Master of Science / Department of Mechanical and Nuclear Engineering / B. Terry Beck / Mohammad H. Hosni / Cavitation is the change of a liquid to a two-phase mixture of liquid and vapor, similar to boiling. However, boiling generates a vapor by increasing the liquid temperature while cavitation generates vapor through a decrease in pressure. Both processes are endothermic, removing heat from the surroundings. Both the phase change and heat absorption associated with cavitation provide many engineering applications, including contributing to a new type of refrigeration cycle under development. Cavitation can occur at or below the vapor pressure; conditions that delay cavitation and allow for a metastable liquid are not well understood. A converging-diverging nozzle was designed and fabricated to create a low pressure region at the nozzle throat. The converging section of the nozzle increased the water velocity and decreased the pressure, according to Bernoulli’s principle. A cavitation front was formed slightly past the nozzle throat. The cavitation location suggested that the water was metastable near the nozzle throat. Flow through the system was controlled by changing the nozzle inlet and outlet pressures. The flowrate of water was measured while the outlet pressure was lowered. The flowrate increased as the outlet pressure dropped until cavitation occurred. Once cavitation initiated, the flow became choked and remained constant and independent of the nozzle outlet pressure. High-speed imagery was used to visualize the flow throughout the nozzle and the formation and collapse of cavitation in the nozzle’s diverging section. High-speed video taken from 1,000 to 35,000 frames per second captured the formation of the cavitation front and revealed regions of recirculating flow near the nozzle wall in the diverging section. Particle Image Velocimetry (PIV) was used to measure the velocity vector field throughout the nozzle to characterize flow patterns within the nozzle. PIV showed that the velocity profile in the converging section and throat region were nearly uniform at each axial position in the nozzle. In the diverging section, PIV showed a transient, high-velocity central jet surrounded by large areas of recirculation and eddy formation. The single-phase experimental results, prior to cavitation onset, were supplemented by Computational Fluid Dynamics (CFD) simulations of the velocity distribution using Fluent software.

Plasma synthetic jet actuators are investigated experimentally, in which the geometrical design of single dielectric barrier discharge (SDBD) plasma actuators is modified to produce zero-mass flux jets similar to those created by mechanical devices. The SDBD plasma actuator consists of two rectangular electrodes oriented asymmetrically and separated by a layer of dielectric material. Under an input of high voltage, high frequency AC or pulsed DC, a region of plasma is created in the interfacial air gap on account of electrical breakdown of the ambient air. A coupling between the electric field in the plasma and the neutral air near the actuator is introduced, such that the latter experiences a net force which results in a horizontal wall jet. This effect of the actuator has been demonstrated to be useful in mitigating boundary layer separation in aerodynamic flows. To increase the impact that a plasma actuator may have on the flow field, this research investigates the development and characterization of a novel flow control device, the plasma synthetic jet actuator, which tailors the residual air in the form of a vertical jet resembling conventional continuous and synthetic jets. This jet can be either three dimensional using annular electrode arrays, or nearly two dimensional using two rectangular strip exposed electrodes and one embedded electrode. Detailed measurements on the isolated plasma synthetic jet reveal that pulsed operation of the actuator results in the formation of multiple counterrotating vortical structures in the flow field. The output jet velocity and momentum are found to be higher for unsteady pulsing as compared to steady operation. In the case of flow over a flat plate, the actuator is observed to create a localized interaction region within which the baseline flow direction and boundary layer characteristics are modified. The efficiency of the actuator in coupling momentum to the neutral air is found to be related to the plasma morphology, pulsing frequency, actuator dimension, and input power. An analytical scaling model is proposed to describe the effects of varying the above variables on the output jet characteristics and actuator efficiency, and the experimental data is used for model validation.

Atherosclerosis and related cardiovascular diseases (CVDs) are amongst the largest causes of morbidity and mortality in the developed world, causing considerable monetary pressure on public health systems worldwide. Atherosclerosis is characterised by the build up of vascular plaque in medium and large arteries and is a direct precursor to acute vascular syndromes such a myocardial infarction, stroke or peripheral arterial diseases. The causative factors leading to CVD still remain relatively poorly understood, but are becoming increasingly identifiable as a dysfunction of the endothelial cells that line the arterial wall. It is well known that the endothelium responds to the prevailing fluid mechanic (i.e. haemodynamic) environment, which plays a crucial role in the localised occurrence of atherosclerosis near vessel bends and bifurcations. In these areas, disturbed haemodynamics lead to flow separation and very low wall shear stress (WSS), which directly affects the functionality of the endothelium and impedes the transport of important blood borne agonists and antagonists. Detailed full field measurements assessing complex haemodynamics are sparse and consequently this thesis aims to address some of the important questions related to arterial haemodynamics and CVD by performing in-vitro flow measurements in physiologically relevant conditions. In particular, this research develops and uses state-of-the-art Particle Image Velocimetry (PIV) techniques to measure three-dimensional velocity and WSS fields in scaled models of the human carotid artery. For this purpose, the necessary theoretical and experimental concepts are developed and in-depth analyses of the underlying factors affecting the local haemodynamics and their relation to CVD are carried out. In the first part, a methodology for the construct of transparent hydraulic flow phantoms from medical imaging data is developed. The arterial geometries are reproduced in optically clear silicone and the flowing blood is modelled with a refractive index matched blood analogue. Subsequently, planar and Stereo-PIV techniques are developed and verified. A novel interfacial PIV (iPIV) technique is introduced to directly measure WSS by inferring the velocity gradient from the recorded particle images. The new technique offers a maximal achievable resolution of 1 pixel and therefore removes the resolution limit near the wall usually associated with PIV. Furthermore, the iPIV performance is assessed on a number of numerical and experimental test cases and iPIV offers a significantly improved measurement accuracy compared to more traditional techniques. Subsequently, the developed methodologies are applied in three studies to characterise the velocity and WSS fields in the human carotid artery under a number of physiological and experimental conditions. The first study focuses on idealised vessel geometries with and without disease and establishes a general understanding of the haemodynamic environment. Secondly, a physiological accurate vessel geometry under pulsatile flow conditions is investigated to provide a more realistic representation of the true in-vivo flow conditions. The prevailing flow structure in both cases is characterised by flow separation, strong secondary flows and large spatial and temporal variations in WSS. Large spatial and temporal differences exist between the different geometries and flow conditions; spatial variations appear to be more significant than transient events. Thirdly, the three-dimensional flow structure in the physiological carotid artery model is investigated by means of stereoscopic and tomographic PIV, permitting for the first time the measurement of the full 3D-3C velocity field and shear stress tensor in such geometries. The flow field within the model is complex and three-dimensional and inherently determined by the vessel geometry and the build up of an adverse pressure gradient. The main features include strong heliocoidal flow motions and large spatial variations in WSS. Lastly, the physiological implications of the current results are discussed in detail and reference to previous work is given. In summary, the present research develops a novel and versatile PIV methodology for haemodynamic in vitro studies and the functionality and accuracy is demonstrated through a number of physiological relevant flow measurements.

The focus of this thesis is the study of turbulent solid-liquid stirred suspensions, which are involved in many common unit operations in the chemical, pharmaceutical and food industries. The studies of two-phase flows present a big challenge to researchers due to the complexity of experiments; hence there is a lack of quantitative solid and liquid hydrodynamic measurements. Therefore, an investigation of turbulence modulation by dispersed particles on the surrounding fluid in stirred vessels has been carried out, via two-phase fluorescent Particle Image Velocimetry (FPIV) and micromixing experiments. The main property of interest has been the local dissipation rate, as well as root-mean-square (rms) velocities and turbulent kinetic energy (TKE) of the fluid. Initially a single-phase PIV study was conducted to investigate the flow field generated by a sawtooth (EkatoMizer) impeller. The purpose of this study was to gain insight into various PIV techniques before moving on to more complex two-phase flows. Subsequently stereo-, highspeed and angle-resolved measurements were obtained. The EkatoMizer formed a good case study as information regarding its hydrodynamics is not readily available in literature, hence knowledge has been extended in this area. An analysis of the mean flow field elucidated the general structure of fluid drawn into the impeller region axially and discharged radially; the latter characterised the impeller stream. The radial rms velocity was considered to represent best the system turbulence, even though the tangential rms velocity was greater close to the blade; however the radial component was more prevalent in the discharge stream. Due to differences in rms velocities, TKE estimates obtained from two and three velocity components deviated, being greater in the latter case. Integral (1-D and 2-D) length scales were overestimated by the quantity W / 2 in the impeller region. Ratios of longitudinal-to-lateral length scales also indicated flow anisotropy (as they deviated from 2:1). The anisotropy tensor showed that the flow was anisotropic close to the blade, and returned to isotropy further away from the impeller. Instantaneous vector plots revealed vortices in the discharge stream, but these were not associated with flow periodicity. Alternatively, the vortex structures were interpreted as low frequency phenomena between 0-200 Hz; macro-instabilities were found to have a high probability of occurrence in the discharge stream. Dissipation is the turbulent property of most interest as it directly influences micromixing processes, and its calculation is also the most difficult to achieve. Its direct determination from definition requires highly resolved data. Alternative methods have been proposed in the literature, namely dimensional analysis, large eddy simulation (LES) analogy and deduction from the TKE balance. All methods were employed using 2-D and 3-D approximations from stereo-PIV data. The LES analogy was deemed to provide the best estimate, since it accounts for three-dimensionality of the flow and models turbulence at the smallest scales using a subgrid scale model. (Continues...).

This thesis project examined the interactive fluid dynamics between a blood pump and the univentricular Fontan circulation. 2-D particle image velocimetry (PIV) measurements were conducted on an idealized total cavopulmonary connection (TCPC) with an axial pump prototype in the inferior vena cava (IVC). Fluid velocity profiles were examined under various physiologic conditions for Fontan patients. The velocity profiles for all cases demonstrated the shunting of flow from the IVC toward the right pulmonary artery. A rotational component in the pump outflow was observed forcing flow to the periphery as compared to the flow profile without a pump present in the IVC. The inclusion of the pump provides a pressure rise of 3 to 9 mmHg. These results demonstrate the ability of the intravascular blood pump to support a Fontan circulation and support the continued optimization and development of the pump.

Abstract An experimental study is conducted to determine the velocity fields, from development to steady state, in a square enclosure due to movement of a constant velocity lid using Particle Image Velocitmetry (PIV). Experiments were conducted with water, seeded with hollow glass sphere particles 10 microns in diameter, at three different lid velocities leading to Reynolds numbers in the high laminar to transitional range. Driven Cavity Flow is a classic fluid dynamics case often used for benchmarking of computational codes. Previous work has primarily focused on improving computational codes, experimental work is lacking and focused on obtaining steady state readings. The test cavity is 1 inch (25.4mm) high by 1 inch (25.4 mm) wide leading to an aspect ratio of 1.0. The depth is taken to be 5 (127mm) inches to reduce the three dimensional effects. Readings are taken from development to steady state allowing for a full spectrum of flow characteristics. PIV technique is successful in capturing the development of driven cavity flow. Circulation is shown to increase strength with time and Reynolds number. PIV capture and processing settings are determined. Keywords: Driven Cavity Flow, Particle Image Velocimeter (PIV)

<p>An accurate understanding of the respiratory fluid dynamics is instrumental for medical applications, such as drug delivery system and treatment of diseases. Substantial research has been done to study such flow. However, a great number of these studies have the prevailing assumption of having a smooth wall, in despite the human trachea and bronchi is sustain by a series of cartilaginous rings, which creates height differences near the wall. To study the effect of including cartilaginous rings in the respiratory flow we developed two experiments, presenting a comparison between a smooth model and a model with cartilaginous rings. First, we present an experimental observation of a simplified Weibel-based model of the human trachea and bronchi with cartilaginous rings. The experiments were carried out with a flow rate comparable with a resting state (trachea-based Reynolds number of Re_D = 2650). In the second experiment, we developed a similar experiment but in a model with a tracheal stenosis (70% in the middle of the model) and no bronchi. In this case we increase the Reynolds number to Re_D= 3350, still a resting breathing state condition.</p> <p>For both experiments, we used transparent models and refractive index-matching methods were used to observe the flow, particularly near the wall. The flow was seeded with tracers to perform particle image velocimetry and particle tracking velocimetry to quantify the effect the rings have on the flow near the trachea and bronchi walls. From the results, we present a previously unknown phenomenon in the cavities between the cartilaginous rings: a small recirculation is observed in the upstream side of the cavities throughout the trachea. This recirculation is due to the adverse pressure gradient created by the expansion, which traps particles within the ring cavity. In addition, we found that the cartilaginous rings induce velocity fluctuations into the flow, which enhances the near-wall momentum of the flow reducing the separation after the stenosis. Size of the recirculation is reduced by 11% and the maximum upstream velocity is reduced by 38%, resulting in a much weaker recirculation because of the rings. Also noticed a delay in the separation from the trachea to bronchi bifurcation. </p> <p>The detection of recirculation zones in the cartilage ring cavities and the perturbation sheds light on the particle deposition mechanism and helps explain results from previous studies that have observed an enhancement of particle deposition in models with cartilage rings. The results highlight the importance to include the cartilaginous rings in respiratory flow studies. Finally, we compared the results from the stenotic case with Reynolds-averaged Navier-Stokes (RANS) models (k – ε, k – ε RNG, k – ω, k – ω SST, k – ω SST LRN and 4-equation Transition SST). In the results, indicate significant discrepancies between the experimental measurements and the simulations, mainly in the area with flow separation after the contraction. Therefore, RANS algorithms should not be considered reliable for research purposes in respiratory fluid dynamics without experimental validation. </p>

Este trabalho representa o início de uma série de pesquisas que visam o estudo da turbulência e de sua relação com processos de mistura e trocas gasosas entre ar e água, através de experimentos com grades oscilantes. Seu objetivo foi o projeto e a construção de um tanque de grade oscilante, equipamento que gera turbulência com intensidade controlável. Após a construção do tanque, experimentos para medições de velocidade turbulenta foram feitos, com uso de uma técnica de velocimetria a laser. Uma grade de 9x9 barras foi usada na agitação do fluido, com uma amplitude de oscilação de 3 cm, para 4 diferentes freqüências de oscilação. Adquiriu-se 9600 imagens do movimento do fluido, em 6 regiões do tanque, para a obtenção dos campos de velocidade turbulenta, calculados através do software Visiflow e de um programa computacional desenvolvido neste trabalho. Os gráficos criados a partir dos campos possibilitaram a observação do decaimento espacial da turbulência e da região de sua produção. Os campos médios de velocidade mostraram-se bem inferiores aos campos instantâneos, indicando a existência de baixo escoamento médio do fluido. As condições de isotropia e a homogeneidade espacial da turbulência são mais aproximadas à medida que se afasta da grade. A intensidade turbulenta produzida é diretamente relacionada com a freqüência de oscilação. O número de imagens para a obtenção de uma média representativa da velocidade turbulenta neste equipamento parece ser dependente da freqüência de oscilação da grade. Isto aponta para a necessidade de estabelecer corretamente as condições para os cálculos estatísticos em escoamentos turbulentos / This work presents the project and construction of a tank with an oscillating grid, equipment which provides for the experimental studies of turbulence and its relation to mixing processes and gas transfer across fluid interface. Experiments were carried out with the use of digital particle image velocimetry technique, to investigate the properties of the produced turbulence. A grid made of 9x9 square bars was used to stir the water, with a stroke of 3 cm and 4 different oscillation frequencies. A number of 9600 images were acquired, in 6 regions of the tank, for the generation of the turbulent velocity fields through the software Visiflow and a computational program developed in this work. The results showed that the current equipment, with a new concept for the grid oscillation system, can be conveniently useful for studies in this research field. Average velocity fields appeared to be considerably smaller than the instantaneous velocity fields, which leads to the existence of nearly-stationary turbulence in the water volume. Nearly-isotropic turbulence and spatial homogeneity were approximate as the measurements distanced from the grid. The turbulent intensity was directly dependent on the oscillation frequency. The spatial decay of the turbulence and the region near the grid where it is produced could also be observed. The number of images necessary for the calculus of reliable root-mean-square turbulent velocities seems to be dependent on the oscillation frequency of the grid. It results in the necessity of establishing correct statistical analysis of turbulent flows

<div>Gas turbine engines have operated on the Brayton cycle for decades, each decade only gaining approximately one to two percent in thermal efficiency as a result of efforts</div><div>to improve engine performance. Pressure-gain combustion in place of constant-pressure combustion in a Brayton cycle could provide a drastic step-change in the thermal efficiency of these devices, leading to reductions in fuel consumption and emissions production. Rotating Detonation Engines (RDEs) have been widely researched as a viable option for pressure-gain combustion. Due to the extremely high frequencies associated with operation of an RDE, the development and application of high-speed diagnostics techniques for RDEs is necessary to further understand and</div><div>develop these devices.</div><div><br></div><div>An application of high-speed diagnostic techniques in a natural gas-air RDE at conditions relevant to land-based power generation is presented. Diagnostics included high-frequency chamber pressure measurements, chemiluminescence imaging of the annulus, and Particle Image Velocimetry (PIV) measurements at the exit plane of the RDE. Results from a case with two detonation waves rotating clockwise (aft looking forward) in the combustor annulus are presented. Detonation surface plots are created from chemiluminescence images and allow for the extraction of properties such as dominant frequency modes and wave number, speed, and direction. The chamber frequency for the case with two co-rotating waves in the chamber is found to be 3.46 kHz and corresponds to average individual wave speeds of 68% Chapman-Jouguet (CJ) velocity. Dynamic Mode Decomposition (DMD) is applied and indicates the presence of two strong detonation waves rotating clockwise and periodically intersecting with weaker, counter-rotating waves in the annulus at certain times during operation. Singular-Spectrum Analysis (SSA) is used to isolate modes corresponding to the detonation frequency in the signals of velocity components obtained from PIV, maintaining instantaneous phase information. Axial and azimuthal components of velocity are observed to remain nearly 180 degrees out of phase. Lastly, approximate angles for the trailing oblique shocks in the combustion chamber are calculated.</div>

The undercutting of toneholes has been practised for centuries with the aim of improving the tuning and playability of woodwind instruments. The influence of undercutting on tuning can be understood in terms of linear acoustic theory. Its effect on other playing characteristics is thought to lie in its reduction of local non-linear flow phenomena (boundary layer separation and the formation of jets and vortices) at the tonehole. Particle Image Velocimetry (PIV) is used to examine the oscillating airflow around a model woodwind tonehole. Velocity and vorticity information is obtained and compared for a square-edged tonehole and an undercut tonehole at a variety of sound levels. The upstream, internal edge of the tonehole is found to be the location of the most significant local non-linear flow behaviour. Undercutting is found to reduce the strength of local non-linear flow phenomena at a given sound level. Microphone measurements carried out in a reverberation chamber show that undercutting the tonehole also reduces the harmonic distortion introduced to the radiated pressure signal by the non-linear flow. Proper Orthogonal Decomposition (POD) is then applied to PIV data of oscillating flow at the end of a tube. It is used to approximately separate the acoustic field from the induced local non-linear flow phenomena. The POD results are then used to approximate the percentage of kinetic energy present in the non-linear flow. POD analysis is applied to the case of flow around the two toneholes. It shows a smaller transfer of kinetic energy to non-linear flow effects around the undercut tonehole at a given sound level. The dependence of the local non-linear flow kinetic energy on Strouhal number is considered.