Dissertations / Theses on the topic 'Vertical cylinder'

The problem addressed in this thesis is that of the behaviour of large offshore structures subjected to ice and earthquake loading. The theoretical formulation of the fluid force and associated added mass and damping coefficients acting on an isolated vertical surface-piercing rigid circular cylinder which is excited by sinusoidal unidirectional ground motions is presented. The closed-form solution is first developed on the basis of potential flow theory for arbitrary values of excitation frequency and, in addition, its asymptotic form for high frequencies is considered. The latter is found to be accurate in predicting the high-frequency added mass only for high structure radius-to-water depth ratios and the high-frequency damping for all radius-to-depth ratios. A computer method for numerical evaluation of the force coefficients is devised and theoretical results for different values of radius-to-depth ratios are thereby generated. An experimental study has been conducted in the Earthquake Engineering Laboratory of the Department of Civil Engineering at the University of British Columbia to verify the theoretical results obtained for the vertical distribution of the force coefficients of a model cylinder which satisfies the large body regime of fluid-structure behaviour for which effects due to fluid viscosity are negligible. Owing to unanticipated technical problems, the current study is unsuccessful and data recorded in the sinusoidal tests are unrealistic, although the calculated coefficients appear to be independent of base displacement (an observation which indicates that viscous effects were insignificant during testing). Nevertheless, values of total force coefficients which were obtained experimentally for a similar model in a previous investigation are found to agree very well with the corresponding theoretical results for frequencies of up to 6 Hz. It is concluded that the theoretical formulation provided for the hydrodynamic force coefficients of a vertical surface-piercing circular cylinder subjected to horizontal sinusoidal base motions of arbitrary frequency may be used to accurately predict the total added mass and damping of real structures satisfying the conditions imposed by the theory.
Applied Science, Faculty of
Civil Engineering, Department of
Graduate

Local scour around bridge piers can cause serious damages and structural failure to the bridge. Correct prediction of the scour is an important criterion for the engineering design. Though the subject has been investigated for many decades, the theoretical developments have been very limited due to the complicated interaction of three-dimensional flow and the sediment transport. This thesis concerns the flow around a vertical bottom mounted cylinder exposed to currents and is considered as the initial phase of a study towards modeling local scour around vertical bottom-mounted structures. The aim of the present study is to obtain a better understanding of the complex three-dimensional flow and the mechanisms related to the scouring. The study started with the development of a three-dimensional numerical model to simulate flow around cylindrical structures. After validation of the model, the model was applied to investigate flow around an isolated rectangular cylinder. Unsteady flows around cylinders of a square cross section (A/D = 1) and a rectangular cross section (A/D = 2) were simulated to understand the flow properties around a cylinder other than a circular cross section. Three-dimensional flow patterns, pressure distribution, forces on the cylinder and vortex-shedding frequencies were discussed. It was found that the present numerical results generally agree well with the experimental data. Flow around a vertical cylinder mounted on a rigid bed was then investigated by the numerical model. A circular and a square cross sections were considered respectively. Flow structures of horseshoe vortex and the wake vortex which are the major mechanisms leading to the scouring around the base of the vertical cylinder were explored. The bed shear stress distributions that directly affect the scour processes were discussed. Finally the numerical model was applied to study the flow around a submerged square cylinder mounted on a bed, which has significant engineering relevance to the local scour around structures, such as bridge pier foundation itself, or a caisson placed underwater around the pier bottom for scour protection. The numerical calculations were carried out at different cylinder heights to investigate the effect of the cylinder height on the flow properties. It was found that for submerged cylinders with the height of less than one and a half of the cylinder side width the maximum bed shear stress amplification is about 60% of the value of an infinite long cylinder. The quantification of the shear stress reduction is important for scour protection design.

Laminar natural convection heat transfer from the vertical surface of a cylinder is a classical subject, which has been studied extensively. Furthermore, this subject has generated some recent interest in the literature. In the present investigation, numerical experiments were performed to determine average Nusselt numbers for isothermal vertical cylinders (103 < RaL < 109, 0.5 < L/D <10, and Pr = 0.7) with and without an adiabatic top in a quiescent ambient environment which will allow for plume growth. Results were compared with commonly used correlations and new average Nusselt number correlations are presented. Furthermore, the limit for which the heat transfer results for a vertical flat plate may be used as an approximation for the heat transfer from a vertical cylinder was investigated.

This thesis investigates water particle kinematics and the wave forces exerted on a slender rigid vertical cylinder in regular bidirectional wave fields. The instrumented portion of this cylinder is partitioned into nine independent segments enabling measurement of the vertical profile of hydrodynamic loading both in-line and transverse to the direction of wave propagation. Experiments conducted at the Hydraulics Laboratory of the National Research Council in Ottawa are described and some results are compared with the predictions of a wave force model based on the Morison equation and linear fluid kinematics. The influence of the crossing angle between the two wave components on the forces experienced by the column is determined. These experiments consider short-crested wave behavior in intermediate and deep water resulting from the interaction of two identical regular wave trains crossing at angles of 30, 60 and 90 degrees. The limit corresponding to unidirectional monochromatic waves is also investigated to provide a reference condition for comparison with the short-crested results. Conditions at the location of maximum short-crested wave height are of primary interest, however, forces at locations between the anti-node and node of the flow are also examined. In all, water surface elevations, flow velocities, and wave forces were measured in 24 short-crested and 8 different long-crested wave conditions spanning the range of Keulegan-Carpenter number between 4 and 24. The results of this study confirm the findings of previous researchers that short -crested waves with a certain period travel faster and rise higher before breaking than do their long-crested counterparts, but that in-line wave forces are not necessarily increased. Lift force maxima equal to half the maximum in-line force were measured; these forces can contribute significantly to the magnitude and direction of the total force resultant.
Applied Science, Faculty of
Civil Engineering, Department of
Graduate

Within LiU Fluid laboratory there had been a number of hydraulic and pneumatic test stands for research projects and teaching. The effort was needed to do more extension within the field of pneumatics. This thesis was about designing, manufacturing, building and testing the Test Stand for vertical application of pneumatic rodless cylinder. It consisted of three main parts; mechanical structure, pneumatic system and electrical and electronics. The design part focused more on the mechanical structure and pneumatic system. Electrical and electronics had to be installed to facilitate actuation and control of the system after mechanical and pneumatic components were assembled together. The mechanical structure consisted of three main parts; cabinet, carriage and ball balancer. For each part three concepts were generated, evaluated and selected to obtain the most promising concept for further development. The design values were given such that the mechanical structure should have total weight not exceeding 500 kg. The pneumatic system would supply maximum pressure of 8 bar. Upstream pressure was considered to be 6 bar after 25% pressure drop assumed to occur in the system. The mass to be lifted was typically 26 kg moving at a maximum speed of 2 m/s. The whole design process was carried out with safety and design for assembly in mind. The methodologies applied in the design included; identification of need, concept generation, concept selection, CAD modeling and simulation and FE Analysis. Development of concepts involved CAD drawings (3D and 2D) in CATIA V5, calculations and selection of appropriate materials for each component. The pneumatic cylinder applied in this design was a rod-less cylinder (Bosch Rexroth; RTC-DA, MNR: R480628571) with a Piston diameter of 50 mm, stroke of 1.2 m and cushioning stroke of 20 mm. More design information on RTC-DA was not available therefore the information in appendix A was based on series RTC-BV which had close similarities with series RTC-DA. The carriage that moves up and down along the cylinder stroke was designed with a mechanism that imparted disturbance to the system. The mechanism could be set to impart either 100.7 N or 151.02 N disturbance. The design was done with the assumptions that:  Spring force Fs and extension x were constant  The system had sonic flow characteristics and critical flow appeared in meter out orifice.  The air was perfect gas which obeyed equation of state  Specific heat was constant  Viscosity was constant  Flow was one dimensional  System pressure drop was 25% of the maximum supply pressure. Dynamics of the system were estimated analytically and by means of simulation. Corresponding parameter values such as pressure, velocity, flow rate, cushioning stiffness, cushioning force and acceleration were obtained and compared. The product in general conformed to the specifications made prior to the design process.

Thesis (M.S. in Mechanical Engineering) Naval Postgraduate School, September 1996.
Thesis advisor(s): Ashok Gopinath. "September 1996." Includes bibliographical references (p. 95-96). Also available online.

In ambiente marino offshore, le onde ripide sono responsabili di episodi di danneggiamento delle strutture tanto quanto gli eventi più estremi. Un tipico fenomeno riconducibile alle onde ripide è il ringing, una risposta risonante ad alta frequenza che è stata spesso osservato in concomitanza alla presenza di un picco nella serie temporale del carico, denominato secondary load cycle (SLC), le cui cause devono essere ancora chiarite. In questa tesi, si presenta uno studio sperimentale volto all’analisi delle forze agenti, della separazione del flusso e della formazione di vortici a valle di un cilindro verticale snello, incernierato alla base, investito da onde ripide, frangenti e non, con lo scopo di chiarire la loro relazione. Si è utilizzata una innovativa e complessa configurazione di prova, che combina l’impiego di tecniche di misurazione ottica (Particle Image Velocimetry, PIV) per l’indagine del flusso a valle del cilindro su quattro piani orizzontali a diverse quote dal fondo alla registrazione sincronizzata di misure della forza agente sul cilindro e di elevazione dell’onda incidente. I risultati hanno mostrato che la separazione del flusso e la formazione di vortici si verifica per molte delle onde frangenti utilizzate e per le onde non frangenti, seppure in una forma completamente diversa. La generazione dei vortici è stata osservata subito dopo il passaggio della cresta dell’onda, a circa un quarto del periodo dell’onda, dove compare un secondo picco di carico, ovvero il SLC. La presenza del SLC è stata osservata per numeri di Froude Fr>0.6, lunghezza d’onda adimensionale kR≥0.1 e ripidità kη≥0.25, valori che ricadono all’interno dei range forniti da precedenti esperienze (Chaplin et al., 1997; Grue and Huseby, 2002; Suja-Thauvin et al., 2017; Riise et al., 2018). È stata trovata una correlazione tra la forza indotta dalla generazione dei vortici e il SLC, tuttavia la formazione dei vortici non può spiegare da sola il fenomeno. La dimensione massima dei vortici osservati è il (20-30)% del diametro del cilindro; questo risulta in disaccordo coi vortici di dimensione circa pari al diametro del cilindro osservati nelle simulazioni numeriche di Paulsen et al. (2014) e Kristiansen and Faltinsen (2017). Infine, la presenza del SLC è stata riscontrata con il ringing, indotto da effetti dovuti alla superficie libera ed alla separazione del flusso, secondo Riise et al. (2018).
Steep water waves may be responsible for damages to offshore structures, as inducing a high-frequency resonant response, commonly known as ringing, found to occur in conjunction with a peak in the load timeseries, named secondary load cycle (SLC), whose causes are still not properly known. In this thesis, an experimental study of the forces generated upon flow separation and vortex formation behind a bottom-hinged, vertical slender cylinder forced by steep waves, both breaking and non-breaking, is presented. An innovative and complex laboratory setup was arranged, this combining the use of optical measurement technique (Particle Image Velocimetry, PIV) for the investigation of the flow downstream the cylinder over four horizontal planes parallel to the bottom at different elevations with the recording of synchronized measurements both of the force acting on the cylinder and of the incoming wave elevation. PIV results showed the occurrence of flow separation and the formation of vortices for many of the breaking waves cases and for all the non-breaking waves, but with a completely different fashion. A correspondence between the SLC and the vortical structures has been found: vortex formation starts just after the wave crest has passed, at a stage corresponding to about one quarter of the wave period after the main load peak, where a second peak occurs i.e. secondary load cycle. The occurrence of a SLC has been identified by some synthetic parameters such as the Froude number Fr>0.6, the dimensionless wavenumber kR≥0.1 and the wave slope kη≥0.25, these falling within the range of limits provided by the experiences of Chaplin et al. (1997), Grue and Huseby (2002), Suja-Thauvin et al. (2017), Riise et al. (2018). A correlation between the vortex-shedding-induced force and the SLC was found, but such contribution is not the only one to the SLC. Generated vortices measure (20-30)% of the cylinder diameter at most, in disagreement with the larger size, about the cylinder diameter, of the vortices observed in the CFD simulations by Paulsen et al. (2014) and Kristiansen and Faltinsen (2017). The SLC occurrence is found to coincide with the ringing response, governed by free surface and flow separation effects, according with Riise et al. (2018).

Oil and gas exploitation in deep waters has become more than just a profit business to be a daily necessity, since the world energy matrix is based on fossil components. Risers are offshore structures that are intimately linked with oil and gas exploitation and those are subjected to a great variety of effects in field, e.g., marine currents, Vortex Induced Vibration (VIV), heave motion caused by gravitational waves, non-linear contact with the sea floor, and many others. Riser dynamics is essentially non-linear and experimental tests in real scale are almost impossible due to a great variety of control parameters acting concomitantly. Small-scale models are a better experimental approach. Nevertheless, there are many structural and hydrodynamical parameters to be evaluated. Considering only vertical risers in the present work, Galerkin\'s modal decomposition is used in order to reduce the dynamics of a vertical flexible cylinder to a few linear modes in which the majority of energy and information are contained. From the modal analysis, added mass and structural parameters damping of a vertical flexible cylinder using data obtained from free-decay tests performed both in water and in air are evaluated. Finally, a modal Mathieu-Hill oscillator with non-linear damping is constructed and, based on aStrutt diagram, modal stability under parametric resonance is discussed.
Exploração de óleo e gás em bacias de águas profundas tem-se tornado mais do que apenas uma economia lucrativa, para ser uma necessidade diária, já que a matriz energética mundial está baseada em componentes fósseis. Risers são estruturas offshore ligadas intimamente com a exploração de óleo e gás e essas estão sujeitas a uma grande variedade de efeitos na operação, e.g., correntes marítimas, Vibrações Induzidas por Vórtices (VIV), movimento de heave causado por ondas gravitacionais, contato não-linear com o solo marinho, entre outros. Dinâmica de risers é essencialmente não-linear e testes experimentais em escala real são praticamente impossíveis devido a uma enorme variedade de parâmetros de controle agindo concomitantemente. Modelos em escala reduzida são uma abordagem experimental mais conveniente. Não obstante, há muitos parâmetros estruturais e hidrodinâmicos a serem determinados. Considerando apenas risers verticais no trabalho presente, a decomposição modal de Galerkin é usada a fim de reduzir a dinâmica de um cilindro fléxivel vertical a alguns modos lineares em que a maior parte da energia e informação estão contidos. A partir da análise modal, parâmetros de massa adicional e amortecimento estrutural de um cilindro flexível vertical são obtidos usando testes de decaimento livre conduzidos na água e no ar. Finalmente, um oscilador modal de Mathieu-Hill com amortecimento não-linear é proposto e, baseado em um diagrama de Strutt, a estabilidade modal sob excitação de ressonânica paramétrica é discutida.

Les travaux portent sur l’étude de la formation de motifs sédimentaires à l’aval d’un cylindre vertical soumis à un courant. Le cylindre simule une fondation monopieu d’éolienne en mer, ou une pile de pont. Une modélisation théorique est développée. Les résultats obtenus sont en bon accord avec les résultats expérimentaux acquis au cours de la thèse. Une étude préliminaire expérimentale et théorique sur le tri sédimentaire au voisinage du cylindre est également effectuée
This work concerns the study of sediment patterns formation downstream a vertical cylinder under a steady current. The cylinder simulates an offshore monopile foundation, or a bridge pile. A theoretical modeling is developed. The results are in good agreement with the experimental results obtained in the framework of this PhD. An experimental and theoretical preliminary study on sediment segregation in the vicinity of the cylinder is also performed

La récupération, grâce aux hydroliennes, de l’énergie cinétique de courants marins et fluviaux constitue une source d’énergie renouvelable considérable et prédictible. La simulation fine, par une description statistique instationnaire de type URANS, de l’écoulement autour d’une hydrolienne isolée à axe vertical, bi-rotor et munie d’un carénage (hydrolienne de type HARVEST) donne accès à une estimation précise de la puissance produite. Cependant, le coût élevé de cette approche URANS la rend inadaptée à la simulation d’un parc de machines. Une analyse de la littérature conduit à retenir un modèle basse-fidélité de type Blade Element Momentum (BEM) pour décrire à moindre coût l’effet du rotor de la turbine sur l’écoulement, dans le contexte d’une description 2D (coupe horizontale). La performance de l’hydrolienne est alors prédite par un calcul RANS incluant des termes sources distribués dans un anneau rotor virtuel et conservant le maillage des parties fixes (carénage). Ces termes sources sont construits grâce à une procédure originale exploitant les conditions locales de l’écoulement en amont des cellules du rotor virtuel et le débit de l’écoulement traversant l’hydrolienne. Les coefficients hydrodynamiques utilisés pour le calcul des termes sources BEM-RANS sont construits une fois pour toutes en exploitant une série de simulations URANS préliminaires ; ils intègrent les effets du carénage et le fonctionnement de chaque rotor à une vitesse de rotation optimale (maximisant la puissance produite) grâce au système de régulation de l’hydrolienne. Le modèle BEM-RANS développé est validé par comparaison avec des simulations URANS de référence : il fournit une estimation fiable de la puissance produite (erreur de quelques % par rapport à l’approche URANS) pour un coût réduit de plusieurs ordres de grandeur. Ce modèle est appliqué à l'analyse de la puissance produite par une rangée d’hydroliennes HARVEST dans un canal pour différents facteurs de blocage et d’espacement latéral ainsi qu’à une ferme marine composée de trois hydroliennes
The capture, thanks to hydrokinetic turbines, of the kinetic energy generated by sea and river currents provides a significant and predictable source of renewable energy. The detailed simulation, using an unsteady statistical description of URANS type, of the flow around an isolated water turbine of HARVEST type (cross flow vertical axis ducted water turbine) provides an accurate estimate of the power output. However, the cost of the URANS approach is much too expensive to be applied to a farm of several turbines. A review of the literature leads to select a low-fidelity model of Blade Element Momentum (BEM) type to describe at a reduced cost the rotor effect on the flow, in a 2D context (horizontal cross-section). The turbine performance is then predicted using a steady RANS simulation including source terms distributed within a virtual rotor ring and preserving the mesh of the turbine fixed parts (duct). These source terms are derived using an original procedure which exploits both the local flow conditions upstream of the virtual rotor cells and the flow rate through the turbine. The hydrodynamic coefficients used to compute the BEM-RANS source terms are built once for all from a series of preliminary URANS simulations; they include the effects of the duct on the flow and the rotor operating at optimal rotational speed (maximizing the power output) thanks to the turbine regulation system. The BEM-RANS model is validated against reference URANS simulations: it provides a reliable prediction for the power output (within a few % of the URANS results) at a computational cost which is lowered by several orders of magnitude. This model is applied to the analysis of the power produced by a row of Vertical Axis Water Turbines in a channel for various values of the blockage ratio and lateral spacing as well as to a 3-machine sea farm

The present work investigates, numerically, the process of melting and solidification in hollow vertical cylinders, filled with air and phase change material (PCM). The PCM used is sodium nitrate, which expands upon melting. Therefore, a void must be present within the cylinder, which is filled with air. The influence of cylinder shape on melting time is determined. The numerical model takes both conductive and convective heat transfer into account during the melting process. The Volume-of-Fluid (VOF) model is used to track the interface between the PCM and air as the PCM melts. Three dimensionless numbers represent the characteristics of the problem, which are the Grashof, Stefan, and Prandtl numbers. The Stefan and Prandtl numbers are held constant, while the Grashof number varies. Inner Aspect Ratio (AR) is used to characterize the shape of the cylinder, which is defined as the ratio of the height to the diameter of the vertical cylinder. In this study, a range of AR values from 0.23 to 10 is investigated. Cylinders with small AR, corresponding to high Grashof numbers, lead to lower melting times compared with cylinders with high AR. The molten PCM velocity was also influenced greatly by this difference between solid PCM shape between high and low AR cases. Cylinders with small AR, corresponding to high Grashof numbers, lead to higher solidification times compared with cylinders with high AR. It was found that the velocity decreased during the solidification process, but the shape of the cylinder had an effect on the decrease. Natural convection velocity was found to decrease during the solidification process and, therefore, its effects diminish as solidification proceeds.

On étudie le problème du déclenchement de la convection thermique d'un fluide conducteur de chaleur avec source interne de chaleur dans le cas d'un fluide confiné verticalement par 2 plans isothermes ou confiné à l'intérieur d'un cylindre de section circulaire. L’étude est faite par une analyse de la stabilité linéaire et par une simulation numérique directe. La solution de l'équation linéarisée est obtenue par une série formelle dans le cas d'un fluide entre deux plans. Dans le cas d'un fluide dans un cylindre, on applique la méthode de Galerkin pour résoudre les équations de perturbations. Pour l'étude numérique de l'instabilité, les équations de conservation sont discrétisées par la méthode des volumes finis et la perturbation est introduite par une faible inclinaison de la cavité. Du point de vue de l'analyse dimensionnelle, les deux groupements qui interviennent sont le nombre de Rayleigh et un paramètre sans dimension relatif à la source de chaleur que nous appellerons nombre de Sparrow. Les critères de stabilité pour différents rapports d'aspect sont déterminés en termes de ces deux nombres. Les effets du rapport d'aspect et des changements des conditions aux limites thermiques de la paroi latérale sur la stabilité ainsi que la structure de la cellule convective sont étudiés. Dans le cas avec source de chaleur interne, l'étude analytique linéaire est en accord avec la simulation numérique. Dans le cas limite sans source de chaleur, on retrouve bien les résultats de la littérature du cas classique d'un fluide chauffé par le bas

La mise en mouvement de fluides induite par la presence de gradients thermiques est commune a de nombreux systemes naturels et industriels. L'analyse et la comprehension des multiples comportement convectifs de fluides purs chauffes par le bas est la source d'un nombre important d'etudes qui ont apporte beaucoup a la comprehension de ces systemes. Lorsque l'on considere un melange de plusieurs constituants, l'existence de couplages entre gradients thermiques et solutaux (l'effet soret) est une source supplementaire de comportements dynamiques complexes, notamment dans les cas de couplages competitifs. Les particularites induites par le fait de considerer un liquide binaire (melange solvant-solute non reactifs) ont mene a de multiples etudes sur les differents regimes convectifs (stationnaires et oscillants) qui apparaissent alors. Ces travaux sont tres majoritairement realises dans le cadre de systemes infinis ou de tres grande extension. Le present travail apporte une contribution sur l'etude des regimes accessibles a ces melanges, en s'interessant en particulier aux consequences induites par un fort confinement sur ceux-ci. Par le biais d'un code numerique pseudo-spectral, les equations regissant la convection thermique (axisymetrique) d'un liquide binaire confine dans un cylindre vertical sont resolues. Une analyse systematique des occurrences, coexistences et transitions entre ces etats pour des rapports d'aspects (rayon sur hauteur du cylindre) de 2, 1 et 1/2 et deux types de conditions aux limites illustre l'importante quantite et diversite des bifurcations qui ont alors lieu. Les resultats issus de la comparaison entre regimes convectifs de fluides purs et binaires soumis a un gradient thermique important, montrent par ailleurs que l'affirmation selon laquelle ces deux systemes se comportent alors de facon equivalente n'est pas toujours verifiee. Le mecanisme physique responsable d'une telle disparite est de plus identifie.

L'écoulement autour d'un objet partiellement immergé comme une carène ou une pile de pont est une configuration fondamentale au regard de la dynamique de la surface libre. La caractérisation de cet écoulement est essentielle dans des applications environnementales, ou pour des systèmes d'énergies marines renouvelables. De telles structures sont soumises à des efforts de traînée et de portance provenant de l'interaction avec le courant, la houle, et la surface libre. Ce travail s'inscrit dans la compréhension des efforts s'exerçant sur des objets partiellement immergés. Notre problématique est simplifiée en considérant une géométrie cylindrique. Nous présentons des résultats expérimentaux obtenus en canal, où le cylindre est tracté ainsi que des résultats numériques obtenus à l'aide du code YALES2, basé sur la méthode des volumes finis. Le cylindre vertical est partiellement immergé et le sillage généré derrière le cylindre est caractérisé par une déformation de la surface libre. L'écoulement derrière le cylindre est gouverné par les nombres de Reynolds et de Froude, caractérisant l'importance des effets inertiels et gravitaires. Ces nombres sans dimension sont définis à l'aide du diamètre du cylindre. La gamme de vitesse balayée et les diamètres utilisés permettent d'atteindre des nombres de Reynolds jusqu'à 240 000, et des nombres de Froude jusqu'à 2.4 correspondant à un sillage turbulent. L'attention est portée sur les fortes déformations de surface libre allant jusqu'à sa rupture et l'entraînement d'air. En particulier, deux modes d'entrainement d'air ont été observés : (i) dans la cavité le long du cylindre et (ii) dans le sillage du cylindre. La vitesse critique à l'entraînement d'air dans la cavité a été mesurée et son évolution est comparée avec une loi d'échelle proposée par Benusiglio. Nous avons observé l'influence de cet entraînement d'air sur les efforts de traînée grâce à des mesures à l'aide de capteurs piézoélectriques. Nos résultats sont comparés avec des résultats expérimentaux à des Reynolds et Froude inférieurs et des résultats sans surface libre. Une comparaison avec des résultats numériques est également réalisée. La présence de la surface libre et de l'entraînement d'air dans la cavité entraîne une diminution des efforts de traînée par rapport au cas monophasique. Numériquement, le code utilise une méthode level-set pour la description de la surface libre et permet de reproduire les phénomènes d'entraînement d'air, la déformation de la surface libre et la dynamique de l'écoulement autour du cylindre. Ce travail étend la gamme de paramètres adimensionnels parcourus expérimentalement et numériquement, et met en évidence l'effet de l'entrainement d'air sur les efforts de traînée
The flow past ships or an emerged body such as bridge pilar, is a fundamental, familiar and fascinating sight.Measurements and modelling of this simple flow can have relevance to offshore structures and renewable energy systems. The interaction of such structures with marine environment lead to drag, lift forces and free-surface effects.Our current problem is simplified by considering cylindrical geometry. This work presents experimental results, in which vertical cylinders are translated at constant speed through water initially at rest, and numerical results using YALES2 computing code based on finite volume method. The cylinders are partially immersed, then the motion induces turbulent wake and free-surface deformation. The flow is governed by the Reynolds and Froude numbers defined with cylinder diameter. The explored range of parameters are in the regime of turbulent wake with experiments carried out for Reynolds number up to 240 000, and Froude number up to 2.4. The focus here is on drag force measurements and strong free-surface deformation up to rupture and air-entrainment. Two modes of air-entraiment have been observed: (i) in the wake of the cylinder and (ii) in a cavity along the cylinder wall. Results are as follows. First, a scaling for the critical velocity for air-entrainment in the cavity proportional to D1/5 proposed by Benusiglio is recovered. Secondly, drag coefficients measured by piezoelectric sensors are smaller in two phase flow compared to monophasic case, and air-entrainment in the cavity enhances this decrease. Numerically, YALES2 uses level-set method for the descirption of the free-surface, and is able to reproduce air-entrainment phenomenon, free-surface deformations and flow dynamics around the cylinder. The present work expands the range of dimensionless parameters and highlights free-surface effects on drag forces

Negli ultimi anni, la presenza in mare di strutture fondate su pali, come piattaforme petrolifere e turbine eoliche offshore, sta diventando sempre più importante. Per questa ragione, guadagna sempre più importanza l’analisi di possibili criticismi e incertezze nella loro progettazione. L’obiettivo di questa tesi è lo studio dell’idro e della morfodinamica indotta da onde regolari e non su un palo di piccolo diametro attraverso una modellazione fisica e numerica. Questa tesi vuole quindi dare un contributo per la comprensione di tali processi fisici. In particolare, si vogliono fornire degli strumenti che possono risultare utili ad evitare valutazioni errate dei parametri che rientrano nella progettazione di tali strutture. Particolare attenzione è riservata alla valutazione della forza dell’azione ondosa sulla struttura, allo scavo generato alla sua base e al run-up sulla sua superficie. Un modello fisico a fondo rigido è stato realizzato per valutare quale fosse il miglior approccio per la stima della forza dovuta ad onde non lineari. Viene quindi proposta un’alternativa al classico metodo di Morison (1950) per il calcolo della forza totale con valori di velocità e accelerazione orbitale calcolati a partire da una serie temporale della superficie libera. Tale metodo consiste in una decomposizione spettrale di Fourier del segnale di livello; il contributo di ogni componente, calcolato mediante la teoria di Airy, viene quindi sommato per ottenere un profilo di velocità orizzontale e verticale in ogni istante. I risultati mostrano che questo approccio fornisce una buona stima della forza totale sia in termini di valore massimo che di valutazione della fase ondosa in cui avviene. Al contrario invece, l’applicazione della teoria lineare a tale scopo (che viene utilizzata usualmente nella progettazione) porta ad una sottostima del valore di picco e ad una cattiva rappresentazione dell’andamento della forza nel periodo. Lo scavo alla base del palo è un altro parametro fondamentale nella sua progettazione. Una campagna sperimentale a fondo mobile è stata quindi realizzata studiando l’effetto di onde regolari e non regolari su un fondale sabbioso in cui è stato inserito un cilindro verticale. I risultati di questo studio sono abbastanza in accordo con quelli ottenuti da un precedente la voro di Sumer et al. (1992) sebbene sia mostrato come il processo di scavo inizi in presenza di condizioni idrodinamiche di minore intensità (parametro di Keulegan-Carpenter KC=4) rispetto a quanto presente in letteratura (KC=6). Diverse formule sono proposte per valutare la profondità di scavo in funzione di parametri adimensionali come KC e Ur. Per lo scavo dovuto ad onde irregolari, è stato modificato l’approccio di Ong et al. (2013). Questo metodo dipende dalla funzione di distribuzione cumulata delle altezze di cresta ma, a causa della sua complessità, è stata semplificata per essere resa più adatta a fini progettuali senza però inficiare la bontà dei risultati. Per migliorare la comprensione sul processo di formazione, crescita e distacco dei vortici, sono state svolte misure di tipo PTV che, integrate con le misure di pressione e coi risultati del fondo mobile, hanno permesso di caratterizzare l’intero processo di scavo. Dai risultati si è osservato come le fasi e le posizioni in cui i gradienti di pressione sono massimi corrispondono a quelle in cui è stato osservato distacco dei vortici. Le mappe di scavo ottenute nella campagna a fondo mobile sono state quindi spiegate dall’interpretazione delle mappe di vorticity e del parametro OW. È stato inoltre realizzato un modello numerico con OpenFOAM per ampliare la comprensione del complesso fenomeno tridimensionale riguardante l’interazione onda-struttura. Una nuova condizione al contorno per la generazione ondosa a partire da una serie temporale misurata di superficie libera è stata implementata nel codice. Il campo di velocità viene calcolato con la scomposizione di Fourier sopra descritta. I test della campagna a fondo rigido sono stati simulati allo scopo di ottenere ulteriori informazioni sul distacco dei vortici e per stimare il run-up sulla struttura che è un parametro fondamentale per garantire l’accesso alle strutture in mare. Il confronto con le misure sperimentali di livello, velocità, pressione e forza è eccellente. L’analisi dei massimi valori di run-up è stata realizzata confrontando i risultati con formule presenti in letteratura. In particolare, l’equazione di Hallermeier (1976) è stata adattata per il calcolo del run-up a partire dall’altezza della cresta e i risultati, ottenuti con un numero di campioni significativo, sono molto buoni. Infine, le strutture vorticose sono visualizzate mediante l’uso del Q-criterion. I risultati numerici mostrano come, a volte, un vortice generato nella prima metà di un periodo può rimanere nell’area di influenza del palo. Dopo l’inversione del flusso cambia direzione insieme al flusso e viene finalmente rimosso nel lato del palo opposto rispetto a dove era stato generato. I risultati sono in accordo con quelli della campagna PTV: sia la fase, la dimensione e la posizione dei vortici sono ben rappresentate. Gli obiettivi raggiunti con il modello numerico aumentano la comprensione di tale complesso processo fisico unendo informazioni di diversa natura come i gradienti di pressioni, le strutture vorticose e le mappe di scavo ad esse associate.
During the last years, the presence of piled structures, such as offshore wind farms or oil platforms, in the marine environment is becoming more important. For this reason, it gains relevance the correct analysis of possible criticisms and uncertainties in the design process. The aim of this thesis is the study of both the hydrodynamics and the morpho-dynamics induced by regular and random waves over a single slender pile by means of laboratory and numerical modelling. Therefore, the objective of the present thesis is to contribute to a better understanding of such physical processes. In particular, it aims to provide some tools that can be useful in the design process, in order to avoid a wrong estimation of the most important parameters and, thus, to ensure a proper design of the piled marine structure and its facilities. Particular attention is paid on the evaluation of the total force due to wave action, on the scour at the base of the pile and on the run-up over its surface. A rigid bed model has been realised to evaluate the best approach for the estimation of the total force over the pile due to nonlinear waves. An alternative method to the classical Morison (1950)'s approach has been proposed for the computation of the total force from the velocity and acceleration of the water particles from a measured water surface elevation time series. Such method consists in a Fourier spectral decomposition of the input free water surface signal; the contribution of each component, computed according to the Airy theory, is summed to obtain the total horizontal and vertical velocity along the vertical profile. The results showed that this approach allowed to give a very good estimate of the total force both in terms of maximum value and of the phase in which it occurs. On the contrary, the application of the linear theory for the calculation of the velocity to be applied in the force calculation (classical Morison approach used in the design process) leads to an underestimation of the peak value and to a wrong representation of the shape of the force. Another primary parameter in the stability of a structure in the marine environment is the scour at the base of the monopile. This experimental campaign is performed by means of a mobile bed model in which a vertical cylinder is placed over a sandy seabed and it is attacked by both regular and random nonlinear waves. The results for regular waves are quite in agreement with the earlier work of Sumer et al. (1992). However, the results of this study reveal that the scour process due to nonlinear regular waves starts for hydrodynamic conditions characterized by a lower intensity Keulegan-Carpenter parameter KC=4) in comparison with those reported in the literature for linear waves (KC=6). Different formulas are proposed for its estimation depending on dimensionless parameters (KC, Ur). For random waves, the approach proposed by Ong et al. (2013) for scour evaluation has been modified. This method depends on the cumulative distribution function of the dimensionless crest heights but, because of its complexity, it has been simplified in order to became suitable for design purposes without affecting the quality of the results. To better understand the process of formation, growth and detachment of vortices, PTV measurements are carried out and the results are integrated with those from pressure sensors and from the mobile bed model for the characterization of the whole process of vortex formation and scour generation. The results showed that the wave phases and positions in which the maximum values of the pressure gradients occur, correspond to those in which the detachment of vortices is observed. From the interpretation of the contour maps of the vorticity and of the OW parameter it is possible to explain the scour patterns obtained in the mobile bed campaign. Furthermore, a numerical model has been realised with the tool OpenFOAM that permitted to contribute to the comprehension of the complex 3D physical phenomenon induced by the wave-structure interaction. A new wave generation boundary condition for the generation of a measured water surface elevation time series has been added to the numerical code. The corresponding velocity field in this BC has been computed with the Fourier decomposition method mentioned above. The nonlinear waves of the rigid bed experimental campaign are here simulated for the better comprehension of the vortex formation process and for the estimation of wave run-up which is very important for the design of the access facility of the marine structures. Excellent results are obtained in comparison with experimental data of force, pressure, velocity and water elevation. The analysis of the maximum values of run-up is performed and the results are compared with some formula available in the literature. An adaptation of the equation of Hallermeier (1976) that compute the run-up depending on the crest height, is proposed and the results, obtained with a significant number of samples, are very good. Finally, the visualization of the three-dimensional vortical structures by means of the Q-criterion has been performed. The numerical results showed that, in some instances, a vortex generated during the first half of the period can remain in the area of influence of the pile. After the reversal of the flow, it changes the direction according to the stream until it is finally removed on the opposite side of the pile with respect to where it was generated. The results are in agreement with those of the PTV campaign: both the phase, size and position of vortices are well represented. The achievements obtained with the numerical model are able to improve the understanding of the complex physical processes by linking the different results, such as the pressure gradients, vortical structures and the associated scour patterns.

Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2006.
The aim of this thesis is to study the stabilising effect which vertical ribs may have on a tall cylindrical structure, with the focus on the proposed Solar Chimney. The report begins with a brief introduction to the Solar Chimney concept. A study is made of flow regimes which exist for flow around a circular cylinder and these flow regimes, together with their characteristics, are described. Various threats to the Solar Chimney are identified and will be investigated. The natural shape and strength of the Saguaro Cactus leads to the investigation of vertically ribbed cylinders. Experimental wind tunnel tests are performed in Tokyo, Japan to obtain external pressure distributions. A smooth cylindrical model, as well as two different configurations of vertically ribbed cylinders are tested. These external pressure distributions are numerically integrated in order to obtain calculated drag coefficient values for the cylinders. The drag coefficients for a smooth cylinder and cylinders with vertical ribs are obtained experimentally by means of total drag force measurements. These tests were performed in the wind tunnel at the University of Stellenbosch. The effect of aspect ratio of a smooth cylinder is also addressed. Computational Fluid Dynamics (CFD) analyses are carried out using the CFD software program, FLUENT. Both the drag coefficient and the external pressure distributions were investigated in this way. The differences between theoretical conditions and those of real life conditions of a smooth cylinder are discussed. The report ends by concluding the stabilising effect of vertical ribs on tall cylindrical structures and how these ribs may eliminate some of the present threats to the proposed Solar Chimney.

[Formulae and special characters can only be approximated here. Please see the pdf version of the abstract for an accurate reproduction.] Wave run-up is the vertical uprush of water when an incident wave impinges on a free- surface penetrating body. For large volume offshore structures the wave run-up on the weather side of the supporting columns is particularly important for air-gap design and ultimately the avoidance of pressure impulse loads on the underside of the deck structure. This investigation focuses on the limitations of conventional wave diffraction theory, where the free-surface boundary condition is treated by a Stokes expansion, in predicting the harmonic components of the wave run-up, and the presentation of a simplified procedure for the prediction of wave run-up. The wave run-up is studied on fixed vertical cylinders in plane progressive waves. These progressive waves are of a form suitable for description by Stokes' wave theory whereby the typical energy content of a wave train consists of one fundamental harmonic and corresponding phase locked Fourier components. The choice of monochromatic waves is indicative of ocean environments for large volume structures in the diffraction regime where the assumption of potential flow theory is applicable, or more formally A/a < Ο(1) (A and a being the wave amplitude and cylinder radius respectively). One of the unique aspects of this work is the investigation of column geometry effects - in terms of square cylinders with rounded edges - on the wave run-up. The rounded edges of each cylinder are described by the dimensionless parameter rc/a which denotes the ratio of edge corner radius to half-width of a typical column with longitudinal axis perpendicular to the quiescent free-surface. An experimental campaign was undertaken where the wave run-up on a fixed column in plane progressive waves was measured with wire probes located close to the cylinder. Based on an appropriate dimensional analysis, the wave environment was represented by a parametric variation of the scattering parameter ka and wave steepness kA (where k denotes the wave number). The effect of column geometry was investigated by varying the edge corner radius ratio within the domain 0 <=rc/a <= 1, where the upper and lower bounds correspond to a circular and square shaped cylinder respectively. The water depth is assumed infinite so that the wave run-up caused purely by wave-structure interaction is examined without the additional influence of a non-decaying horizontal fluid velocity and finite depth effects on wave dispersion. The zero-, first-, second- and third-harmonics of the wave run-up are examined to determine the importance of each with regard to local wave diffraction and incident wave non-linearities. The modulus and phase of these harmonics are compared to corresponding theoretical predictions from conventional diffraction theory to second-order in wave steepness. As a result, a basis is formed for the applicability of a Stokes expansion to the free-surface boundary condition of the diffraction problem, and its limitations in terms of local wave scattering and incident wave non-linearities. An analytical approach is pursued and solved in the long wavelength regime for the interaction of a plane progressive wave with a circular cylinder in an ideal fluid. The classical Stokesian assumption of infinitesimal wave amplitude is invoked to treat the free-surface boundary condition along with an unconventional requirement that the cylinder width is assumed much smaller than the incident wavelength. This additional assumption is justified because critical wavelengths for wave run-up on a fixed cylinder are typically much larger in magnitude than the cylinder's width. In the solution, two coupled perturbation schemes, incorporating a classical Stokes expansion and cylinder slenderness expansion, are invoked and the boundary value problem solved to third-order. The formulation of the diffraction problem in this manner allows for third-harmonic diffraction effects and higher-order effects operating at the first-harmonic to be found. In general, the complete wave run-up is not well accounted for by a second-order Stokes expansion of the free-surface boundary condition and wave elevation. This is however, dependent upon the coupling of ka and kA. In particular, whilst the modulus and phase of the second-harmonic are moderately predicted, the mean set-up is not well predicted by a second-order Stokes expansion scheme. This is thought to be caused by higher than second-order non-linear effects since experimental evidence has revealed higher-order diffraction effects operating at the first-harmonic in waves of moderate to large steepness when k < < 1. These higher-order effects, operating at the first-harmonic, can be partially accounted for by the proposed long wavelength formulation. For small ka and large kA, subsequent comparisons with measured results do indeed provide a better agreement than the classical linear diffraction solution of Havelock (1940). To account for the complete wave run-up, a unique approach has been adopted where a correction is applied to a first-harmonic analytical solution. The remaining non-linear portion is accounted for by two methods. The first method is based on regression analysis in terms of ka and kA and provides an additive correction to the first-harmonic solution. The second method involves an amplification correction of the first-harmonic. This utilises Bernoulli's equation applied at the mean free-surface position where the constant of proportionality is empirically determined and is inversely proportional to ka. The experimental and numerical results suggest that the wave run-up increases as rc/a--› 0, however this is most significant for short waves and long waves of large steepness. Of the harmonic components, experimental evidence suggests that the effect of a variation in rc/a on the wave run-up is particularly significant for the first-harmonic only. Furthermore, the corner radius effect on the first-harmonic wave run-up is well predicted by numerical calculations using the boundary element method. Given this, the proposed simplified wave run-up model includes an additional geometry correction which accounts for rc/a to first-order in local wave diffraction. From a practical view point, it is the simplified model that is most useful for platform designers to predict the wave run-up on a surface piercing column. It is computationally inexpensive and the comparison of this model with measured results has proved more promising than previously proposed schemes.

Deux importants problemes de l'hydrodynamique navale ont ete traites. Le premier est la diffraction-radiation avec petite vitesse d'avance (interaction houle-courant) qui permet le calcul de l'effort de derive et de l'amortissement de derive necessaires dans l'etude du mouvement de derive lente des structures flottantes. Le deuxieme est la diffraction non-lineaire aux ordres superieurs qui permet le calcul des efforts aux frequences plus grandes que la frequence de la houle incidente, ce qui est important dans l'etude des phenomenes de resonance haute frequence (springing et ringing). Dans les deux cas l'ecoulement est suppose etre potentiel et le cas particulier d'un cylindre vertical circulaire reposant sur le fond a ete traite. La methode de calcul du potentiel est semi-analytique et est basee sur le developpement de la solution sur la base des fonctions propres du probleme et sur l'utilisation de la methode des singularites

Il s'agit d'etudier l'influence des modulations de la microgravite sur le seuil d'apparition de la convection naturelle. L'etude est menee dans le cas d'un cylindre d'extension finie, soumis a un gradient de temperature axial antiparallele au vecteur de la gravite. Les parois laterales du cylindre ont ete considerees comme parfaitement conductrices ou adiabatiques. Dans le cas d'une gravite constante, la methode de galerkin, appliquee au systeme d'equations lineaires, permet de determiner les seuils de bifurcation caracterises par le nombre de rayleigh, pour l'apparition de mouvements axisymetriques et asymetriques. Dans le cas de modulations sinusoidales de la gravite, la methode de resolution utilise la theorie de floquet en considerant le cas de petites amplitudes de modulation de la gravite. Cette methode, qui repose sur le fait que l'effet des g-jitter est d'alterer les seuils de transition en provoquant un retard ou une avance substanciel de tranfert de chaleur et de masse, fait apparaitre un facteur d'alteration k. L'etude de ce facteur d'alteration permet de degager l'effet des differents parametres de controle (nombre de prandtl, rapport de forme du cylindre, frequence de modulation de la microgravite) sur l'alteration des seuils de bifurcation par une tendance stabilisatrice ou destabilisatrice

This master thesis project was performed in collaboration with Scania CV AB, Engine Materials group. The purpose with the project was to investigate different ceramic TBC (Thermal Barrier Coating) thermal insulation properties inside the combustion chamber. Experimental testing was performed with a Single-Cylinder engine with TBC deposited on selected components. A dummy-valve was developed and manufactured specifically for this test in order to enable a water cooling system and to ease the testing procedure. The dummy-valve consists of a headlock, socket, valve poppet and valve shaft. Additionally, a copper ring is mounted between the cylinder head and the valve poppet to seal the system from combustion gases. Thermocouples attached to the modified valve poppet and valve shaft measured the temperature during engine test to calculate the heat flux. The TBCs consisted of three different materials: 7-8% yttrium-stabilized zirconia (8YSZ), gadolinium zirconia and lanthanum zirconia. The 8YSZ TBC was tested as standard, but also with microstructural modifications. Modifications such as pre-induced segmented cracks, nanostructured zones and sealed porosity were used. The results indicated that the heat flux of 8YSZ-standard, 8YSZ-nano and 8YSZ-segmented cracks was in level with the steel reference. In the case of 8YSZ-sealed porosity the heat flux was measured higher than the steel reference. Since 8YSZ-standard and 8YSZ-sealed porosity are deposited with the same powder it is believed that the high heat flux is caused by radiative heat transfer. The remaining samples have had some microstructural changes during engine testing. 8YSZ-nano had undergone sintering and its nanostructured zones became fewer and almost gone after engine testing leading to less heat barrier in the top coat of the TBC. However, for 8YSZ-segmented cracks and gadolinium zirconia lower heat flux was measured due to the appearance of horizontal cracks. These cracks are believed to act as internal barriers as they are orientated perpendicular to the heat flow. During long-time (5 hour) engine tests the 8YSZ-standard exhibited the same phenomena: a decrease in heat flux due to propagation of horizontal cracks. One-dimensional heat flux was not achieved and the main reason for that was caused by heating and cooling of the shafts outer surface. However, the dummy-valve system has proven to be a quick, easy and stable to perform tests with a Single-Cylinder engine. Both water-cooling and long-time engine tests were conducted with minor issues. The dummy-valve has been further developed for future tests. Changes to the valve shaft are the most remarkable: smaller diameter to reduce heat transfer and smaller pockets to ensure better thermocouple positioning. Another issue was gas leakage from the combustion chamber through the copper ring and valve poppet joint. The copper ring will be designed with a 1 mm thick track to improve sealing, hence better attachment to the valve poppet.

On s'interesse aux ecoulements de convection naturelle en cavite annulaire verticale differentiellement chauffee. Dans un premier temps, on met en evidence, dans le cas d'une fente de hauteur infinie, une classe de solutions analytiques mono-dimensionnelles, qui depend d'un parametre appele parametre de stratification. Les solutions obtenues pour differentes valeurs de ce parametre reproduisent les differents regimes d'ecoulement (conduction, transition et couche limite), rencontres dans une cavite annulaire de rapport de forme fini, chauffee et refroidie lateralement a flux constant. On etudie ensuite la stabilite lineaire de cette classe de solutions, en discretisant l'equation d'orr-sommerfeld par une methode pseudo-spectrale de collocation chebyshev et en resolvant le probleme aux valeurs propres correspondant. On examine les effets de la stratification, de la courbure et du nombre de prandtl sur la stabilite de cette solution. Une analyse du bilan energetique des perturbations permet de conclure sur l'origine hydrodynamique ou thermique de l'instabilite. On effectue enfin quelques simulations numeriques du probleme non-lineaire en integrant les equations de navier-stokes completes par voie pseudo-spectrale de collocation chebyshev et une technique de type uzawa pour le maintien de l'incompressibilite. On confirme ainsi les resultats de l'etude de stabilite lineaire. On s'interesse alors a la dynamique des solutions obtenues pour des valeurs tres supercritiques du nombre de rayleigh

L'auteur etudie les transferts de chaleur et d'impulsion instationnaires par convection naturelle a l'interieur d'un ellipsoide de revolution rempli d'air et dont la paroi est soit portee a une temperature constante, soit traversee par un flux de chaleur de densite constante et uniforme ainsi qu'a l'interieur d'un cylindre horizontal de section droite elliptique dont la paroi est isotherme. Il donne les profils des isothermes et des lignes de courant, ainsi que les variations des coefficients de transfert de chaleur locaux et moyens instantanes pour differentes valeurs du nombre de grashof et du facteur de forme. L'existence de structures multicellulaires pour certaines valeurs du nombre de grashof et du facteur de forme est mise en evidence

L'auteur presente une etude numerique des transferts de chaleur par convection naturelle et mixte en regime laminaire dans un espace delimite par deux cylindres concentriques verticaux. Il analyse egalement l'influence de protuberances sinusoidales disposees sur les surfaces exterieure et interieure, sur les transferts de chaleur. L'auteur complete ce memoire par une etude numerique et experimentale du meme phenomene dans la meme configuration muni cette fois-ci de deux protuberances sinusoidales sur la surface exterieure du cylindre interieur et une autre a faible amplitude sur la surface interieure du cylindre exterieur. La discretisation des equations qui regissent les phenomenes est basee sur la technique des volumes de controle aux differences finies. Le couplage entre la vitesse et la pression est effectue a l'aide de l'algorithme simple. Les resultats sont presentes sous forme de fonctions de courant, d'isothermes, de profils de temperature, de vitesses et de pressions et sous forme de correlations. Ils mettent en evidence les effets des forces de gravitation sur l'ecoulement et de l'interaction entre ces forces et les forces centrifuges. Les protuberances intensifient le transfert de chaleur et perturbent l'ecoulement. Les resultats experimentaux caracterises par une visualisation de l'ecoulement en utilisant de la fumee d'encens et par les mesures de temperatures de surface du cylindre exterieur a l'aide de thermocouples et d'une camera de thermographie infrarouge sont en bon accord avec les resultats theoriques

On considere ici des ecoulements potentiels a surface libre. Deux cas d'application sont envisages. On tente de resoudre le probleme complet des non-linearites de surface libre dans un bassin rectangulaire par une methode d'elements finis. Ce probleme, formule en terme de potentiel des vitesses et elevation de surface libre, est instationnaire; on discretise le systeme differentiel couple par un schema implicite. On suit les mouvements de la surface libre en remaillant, a chaque pas de temps, le domaine de calcul. Il en resulte des instabilites numeriques. Les traitements envisages peuvent entrainer une baisse du niveau moyen ou une diminution importante de l'energie du systeme; la methode n'est validee que pour des regimes faiblement non lineaires. La deuxieme partie est consacree a la diffraction au second ordre d'une onde plane progressive sur un groupe de cylindres verticaux. Le probleme aux limites est resolu dans le cadre d'une methode integrale. La recherche de conditions de radiation permet d'introduire les notions d'ondes liees et libres. On raccorde la solution interieure aux deux composantes exterieures par une methode des moindres carres. Des criteres permettent d'optimiser ce raccord. Des resultats numeriques sont presentes pour les cas d'un ou plusieurs cylindres.

碩士
國立中央大學
機械工程研究所
88
Unsteady numerical computation of natural convection are carry out in this paper. Our modeling configuration is three-dimensional vertical cylinder heated from below and adiabatic at sidewall. We use H2O(Pr=6.7) as melt and various three different types of aspect ratio(A=2,1,0.5, diameter D/height H) on the Rayleigh-Benard convection. As for A=2, steady flow structure would change to m1(non-axial symmetry) type form m0(axial symmetry) type once Ra(Rayleigh number) exceed 15000. However, we found that for A=1, 20000