An experimental study of an aerodynamic ramp (aeroramp) injector was conducted at Virginia Tech. The aeroramp consisted of an array of two rows with two columns of flush-wall holes that induce vorticity and enhance mixing. For comparison, a single-hole circular injector with the same area angled downstream at 30 degrees was also examined. Test conditions involved sonic injection of helium heated to 313 K, to safely simulate hydrogen into a Mach 4 air cross-stream with average Reynolds number 5.77 e+7 per meter at a jet to freestream momentum flux ratio of 2.1. Sampling probe measurements were utilized to determine the local helium concentration. Pitot and cone-static pressure probes and a diffuser thermocouple probe were employed to document the flow. The main results of this work was that the mixing efficiency value of this aeroramp design which was optimized at Mach 2.4 for hydrocarbon fuel was only slightly higher than that of the single-hole injector at these flow conditions and the mass-averaged total pressure loss parameter showed that the aero-ramp and single-hole injectors had the same overall losses. The natural extension of the investigation was then to look in detail at two major physical phenomena that occurs in a complex injector design such the Aeroramp: the jet-shock interaction and the interaction of the vortical structures produced by the jets injection into a supersonic cross flow. Experimental studies were performed to investigate the effects of impinging shocks on injection of heated helium into a Mach 4 crossflow. It was found that the addition of a shock behind gaseous injection into a Mach 4 crossflow enhances mixing only if the shock is closer to the injection point where the counter-rotating vortex pair (always associated with transverse injection in a crossflow) is not yet formed, and the deposition of baroclinic generated of vorticity is the highest. The final investigation concerned with the interaction of the usual vortex structure produced by jet injection into a supersonic crossflow and an additional axial vortex typical of those that might be produced by the inlet of a scramjet or the forebody of a vehicle to be controlled by jet interaction phenomena. The additional axial vortices were generated by a strut-mounted, diamond cross-section wing mounted upstream of the injection location. The wing was designed to produce a tip vortex of a strength comparable to that of one of the typical counter-rotating vortex pair (CVP) found in the plume of a jet in a crossflow. The profound interaction of supersonic vortices supported by a quantitative description and characterization of the flowfield has been demonstrated. / Ph. D.

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.

Further understanding the small scale coherent structures which occur in high Reynolds number turbulence would be of enormous benefit. Therefore, the aim of the current project was to make well resolved three-dimensional flow measurements of the mixing flow between counter rotating impellers, using Tomographic Particle Image Velocimetry (TPIV).TPIV software was developed, with a novel approach permitting a significant reduction in processing time, and a series of numerical accuracy studies contributing to the fundamental understanding of this new technique. Basic flow characterisation determined the local isotropy, homogeneity and expected Reynolds number scaling. A favourable comparison between planar PIV and TPIV increased confidence in the latter, which was used to assess the dynamics and topology of the dissipation scale structures. In support of previous investigations similar topology, strain rate alignment, scale-invariance, and clustering behaviours are demonstrated. Correlated high enstrophy and dissipation regions occur in the periphery of larger structures, resulting in intermittency. Geometry characterisation indicates a predominance of tube-like structures, which are observed to form from larger ribbon-like structures through unsteady breakdown and vortex roll-up. Significant correlation between intermittent fields of dissipation and enstrophy describe the fine scales effects. These relationships should pave the way for more accurate models, capable of relating small scales and large scales during the prediction of dynamically important quantities.

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 accuracy of existing methods for estimating the drag based on experimental flow field measurements were assessed for two-dimensional bodies. The effects of control volume boundary placement and inherent simplifying assumptions were also investigated. Wind tunnel experiments were performed on a circular cylinder operating at a Reynolds number of 8,000 and 20,000, and on a NACA 0018 airfoil operating at a chord Reynolds number of 100,000 for three angles of attack (α), specifically, 5◦, 10◦, and 15◦. The circular cylinder experiments fall within the the shear layer transition flow regime. Airfoil investigations span both types of flow development common to low Reynolds number airfoil operation. For α = 5◦ and 10◦, a separation bubble forms on the upper surface of the airfoil, while, for α = 15◦, the flow separates without reattaching, resulting in a stalled airfoil. Wake velocity and pressure measurements were performed at several downstream locations to investigate the impact of control volume boundary placement. Wake profiles were measured between 3 and 40 diameters downstream from the circular cylinder axis and between 1 and 4.5 chord lengths from the trailing edge of the airfoil. In addition to wake profiles, the outer flow velocity variation was quantified to investigate the appropriate location to measure freestream flow characteristics in a test section with streamwise-varying outer flow conditions. The results show that drag estimates are strongly dependent on the streamwise position of the measured wake profile for all methods investigated. Drag estimates improved, and streamwise variation decreased, with increasing streamwise position of the flow measurements. For the pressure based method examined, wake measurements should be taken at least 10 times the projected model height downstream of the model. In the case of the circular cylinder, this is equivalent to 10 diameters and, for the airfoil investigated, it is approximately 1 chord length from the trailing edge. For the methods relying on velocity measurements, acceptable estimates of drag were possible when based on measurements taken at least 30 projected heights downstream, i.e., 30 diameters for the circular cylinder and 3 chord lengths for the airfoil model investigated. The findings highlight the importance of providing a detailed description of the methodology and experimental implementation for drag estimates based on flow field measurements. Finally the study offers guidelines for implementing momentum integral based drag calculations in future investigations.

The extension of the theory for two dimensional turbulent boundary layers into three dimensional flows has met with limited success. The failure of the extended models is attributed to the anisotropy of the turbulence. This is seen by the turbulent shear stress angle lagging the flow gradient angle and by the behavior of the Reynolds shear stresses lagging that of the mean flow. Transport equations for the turbulent shear stresses were proposed to be included in a modeling effort capable of accounting for the lags seen in the flow. This study is aimed at developing algebraic relationships between the various Reynolds-averaged terms in these modeling equations. Particular emphasis was placed on the triple products that appear in the transport equations. Eleven existing experimental data sets were acquired from the original authors and re-examined with respect to developed and existing parameters. A variety of flow geometries were collected for comparison. Emphasis was placed on experiments that included all six components of the Reynolds stress tensor and triple products. Parameters involving the triple products are presented that appear to maintain a relatively constant value across regions of the boundary layer. The variation of these parameters from station to station and from flow to flow is discussed. Part of this study was dedicated to parameters that were previously introduced, but never examined with respect to the data that was collected. Results of these parameters are presented and discussed with respect to agreement or disagreement with the previous results. The parameters presented will aid in the modeling of three dimensional turbulent boundary layers especially with models that employ the transport equations for the Reynolds stresses. / Master of Science

No description available.

Fully turbulent inflow past symmetrically located side branches mounted in a duct can give rise to pronounced flow oscillations due to coupling between separated shear layers and standing acoustic waves. Experimental investigation of acoustically-coupled flows was conducted using digital particle image velocimetry (DPIV) in conjunction with unsteady pressure measurements. Global instantaneous, phase- and time-averaged flow images, as well as turbulence statistics, were evaluated to provide insight into the flow physics during flow tone generation. Onset of the locked-on resonant states was characterized in terms of the acoustic pressure amplitude, frequency and the quality factor of the resonant pressure peak. Structure of the acoustic noise source is characterized in terms of patterns of generated acoustic power. In contrast to earlier work, the present study represents the first application of vortex sound theory in conjunction with global quantitative flow imaging and numerical simulation of the 2D acoustic field. In addition to the basic side branch configuration, the effects of bluff rectangular splitter plates located along the centerline of the main duct was investigated. The first mode of the shear layer oscillation was inhibited by the presence of plates, which resulted in substantial reduction of the amplitude of acoustic pulsations and the strength of the acoustic source. These results can lead to the development of improved control strategies for coaxial side branch resonators. Motivation for the second part of this study stems from the paper manufacturing industry, where air clamp devices utilize high-speed jets to position paper sheets with respect to other equipment. Thus, vibration of the paper sheet and turbulent flow that emerged from a planar curved nozzle between a flexible wall and a solid surface containing a backward-facing step (BFS) were investigated using high-speed photography and DPIV, respectively. The emphasis was on the characterization of the flow physics in the air clamp device, as well as of the shape of the paper sheet. For the control case, that involved a solid wall with a geometry that represented the time-averaged paper profile, hydrodynamic oscillation frequencies were characterized using unsteady pressure measurements. Experimentally obtained frequencies of the paper sheet vibration were compared to the hydrodynamic frequencies corresponding to the oscillations of the shear layer downstream of the BFS.

No description available.

L'étude décrite dans cette thèse porte sur l'injection ponctuelle d'une solution saline au sein d'une cellule dite de Hele-Shaw, afin de caractériser le comportement dispersif d'un polluant en milieu poreux. L'approche expérimentale employée est basée sur l'implémentation originale d'un dispositif de Fluorescence Induite par Laser (LIF) dans la cellule. La mise en place d'un protocole de mesure efficace permet de mener une analyse quantitative des résultats expérimentaux. En outre, en appliquant la méthode des moments, il est possible de caractériser avec précision le comportement dispersif de la zone de mélange de la solution injectée. Parallèlement aux expériences, à l'aide du code numérique FRIPE, les injections ont été simulées numériquement. L'analyse quantitative a été appliquée à ces dernières. Une comparaison poussée des résultats expérimentaux et numériques a donc été effectuée, du point de vue qualitatif mais aussi sur l'expression de la dispersion du panache de la zone de mélange de la solution / The study described in this thesis is about punctual injection of a saline solution inside a "Hele-Shaw cell" in order to characterize the dispersive behavior of a pollutant in porous media. The chosen experimental approach is based on the setup of an original Laser Induced Fluorescence (LIF) in the Hele-Shaw cell. The setting of the experimental apparatus allows quantitative data reduction of the experimental results. Moreover the "Moments Method" studied precisely the solution mixing dispersive behavior. Using the numerical code FRIPE the same injections have been simulated. The same quantitative data reductions have been applied to the numerical results. This led to an extensive comparison of the numerical and the experimental results, qualitatively but also of the dispersion in the mixing area of the injected solution