The main aim of this research was to investigate the mechanism of cavitation in conforming and partially conforming tribological conjunctions. The effect of cavitation on load carrying capacity and frictional performance of is also investigated. This is important with regards to fuel efficiency in internal combustion (IC) engines. Friction accounts for 15–20% of IC engine losses. The piston–cylinder system contributes to 40–50% of these, with the compression ring(s) being responsible for most of this. This is because the primary function of the ring is to seal the combustion chamber, thus small emerging gaps lead to increased friction. In fact, compression ring(s) expend 3–5% of engine input fuel energy. The share of frictional losses of engine bearings is approximately 20–25%. Traditionally, prediction of performance of tribological conjunctions has been studied using Reynolds equation. When the effect of cavitation is considered, various cavitation algorithms with associated boundary conditions for lubricant rupture and reformation are proposed. These include Elrod, and Elrod and Coyne algorithms, as well as boundary conditions such as Swift-Stieber, JFO and Prandtl-Hopkins. There are a number of assumptions embodied in these approaches, as well as the use of Reynolds equation itself. These approaches do not uphold the continuity of mass and momentum in multi-phase flow, in cavitation beyond the lubricant film rupture. A detailed methodology for multi-phase flow, comprising simultaneous solution of Navier-Stokes, energy and lubricant rheological state equations is developed.

Esta dissertação trata da aeração induzida em vertedores com objetivo de mitigar os efeitos da cavitação sobre os mesmos. A análise bibliográfica do tema está calcada na introdução ao mecanismo da cavitação e no levantamento do estado da arte a partir dos trabalhos pioneiros, nos clássicos e nos recentes. Com base nos princípios da física: Conservação de Massa e Primeira Lei da Termodinâmica, foi desenvolvido um modelo matemático para aeração induzida em vertedores. O modelo proposto, após ser analisado do ponto de vista de sua consistência, foi avaliado em comparações com resultados fornecidos por outros pesquisadores, obtidos através de modelos físicos reduzidos. Os resultados obtidos pelo modelo matemático proposto correspondem a boas estimativas das grandezas envolvidas na aeração induzida em vertedores, o que credencia o modelo proposto como uma ferramenta apropriada para projetos de engenharia hidráulica e futuros desenvolvimentos científicos. / This study adress prompt aeration in spillways reaching to reduce the cavitation effects on them. References were based on introduction to cavitation machanism and, state of art survey, to leave on not only earlier studies, but also on classical and most recent ones. Based on principles of Physics, such as Mass Conservation and the First Law of Thermodynamics, a mathematical model has been developed as an example of prompt aeration in spillways. After extensive consistency analyses the proposed model has been validated by comparing different results furnished by studies on physical reduced models by other researchers. Results obtained from the mathematical model proposed here correspond to good estimates of greatnesses involved in prompt aeration is spillways and that turns the proposed model into an adequate tool for Hydraulic Engineering projects and for future scientifics stydies.

Historically inducer pumps have been designed with low inlet diffusion that allows for a gradual pressure rise through the machine that has the ability to slowly collapse any cavitation bubbles that may be present. A novel cavitation control device has been developed by researchers at ConceptsNREC that has been shown in previous experimental work to greatly improve the suction performance of a traditionally designed machine. Computational fluid dynamics (CFD) has been employed to understand the effectiveness of the cavitation control device (CCD) at controlling the conditions that lead to cavitation inception and to determine the impact that the CCD has on the flow. Also the upper limit of design incidence ratio where the CCD is no longer able to control the factors that lead to cavitation inception was to be determined through the CFD approach. All machine geometries and test data were provided by researchers at ConceptsNREC. Two cases were selected for validation work and 32 additional designs were employed in a parametric study where the flow coefficient and design incidence ratio were varied over a typical range of interest for a turbopump application. The results of this computational work show that the CCD is able to control the factors that lead to early cavitation inception. The research shows that the addition of the CCD has an overall stabilizing affect on the flow by significantly decreasing the incidence at the leading edge of the blade. It has been determined that the maximum design incidence ratio where the CCD is able to effectively control the factors that lead to cavitation inception is dependent on the flow coefficient and in general the maximum design incidence ratio decreases as the flow coefficient is increased.

The performance of an inducer with the integration of an inlet cover bleed system known as a stability control device (SCD) is investigated using computational fluid dynamics. Inducers are the first stage of high suction performance pumps and are designed to operate under cavitating conditions. Improvements in design have allowed inducers to operate stably with low inlet head conditions, however, cavitation instabilities ultimately lead to pump failure. It has been shown that inducers that employ an SCD fully suppress cavitation instabilities.The performance of an inducer is explored at both on- and off-design flow coefficients, where the flow coefficient is a normalized flow rate through the inducer. Both the cavitating and non-cavitating performance of the inducer are analyzed. Improved stability is observed when the SCD is implemented, particularly at flow coefficients below the design value. The stabilizing effect of the SCD allows the inducer to operate stably at much lower flow coefficients, which allows for significant improvements in the pumps ability to operate with minimal inlet head. Cavitation instabilities, such as rotating cavitation, are also suppressed when the SCD is implemented.As part of this work, the design space created by the SCD is explored. Variations in the SCD geometry as well as the inlet blade angle of the inducer are explored. High suction performance pumps are required to operate at very low flow coefficients in order to have the best suction performance. Traditionally, only inducers with small inlet blade angles can maintain stable operation at very low flow coefficient. Because of the stabilizing effect of an SCD, inducers with larger inlet blade angles can now operate stably at the low flow rates require for high suction performance pumps. The influence of varying the inlet blade angle is explored in inducers that employ an SCD. This provides a better understanding of the flow physics in inducers that employ an SCD and help to define their design criteria. Stable operation at low flow coefficients is achieved with the larger inlet blade angles, confirming that inducers with larger inlet blade angles that employ an SCD can be used in high suction performance pumps. Modifications to the SCD geometry are considered to better optimize the design. Variations in the SCD geometry have almost no effect on the cavitation breakdown curve for each inducer, however, the stability of the pumps is greatly influenced by the SCD geometry. Some cavitation instabilities are observed in inducers that operate with an SCD. The physics that leads to the generation of these instabilities is unique to an inducer with an SCD. Modifications to the SCD geometry can allow inducers that employ an SCD to suppress traditional cavitation instabilities that occur without an SCD as well as the new instabilities that are observed when an SCD is implemented.

Proper aeration of cavitating hydraulic equipment can greatly reduce cavitation intensity, noise, and damage. This thesis quantifies the benefit, in terms of damage and noise, from aerating six inch butterfly valve. The incipient damage level of cavitation was obtained for both aerated and non -ae ra ted conditions. The level is defined as one pit per square inch of a soft aluminum test specimen per one minute of operation. A description of the cavitation pits that occurred plus where they appeared is presented. A graph showing the aerated and non-aerated limits of incipient damage is given along with a table showing the percent reduct ion of damage from aeration. A graph and table are also given depicting the reduction in noise. The proper location of aeration ports to allow natural aeration is outlined .

This thesis evaluates the application of Reynolds equation in calculating fluid film forces (FFFs) in circumferential groove journal bearings (CGJBs) with both balanced and unbalanced rotors. The existing rig was fabricated at UNSW for that purpose. Unfortunately the rig was unsuitable because it was unstable at speeds as low as 500 rpm. This occurred because, when designing the rig, the traditional ??-film cavitation boundary model (CBM) was used. Consequently a modified cavitation boundary model (MCBM) was proposed which correctly predicted the instability threshold of the rig. Using the MCBM, new bearings were installed and the modified rig was stable in the required speed range up to 2400 rpm. Two existing computer programs to calculate bearing stiffness and damping coefficients and FFFs were modified for the purposes of this study. In addition, only dynamic components (AC components) of experimental FFFs could be measured accurately. The vertical mean value (DC component) of experimental FFF was approximated to the vertical reaction force and the horizontal DC component of experimental FFF was approximated to zero at each bearing. This thesis concludes that the numerical solution of Reynolds equation to calculate DC components of FFFs in a CGJB is valid with a proper selection of CBM. The agreement of AC components of the FFFs were good with a balanced rotor, but were poor with an unbalanced rotor. The modified CBM is more accurate than the existing ??-film CBMs. The cavity region is important to obtain accurate numerical results and depends on test conditions, bearing dimensions, etc. To design for critical speeds, cavitation pressure could be either oil vapour pressure or atmospheric pressure; however, to design for stability, cavitation pressure should be oil vapour pressure, or even much lower. Two halves of a CGJB carried different loads because of misalignment and different clearances. In addition, reducing length and increasing clearance significantly increased the stability of the rig. Finally, to obtain perfect agreement between numerical and experimental FFFs, short bearings and a smaller clearance ratio are strongly recommended. A special design to measure cavitation pressure is suggested.

Cette thèse s'inscrit dans le cadre des énergies renouvelables au sein du programme HARVEST centré sur le développement d'un concept d'hydrolienne dérivé des turbines Darrieus et Gorlov. L'ajout d'un dispositif appelé carénage à la turbine permet à celle-ci d'extraire une portion de l'énergie cinétique du courant plus grande. Toutefois ce dernier peut favoriser la cavitation qui nuit à la turbine. Parmi les différents axes du programme, les travaux de thèse se situent dans cette problématique. En régime subcavitant et cavitant, l'analyse de l'hydrolienne a été menée suivant une approche numérique et expérimentale. Pour ce faire deux outils ont été mis en place. Du coté expérimental, le tunnel hydrodynamique du LEGI a été équipé d'une balance qui donne la mesure instantanée des forces et du couple qui s'exercent sur la turbine. Du coté numérique, les efforts ont été orientés sur l'amélioration et le développement du code de calcul universitaire, CAVKA. L'utilisation intensive de ces deux moyens, couplée à des modèles théoriques, a permis de mettre en évidence d'une part le fonctionnement de la turbine libre ou carénée et, d'autre part, les limites de fonctionnement vis-à-vis de la cavitation.

La vitesse du son dans les écoulements cavitants varie considérablement en fonction du taux de vide local. Par exemple, elle est proche de 1500 mis dans l'eau pure, 400 mis dans la vapeur d'eau et peut diminuer bien en dessous de ces valeurs dans le mélange liquide-vapeur. Les écoulements cavitants sont donc quasiment incompressibles dans les zones purement liquides, et fortement compressibles dans les zones diphasiques. Il est nécessaire de prendre en compte la compressibilité du fluide dans les simulations numériques afin de reproduire certains mécanismes complexes associés aux instabilités de cavitation. Cela nécessité une estimation correcte de la vitesse du son dans un milieu cavitant. Ce travail de thèse avait donc pour principal objectif la création d' une zone de cavitation homogène afin d'y mesurer la célérité du son en fonction du taux de vide. Un dispositif expérimental a donc été développé pour créer une telle zone. Deux méthodes pour mesurer la vitesse du son sont examinées: J- avec 3 capteurs de pression équidistants le long de la veine d'essais TI- avec 2 hydrophones en vis-à-vis dans une section de la veine. Le taux de vide est mesuré à l 'aide d' une sonde optique. L'étalonnage de la sonde optique et la validation des mesures des capteurs et des hydrophones, sont fait en écoulement eau-air. La méthode J étant plus adaptée pour la mesure de la vitesse du son, est appliquée pour un écoulement cavitant. Pour chaque écoulement (eau-air et cavitant), les résultats sont comparés avec les modèles théoriques existants et une discussion est menée. Un bon accord se révèle entre les expériences et la théorie.

Dans les turbomachines et plus particulièrement dans les inducteurs, la cavitation reste un sujet de préoccupation majeure. L'évolution rapide de l'industrie des turbomachines combinée aux récents progrès réalisés en CFD, permettent une investigation, une analyse et un diagnostic numériques de ces machines par une meilleure connaissance des caractéristiques des écoulements dont elles sont le siège. La cavitation est un phénomène physique qui apparaît fréquemment dans les machines hydrauliques soumises aux conditions d'aspiration spécifiques. Elle entraîne plusieurs effets non souhaitables dans les systèmes industriels où elle a lieu. On peut mentionner le bruit généré, l'érosion, la perte de performances de la machine et les vibrations. Dans ces travaux, diverses études numériques et expérimentales, permettant l'analyse de l'écoulement cavitant stationnaire et instationnaire dans les inducteurs axiaux ont été réalisées. L'identification des instabilités de type cavitation alternée et cavitation tournante a été poursuivie.

Therapeutic strategies of stroke can take two directions: to prevent brain damage from stroke or aid in its repair after a stroke. In this thesis, a rat stroke model, which mimics the human small vessel stroke, was used. Two potential repair strategies were investigated with this model, reduction of inflammatory processes with the aid of minocycline treatment and replacing necrotic neurons with new ones with the aid of neurogenesis of endogenous progenitor cells. <p>The stroke model is induced by disrupting the medium-size pial vessels within a 5mm-circular brain surface of adult Wistar rats. This leads to a cone-shaped cortical lesion. Therefore it mimics the clinical situation of lacunar infarction, the most frequent outcome of small vessel stroke. <p>Minocycline, a second-generation tetracycline, prevented cavitation and facilitated the repopulation of the lesion by reactive astrocytes. However, I could not identify the molecular target as the number of activated microglia, infiltrating leukocytes and CD3+ lymphocytes as well as interleukin-1β expression were not significantly altered. Doublecortin (DCX) is a microtubule-associated protein expressed by migrating neuroblasts and immature neurons. After injury, DCX-positive cells appeared in the neocortex at the base of the lesion. These cells exhibit a morphology resembling differentiated post-migratory neurons with long branched processes. Some of the DCX-positive cells were also immunoreactive for βIII-tubulin, another marker of immature neurons. This might indicate a migratory pathway for developing neuroblasts from the subventricular zone (SVZ) through the corpus callosum to the lesion. SVZ cells were labeled with carboxyfluorescein diacetate, succinimidyl ester (CFSE) stereotaxical injections. Although rostral migratory stream and olfactory bulb were intensely labeled, no CFSE containing cells were found in the cortex underneath the lesion. These results suggest that the DCX-positive cells may not originate from neural precursors from the SVZ, but might be generated from local progenitor cells. In summary, using the PVD II model, which mimics the lacunar stroke, I found that neuroblasts appeared spontaneously near the lesion in the cerebral cortex and were attempting to upregulate neuronal properties. Reducing inflammation with post-stroke minocycline treatment prevented cavitation. I think both findings open up exciting new avenues for treatment of lacunar infarctions.