The fundamental nature of the flow oscillations which are generated by two opposing planar jets is investigated. Particular attention is given to the underlying mechanism which sustains the oscillations over a wide range of flow parameters. The jet columns are observed to undergo large lateral deflection oscillations once in each direction per cycle, in an asymmetric manner. Extensive characterization of the jet oscillations over a wide range of flow parameters is established, including both the aeroacoustic response, as well as the unique flow features which are synchronized with the oscillations. The impingement region and circulation regions in each quadrant of the flow field are shown to play essential roles in sustaining the oscillations, as the pressurization of the impingement region causes the jets to initially deflect away from the centerline, while the low-pressure regions which form in the circulation zones drive the jet columns back towards, and ultimately across, the centerline. A number of interesting observations are made regarding the oscillation characteristics, including a dependence of the oscillation frequency on the jet aspect ratio, which helps explain much of the discrepancy in the Strouhal numbers reported in the literature to date. Furthermore, the nature of the sound-source field is investigated including the directionality of the various frequency components which are radiated. Unique mitigation strategies of the opposing planar jet oscillations are also explored by attempting to disrupt the circulation regions through the use of splitter plates. The oscillations are weakened considerably as the development and convection of the circulation zones is impeded. Preventing the circulation flow from interacting with the jet exit region drastically increases the effectiveness of the splitter plates, as even short splitter plates are shown to completely eliminate the oscillations. This demonstrates a very effective mitigation strategy of the opposing planar jet oscillator which is ideal for a variety of practical applications. One of the main challenges of the current investigation into the opposing planar jet oscillator is the extent to which the detailed time-varying pressure field can be resolved. Since it is not possible to experimentally detail the time-varying pressure field of the opposing planar jets, a novel PIV-based pressure field mapping technique is developed and benchmarked. A separate apparatus consisting of a planar jet impinging on a v-shaped plate is utilized to benchmark the proposed technique. This technique effectively resolves the features of the time-varying pressure field which are synchronized with the flow oscillations and helps circumvent many of the challenges which existing PIV pressure field mapping techniques face. It also provides a valuable tool for researchers to simultaneously determine the kinematic and dynamic aspects of various flow phenomena in a variety of fields, especially those in the area of aeroacoustics and fluid-structure-interaction. / Thesis / Doctor of Philosophy (PhD)

Particle Image Velocimetry (PIV) is a powerful technique to measure the velocity at many points in a flow simultaneously by performing correlation analysis on images of particles being transported by the flow. These images are acquired by illuminating the flow with two light pulses so that each particle appears once on each image. <p> The spatial resolution is an important parameter of this measuring system since it determines its ability to resolve features of interest in the flow. The super-resolution technique maximises the spatial resolution by augmenting the PIV analysis with a second pass that identifies specific particles and measures the distance between them. <p> The accuracy of the procedure depends on both the success with which the proper pairings are identified and the accuracy with which their centre-to-centre distance can be measured. This study presents an analysis of both the systematic uncertainty and random uncertainty associated with this process. The uncertainty is analysed as a function of several key parameters that define the quality of the image. The uncertainty analysis is performed by preparing 4000 member ensembles of simulated images with specific setpoints of each parameter. <p> It is shown that the systematic uncertainty is negligible compared to the random uncertainty for all conditions tested. Also, the image contrast and the selection of a threshold for the particle search are the most critical parameters influencing both success rate and uncertainty. It is also shown that high image intensities still yield accurate results. The search radius used by the super-resolution algorithm is shown to be a critical parameter also. By increasing the search radius, the success rate can be increased although this is accompanied by an increase in random uncertainty.

Particle Image Velocimetry (PIV) is a powerful technique to measure the velocity at many points in a flow simultaneously by performing correlation analysis on images of particles being transported by the flow. These images are acquired by illuminating the flow with two light pulses so that each particle appears once on each image. <p> The spatial resolution is an important parameter of this measuring system since it determines its ability to resolve features of interest in the flow. The super-resolution technique maximises the spatial resolution by augmenting the PIV analysis with a second pass that identifies specific particles and measures the distance between them. <p> The accuracy of the procedure depends on both the success with which the proper pairings are identified and the accuracy with which their centre-to-centre distance can be measured. This study presents an analysis of both the systematic uncertainty and random uncertainty associated with this process. The uncertainty is analysed as a function of several key parameters that define the quality of the image. The uncertainty analysis is performed by preparing 4000 member ensembles of simulated images with specific setpoints of each parameter. <p> It is shown that the systematic uncertainty is negligible compared to the random uncertainty for all conditions tested. Also, the image contrast and the selection of a threshold for the particle search are the most critical parameters influencing both success rate and uncertainty. It is also shown that high image intensities still yield accurate results. The search radius used by the super-resolution algorithm is shown to be a critical parameter also. By increasing the search radius, the success rate can be increased although this is accompanied by an increase in random uncertainty.

With the ever more stringent requirements of emissions and fuel economy imposed on the automotive industry, there is a need to understand more fully all aspects of the internal combustion engine to meet these requirements and at the same time the desire of the customer for acceptable performance. This research was aimed at investigating one part of the engine behaviour i.e. induction of the fresh charge to the engine cylinder. Conventionally, these measurements have been performed on a steady state flow rig, where bulk, integral measurements for mass flow rate and swirl or tumble ratio are performed. However, for some of the combustion strategies now being implemented on modern engines, the flow structure is becoming more important necessitating the use of techniques that can measure the flow field and its interaction with spray systems. This piece of work compares engine flow measurements on both a standard steady flow rig and in the cylinder of a motored engine. The flow bench measurements are both easier and cheaper to implement, but serve no real purpose unless there is a correspondence between the flow measured under steady state conditions and that measured in the transient environment of an engine cylinder. On the steady flow bench, both conventional measurements and also measurements of the detailed flow using laser Doppler anemometry have been made. This allowed a direct comparison to be performed between these two sets of measurements. Laser Doppler anemometry measurements were than performed in the cylinder of a motored engine, allowing a direct comparison between the results from the steady flow rig and the engine. Additionally, particle image velocimetry was used to investigate the data on the steady flow bench. It was found that the laser Doppler anemometry measurements were no substitute in terms of accuracy, when compared to the integral measurement of mass flow rate. They did however give some insight into the flow patterns being generated within the cylinder under these conditions. When compared to similar measurements in the engine, in most instances a high degree of correlation was found between the air velocity measurements, although the tumble ratio calculated from the engine was generally higher than that from the steady flow bench. A comparison of vector flows fields from the particle image velocimetry for the steady state and laser Doppler anemometry for the engine measurements, suggested that the influence of the piston on the flows, not present for steady state measurements, was only relevant in the neighbourhood of the piston itself. The transient nature of the flow in engine also seemed to show very little differences between the two sets of measurements. It was concluded that ideally both sets of measurements are required, but that a lot of the detail, with some additional work, could be extracted from the steady flow measurements, but only by using laser diagnostics to measure the flow fields. It was also observed that more than one plane of measurements is required using laser diagnostics to fully characterise the tumble flow field, which is not uniform across the cylinder. This also led to a simple form of weighting of the data in different planes which could be improved with a more detailed set of measurements to gain better insight into the weighting factors required.

This present study seeks to contribute detailed visualization data on a pool boiling experiments using HFE-7000. Particle Image Velocimetry (PIV) was used to measure the time resolved whole field liquid velocity. Bubble dynamic parameters such as nucleation site density, bubble departure diameter, contact angles and frequency were obtained in shadowgraphy measurements. Infrared thermometry with an IR camera was used for observation of temperature fluctuations of nucleation sites. The experiments were taken for the heat flux from 0.042 kW/m^2 to 0.266 kW/m^2, six experimental conditions in total. To provide a supplementary description of heat transfer mechanism, a novel bubble characterization technique, reflection interference contrast microscopy (RICM), was used to obtain detailed information on bubble dynamic parameters on the microscopic scale. Bubble diameter was obtained from RICM pictures. Comparison between the experiments results and previous empirical correlation were made. Agreements and discrepancies were discussed.

Surface-mounted finite-height circular cylinders and square prisms can be found in many industrial and engineering applications. The local flow fields around these bluff bodies are not yet well understood due to lack of experimental and numerical data close to the cylinder and prism. The aim of this thesis was therefore to gain an improved physical description of the flow field above the free end surface and around the cylinders and prisms. In the present experimental study, the particle image velocimetry (PIV) technique was used to measure the flow field very close to these bluff bodies in the test section of a low-speed wind tunnel. Four finite circular cylinders and square prisms of aspect ratios AR = 9, 7, 5 and 3 were tested at a Reynolds number of ReD = 4.2×104. At the location of the cylinder or prism, the boundary layer thickness relative to the cylinder diameter or prism width (D) was δ/D = 1.6. PIV velocity field measurements in the near-wake region were made in a vertical plane parallel to the mean flow direction on the flow centreline (the symmetry plane), within 2D upstream and 5D downstream of the cylinder or prism. Additional PIV measurements were carried out in three orthogonal x-z, x-y, and y-z planes above the free end surface of the models. In the near-wake region of the finite circular cylinders, the large recirculation zone contained a vortex immediately behind and below the free end; this vortex was found for all four aspect ratios. A second vortex was found behind the cylinder near the cylinder-wall junction; this vortex was not observed for the cylinder of AR = 3, indicating a distinct wake structure for this cylinder. Similar to the circular cylinder case, in the near-wake region of the square prisms, a vortex was observed immediately behind and below the free end in the recirculation zone. The size and strength of this vortex increased as the aspect ratio of the prism decreased. Also, a second vortex was found near the prism-wall junction downstream of the prisms of AR = 9 and 7, while this vortex was not observed for the prisms of AR = 5 and 3. The PIV results in the near-wake regions of the circular cylinders and square prisms show that the effect of the bluff body shape (circular or square cross-section) is evident in the maximum length of the mean recirculation zone. A considerable difference was seen between the maximum length of the mean recirculation zones of the circular cylinder and square prism of AR = 9, while the shape of the bluff body does not considerably affect the length of the recirculation zones for the bodies of AR = 7, 5, and 3. The present PIV results also provided insight into the separated flow above the free ends, including the effects of AR and body shape. Above the free end of the cylinders, flow separation from the leading edge led to the formation of a mean recirculation zone on the free-end surface. The point of reattachment of the flow onto the free-end surface moved towards the trailing edge as the cylinder aspect ratio was decreased. Large regions of elevated turbulence intensity and Reynolds shear stress were found above the free end. For the finite circular cylinders, the flow pattern above the free end was similar in all three x-z planes for all aspect ratios, consisting of a cross-stream vortex at approximately x/D = 0. According to the PIV results in the x-y planes, one of the main characteristics of the flow over the free end surface of the circular cylinders was a pair of focal points at x/D ≈ 0 and near the edge of the free end. As the cylinder aspect ratio increased, the size and strength of these vortices decreased. Also, the centers of the vortices moved downstream as the aspect ratio increased. For the finite square prism, the large, separated, recirculating flow region extended into the near wake. For the square prism of AR = 3, considerable difference was seen in the free-end flow pattern compared to the more slender prisms of AR = 9, 7 and 5. In particular, a cross-stream vortex formed due to interaction between the separated flow from the leading edge of the prism and the reverse flow over the trailing edge of the free end. This vortex was seen in all three planes at different cross-stream locations for AR = 3 but only in the symmetry plane for AR = 9. Hence, the present PIV results in the x-z planes revealed the effect of the near-wake flow on the flow above the prism free end. The results also showed a considerable effect of the aspect ratio on the mean velocity field as well as the Reynolds stress fields. The results in the x-y planes showed different flow patterns for the prism of AR = 3 including wall-normal vortices close to the free end at the sides of the prism as well as two saddle points close to the corners of the trailing edge and one node downstream of the trailing edge, while for AR = 9, no vortices and node were observed. Two streamwise vortices with opposite sign of rotation were seen in the y-z plane at x/D = 0.2 for all aspect ratios. The present results illustrate in-plane vorticities originating from the vertices of the leading edge of the prism for all aspect ratios.

The Richtmyer-Meshkov Instability (RMI) (Commun. Pure Appl. Math 23, 297-319, 1960; Izv. Akad. Nauk. SSSR Maekh. Zhidk. Gaza. 4, 151-157, 1969) occurs due to an impulsive acceleration acting on a perturbed interface between two fluids of different densities. In the experiments presented in this thesis, single mode 3D RMI experiments are performed. An oscillating speaker generates a single mode sinusoidal initial perturbation at an interface of two gases, air and SF6. A Mach 1.19 shock wave accelerates the interface and generates the Richtmyer-Meshkov Instability. Both gases are seeded with propylene glycol particles which are illuminated by an Nd: YLF pulsed laser. Three high-speed video cameras record image sequences of the experiment. Particle Image Velocimetry (PIV) is applied to measure the velocity field. Measurements of the amplitude for both spike and bubble are obtained, from which the growth rate is measured. For both spike and bubble experiments, amplitude and growth rate match the linear stability theory at early time, but fall into a non-linear region with amplitude measurements lying between the modified 3D Sadot et al. model (Phys. Rev. Lett. 80, 1654-1657, 1998) and the Zhang & Sohn model (Phys. Fluids 9. 1106-1124, 1997; Z. Angew. Math Phys 50. 1-46, 1990) at late time. Amplitude and growth rate curves are found to lie above the modified 3D Sadot et al. model and below Zhang & Sohn model for the spike experiments. Conversely, for the bubble experiments, both amplitude and growth rate curves lie above the Zhang & Sohn model, and below the modified 3D Sadot et al. model. Circulation is also calculated using the vorticity and velocity fields from the PIV measurements. The calculated circulation are approximately equal and found to grow with time, a result that differs from the modified Jacobs and Sheeley's circulation model (Phys. Fluids 8, 405-415, 1996).

An experimental investigation was conducted using particle image velocimetry to study the effect of nozzle geometry on turbulent free orifice jets. The nozzle geometries studied include the round, cross, flower, star, rectangular and elliptical nozzles (aspect ratio 2). The spread rate of the rectangular nozzle was 61% greater than the square nozzle while the elliptical nozzle was 45% greater than the round nozzle using the conventional half velocity width. The superior mixing capacity of the rectangular and elliptical nozzles is attributed to the axis-switching mechanism. Evaluation of the energy budget showed a higher level of production of turbulence and convection of the mean flow for the rectangular nozzle compared with the round nozzle. Two-point auto-correlation function revealed larger structures in the non-circular nozzles and in particular the rectangular nozzle. The Kolmogorov and Taylor microscales however, did not show any significant dependency on nozzle geometry. / October 2017

An inverted airfoil mounted above a flat plate was used to create laminar separation bubbles on a flat plate in water tunnel experiments at low Reynolds numbers. Boundary layer suction ensured that the flow remained attached to the wing. Two-dimensional PIV measurements were used to qualitatively and quantitatively characterize the spanwise bubble variation on an unswept wing and on the same wing featuring a 22 degree sweep. The separation bubbles were recorded at varied span-wise locations in a 31.5 cm wide region of the flow. The limitations of this measurement region were dictated by the focal length of the laser optic used for PIV measurements. The straight wing exhibited approximately uniform time averaged separation positions across the span of the wing. The reattachment locations varied only slightly which was expected due to the transition to turbulent flow before reattachment. A form of bubble "breathing" was observed in the laminar separation bubbles on the straight wing and is believed to have affected the mean reattachment locations for two data points recorded. The shedding frequencies on the straight wing were slightly higher than those obtained from CFD simulations. The swept wing planform showed significantly more variation in the mean separation and reattachment locations with respect to the leading edge of the wing. There is a general trend of the separation locations moving upstream in the direction of the aft leading edge. The reattachment points are shown to move downstream as the separation points move upstream relative to the leading edge and visa versa, displaying an inverse relationship between the two. The bubble lengths were found to be slightly longer on the swept wing compared to the straight wing usually by about 10%. The shedding frequencies on the swept wing were found to be lower than the straight wing. The quality of flow in the water tunnel may have degraded over time, showing signs of increased free stream turbulence. After data collection, it was also discovered that the boundary layer suction on the wing was not constant at all span-wise locations. It is believed that the introduction of wing sweep intensified the effect of insufficient suction on the structure of the bubbles observed. The present results were in agreement with previous research for bubble structure but the dynamic instabilities were found to differ slightly.

An experimental investigation into the form of the notchback wake topology, its temporal behaviour, and how this changes with the underlying geometry has been undertaken to further understanding of this flow regime pertaining to a popular automotive body type. Whilst this work has been performed at model scale on a simplified body a sufficiently complex design of backlight header and trailing pillar have been utilised. Thereby allowing the systematic study of the wake structure of a family of production representative geometries to be undertaken enabling the flow topology across bodies with parameters representative of vehicles produced from the 1960s to the present day to be investigated. Body force measurements showed both drag and rear lift to increase with backlight angle in a manner which was largely expected due to these designs being representative of older production notchback vehicles. Manufacturers knowledge and understanding of how drag changes with this parameter, combined with on going shape optimisation studies, have led to the shallower backlight angles common to modern designs. Detailed flow field measurements were subsequently used to determine the form and temporal behaviour of the flow topologies responsible for this force behaviour. Across the range of geometries tested, the in-notch structures were shown to undergo significant variation, both their time-averaged form and time-variant behaviour changing. Common to all configurations were the presence of a pair of strong trailing vortex structures which flanked the edges of the backlight and bootdeck. However, flow in the centre of the backlight underwent the greatest variation. This region was shown to develop from a largely attached form at shallower backlight angles before developing into an increasingly strong hairpin like structure. As backlight angle increased further the topology ultimately took a highly asymmetric form. With these changes of the flow topology also came changes of the temporal behaviour which revealed vortex shedding, flow structure oscillation and the switching of bi-stable structures as backlight angle increased. It is hoped that in thoroughly understanding the range of notchback flow topologies typically generated by production vehicles that this work will form the vital foundation upon which future investigations looking to reduced drag can be based.