Die moderne Gefäßchirurgie bedient sich bei hohen Stadien der pAVK, spezieller Gefäßrekonstruktionen in Form von distalen End-zu-Seit-Gefäßanastomosen. Das langfristige Versagen der Gefäßanastomose hängt primär von der Entstehung einer subendothelialen Intimahyperplasie (IH) ab. Diese IH-Gebiete befinden sich je nach Anastomosengeometrie im Gebiet der Hauben- und Fersenzone sowie am Boden der Anastomose. // Mit Hilfe der Particle Image Velocimetry-Technik wird eine Taylor-Patch-, eine Miller-Cuff-Anastomose und eine femoro-crurale Patch-Prothese bezüglich ihrer Flussmuster sowie ihrer hämodynamischen Eigenschaften wie Geschwindigkeit, Scherstress und Rotation in z-Richtung (Vorticity) untersucht. // In einem hydrodynamischen Kreislaufmodell werden elastische, transparente Silikonmodelle der Anastomosen hergestellt und mit einem blutanalogen Newtonschen Fluid (Glycerol-Wasser-Gemisch) unter Simulation der femorocruralen Druckkurve, pulsatil bei Variation der Strömungsbedingung perfundiert. Der periphere Widerstand beträgt 0,5 mmHg/ml/min (PRU) und die Phasenverschiebung -12 Grad (zwischen Druck- und Flusskurve). // Die Flussmuster variieren zwischen den unterschiedlichen Ausstromverhältnissen erheblich. Bei den unterschiedlichen Flussstärken hingegen ähneln sich die Flussmuster. Alle drei Modelle zeigen ausgeprägte Flussseparationszonen im Hauben- und Fersengebiet sowie geometrieabhängig auch eine Stagnationszone am Boden. Diese Bereiche wiesen die geringsten Fluidgeschwindigkeiten, deutlich unter normalem Wandscherstressniveau liegende Scherstressverhältnisse sowie geringe Vorticitywerte auf. Im Bereich der Übergangszonen finden sich hohe Scherstress- sowie Vorticitywerte. Geschwindigkeitsunterschiede des Fluids zeigten sich im Bereich der Ausstromsegmente. Variable Stressverteilungen zeigen sich auch innerhalb der Separationszonen. Eine Erklärung für die unterschiedlich beschriebenen Offenheitsraten der drei Anastomosenformen wird durch diese Arbeit nicht gefunden. / Modern vascular surgery uses special termino-lateral anastomoses for treating high levels of peripheral arterial disease (PAD). Long term stenoses and occlusions of vascular anastomoses mostly depend on the development of subendothelial myointimal hyperplasia (MIH). There are characteristic areas within the anastomoses, where this process can be examined: The heel, the tow and the floor zone. // This examination observes local hemodynamics like velocity, shear stress and vorticity (rotation in z-direction) and flow patterns of a Taylor-Patch-, a Miller-Cuff-Anastomosis and a feroro-crural patch prothesis (FCPP) with the usage of a Particle Image Velocimetry. In a hydrodynamic circulation model various elastic, transparent silicon phantoms of termino-lateral anastomoses are perfused with a Newton fluid blood analogon (glycerol-water mixture) while simulating the femorocrural pressure curve in a pulsatile manner under variation of the flow conditions. The outflow resistance is 0.5 mmHg/ml/min (PRU, peripheral resistance units) and a phase shift of -12° between flow and pressure curve is simulated. // The flow patterns differed extremely in accordance of the various outflow ratios. Using different flow intensity, the flow patterns are very similar. // All three anastomoses show characteristic heel and toe separation zones. In the FCPP centre a stagnation zone on the floor can not be examined. Shear stress inside the flow separations was significantly lower than normal wall shear stress. High shear stress levels were found inside the transition zones between flow separation and high velocity mainstream. An explanation for the different stenoses and occlusions time of the three different anastomoses can not be found.

Durch die intraoperative Anwendung der extrakorporalen Bypassflow-Messung wurde zum ersten Mal die Messung und Berechnung der hydraulischen Impedanz möglich. Von den in der vorgelegten Untersuchung berücksichtigten Variablen des hydraulischen Impedanzkonzeptes zeigte sich, dass die Phasenverschiebung zwischen Druck- und Flusskurven, gemessen als erster Impedanzphasenwinkel, die größte klinische Bedeutung hatte. Mit Hilfe der Phasenverschiebung wurde die Langzeitprognose von femoro-poplitealen und insbesondere cruralen Bypässen eindeutig vorhersagbar. Sämtliche Bypässe zu Ausstromgebieten, die einen hohen Wellenwiderstand von < -40° aufwiesen, waren innerhalb von 16 Monaten nach der Operation verschlossen. Das Verhalten der Impedanzparameter nach Prostaglandinapplikation bestätigte die bisherigen tierexperimentellen Erfahrungen. Durch die induzierte Vasodilatation kam es neben der Senkung des linearen Strömungswiderstandes auch zu einem deutlichen Abfall des pulsatilen Wellenwiderstandes. Es zeigte sich, dass die isolierte Prostaglandinreaktion ein weiterer Prognosefaktor von cruralen Bypässen war. Sämtliche Rekonstruktionen, die zu Anschlussgefässen geführt wurden, welche keine Prostaglandinreaktion mehr zeigten, waren innerhalb von 9 Monaten postoperativ verschlossen. Hierdurch konnte die Genauigkeit der Vorhersagbarkeit von Bypassverschlüssen noch verbessert werden. Es zeigte sich außerdem in der vorgelegten Studie, dass konventionelle Angiographien weder mit der Prognose der peripheren Rekonstruktionen noch mit den intraoperativ gemessenen linearen oder pulsatilen Widerständen korrelierten. Dieses ist um so mehr von Bedeutung, da die Operationsplanung in der Regel anhand der Angiographien erfolgt. In kritischen Fällen, in denen der periphere Abstrom aufgrund einer schwachen Kontrastierung der Anschlussgefässe nicht sicher zu bestimmen ist, sollten aus diesem Grund zusätzliche dynamische Untersuchungen, wie die farbkodierte Duplexsonographie oder Magnetresonanz-Angiographie erfolgen. Der hohe lineare und pulsatile Widerstand ist zusammen mit der Anastomosengeometrie weiterhin dafür verantwortlich, dass es innerhalb der Anastomosen zu ausgeprägten Flussseparationen kommt. Wie mit Hilfe der Particle Image Velocimetry-Untersuchung gezeigt wurde, finden diese sich innerhalb der klassischen End-zu-Seit-Anastomosen im Bereich der Haube und der Fersenregionen. Darüber hinaus kommt es in Abhängigkeit von der Geometrie zu einer Stagnationszone auf dem Boden der Anastomose. Innerhalb dieser Flussablösungszonen fanden sich Scherstressmuster, die deutlich unterhalb des normalen Wandscherstressniveaus lagen. Zwischen dem Zentralstrom und den Separationszonen ließen sich darüber hinaus Übergangszonen mit deutlich höherem Scherstress nachweisen. Diese Befunde unterstützen die sogenannte low-shear Theorie, welche zur Erklärung der subendothelialen Intimahyperplasie herangezogen wird. Die räumliche Korrelation der typischen Prädilektionsstellen der Intimahyperplasie mit den durch PIV nachgewiesenen Flussablösungen zeigte eine sehr hohe Übereinstimmung. Neben den Hauben- und Fersenseparationen entwickelte sich eine Stagnationszone am Boden des Empfängergefässes. Ein weiterer wichtiger Aspekt in diesem Zusammenhang war der erstmals erhobene Befund, dass in der Übergangszone zwischen Zentralstrom und Separationen normale Wandscherbedingungen vorlagen. Hierbei könnte es sich um das erforderliche Stopsignal für die Endothelzellen handeln, die Stimulation der subendothelialen Myozyten einzustellen. / The intraoperative use of the extracorporeal bypass flow method allowed for the first time measurement and calculation of hydraulic impedance. The phase shift between flow and pressure waves, as represented by the first phase angle, showed a strong correlation to graft patency. All grafts presenting with high impedance values of < -40°, were occluded within 16 month after the operation. The reaction of the impedance parameters following application of prostaglandin E1 correlated with the previously known experimental results. The induced vasodilation led to a significant decrease of the first phase angle. The prostaglandin reaction itself also proved to be a valuable prognostic factor. All reconstructions to a vasculature with a negative prostaglandin reaction were occluded within 9 months after the operation. The prostaglandin response therefore provided an enhancement of the prognostic tools associated with graft patency. In addition it was shown, that the conventional angiographies did not correlate with graft prognosis. This is particularly important, since angiographies are used for the preoperative planning of the operation. In critical cases, additional assessment of runoff by means of duplex sonography or magnetic resonance angiography is necessary. The high pulsatile resistance is responsible for flow separation inside distal anastomoses of peripheral bypasses. Using Particle Image Velocimetry it was shown, that significant separation areas were present at the heel, the hood and the floor of the anastomoses. The shear stress inside these areas was significantly lower than normal wall shear stresses. At the transition between separation and mainstream very high shear stresses were present, which could provide the stop signal for the signal cascades, leading to subendothelial hyperplasia, which leads to graft stenoses.

A stacked microchannel heat sink was developed to provide efficient cooling for microelectronics devices at a relatively low pressure drop while maintaining chip temperature uniformity. Microfabrication techniques were employed to fabricate the stacked microchannel structure, and experiments were conducted to study its thermal performance. A total thermal resistance of less than 0.1 K/W was demonstrated for both counter flow and parallel flow configurations. The effects of flow direction and interlayer flow rate ratio were investigated. It was found that for the low flow rate range the parallel flow arrangement results in a better overall thermal performance than the counter flow arrangement; whereas, for the large flow rate range, the total thermal resistances for both the counter flow and parallel flow configurations are indistinguishable. On the other hand, the counter flow arrangement provides better temperature uniformity for the entire flow rate range tested. The effects of localized heating on the overall thermal performance were examined by selectively applying electrical power to the heaters. Numerical simulations were conducted to study the conjugate heat transfer inside the stacked microchannels. Negative heat flux conditions were found near the outlets of the microchannels for the counter flow arrangement. This is particularly evident for small flow rates. The numerical results clearly explain why the total thermal resistance for counter flow arrangement is larger than that for the parallel flow at low flow rates. In addition, laminar flow inside the microchannels were characterized using Micro-PIV techniques. Microchannels of different width were fabricated in silicon, the smallest channel measuring 34 mm in width. Measurements were conducted at various channel depths. Measured velocity profiles at these depths were found to be in reasonable agreement with laminar flow theory. Micro-PIV measurement found that the maximum velocity is shifted significantly towards the top of the microchannels due to the sidewall slope, a common issue faced with DRIE etching. Numerical simulations were conducted to investigate the effects of the sidewall slope on the flow and heat transfer. The results show that the effects of large sidewall slope on heat transfer are significant; whereas, the effects on pressure drop are not as pronounced.

Im Rahmen des Transferprojektes T01 „Textilbeschichtung mit hochviskosen Massen“ des Sonderforschungsbereiches 532 (SFB 532) wird die Realisierung und Bewertung eines integrierten Beschichtungskonzeptes zur nachhaltigen Verbesserung der Tragfähigkeit von textilbewehrten Betonbauteilen an der RWTH Aachen University untersucht. Dazu wird eine neue Auftragstechnik für hochviskose Beschichtungsmassen entwickelt, die eine vollständige Penetration von Multifilamentgarnen mit großen Garntitern und einer hohen Anzahl an Filamenten in textilen Gelegen erzielt. Des Weiteren werden aktive Beschichtungsmassen auf der Basis von Polyvinylalkohol-Zement-Compositen, die eine homogene Anbindung aller Einzelfilamente an die Zementmatrix ermöglichen, erforscht.

Cette thèse porte sur l’étude expérimentale de la réduction de traînée par injection de bulles. Le contexte de l’étude est lié à la propulsion navale. Les bulles peuvent, lorsqu’elles sont injectées dans la couche limite en développement le long des carènes de navires, contribuer à réduire significativement la résistance de frottement, en allégeant d’une part le fluide le long de la coque et d’autre part en interagissant avec les structures turbulentes de proche paroi. La configuration expérimentale retenue pour cette étude est l’écoulement de Taylor-Couette, avec cylindre extérieur fixe et injection de bulles calibrées. Nos investigations portent sur les régimes de transition, de turbulence naissante et turbulent avec persistance des cellules de Taylor (Re≤20.104). La taille des bulles varie avec le capillaire et le mélange utilisés, entre 0,05 et 0,12 fois la largeur de l’entrefer. Une méthode originale de suivi de la phase dispersée dans un plan méridien, couplée à des mesures de couple global appliqué au cylindre intérieur ont permis de mettre en évidence deux régimes de réduction de traînée et plusieurs types d’arrangements des bulles, en fonction de leur taille et du nombre de Reynolds. Les bulles peuvent avoir une trajectoire glissante, oscillante, être capturées par les cellules de Taylor ou en proche paroi du cylindre intérieur dans les zones de jets sortants. La caractérisation par PIV des vitesses de la phase liquide en monophasique et diphasique a permis d’étudier les modifications induites par les bulles sur la phase liquide et de discuter des mécanismes impliqués dans la modification du couple global par la présence des bulles. Il ressort de cette étude que pour les nombres de Reynolds en deçà de la capture, les bulles contribuent à stabiliser l’écoulement en accord avec une réduction du couple visqueux pouvant atteindre -30% pour des taux de vide très faible (< 1%). Pour des nombres de Reynolds plus élevés, la capture dans les cellules conduit à une réduction de la longueur d’onde axiale et une augmentation de la vorticité des cellules, associée à une augmentation des vitesses rms. Cette configuration est favorable à une augmentation du couple visqueux. A l’inverse, la capture des bulles dans le jet sortant conduit à une augmentation de la longueur d’onde axiale, associée à une diminution de la vorticité. Cette configuration est favorable à une réduction du couple visqueux, moins marquée qu’en absence de capture. / This work presents experimental study of drag reduction by injection of bubbles. Injection of bubbles into the developing boundary layer along the hulls of ships could help to reduce significantly the frictional resistance by lowering a fluid along the hull and interacting with the near-wall turbulent structures. We investigate the interactions between bubbles, the coherent motion and the viscous torque in a Taylor-Couette flow for outer cylinder at rest, while bubbles are injected constantly through a needle. The Reynolds number ranges up to Re≤20.104, for these values of the Reynolds number, Taylor vortices are persistent leading to an axial periodicity of the flow. Bubbles size varies between 0.05 and 0.12 times the width of the gap, depending on the needle and the liquid used. An original method for tracking bubbles in a meridian plane coupled with measures of overall torque applied to the inner cylinder helped to highlight two regimes of drag reduction and various types of arrangements of bubbles, depending on their size and Reynolds number. Bubbles could have a sliding motion, wavering, be captured by the Taylor cells or in the outflow areas near the inner cylinder. Characterization of the liquid velocity by PIV both in single phase and two-phase flow helped to study the modifications induced by the bubbles on the liquid phase and to discuss about the mechanisms involved in the changes induced by the bubbles in the overall torque. The study show that for the Reynolds number before the capture, bubbles could help to stabilize the flow in agreement to the reduction of the viscous torque up to -30% for lowest void fraction (<1%). For the Reynolds number after the capture, bubbles trapped by the Taylor cells lead to a reduction of the axial wavelength and increasing of the vorticity of the cells, associated to an increasing of the rms. This configuration leads to an increasing of the viscous torque. However, bubbles trapped in the outflow areas near the inner cylinder lead to an increasing of the axial wavelength, associated to a decreasing of the vorticity. The configuration supports a smaller reduction of the viscous torque than in the case without captured.

Im Rahmen des Transferprojektes T01 „Textilbeschichtung mit hochviskosen Massen“ des Sonderforschungsbereiches 532 (SFB 532) wird die Realisierung und Bewertung eines integrierten Beschichtungskonzeptes zur nachhaltigen Verbesserung der Tragfähigkeit von textilbewehrten Betonbauteilen an der RWTH Aachen University untersucht. Dazu wird eine neue Auftragstechnik für hochviskose Beschichtungsmassen entwickelt, die eine vollständige Penetration von Multifilamentgarnen mit großen Garntitern und einer hohen Anzahl an Filamenten in textilen Gelegen erzielt. Des Weiteren werden aktive Beschichtungsmassen auf der Basis von Polyvinylalkohol-Zement-Compositen, die eine homogene Anbindung aller Einzelfilamente an die Zementmatrix ermöglichen, erforscht.

Cette thèse concerne l’aérodynamique de pales d'extrémité transsonique. Ces pales sont conçues pour maximiser le rendement en croisière, tout en générant la traction requise au décollage. Elles ont des profils fins et peu cambrés, travaillant à forte incidence au décollage, ce qui peut entraîner l’apparition d’un tourbillon de bord d’attaque (TBA). Or ce TBA présente des similitudes avec les tourbillons d’apex d’aile Delta, connus pour leur capacité à générer de la portance tourbillonnaire.Cette étude consiste à examiner l’intérêt du TBA pour les performances aérodynamiques.La démarche a consisté dans un premier temps à caractériser la topologie du TBA sur une maquette représentative d’une pale d’ Open Rotor, à l'aide d'essais PIV résolus en temps et de calculs RANS k-omega SST, et à évaluer la capacité de la simulation RANS à reproduire les caractéristiques d’intérêt pour cette étude. Un algorithme a été développé afin d'estimer la contribution de ce TBA à la portance à partir du champ de pression pariétal RANS.Afin d'expliciter l'influence des paramètres géométriques et de fonctionnement de la pale sur la portance tourbillonnaire, un modèle 1D de la portance tourbillonnaire a été développé puis couplé à la méthode de l'élément de pale.Les premières comparaison de géométries à iso-traction ont montré que la portance tourbillonnaire permet de générer la traction requise au décollage avec une surface alaire plus faible. Ces résultats ouvrent de nouvelles perspectives pour la conception de géométries avec un meilleur rendement en croisière. / This thesis deals with the aerodynamic properties of propeller blades. Those blades are designed to maximise cruise efficiency, while achieving the target thrust at take-off. Their thin, low-cambered profiles must work at high incidence at take-off, which may give rise to a leading-edge vortex (LEV).The topology of this LEV looks similar to Delta wing LEVs, which are known to generate vortex lift.the aim of this study is to explore the probable impact of the LEV on lift at take-off in order to reconsider propeller blade designs. The approach first consisted in caracterising the LEV topology on a model blade representative of an Open Rotor front blade, using both Time-Resolved PIV and RANS k-omega SST calculations. The comparison between both methods demonstrated the ability of RANS calculations to reproduce the LEV characteristics of interest to this study.Then, the LEV contribution to lift was evaluated thanks to an algorithm developed to estimate vortex lift contribution from RANS wall pressure fields.In order to explicit the influence of the blade's geometrical and functioning parameters on vortex lift, a 1D vortex lift model was developed and coupled to the Blade Element Momentum Theory.The first blade geometry comparative studies at iso-thrust showed that vortex lift enables to generate target thrust at take-off with a lower blade surface. This opens new perspectives for the design of blade geometries with enhanced cruise efficiency.

Current research within the field of ultrasound contrast agents (UCAs) aims at developing capsules which are not only acoustically active, but also have a chemically modifiable surface. This enables use in new areas, including targeted drug delivery and theranostics. For such purposes, air-filled microbubbles (MBs) with a polyvinyl alcohol (PVA) shell are being studied. Ultrasound imaging velocimetry (UIV) is a technique used to evaluate various types of liquid flows by tracking patterns caused by UCAs across ultrasound images, and has shown great potential for flow measurements in terms of accuracy. The aim of this thesis was to implement a basic UIV program in Matlab to investigate the flow behavior of air-filled PVA MBs being pumped through a phantom, mimicking a blood vessel. The images were acquired using the programmable Verasonics research system by plane wave imaging with coherent compounding, and UIV was implemented as a post-processing technique. Three parameters were varied to study how the UIV performance and flow behavior of the MBs were affected: the concentration of MBs, the flow velocity, and the transducer voltage. The resulting velocity vector fields showed that it is possible to track PVA MBs using the implemented UIV program, and that the concentration 5·106 MBs/ml gave the best results out of the five concentrations tested. The generated velocity vector fields indicated a turbulent and pulsatile flow behavior, which was in line with the predicted flow behavior, although there was a disparity between the measured average flow velocity of the MBs and the predicted flow velocity. It was also observed that the MBs were increasingly pushed in the axial direction with increasing voltage, as according to theory. Even though a more advanced UIV algorithm could improve the accuracy of the velocity measurements, the results show possible use of air-filled PVA MBs in combination with UIV. / Nuvarande forskning inom ultraljudskontrastmedel syftar till att utveckla kapslar som inte bara är akustiskt aktiva, utan som även har en kemiskt modifierbar yta. Detta möjliggör användning inom nya områden, så som målinriktade läkemedel och theanostics. För detta syfte studeras luftfyllda mikrobubblor med ett skal av polyvinylalkohol (PVA). Ultrasound imaging velocimetry (UIV) är en teknik som används för att analysera olika typer av vätskeflöden genom att spåra mönster orsakade av ultraljudskontrastmedel över ett antal ultraljudsbilder. Metoden har visats ha stor potential för flödesmätningar, och hög noggrannhet har uppnåtts. Detta projekt syftade till att implementera ett grundläggande UIV-program i Matlab för att undersöka flödesbeteenden hos luftfyllda PVA-mikrobubblor som pumpas genom en modell av ett blodkärl. Ultraljudsbilderna togs med hjälp av det programmerbara forskningssystemet Verasonics, genom att använda planvågsavbildning och coherent compounding, och UIV implementerades som ett efterbearbetningsprogram. Tre parametrar varierades för att studera hur prestandan av UIV-programmet och flödesbeteendet hos mikrobubblorna påverkades: koncentrationen av mikrobubblor, flödeshastigheten, och spänningsamplituden hos ultraljudsproben. De resulterande hastighetsvektorfälten visade det möjligt att evaluera flödesbeteenden hos PVA-mikrobubblor med hjälp av det implementerade UIV-programmet. Bäst resultat erhölls genom att använda koncentrationen 5·106 mikrobubblor/ml, av de fem testade koncentrationerna. De genererade hastighetsvektorfälten indikerade ett turbulent och pulserande flöde, vilket överensstämde med teorin, trots att det fanns skillnader mellan genomsnittliga uppmätta flödeshastigheter och den beräknade flödeshastigheten. Det kunde också observeras att mikrobubblorna trycktes i den axiella riktningen när spänningsamplituden ökade, vilket överensstämde med teorin. Trots att metodens noggrannhet skulle kunna ökas genom att använda ett mer avancerat UIV-program, visade resultaten på möjligheten att använda luftfyllda PVA-mikrobubblor i kombination med UIV.

Master of Science / Department of Grain Science and Industry / Kingsly Ambrose / Consistent and reliable flow of bulk wheat from hoppers and silos is very significant in wheat handling and processing. Bulk wheat flow challenges such as inconsistent flow, arching, etc., are common during handling. The irregular size and non-uniformity of physical properties, the presence of impurities affects the flow behavior during discharge. Chaff and insects infested kernels are the two most common impurities present in wheat. In this research, the effect of these two impurities on their physical and flow properties of wheat were studied. Physical and flow indicators, such as bulk, tapped, particle densities, angle of repose, Hausner’s ratio, Carr index, and porosity measures the flowability of uncompacted bulk solids. Meanwhile, flow properties tested by shear testing principle based on Jenike’s method, simulated bulk wheat under pressure in bins/hoppers. The dynamic properties tested quantify the energy required to flow, compressibility and permeability at dynamic handling situations. Due to the presence of impurities and moisture content differences, bulk density and angle of repose of wheat varied from 801.54kg/m3 to 718.36kg/m3, and 23.6° to 38.4°, respectively. Angle of internal friction and wall friction angle that reflect interaction between particles and particle with bins/hopper walls, ranged from 23.95° to 43.13° and 15.46° to 20.33°, respectively. In addition to instrumental flow property evaluation, the flow profile, discharge rate, and particle velocity during hopper flow of bulk wheat was studied using Particle Image Velocimetry method. Mass flow and funnel flow hopper dimensions were used for this flow profile analysis. The discharge rate decreased from 1.67 to 1.12 kg/s for mass flow and 1.42 to 0.86 kg/s for funnel flow when the chaff in bulk wheat increased from 0% to 7.5% (weight basis). Analysis of the active flow zone indicated that bulk wheat without chaff had a uniform flow compared to wheat with chaff in the bulk. The findings from this study will be useful for design of hopper bottom bins and handling equipment based on the wheat quality and percent moisture content.

Cardiovascular disease, mostly atherosclerosis, is responsible for one third of all deaths globally, rising to more than 50% in the Western World. Risk factors include smoking, diet, and familial history. Doppler ultrasound can provide estimates of blood velocity and wall shear rate. Clinically, maximum velocity is used to categorise patients for surgery, although Doppler velocity measurement is prone to errors and in need of validation. Wall shear stress—which can be derived from wall shear rate—plays a role in disease initiation and progression, although its clinical utility is unclear due to difficulties associated with its measurement. This thesis investigates the use of Doppler ultrasound as a tool to estimate blood velocity and wall shear rate. A simplified method for estimation of wall shear rate in healthy arteries is developed that uses spectral Doppler ultrasound. This method is based upon the theory of oscillatory flow in rigid pipes, requiring two measurements that are readily available with clinical ultrasound machines. This method is compared to a similar method based on colour flow imaging. The spectral Doppler method underestimated the theoretic value of wall shear rate by between 7 and 22%, with results varying between phantoms. Errors for the colour method were on average 35% greater. Test measurements from one healthy volunteer demonstrated that this method can be applied in-vivo. In more advanced stages of disease, peak velocity distal to a stenosis is of clinical interest and the simplified method for wall shear rate estimation is invalid. Steady flow in a series of simplified stenosis geometries was studied using a dual-beam Doppler system to obtain velocity vectors. These measurements were compared with data from an equivalent system that used particle image velocimetry (PIV) and was considered the gold standard. For Reynolds numbers at the stenosis throat of less than 800, flow remained laminar over the region studied, although distal flow separation did occur. For higher throat Reynolds numbers—corresponding to more severe stenoses or increased flow rates—asymmetric recirculation regions developed; the transition to turbulence occurred more proximally, with a corresponding reduction in stenotic jet and recirculation length. Qualitative agreement was observed in the velocity profile shapes measured using ultrasound and PIV at throat Reynolds numbers less than 800. Above this threshold the qualitative agreement between the velocity profiles became poorer as both downstream distance and the degree of stenosis increased. Peak axial velocity distal to the stenosis was underestimated, on average, by 15% in the ultrasound system. Estimation of shear rate remained difficult with both experimental techniques. Under a Newtonian approximation, the normalised wall shear stresses agree qualitatively. Under pulsatile flow conditions using an idealised flow waveform, superior qualitative agreement was observed in the velocity profiles at diastole than at systole. Similar to the steady flow behaviour, this agreement deteriorated with stenosis severity. The current generation of clinical ultrasound machines are capable of estimating the wall shear rate in healthy arteries. In the presence of significant arterial disease, errors in the peak velocity may result in mis-selection of patients for surgery, while estimation of the wall shear stress remains extremely problematic; particularly with identifying the wall location and measuring velocities close to the wall.