In Particle Image Velocimetry (PIV), the number of particle images per interrogation region, or particle image density, impacts the strength of the correlation and, as a result, the number of valid vectors and the measurement uncertainty. Therefore, any a-priori estimate of the accuracy and uncertainty of PIV requires knowledge of the particle image density. An autocorrelation-based method for estimating the local, instantaneous, particle image density is presented. Synthetic images were used to develop an empirical relationship based on how the autocorrelation peak magnitude varies with particle image density, particle image diameter, illumination intensity, interrogation region size, and background noise. This relationship was then tested using images from two experimental setups with different seeding densities and flow media. The experimental results were compared to image densities obtained through using a local maximum method as well as manual particle counts and are found to be robust. The effect of varying particle image intensities was also investigated and is found to affect the particle image density.

Thesis (M.S.)--University of Delaware, 2007. / Principal faculty advisor: Fabrice Veron, College of Marine and Earth Studies. Includes bibliographical references.

According to the American Heart Association about 795,000 people suffer a stroke each year. Of those strokes almost 140,000 are fatal; this makes Stroke the third leading cause of death in the United States behind coronary heart disease and cancer. Hemorrhagic strokes are caused when an artery in the brain ruptures, such as a ruptured aneurysm. One possible treatment for cerebral aneurysm is a porous tubular structure, similar to a stent, called a flow diverter. A flow diverter can be placed across the neck of a cerebral aneurysm to induce the cessation of flow and initiate the formation of an intra-aneurysmal thrombosis. This excludes the aneurysm from the parent artery and returns the flow of blood to normal. The process of flow diversion alone has been shown to take months to fully exclude the aneurysm. It is possible however with an adjuvant therapy called photothrombosis to accelerate this process so that the aneurysm is excluded within minutes. Previous flow diverting devices have been analyzed to determine optimal characteristics, such as braiding angle and wire diameter. From this information a new optimized device was designed and is now in the process of being tested. In order to evaluate the effect of the device, a model must be created. One such model is the rabbit elastase induced aneurysm, which was characterized so that elastomer models could be created for in vitro studies. Particle Image Velocimetry (PIV) is a method of analysis that utilizes very small glass spheres (between 8 mu m and 12 mu m in diameter) to determine the velocity vectors of fluid flow in an in vitro model. These velocities can be used to calculate hydrodynamic circulation and kinetic energy inside an elastomer model of the elastase induced aneurysm. By comparing these values inside the aneurysm with values for previously developed diverters and a control without a diverter, it can be shown that despite changes in the braiding angle and individual wire thickness that the behavior of the devices is not significantly different (P > 0.05). Flow diversion is also being used in concert with photothrombosis. A flow diverter is used to exclude the neck remnant from the parent vessel and to provide a scaffold for the remodeling of the neck. This combination of techniques allows for very fast and near complete occlusion of the aneurysm thereby excluding the aneurysm from the parent vessel and eliminating the risk of a rupture.

La thèse porte sur l'étude de la convection capillaire (effet Marangoni) et son couplage avec la convection thermogravitationnelle. Le travail met en oeuvre des mesures de champ de vitesse par vélocimétrie laser, d'une part, et des simulations numériques de ces expériences, d'autre part. Des solutions asymptotiques sont également utilisées, et la transition de la convection stationnaire vers un état d'oscillations spatiales ou spatio-temporelles a été analysée. Le manuscript aborde également des situations impliquant l'effet de thermodiffusion, ainsi que les mouvements dans une bicouche de fluides immiscibles.

Engineering and Physical Science Research Council (EPSRC) / This thesis describes the development of the two-frequency Planar Doppler Velocimetry (2n-PDV) flow measurement technique. This is modification of the Planar Doppler Velocimetry (PDV) technique that allows the measurement of up to three components of the flow velocity across a plane defined by a laser light sheet. The 2n-PDV technique reduces the number of components required to a single CCD camera and iodine cell from the two CCDs in conventional PDV. This removes the error sources associated with the misalignment of the two camera images and polarisation effects due to the beam splitters used in conventional PDV. The construction of a single velocity component 2n-PDV system is described and measurements made on the velocity field of a rotating disc and an axisymmetric air jet. The system was then modified to make 3D velocity measurements using coherent imaging fibre bundles to port multiple views to a single detector head. A method of approximately doubling the sensitivity of the technique was demonstrated using the measurements made on the velocity field of the rotating disc and was shown to reduce the error level in the final orthogonal velocity components by ~40 to 50%. Error levels of between 1.5ms-1 and 3.1ms-1 depending upon observation direction are demonstrated for a velocity field of ±34ms-1. The factors that will influence the selection of a viewing configuration when making 3D PDV measurements are then investigated with the aid of a computer model. The influence of the observation direction, the magnitude of the flow velocity, and the transformation to orthogonal velocity components are discussed. A new method using additional data in this transformation is presented and experimental results calculated using four-measured velocity components are compared to those found conventionally, using only three components. The inclusion of additional data is shown to reduce the final error levels by up to 25%.

This study investigated the applicability of Particle Image Velocimetry (PIV) as a velocity measurement technique for use in wind tunnel flows. To carry out the investigation, a small scale wind tunnel was designed and built to be used specifically with PIV. The tunnel employed a novel contraction geometry which was compared to six other contraction designs using a computational fluid dynamics (CFD) software package. The wind tunnel configuration allowed for full optical access in the test section to allow for PIV measurements in three dimensions. The calibration and characterization of the flow quality within the wind tunnel were performed using PIV. Velocity measurements were obtained in the empty test section to assess the degree of uniformity, alignment, and turbulence at various test speeds. The longitudinal velocities were found to deviate by an average of 1. 8% along any given velocity profile. The flow was found to be well aligned with the test section walls, deviating by no more than &plusmn;0. 20° in most cases. As well, the turbulence levels in the test section were found to be low, with average intensities of 2. 0% and 0. 5% in the longitudinal and transverse directions, respectively. Following the characterization of the flow in the empty wind tunnel, a square cylinder was placed in the test section and PIV measurements were performed at a Reynolds number of 21,400. Mean velocities and turbulence intensities measured around the square cylinder were found to compare well with previous works conducted at similar Reynolds numbers in water flows. As a final validation of the wind tunnel/PIV system, measurements were made of the flow over a 1:18 scale Formula One racecar model at a free stream velocity of 40 <i>m/s</i>. The PIV system collected a large quantity of velocity information around the model, providing insight into the aerodynamic aspects of racecars such as downforce devices and vehicle draughting. The experiments performed in this study led to the conclusion that PIV is indeed a measurement technique with high potential for use in small wind tunnels, providing more spatially resolved velocity data than any other known measurement technique. The advancement of digital camera technology will make PIV a more practical measurement technique for use in larger wind tunnels as well.

This study investigated the applicability of Particle Image Velocimetry (PIV) as a velocity measurement technique for use in wind tunnel flows. To carry out the investigation, a small scale wind tunnel was designed and built to be used specifically with PIV. The tunnel employed a novel contraction geometry which was compared to six other contraction designs using a computational fluid dynamics (CFD) software package. The wind tunnel configuration allowed for full optical access in the test section to allow for PIV measurements in three dimensions. The calibration and characterization of the flow quality within the wind tunnel were performed using PIV. Velocity measurements were obtained in the empty test section to assess the degree of uniformity, alignment, and turbulence at various test speeds. The longitudinal velocities were found to deviate by an average of 1. 8% along any given velocity profile. The flow was found to be well aligned with the test section walls, deviating by no more than &plusmn;0. 20° in most cases. As well, the turbulence levels in the test section were found to be low, with average intensities of 2. 0% and 0. 5% in the longitudinal and transverse directions, respectively. Following the characterization of the flow in the empty wind tunnel, a square cylinder was placed in the test section and PIV measurements were performed at a Reynolds number of 21,400. Mean velocities and turbulence intensities measured around the square cylinder were found to compare well with previous works conducted at similar Reynolds numbers in water flows. As a final validation of the wind tunnel/PIV system, measurements were made of the flow over a 1:18 scale Formula One racecar model at a free stream velocity of 40 <i>m/s</i>. The PIV system collected a large quantity of velocity information around the model, providing insight into the aerodynamic aspects of racecars such as downforce devices and vehicle draughting. The experiments performed in this study led to the conclusion that PIV is indeed a measurement technique with high potential for use in small wind tunnels, providing more spatially resolved velocity data than any other known measurement technique. The advancement of digital camera technology will make PIV a more practical measurement technique for use in larger wind tunnels as well.

Dynamic stall has proven to be a complex problem in helicopter aerodynamics because it limits the helicopter flight regime. Dynamic stall is characterized by drastic increases in lift and a delay of stall due to rapid pitching motions of aerodynamic surfaces. Prediction and control of dynamic stall requires an understanding of the leading edge flow structure. An investigation was conducted of dynamic stall near the leading edge of a large-scale Sikorsky SSC-A09 airfoil, dynamically pitching about its quarter chord, under realistic helicopter flight conditions (M_infinity = 0.1, k = 0.1, Re_c = 1.0 x 10^6). A testing model with a chord of 0.46 m and a span of 2.13 m was designed and constructed for experimentation in the Dynamic Stall Facility at Texas A&M University. Particle image velocimetry data were recorded for the first 15% of the airfoil chord. Mean velocities, Reynolds stresses, and vorticity were computed. Analyses revealed that during the upstroke, stall onset is delayed in the leading edge region and the first indications of separation are observed at 18 degree angle of attack. The edge of the boundary layer has been characterized for alpha = 18 degrees. The roles of the Reynolds stresses and vorticity are examined.

Zugl.: Karlsruhe, Univ., Diss., 2009

Zugl.: Aachen, Techn. Hochsch., Diss., 2009.