detailed flow features obtained from the ensemble averaged flow fields and the instantaneous flow fields could be different in a substantial way. The flow behaviour, its kinematics, dynamics and scales of turbulence, therefore are further investigated by using the classical Reynolds decomposition to separate the instantaneous velocity fields into ensemble-averaged mean velocity fields and fluctuations in a set of predetermined phases within an oscillation cycle. The mean velocity field and the fluctuation intensity distributions are investigated over the acoustic oscillation cycle. By using fast Fourier transform (FFT) spatial filtering techniques, the velocity fluctuation is further divided into large- and small-scale fluctuations, and their physical significance is discussed. The physics behind the flow phenomena are further studied by carrying out an analysis of the wake flow during the ejection part of the flow cycle, where either closed re-circulating vortices or alternating vortex shedding can be observed. A similarity analysis of the governing Navier-Stokes equations is then undertaken in order to derive the similarity criteria governing the wake flow behaviour. Similarity numbers including two types of Reynolds number, the KC number and a non-dimensional stack configuration parameter are considered. The influence of these parameters on the flow behaviour is discussed by investigating the experimental data obtained, along with additional data from literature.

Tomographic Particle Image Velocimetry Using Colored Shadow Imaging by Meshal K Alarfaj, Master of Science King Abdullah University of Science & Technology, 2015 Tomographic Particle image velocimetry (PIV) is a recent PIV method capable of reconstructing the full 3D velocity field of complex flows, within a 3-D volume. For nearly the last decade, it has become the most powerful tool for study of turbulent velocity fields and promises great advancements in the study of fluid mechanics. Among the early published studies, a good number of researches have suggested enhancements and optimizations of different aspects of this technique to improve the effectiveness. One major aspect, which is the core of the present work, is related to reducing the cost of the Tomographic PIV setup. In this thesis, we attempt to reduce this cost by using an experimental setup exploiting 4 commercial digital still cameras in combination with low-cost Light emitting diodes (LEDs). We use two different colors to distinguish the two light pulses. By using colored shadows with red and green LEDs, we can identify the particle locations within the measurement volume, at the two different times, thereby allowing calculation of the velocities. The present work tests this technique on the flows patterns of a jet ejected from a tube in a water tank. Results from the images processing are presented and challenges discussed.

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Understanding the dynamics of micro- and nanometer-sized objects like molecules, particles, and living cells in biological systems and biomaterials has become a key component in biomedical research. Consequently, significant progress has been made for the development of imaging platforms, fluorescent probes, and computational tools to visualize and quantify biological processes at different length and time scales. However, despite such advances, achieving a reliable measurement accuracy on the dynamic behavior of these microscopic vehicles in diverse biological contexts is challenging. Subsequently, the motivation behind this dissertation is to develop new robust microfluidic velocimetry techniques to investigate molecular transport and cell migration within an in-vitro microfluidic platform.

A pressurized vortex tube is used to generate streamwise vortices in a wind tunnel and the resulting flow behavior is analyzed. The apparatus is intended to verify computational data from the AFRL by offering a method of conducting real-world counterpart experiments. The apparatus design process and other considered approaches are discussed. The vortex tube is operated at pressures of 20, 30 and 40 psi while the wind tunnel is operated at 3, 5, 10 and 20% capacity. Flow measurements are performed using particle image velocimetry to observe vortices and freestream interactions from which velocity and vorticity data is comparatively analyzed. Results indicate that vortex velocity greater than freestream flow velocity is a primary factor in maintaining vortex structures further downstream, while increased supply pressure and reduced freestream velocity also reduce vortex dissipation rate. A brief analysis of the vortex interaction with a downstream airfoil is presented to support future work.

Interaction of flow structures from a three-cylinder system is complex and important for fundamental and engineering applications. In this study, experiments using hotwire, 2D PIV, and Tomography are to be conducted to characterize the fluid flow at various Re number and rotation speeds. The Reynolds number considered based on the diameter of the single-cylinder ranges from 37 to 1700. The peaks in the frequency spectrum obtained from the hotwire study show a unique relation of Strouhal number as a function of static incident angle, RPM, and Reynolds number. From the 2D PIV and 3D tomography experiment, vorticity and velocity results characterize the interaction of wake flow from individual cylinders and as a function of the rotational speeds. Besides, the Standard deviation map shows the turbulence intensity variation at the various static and rotating conditions. The obtained results at static conditions are found to be consistent with the previous computational study.

Internal waves are generated in stratified fluids, like the ocean, where density increases with depth. Tides are one of the major generation mechanisms of internal waves. As the tides move water back and forth over underwater topography, internal waves can be generated. The shape of the topography plays a major part in the properties of the generated internal wave and the type of wave and energy is known for multiple symmetric topographies, such as Gaussian or sinusoidal. In order to further understand the effects topographic shape plays, the effect of asymmetry on internal waves is investigated. First, two experimental methods are compared to evaluate which will capture the relevant information for comparing waves generated from oscillating asymmetric topographies. Two experimental methods are often used in internal wave research, Synthetic Schlieren (SS) and Particle Image Velocimetry (PIV). Both SS and PIV experimental methods are used to analyze a set of experiments in a variety of density profiles and with a variety of topographies. The results from these experiments are then compared both qualitatively and quantitatively to decide which method to use for further research. In the setup, the larger field of view of SS results in superior resolution in wavenumber analysis, when compared to PIV. In addition, SS is 25% faster to setup and significantly cheaper. These are the deciding factors leading to the selection of SS as the preferred experimental method for further tests regarding tidally generated internal waves from asymmetric topographies. Previous experimental and theoretical research on tidally generated internal waves has most often used symmetric topographies. However, due to the complex nature of real ocean topography, the effect of asymmetry can not be overlooked. A few studies have shown that asymmetry can have a significant effect on internal wave generation, but topographic asymmetry has not been studied in a systematic manner up to this point. This work presents a comparison of tidally generated internal waves from nine different asymmetric topographies, consisting of a steeper Gaussian curve on one side, and a wider Gaussian curve on the other. The wider curve has varying amplitude from 1 to 0.6 of the steeper curve's amplitude, and two oscillation frequencies are explored. First, kinetic energy density in tidally generated internal waves is compared qualitatively and quantitatively, in both physical and Fourier space. When compared to similar symmetric topographies, the asymmetric topographies varied distinctly in the amount of internal wave kinetic energy generated. In general, internal wave kinetic energy generated from asymmetric topographies is higher for waves generated at a lower frequency than at a higher frequency. Also, kinetic energy is higher in internal waves on the relatively steeper side of the topography. There is very little kinetic energy in the higher wavenumbers, with most of the internal waves being generated at the lower wavenumbers. The amplitude does not make an appreciable difference in the wavenumber at which the internal waves are generated. Thus, the differences quantified here are due solely to changing slope, showing a significant impact of a relatively slight asymmetry.

A water droplet that impacts on a cold surface will start to freeze and in time ice will accumulate. To exemplify, effects of ice accretion is important in areas such as power generation e.g. wind power and vehicles located in a cold climate e.g. aircraft, cars, and boats. The common denominator for these examples is that ice accumulation can lead to a loss of efficiency and in some cases danger. Most studies have so far focused on investigating freezing water droplets visually in experiments or numerically in regards to how the freezing process behaves in terms of shape or freezing time for either a sessile or impacting droplet. It has been observed that the surface material and structures of the substrate is of importance. One part of the freezing process that has been less investigated is the internal flow and how it affects the freezing process. In this thesis, the internal flow in a freezing water droplet has been investigated experimentally. The internal flow inside a droplet is calculated by using Particle Image Velocimetry. A metal plate with a groove filled with ice was used to generate an area for the nucleation to start and to be able to control the shape of the droplet.  Previous work indicate that the substrate is of importance for the freezing process. The influence of the substrate material on the internal flow for similar shaped droplets is therefore investigated in Paper A, for a substrate temperature of -8°C. The results show that the substrate material, here in terms of metals such as aluminum, copper and steel, affect the magnitude of the internal velocity. In paper B it is investigated how the contact angle influence the internal flow. The vector field is examined at 9% of the total freezing time for water droplets at five different contact angles. A droplet with a higher contact angle will have a higher internal velocity in the center. A lower contact angle will barely show any movement in the center, however a higher velocity magnitude is observed close to the free surface compared to a droplet with a higher contact angle. Paper C studies the time until the directional change of the internal flow in a water droplet. Experiments at -8°C as in Paper B are used as well as experiments at -12°C for the five different contact angles. The time until the directional change is similar in time for both -8°C and -12°C while the total freezing time and also the time of the directional change varies with contact angles. A droplet with a lower contact angle will have a shorter time until the directional change occure while an increase in contact angle prolongs both freezing time and the time until the directional change.