The air-water flow reqime transitions in a horizontal pipe under the influence of a stronq electric field perpendicular to the interface are studied. The separated flow model to predict flow regime transitions has been developed. The present version of the model is a modification of Taitel and Dukler's separated flow model. This assumes that all flow reqimes are perturbations from stratified smooth flow. Expressions for the electrical force are derived and added to the conservation and constitutive equations to obtain new transition criteria. The theoretical results are compared with observations of air-water flow in a 1.27 cm. and 1.9 cm. internal diameter pipe. Good agreement was found when no electric field was applied, however, the experimentally observed effect of the electric field was not as pronounced as predicted by theory. Further experiments to refine the theoretical model are presented. / Thesis / Master of Engineering (ME)

Apresenta-se um modelo matemático que avalia numericamente a estabilidade do estado estacionário para escoamentos água-ar em sistemas pipeline-riser de geometria variável. Uma análise a partir da teoria de estabilidade linear é aplicada a um modelo matemático adequado ao escoamento água-ar no sistema pipeline-riser. O modelo considera equações de continuidade para a fase líquida e para a fase gasosa, admite-se escoamento unidimensional e em condição isotérmica. O líquido é considerado incompressível enquanto que a fase gasosa é considerada um gás ideal. Admite-se uma equação de momento simplificada para mistura onde despreza-se a inércia (NPW - Modelo No Pressure Wave) e o padrão de escoamento local é definido com base nas condições do escoamento e na inclinação local. Assim, a intermitência severa é controlada principalmente pela gravidade no riser e pela compressibilidade do gás no pipeline. Tanto a correlação de fluxo de deriva quanto o cálculo da queda de pressão por atrito, adotados como lei de fechamento do modelo, são determinados em função do padrão de escoamento. Injeção de gás e válvula de choke são consideradas, respectivamente, na base e no topo do riser. O modelo é aplicado à sistemas pipeline-riser com escoamento água-ar citados na literatura. Os resultados da análise de estabilidade linear numérica são comparados aos resultados experimentais e numéricos apresentando uma excelente concordância. / A mathematical model that numerically evaluates the stability of the stationary state for hilly terrain air-water flows systems is presented. Numerical linear stability analysis is performed to a suitable mathematical model for the two-phase flows in a pipeline-riser system. The mathematical model considers the continuity equations for the liquid and gas phases, one-dimensional flow and isothermal conditions. The liquid is assumed incompressible while the gas phase is considered as an ideal gas. A simplified momentum equation for the mixture, neglecting inertia (NPW - No pressure wave model) is considered and the local flow pattern is defined based on the flow conditions and the local inclination. In this way, severe slugging is controlled mainly by gravity in the riser and compressibility in the pipeline. The void fraction and friction pressure drop, utilized as closure laws, are determined based on the local flow pattern. Gas injection at the bottom of the riser and a choke valve at the top are considered. The model is applied to air-water pipeline-riser systems reported in the literature. Numerical linear stability analysis results are compared with experimental and numerical results reported in the literature with excellent agreement.

Apresenta-se um modelo matemático que avalia numericamente a estabilidade do estado estacionário para escoamentos água-ar em sistemas pipeline-riser de geometria variável. Uma análise a partir da teoria de estabilidade linear é aplicada a um modelo matemático adequado ao escoamento água-ar no sistema pipeline-riser. O modelo considera equações de continuidade para a fase líquida e para a fase gasosa, admite-se escoamento unidimensional e em condição isotérmica. O líquido é considerado incompressível enquanto que a fase gasosa é considerada um gás ideal. Admite-se uma equação de momento simplificada para mistura onde despreza-se a inércia (NPW - Modelo No Pressure Wave) e o padrão de escoamento local é definido com base nas condições do escoamento e na inclinação local. Assim, a intermitência severa é controlada principalmente pela gravidade no riser e pela compressibilidade do gás no pipeline. Tanto a correlação de fluxo de deriva quanto o cálculo da queda de pressão por atrito, adotados como lei de fechamento do modelo, são determinados em função do padrão de escoamento. Injeção de gás e válvula de choke são consideradas, respectivamente, na base e no topo do riser. O modelo é aplicado à sistemas pipeline-riser com escoamento água-ar citados na literatura. Os resultados da análise de estabilidade linear numérica são comparados aos resultados experimentais e numéricos apresentando uma excelente concordância. / A mathematical model that numerically evaluates the stability of the stationary state for hilly terrain air-water flows systems is presented. Numerical linear stability analysis is performed to a suitable mathematical model for the two-phase flows in a pipeline-riser system. The mathematical model considers the continuity equations for the liquid and gas phases, one-dimensional flow and isothermal conditions. The liquid is assumed incompressible while the gas phase is considered as an ideal gas. A simplified momentum equation for the mixture, neglecting inertia (NPW - No pressure wave model) is considered and the local flow pattern is defined based on the flow conditions and the local inclination. In this way, severe slugging is controlled mainly by gravity in the riser and compressibility in the pipeline. The void fraction and friction pressure drop, utilized as closure laws, are determined based on the local flow pattern. Gas injection at the bottom of the riser and a choke valve at the top are considered. The model is applied to air-water pipeline-riser systems reported in the literature. Numerical linear stability analysis results are compared with experimental and numerical results reported in the literature with excellent agreement.

Two-phase flow in small tubes and channels is becoming a common phenomenon in industrial processes. However, the study of two-phase flow regimes in small tubes is still at its infancy. The previous studies are reviewed and discussed in the literature section. The problems and inconsistencies encountered in the earlier studies are presented and discussed. The experimental facility is introduced in the chapters that follow. They include a section on the design of the experimental system and the test sections, the selection of the experimental parameters and the introduction of the purposely-developed programs to control the experiments and collect and process the data. The methodology of the calibration and the uncertainty analysis, the problems encountered and their solutions and the single-phase validation experiments are also described. In this project we studied the effect of tube diameter and fluid flow parameters on flow patterns in small tubes using R134a as the working fluid. The tested tube diameters were 1.10, 2.01, 2.88 and 4.26 mm; the fluid pressures were 6, 10 and 14 bar; the liquid and gas superficial velocities covered a range of 0.04-5.0 m/s and 0.01-10.0 m/s respectively. The observed flow patterns included bubbly, dispersed bubble, confined bubble, slug, chum, annular and mist flow. Twelve integrated flow maps are sketched in this report. The obtained results were compared with earlier experiments by other workers and with existing models, with obvious differences in the prediction of the transition boundaries. A set of new models and correlations were developed, based on the new data for boiling R134a presented in this thesis, to predict the effect of tube diameter and fluid properties on the transition boundaries. Some also agreed with the limited data available from earlier studies for adiabatic air-water flow in small to normal size tubes.

If air is entrained in a bottom outlet of a dam in an uncontrolled way, the resulting air pockets may cause problems such as blowback, blowout and loss of discharge capacity. In order to provide guidance for bottom outlet design and operation, this study examines how governing parameters affect air entrainment, air-pocket transport and de-aeration and the surrounding flow structure in pipe flows. Both experimental and numerical approaches are used. Air can be entrained into the bottom outlet conduit due to vortex formation at the intake if the intake submergence is not sufficient. The influent of the intake entrance profiles and channel width on the critical submergence were studied in the experiment. The experimental study was performed to investigate the incipient motion of air pockets in pipes with rectangular and circular cross sections. The critical velocity is dependent on pipe slope, pipe diameter, pipe roughness and air-pocket volume. If the pipe is horizontal, air removal is generally easier in a rectangular pipe than in a circular pipe. However, if the pipe is downward-inclined, air removal is easier in a circular pipe. When a bottom outlet gate opens, air can become entrained into the conduit in the gate shaft downstream of the gate. Using FLUENT software, the transient process of air entrainment into a prototype bottom outlet during gate opening is simulated in three dimensions. The simulations show in the flow-pattern changes in the conduit and the amount of air entrainment in the gate shaft. The initial conduit water level affects the degree of air entrainment. A de-aeration chamber is effective in reducing water surface fluctuations at blowout. High-speed particle image velocimetry (HSPIV) were applied to investigate the characteristics of the flow field around a stationary air pocket in a fully developed horizontal pipe flow. The air pocket generates a horseshoe vortex upstream and a reverse flow downstream. A shear layer forms from the separation point. Flow reattachment is observed for large air pockets. The air―water interface moves with the adjacent flow. A similarity profile is obtained for the mean streamwise velocity in the shear layer beneath the air pocket. / <p>QC 20140211</p>

Due to the high-speed flow in a chute spillway, cavitation damages often occur. This undesired phenomenon threatens the safety of the structure. For the purpose of eliminating the damages, an aerator is often installed in the spillway. To understand its characteristics, physical model tests are a popular method. To complement the model tests, computation fluid dynamics (CFD) simulations are used to study aerator flows. To represent the two-phase flows, multiphase models should be employed. This thesis examines two of them, namely, the Volume-Of-Fluid model (VOF) and Two-Fluid model. Based on the background of the Bergeforsen dam, the aerator flow is modelled by means of the VOF model. The simulated spillway discharge capacity is in accordance with the experimental data. Compared with the results, empirical formulas fail to evaluate the air supply capacity of aerator as it is wider than the conventional width. A hypothetical vent modification is proposed. For the original and proposed layouts, the study illustrates the difference in the air-flow conditions. The results show that a larger vent area is, for a large-width aerator, preferable in the middle of the chute. To study the flip bucket-shaped aerators in the Gallejaur dam, physical model tests and prototype observations are conducted. The results lead to contradicting conclusions in terms of jet breakup and air entrainment. A CFD model is, as an option, employed to explain the reason of the discrepancy. The numerical results coincide with the prototype observations. The jet breakup and air entrainment are evaluated from air cavity profiles; the air-pressure drops are small in the cavity. The discrepancy is due to overestimation of the surface-tension effect in the physical model tests. Based on the experimental data of an aerator rig at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, the Two-Fluid model is used to predict air concentration distributions in the aerated flow. The model includes relevant forces governing the motion of bubbles and considers the effects of air bubble size. The numerical results are conformable to the experiments in the air cavity zone. Downstream of the cavity, the air concentration near the chute bottom is higher, which is presumably caused by the fact that the interfacial forces in the Two-Fluid model are underestimated. / <p>QC 20170224</p>

The rapid depletion of hydrocarbon fields around the world has led the industry to search for these resources in ever increasing water depths. In this context, the large diameter (D > 100mm) vertical riser has become a subject of great interest. In this research work, a major investigation was undertaken to determine the two phase flow hydrodynamics in a 254mm vertical riser. Two types of experiments were performed for range of air-water superficial velocities. The first experimental campaign addresses the issue of the two gas injector’s performances (conventional vs. novel design gas injector) in the large diameter vertical riser. The experimental results show that the novel design gas injector should be the preferential choice. The second set of the experimental work investigates the two phase flow hydrodynamics in the vertical riser in detail. The two phase flow patterns and their transitions were identified by combination of visual observations and statistical features. Based on the results, the experimental flow regime map was developed and compared with the existing vertical upflow regime maps/models. None of the flow regime transition models adequately predicted the flow regimes transitions in large diameter vertical risers as a whole. In this regard, the Taitel et al. (1980) bubble to slug flow transition model has been modified for large diameter vertical upflow conditions, based on the physical mechanism observed. The general trends of modified criteria agreed well with the current and other large diameter experimental results. The effect of upstream conditions on the vertical riser flow behaviour was also investigated in detail by two different inlet configurations (i) near riser base injection and (ii) upstream flowline injection. It was found that no significant differences exist in flow behaviour at low air-water superficial velocities for both the inlet configuration, at high air-water superficial velocities, the intermittent flow behavior in flowline influences the riser flow pattern characteristics and thereby controls the riser dynamics. It is found that liquid slugs from the flowline naturally dissipate to some extent in the riser as a consequence of compression of succeeding bubble that rapidly expands and break through the liquid slug preceding it when it enters the riser. The experimental work corroborates the general consensus that slug flow does not exist in large diameter vertical upflow condition. Experimental data has been further compared to increase the confidence on the existing two phase flow knowledge on large diameter vertical riser: (a) by comparing with other experimental studies on large diameter vertical upflow in which generally, a good agreement was found, (b) by assessing the predictive capability of void fraction correlations/pressure gradient methods. The important implication of this assessment is that the mechanistic approach based on specific flow regime in determining the void fraction and pressure gradient is more successful than conventional empirical based approaches. The assessment also proposes a proposed set a of flow regime specific correlations that recommends void fraction correlations based on their performances in the individual flow regimes. Finally, a numerical model to study the hydrodynamic behaviour in the large diameter horizontal flowline-vertical riser system is developed using multiphase flow simulator OLGA. The simulated results show satisfactory agreement for the stable flows while discrepancies were noted for highly intermittent flows. The real time boundary application was partially successful in qualitatively reproducing the trends. The discrepancies between the predicted results and experimental data are likely to be related to the incorrect closure relations used based on incorrect flow regimes predictions. The existence of the multiple roots in the OLGA code is also reported for the first time.

The rapid depletion of hydrocarbon fields around the world has led the industry to search for these resources in ever increasing water depths. In this context, the large diameter (D > 100mm) vertical riser has become a subject of great interest. In this research work, a major investigation was undertaken to determine the two phase flow hydrodynamics in a 254mm vertical riser. Two types of experiments were performed for range of air-water superficial velocities. The first experimental campaign addresses the issue of the two gas injector’s performances (conventional vs. novel design gas injector) in the large diameter vertical riser. The experimental results show that the novel design gas injector should be the preferential choice. The second set of the experimental work investigates the two phase flow hydrodynamics in the vertical riser in detail. The two phase flow patterns and their transitions were identified by combination of visual observations and statistical features. Based on the results, the experimental flow regime map was developed and compared with the existing vertical upflow regime maps/models. None of the flow regime transition models adequately predicted the flow regimes transitions in large diameter vertical risers as a whole. In this regard, the Taitel et al. (1980) bubble to slug flow transition model has been modified for large diameter vertical upflow conditions, based on the physical mechanism observed. The general trends of modified criteria agreed well with the current and other large diameter experimental results. The effect of upstream conditions on the vertical riser flow behaviour was also investigated in detail by two different inlet configurations (i) near riser base injection and (ii) upstream flowline injection. It was found that no significant differences exist in flow behaviour at low air-water superficial velocities for both the inlet configuration, at high air-water superficial velocities, the intermittent flow behavior in flowline influences the riser flow pattern characteristics and thereby controls the riser dynamics. It is found that liquid slugs from the flowline naturally dissipate to some extent in the riser as a consequence of compression of succeeding bubble that rapidly expands and break through the liquid slug preceding it when it enters the riser. The experimental work corroborates the general consensus that slug flow does not exist in large diameter vertical upflow condition. Experimental data has been further compared to increase the confidence on the existing two phase flow knowledge on large diameter vertical riser: (a) by comparing with other experimental studies on large diameter vertical upflow in which generally, a good agreement was found, (b) by assessing the predictive capability of void fraction correlations/pressure gradient methods. The important implication of this assessment is that the mechanistic approach based on specific flow regime in determining the void fraction and pressure gradient is more successful than conventional empirical based approaches. The assessment also proposes a proposed set a of flow regime specific correlations that recommends void fraction correlations based on their performances in the individual flow regimes. Finally, a numerical model to study the hydrodynamic behaviour in the large diameter horizontal flowline-vertical riser system is developed using multiphase flow simulator OLGA. The simulated results show satisfactory agreement for the stable flows while discrepancies were noted for highly intermittent flows. The real time boundary application was partially successful in qualitatively reproducing the trends. The discrepancies between the predicted results and experimental data are likely to be related to the incorrect closure relations used based on incorrect flow regimes predictions. The existence of the multiple roots in the OLGA code is also reported for the first time.

Ce travail est consacré à la modélisation mathématique et numérique des écoulements eau-air en conduite qui interviennent notamment dans les centrales de production d’électricité ou les réseaux d’eaux usées. On s’intéresse particulièrement aux écoulements mixtes caractérisés par la présence de régimes stratifiés pilotés par des ondes gravitaires lentes, de régimes en charge ou secs (conduite remplie d’eau ou d’air) pilotés par des ondes acoustiques rapides, et de poches d’air piégées. Une modélisation précise de ces écoulements est nécessaire afin de garantir le bon fonctionnement du circuit hydraulique sous-jacent. Alors que la plupart des modèles disponibles dans la littérature se concentrent sur la phase eau en négligeant la présence de l’air, un modèle bicouche compressible prenant en compte les interactions eau-air est proposé dans cette thèse. Sa construction réside dans l’intégration des équations d’Euler barotropes sur la hauteur de chaque phase et dans l’application de la contrainte hydrostatique sur le gradient de pression de l’eau. Le modèle obtenu est hyperbolique et satisfait une inégalité d’entropie en plus d’autres propriétés mathématiques notables, telles que l’unicité des relations de saut ou la positivité des hauteurs et densités de chaque phase. Au niveau discret, la simulation d’écoulements mixtes avec le modèle bicouche compressible soulève plusieurs défis en raison de la disparité des vitesses d’ondes caractérisant chaque régime, des processus de relaxation rapide sous-jacents, et de la disparition de l’une des phases dans les régimes en charge ou sec. Une méthode à pas fractionnaires implicite-explicite est alors développée en s’appuyant sur la relaxation rapide en pression et sur le mimétisme avec les équations de Saint-Venant pour la dynamique lente de la phase eau. En particulier, une approche par relaxation permet d’obtenir une stabilisation du schéma en fonction du régime d’écoulement. Plusieurs cas tests sont traités et démontrent la capacité du modèle proposé à gérer des écoulements mixtes incluant la présence de poches d’air piégées. / The present work is dedicated to the mathematical and numerical modelling of transient air-water flows in pipes which occur in piping systems of several industrial areas such as nuclear or hydroelectric power plants or sewage pipelines. It deals more specifically with the so-called mixed flows which involve stratified regimes driven by slow gravity waves, pressurized or dry regimes (pipe full of water or air) driven by fast acoustic waves and entrapped air pockets. An accurate modelling of these flows is necessary to guarantee the operability of the related hydraulic system. While most of available models in the literature focus on the water phase neglecting the air phase, a compressible two-layer model which accounts for air-water interactions is proposed herein. The derivation process relies on a depth averaging of the isentropic Euler set of equations for both phases where the hydrostatic constraint is applied on the water pressure gradient. The resulting system is hyperbolic and satisfies an entropy inequality in addition to other significant mathematical properties, including the uniqueness of jump conditions and the positivity of heights and densities for each layer. Regarding the discrete level, the simulation of mixed flows with the compressible two-layer model raises key challenges due to the discrepancy of wave speeds characterizing each regime combined with the fast underlying relaxation processes and with phase vanishing when the flow becomes pressurized or dry. Thus, an implicit-explicit fractional step method is derived. It relies on the fast pressure relaxation in addition to a mimetic approach with the shallow water equations for the slow dynamics of the water phase. In particular, a relaxation method provides stabilization terms activated according to the flow regime. Several test cases are performed and attest the ability of the compressible two-layer model to deal with mixed flows in pipes involving air pocket entrapment.

Tesis por compendio / The new legal regulations derived from climate change dictate that hydraulic structures must be designed to handle flood events associated with return periods up to 10,000 years. This obviously involves adapting the existing infrastructure to meet such requirements. In order to avoid risks in the restitution of the flow discharged to rivers, such as bank overflows or streambed erosion and scour processes, hydraulic design must be supported by reliable tools capable of reproducing the behavior of hydraulic structures. In the work presented herein, a fully three-dimensional CFD model to reproduce the behavior of different types of air-water flow in hydraulic structures is presented. The flow is assumed to be turbulent, isotropic and incompressible. Several RANS turbulence models are tested and structured rectangular meshes are employed to discretize the analyzed domain. The presence of two fluids is modeled using different VOF approaches and simulations are run using the PIMPLE algorithm. The model is implemented using the open-source platform OpenFOAM and its performance is compared to the commercial code FLOW-3D. The analysis is conducted separately on two different parts of hydraulic structures, namely: the spillway and the stilling basin. Additionally, a case of practical application, where the model reproduces the flow of a real-life case, is also presented in order to prove the suitability of the model to actual design cases. Mesh independence and model validation using experimental data are checked in the results of all the case studies. The sensitivity of the presented model to certain parameters is extensively discussed using different indicator variables. Among these parameters are turbulence closure, discretization scheme, surface tracking approach, CFD code or boundary conditions. Pros and contras of each of them are addressed. The analyzed turbulence models are the Standard k ¿ ¿, the Realizable k ¿ ¿, the RNG k ¿ ¿, and the SST k ¿ ¿. The discretization schemes under study are: a first-order upwind method, the second-order limited Van Leer method, and a second-order limited central difference method. The VOF approaches analyzed are the Partial VOF, as implemented in OpenFOAM, and the TruVOF, as implemented in FLOW-3D. In most cases, the Standard k ¿ ¿ model provides the most accurate estimations of water free surface profiles, although the rest of variables, with few exceptions, are better predicted by the RNG k ¿ ¿. The latter model generally requires slightly longer computation times. The SST k ¿ ¿ reproduces correctly the phenomena under study, although it generally turned out to be less accurate than its k ¿ ¿ counterparts. As regards the comparison among VOF approaches and codes, it is impossible to determine which one performs best. E.g. OpenFOAM, using the Partial VOF, managed to reproduce the in- ternal hydraulic jump structure and all derived variables better than FLOW-3D, using the TruVOF, although the latter seems to capture better the momentum transfer and so all derived variables. In the case of flow in stepped spillways, OpenFOAM captures better the velocity profiles, although FLOW-3D is more accurate when estimating the water free surface profile. It is worth remark- ing that not even their response to certain model parameters is comparable. E.g. FLOW-3D is significantly less sensitive to mesh refinement than OpenFOAM. Given the result accuracy achieved in all cases, the proposed model is fully applicable to more complex design cases, where stilling basins, stepped spillways and hydraulic structures in general must be investigated. / Las nuevas disposiciones legales derivadas del cambio climático dictaminan que las estructuras hidráulicas sean capaces de funcionar correctamente con eventos de inundación asociados a periodos de retorno de hasta 10,000 años. Esto, obviamente, implica adaptar la infraestructura existente para satisfacer dichos requerimientos. A fin de evitar riesgos en la restitución de los caudales vertidos al río, como desbordamientos o procesos erosivos y de socavación, el diseño hidráulico ha de sustentarse en herramientas fiables capaces de reproducir el comportamiento de las estructuras hidráulicas. En este trabajo, se presenta un modelo numérico CFD completamente tridimensional para reproducir el comportamiento de diferentes tipos de flujo aire-agua en estructuras hidráulicas. Se asume que el flujo es turbulento, isotrópico e incompresible. Diversos modelos de turbulencia RANS son contrastados y se emplean mallas estructuradas rectanuglares para discretizar el dominio analizado. La presencia de dos fluidos es modelada utilizando diferentes enfoques VOF y las simulaciones son ejecutadas empleando el algoritmo PIMPLE. El modelo es implementado mediante la plataforma de código abierto OpenFOAM y su respuesta es comparada con la del modelo comercial FLOW-3D. El análisis se lleva a cabo sobre dos partes diferentes de una estructura hidráulica, a saber, el aliviadero y el cuenco amortiguador, de forma separada. Además, un caso de aplicación práctica, donde el modelo reproduce el flujo en una estructura real, es presentado también a fin de probar la adecuación del modelo a casos de diseño aplicado. Se comprueban la independencia de la malla y la validación con datos experimentales de los resultados de todos los casos de estudio. La sensibilidad del modelo presentado a ciertos parámetros es analizada de forma exhaustiva empleando diferentes variables indicadoras. Los pros y contras de cada uno de éstos son planteados. Los modelos de turbulencia analizados son el Standard k-epsilon, el Realizable k-epsilon, el RNG k-epsilon y el SST k-omega. Los esquemas de discretización estudiados son: un método de primer orden upwind, uno de Van Leer de segundo orden y un esquema de segundo orden limitado de diferencias centradas. Los enfoques VOF analizados son el Partial VOF, implementado en OpenFOAM, y el TruVOF, implementado en FLOW-3D. En la mayoría de casos, el modelo k-epsilon aporta las estimaciones más precisas de perfiles de lámina libre de agua, pese a que el resto de variables, con alguna excepción, son mejor predichas por el RNG k-epsilon. Este modelo generalmente requiere mayores tiempos de cálculo. El k-omega reproduce correctamente los fenómenos bajo estudio, pese a que su precisión es generalmente más baja que la de los modelos k-epsilon. En lo que respecta a la comparación entre enfoques VOF y códigos, es imposible determinar cuál es el mejor. Por ejemplo, OpenFOAM, empleando el Partial VOF, logra reproducir la estructura interna del resalto hidráulico y todas las variables derivadas mejor que FLOW-3D, empleando el TruVOF, a pesar de que este último parece capturar mejor la transferencia de cantidad de movimiento y, por tanto, todas las variables derivadas. En el caso del flujo en aliviaderos escalonados, OpenFOAM captura mejor los perfiles de velocidad, pese a que FLOW-3D es más preciso en la estimación de los perfiles de lámina libre de agua. Conviene recalcar que ni tan sólo su respuesta a ciertos parámetros del modelo es comparable. Por ejemplo, FLOW-3D es significativamente menos sensible al refinado de malla que OpenFOAM. A la luz de la precisión de los resultados obtenidos en todos los casos, el modelo propuesto es completamente aplicable a casos de diseño más complejos, donde cuencos amortiguadores, aliviaderos escalonados y estructuras hidráulicas en general han de ser investigadas. / Les noves disposicions legals derivades del canvi climàtic dictaminen que cal que les estructures hidràuliques siguen capaces de funcionar correctament amb esdeveniments d'inundació associats a períodes de retorn de fins a 10,000 anys. Això, òbviament, implica adaptar la infraestrctura existent per satisfer aquests requeriments. A fi d'evitar riscs en la restitució dels cabals vessats al riu, com desbordaments o processos erosius i de socavació, el disseny hidràulic ha de recolzar-se en ferramentes fiables capaces de reproduir el comportament de les estructures hidràuliques. En aquest treball, es prsenta un model numèric CFD completament tridimensional per a reproduir el comportament de diferents tipus de flux aire-aigua en estructures hidràuliques. S'assumeix que el flux és turbulent, isotròpic i incompressible. Diferents models de turbulència RANS són contrastats i s'empren malles estructurades rectangulars per discretitzar el domini analitzat. La presència de dos fluids és modelada utilitzant diferents enfocaments VOF i les simulacions són executades emprant l'algorisme PIMPLE. El model és implementat mitjançant la plataforma de codi obert OpenFOAM i la seua resposta és comparada amb la del codi comercial FLOW-3D. L'anàlisi es du a terme sobre les diferents parts d'una estructura hidràulica, a saber, sobreeixidors esgraonats i vas esmorteïdor, de forma separada. A més, un cas d'aplicació pràctica, on el model reprodueix el flux a una estructura real, és presentat també a fi de provar l'adequació del model a casos de disseny aplicat. Es comproven la independència de la malla i la validació amb dades experimentals dels resultats de tots els casos d'estudi. La sensibilitat del model presentat a certs paràmetres és analitzada de forma exhaustiva emprant diferents variables indicadores. Els pros i contres de cadascun d'aquests són plantejats. Els models de turbulència analitzats són l'Standard k-epsilon, el Realizable k-epsilon, el RNG k-epsilon i l'SST k-omega. Els esquemes de discretització estudiats són: un mètode de primer ordre upwind, un de Van Leer de segon ordre i un esquema de segon ordre limitat de diferències centrades. Els enfocaments VOF analitzats són el Partial VOF, implementat en OpenFOAM, i el TruVOF, implementat en FLOW-3D. En la majoria de casos, el model Standard k-epsilon aporta les estimacions més precises de perfils de làmina lliure d'aigua, tot i que la resta de variables, amb alguna excepció, són millor predites pel RNG k-epsilon. Aquest model generalment requereix majors temps de càlcul. El k-omega reprodueix correctament els fenòmens sota estudi, tot i que la seua precisió és generalment més baixa que la dels models k-epsilon. Pel que fa la comparació entre enfocaments VOF i codis, és impossible determinar quin és el millor. Per exemple, OpenFOAM, emprant el Partial VOF, aconsegueix reproduir l'estructura interna del ressalt hidràulic i totes les variables derivades millor que FLOW-3D, emprant el TruVOF, tot i que aquest últim pareix capturar millor la transferència de quantitat de moviment i, per tant, totes les variables derivades. En el cas del flux en sobreeixidors esgraonats, OpenFOAM captura millor els perfils de velocitat, tot i que FLOW-3D és més precís en estimar els perfils de làmina lliure d'aigua. Cal deixar palès que ni tan sols la seua resposta a certs paràmetres del model és comparable. Per exemple, FLOW-3D és significativament menys sensible al refinament de malla que OpenFOAM. En base a la precisió dels resultats obtinguts en tots els casos, el model proposat és completament aplicable a casos de disseny més complexos, on vassos esmorteïdors, sobreeixidors esgraonats i estructures hidràuliques en general han de ser investigades. / Bayón Barrachina, A. (2017). Numerical analysis of air-water flows in hydraulic structures using computational fluid dynamics (CFD) [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90440 / Compendio