<P> The work presented m this thesis focused on using the hybrid Unsteady Reynolds-Averaged Navier-Stokes (URANS)/Large Eddy Simulation (LES) methodology to investigate the flow pulsation phenomenon in compound rectangular channels for isothermal flows. The specific form of the hybrid URANS/LES approach that was used is the Strelets (2001) version of the Detached Eddy Simulation (DES). It is of fundamental interest to study the problem of flow pulsations, as it is one of the most important mechanisms that directly affect the heat transfer occurring in sub-channel geometries such as those in nuclear fuel bundles. The predictions associated with the heat transfer and fluid flow in sub-channel geometry can be used to develop simplified physical models for sub-channel mixing for use in broader safety analysis codes. The primary goal of the current research work was to determine the applicability of the DES approach to predict the flow pulsations in sub-channel geometries. It was of interest to see how accurately the dynamics associated with the flow pulsations can be resolved from a spatial-temporal perspective using the specific DES model. The research work carried out for this thesis was divided into two stages. </p> <p> In the first stage of the research work, effort was concentrated to primarily understand the field of sub-channel flow pulsations and its implications from both an experimental and numerical point of view. It was noted that unsteady turbulence modeling approaches have great potential in providing insights into the fundamentals of sub-channel flow pulsations. It was proposed that for this thesis work, the Shear Stress Transport (SST) based DES model be used to understand the dynamics associated with sub-channel flow pulsations. To the author's knowledge the DES-SST based turbulence model has never been used for resolving the effects of sub-channel flow pulsations. Next, the hybrid URANS/LES turbulence modeling technique was reviewed in great detail to understand the philosophy of the hybrid URANS/LES technique and its ability to resolve fundamental flows of interest. Effort was directed to understand the switching mechanism (which blends the URANS region with the LES region) in the DES-SST model for fully wall bounded turbulent flows without boundary layer separation. To the author's knowledge, the DES-SST model has never been used on a fully wall bounded turbulent flow problem without boundary layer separation. Thus, the DES-SST model was first completely validated for a fully developed turbulent channel flow problem without boundary layer separation. </p> <p> In the second stage of the research work, the DES-SST model was used to study the flow pulsation phenomena on two rectangular sub-channels connected by a gap, on which extensive experiments were conducted by Meyer and Rehme (1994). It was found that the DES-SST model was successful in resolving significant portion of the flow field in the vicinity of the gap region. The span-wise velocity contours, velocity vector plots, and time traces of the velocity components showed the expected cross flow mixing between the sub-channels through the gap. The predicted turbulent kinetic energy showed two clear peaks at the edges of the gap. The dynamics of the flow pulsations were quantitatively described through temporal auto-correlations, spatial cross-correlations and power spectral functions. The numerical predictions were in general agreement with the experiments in terms of the quantitative aspects. From an instantaneous time scale point of view, the DES-SST model was able to identify different flow mixing patterns. The pulsating flow is basically an effect of the variation of the pressure field which is a response to the instability causing the fluid flow pulsations. Coherent structures were identified in the flow field to be comprised of eddies, shear zones and streams. Eddy structures with high vorticity and low pressure cores were found to exist near the vicinity of the gap edge region. A three dimensional vorticity field was identified and found to exist near the gap edge region. The instability mechanism and the probable cause behind the quasi-periodic fluid flow pulsations was identified and related to the inflectional stream-wise velocity profile. Simulations were also performed with two different channel lengths in comparison to the reference channel length. Different channel length studies showed similar statistical description of the flow field. However, frequency independent results were not obtained. In general, simulations performed using the DES-SST model were successful in capturing the effects of the fluid flow pulsations. This modeling technique has great potential to be used for actual rod bundle configurations. </p> / Thesis / Doctor of Philosophy (PhD)

L'étude porte sur le contrôle actif du phénomène de tremblement transsonique sur une voilure supercritique par l'utilisation de deux stratégies de contrôle : thermique et pneumatique. Des simulations numériques de l'écoulement naturel obtenues par résolution des équations de Navier-Stokes moyennées ont permis la restitution des différents régimes (avec ou sans tremblement) en bonne cohérence avec les données expérimentales. Les observations des champs moyens, des champs turbulents et des signaux de pression permettent d'établir le principe de fonctionnement et les performances des actionneurs. Le tremblement transsonique est alors soit repoussé soit supprimé autorisant ainsi l'élargissement du domaine de vol des aéronefs. L'étude paramétrique des deux actionneurs est réalisée aussi bien en mode de fonctionnement continu que rétroactif. Une campagne d'essai consolide et approfondie les résultats obtenues avec la stratégie pneumatique.

Vegetation is commonly modelled as emergent arrays of rigid, circular cylinders. However, the drag coefficient (CD) of real stems or trunks is closer to that of cylinders with a square cross-section. In this thesis, vegetation has been idealised as square cylinders in laboratory experiments with a turbulence intensity of the order of 10% which is similar to that of typical river flows. These cylinders may also represent other obstacles such as architectural structures. This research has determined CD of an isolated cylinder and cylinder pairs as a function of position as well as the average drag coefficient (CDv) of larger arrays. A strain gauge was used to measure CD whilst CDv was computed with a momentum balance which was validated by strain gauge measurements for a regularly spaced array. The velocity and turbulence intensity surrounding a pair of cylinders arranged one behind the other with respect to mean flow (in tandem) were also measured with an Acoustic Doppler Velocimeter. The isolated cylinder CD was found to be 2.11 in close agreement with other researchers. Under fixed flow conditions CD for a cylinder in a pair was found to be as low as -0.40 and as high as 3.46 depending on their relative positioning. For arrays, CDv was influenced more by the distribution of cylinders than the flow conditions over the range of conditions tested. Mean values of CDv for each array were found to be between 1.52 and 3.06. This new insight therefore suggests that CDv for vegetation in bulk may actually be much higher than the typical value of 1 which is often assumed to apply in practice. If little other information is available, a crude estimate of CDv = 2 would be reasonable for many practical applications. The validity of a 2D realizable k-epsilon turbulence model for predicting the flow around square cylinders was evaluated. The model was successful in predicting CD for an isolated cylinder. In this regard the model performed as well as Large Eddy Simulations by other authors with a significant increase in computational efficiency. However, the numerical model underestimates CD of downstream cylinders in tandem pairs and overestimates velocities in their wake. This suggests it may be necessary to expand the model to three-dimensions when attempting to simulate the flow around two or more bluff obstacles with sharp edges.

This thesis is concerned with studies on spatially developing high speed mixing layers with twin objectives: (a) to provide enhanced and detailed understanding of spatial development of two-dimensional mixing layer emanating from splitter plate through large eddy simulation (LES, from now on) technique and (b) to evolve a consistent strategy for Unsteady Reynolds Averaged Navier-Stokes (URANS) approach to mixing layer calculations. The inspiration for this work arose out of the explanations that were being developed for the reduction in the mixing layer thickness with compressibility (measured by a parameter called convective Mach number, Mc). The reasons centered around increased stability, increase in compressible dissipation that was later discounted in favor of reduction in production and pressure-strain terms (with Mc, of course). These were obtained with direct numerical simulations (DNS) or LES techniques with homogeneous shear flow or temporal mixing layer. As apart, there was also a wide held view that using RANS (steady) techniques did not capture the compressibility effects when used in a way described above and so classical industrial codes for computing mixing- layer-embedded flows are unsuitable for such applications. Other important aspects that come out of the examination of literature are: the mixing layer growth is controlled in the initial stages by the double- boundary layer profile over the splitter plate and results in the mixing layer growth that is somewhat irregular due to doubling and merging of vertical structures. The view point of a smooth growth of the mixing layer is a theo- retical approximation arising out of the use of a smooth tan-hyperbolic profile that results at larger distances from the splitter plate. For all practical applications, it is inferred that the initial development is what is important because the processes of ignition and stable combustion occur close to the splitter plate. For these reasons, it was thought that understanding the development of the mixing layer is best dealt with using accurate spatial simulation with the appropriate initial profile. The LES technique used here is drawn from an OpenFOAM approach for dissimilar gases and uses one-equation Eddy Model for SGS stresses. The temporal discretization is second order accurate backward Euler and spatial discretization is fourth order least squares; the algorithm used for solving the equations is PISO and the parallelized code uses domain decomposition approach to cover large spatial domain. The calculations are performed with boundary layer profiles over the splitter plate and an initial velocity field with white noise-like fluctuations to simulate the turbulence as in the experiments. Grid independence studies are performed and several experimental cases are considered for comparison with measured data on the velocity and temperature fields as well as turbulent statistics. These comparisons are excellent for the mean field behavior and moderately acceptable for turbulent kinetic energy and shear stress. To further benefit from the LES approach, the details of the mixing layer are calculated as a function of four independent parameters on which the growth depends: convective Mach number (Mc = (U1 -U2)/ (a1 +a2)), stream speed ratio (r = U2=U1), stream density ratio (s = p2/p1) and the average velocity of the two streams ((U1+U2)=2) and examine the various terms in the equations to enable answering the questions discussed earlier. It is uncovered that r has significant influence on the attainment of self similarity (which also implies on the rate of removal of velocity defect in the double-boundary layer profile) and other parameters have a very weak influence. The minimum velocity variation with distance from the splitter plate has the 1/paxial distance behavior like in wakes; however, after a distance, departure to linear rise occurs and the distance it takes for this to appear is delayed with Mc. Other features such as the coherent structures, their merger or break up, the area of the structures, convective velocity information extraction from the coherent structures, the behavior of the pressure field in the mixing layer through the field are elucidated in detail; the behavior of the correlations between parameters (like pressure, velocity etc) at different points is used to elucidate the coherence of their fluctuating field. The effects of the parameters on the energy spectra have expected trends. An examination of the kinetic energy budget terms reveals that • the production term is the main source of the xx turbulence stress, whereas it is not significant in the yy component. • A substantial portion of this is carried by the pressure-velocity coupling from the xx direction to the yy direction, which becomes the main source term in the yy component. • Both, the production term as well as the pressure-velocity term show a clear decrease with increase in Mc. The high point of the thesis is related to using the understanding derived from an analysis of various source terms in the kinetic energy balance to evolve an unsteady Reynolds Averaged Navier Stokes (URANS) model for calculating high speed mixing layers, a subject that has eluded international research till now. It recognizes that the key feature affected by ompressibility is related to the anisotropy of the stress tensor. The relationship between stress component (_Txy) and the velocity gradient (Sxy) as obtained from LES is set out in the form of a simple relationship accounting for the effects of other parameters obtained earlier in this thesis. A minor influence due to _Tyy is extracted by describing its dependence on Sxy again as gleaned from LES studies. The needed variation of Prandtl and Schmidt numbers through the field is extracted. While the detailed variations can in fact be taken into account in URANS simulations, a simple assumption of these values being around 0.3 is chosen for the present simulations of URANS. Introduction of these features into the momentum equation gives the much expected variation of the reduction in the growth rate of the mixing layer with convective Mach number as in experiments. The relationships that can be used in high speed mixing layers are Introduction of these features into the momentum equation gives the much expected variation of the reduction in the growth rate of the mixing layer with convective Mach number as in experiments. This is then a suggested new approach to solve high speed mixing layers. While it can be thought that the principal contributions of the thesis are complete here, an additional segment is presented related to entropy view of the mixing layer. This study that considers the mixing layer with two different species expresses various terms involved in the entropy conservation equation and obtains the contribution of various terms on the entropy change for various Mc. It is first verified that the entropy derived from the conservation equation matches with those calculated from fluid properties, entropy being a state variable. It is shown that irreversible diffusion comes down the most with convective Mach number. Left: This image shows pictorially the flow of source of turbulent stress from the axial to the cross wise turbulent stress. Production (Σ) of turbulence happens mainly in the xx direction, a part of it is carried by the pressure-velocity correlation to the yy direction, which itself has a low production. With increasing Mc, both the production as well as the pressure-velocity correlation decrease. Right: This image shows the growth rate obtained from simulations scaled with the incompressible growth rate, of LES and RANS in the background of experiments (others). As is clear, the growth rate obtained is well within the band of experimental results.

The aim of this master thesis is to characterize the fluid forces applied to a fuel assembly inthe core of a nuclear power plant in case of seism. The forces are studied with a simplifiedtwo-dimensional model constituted of an array of 3 by 3 infinite cylinders oscillating in aclosed box. The axial flow of water, which convects the heat in the core of a nuclear powerplant, is also taken into account. The velocity of the axial flow reaches 4m/s in the middle ofthe assembly and modifies the forces features when the cylinders move laterally.The seism is modeled as a lateral displacement with high amplitude (several cylinderdiameters) and low frequencies (below 20 Hz). In order to study the effects of the amplitudeand of the frequency of the displacement, the displacement taken is a sine function withboth controlled amplitude and frequency. Four degrees of freedom of the system will bestudied: the amplitude of the displacement, its frequency, the axial velocity amplitude andthe confinement (due to the closed box).The fluid forces exerted on the cylinders can be seen as a combination of three terms: anadded mass, related to the acceleration of cylinders, a drift force, related to the damping ofthe fluid and a force due to the interaction of the cylinder with residual vortices. The firsttwo components will be characterized through the Morison expansion, and their evolutionwith the variation of the degree of freedom of the system will be quantified. The effect ofthe interaction with the residual vortices will be observed in the plots of the forces vs. timebut also in the velocity and vorticity map of the fluid.The fluid forces are calculated with the CFD code Code_Saturne, which uses a second orderaccurate finite volume method. Unsteady Reynolds Averaged Navier Stokes simulations arerealized with a k-epsilon turbulence model. The Arbitrary Lagrange Euler model is used todescribe the structure displacement. The domain is meshed with hexahedra with thesoftware gmsh [1] and the flow is visualized with Paraview [2]. The modeling techniquesused for the simulations are described in the first part of this master thesis.

[ES] El principal desafío en los motores turbina de gas empleados en aviación reside en aumentar la eficiencia del ciclo termodinámico manteniendo las emisiones contaminantes por debajo de las rigurosas restricciones. Ésto ha conllevado la necesidad de diseñar nuevas estrategias de inyección/combustión que operan en puntos de operación peligrosos por su cercanía al límite inferior de apagado de llama. En este contexto, el concepto Lean Direct Injection (LDI) ha emergido como una tecnología prometedora a la hora de reducir los óxidos de nitrógeno (NOx) emitidos por las plantas propulsoras de los aviones de nueva generación. En este contexto, la presente tesis tiene como objetivos contribuir al conocimiento de los mecanismos físicos que rigen el comportamiento de un quemador LDI y proporcionar herramientas de análisis para una profunda caracterización de las complejas estructuras de flujo de turbulento generadas en el interior de la cámara de combustión. Para ello, se ha desarrollado una metodología numérica basada en CFD capaz de modelar el flujo bifásico no reactivo en el interior de un quemador LDI académico mediante enfoques de turbulencia U-RANS y LES en un marco Euleriano-Lagrangiano. La resolución numérica de este problema multi-escala se aborda mediante la descripción completa del flujo a lo largo de todos los elementos que constituyen la maqueta experimental, incluyendo su paso por el swirler y entrada a la cámara de combustión. Ésto se lleva a cabo través de dos códigos CFD que involucran dos estrategias de mallado diferentes: una basada en algoritmos de generación y refinamiento automático de la malla (AMR) a través de CONVERGE y otra técnica de mallado estático más tradicional mediante OpenFOAM. Por un lado, se ha definido una metodología para obtener una estrategia de mallado óptima mediante el uso del AMR y se han explotado sus beneficios frente a los enfoques tradicionales de malla estática. De esta forma, se ha demostrado que la aplicabilidad de las herramientas de control de malla disponibles en CONVERGE como el refinamiento fijo (fixed embedding) y el AMR son una opción muy interesante para afrontar este tipo de problemas multi-escala. Los resultados destacan una optimización del uso de los recursos computacionales y una mayor precisión en las simulaciones realizadas con la metodología presentada. Por otro lado, el uso de herramientas CFD se ha combinado con la aplicación de técnicas de descomposición modal avanzadas (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificación numérica de los principales modos acústicos en la cámara de combustión ha demostrado el potencial de estas herramientas al permitir caracterizar las estructuras de flujo coherentes generadas como consecuencia de la rotura de los vórtices (VBB) y de los chorros fuertemente torbellinados presentes en el quemador LDI. Además, la implementación de estos procedimientos matemáticos ha permitido tanto recuperar información sobre las características de la dinámica de flujo como proporcionar un enfoque sistemático para identificar los principales mecanismos que sustentan las inestabilidades en la cámara de combustión. Finalmente, la metodología validada ha sido explotada a través de un Diseño de Experimentos (DoE) para cuantificar la influencia de los factores críticos de diseño en el flujo no reactivo. De esta manera, se ha evaluado la contribución individual de algunos parámetros funcionales (el número de palas del swirler, el ángulo de dichas palas, el ancho de la cámara de combustión y la posición axial del orificio del inyector) en los patrones del campo fluido, la distribución del tamaño de gotas del combustible líquido y la aparición de inestabilidades en la cámara de combustión a través de una matriz ortogonal L9 de Taguchi. Este estudio estadístico supone un punto de partida para posteriores estudios de inyección, atomización y combus / [CA] El principal desafiament als motors turbina de gas utilitzats a la aviació resideix en augmentar l'eficiència del cicle termodinàmic mantenint les emissions contaminants per davall de les rigoroses restriccions. Aquest fet comporta la necessitat de dissenyar noves estratègies d'injecció/combustió que radiquen en punts d'operació perillosos per la seva aproximació al límit inferior d'apagat de flama. En aquest context, el concepte Lean Direct Injection (LDI) sorgeix com a eina innovadora a l'hora de reduir els òxids de nitrogen (NOx) emesos per les plantes propulsores dels avions de nova generació. Sota aquest context, aquesta tesis té com a objectius contribuir al coneixement dels mecanismes físics que regeixen el comportament d'un cremador LDI i proporcionar ferramentes d'anàlisi per a una profunda caracterització de les complexes estructures de flux turbulent generades a l'interior de la càmera de combustió. Per tal de dur-ho a terme s'ha desenvolupat una metodología numèrica basada en CFD capaç de modelar el flux bifàsic no reactiu a l'interior d'un cremador LDI acadèmic mitjançant els enfocaments de turbulència U-RANS i LES en un marc Eulerià-Lagrangià. La resolució numèrica d'aquest problema multiescala s'aborda mitjançant la resolució completa del flux al llarg de tots els elements que constitueixen la maqueta experimental, incloent el seu pas pel swirler i l'entrada a la càmera de combustió. Açò es duu a terme a través de dos codis CFD que involucren estratègies de mallat diferents: una basada en la generación automàtica de la malla i en l'algoritme de refinament adaptatiu (AMR) amb CONVERGE i l'altra que es basa en una tècnica de mallat estàtic més tradicional amb OpenFOAM. D'una banda, s'ha definit una metodologia per tal d'obtindre una estrategia de mallat òptima mitjançant l'ús de l'AMR i s'han explotat els seus beneficis front als enfocaments tradicionals de malla estàtica. D'aquesta forma, s'ha demostrat que l'aplicabilitat de les ferramente de control de malla disponibles en CONVERGE com el refinament fixe (fixed embedding) i l'AMR són una opció molt interessant per tal d'afrontar aquest tipus de problemes multiescala. Els resultats destaquen una optimització de l'ús dels recursos computacionals i una major precisió en les simulacions realitzades amb la metodologia presentada. D'altra banda, l'ús d'eines CFD s'ha combinat amb l'aplicació de tècniques de descomposició modal avançades (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificació numèrica dels principals modes acústics a la càmera de combustió ha demostrat el potencial d'aquestes ferramentes al permetre caracteritzar les estructures de flux coherents generades com a conseqüència del trencament dels vòrtex (VBB) i dels raigs fortament arremolinats presents al cremador LDI. A més, la implantació d'estos procediments matemàtics ha permès recuperar informació sobre les característiques de la dinàmica del flux i proporcionar un enfocament sistemàtic per tal d'identificar els principals mecanismes que sustenten les inestabilitats a la càmera de combustió. Finalment, la metodologia validada ha sigut explotada a traves d'un Diseny d'Experiments (DoE) per tal de quantificar la influència dels factors crítics de disseny en el flux no reactiu. D'aquesta manera, s'ha avaluat la contribución individual d'alguns paràmetres funcionals (el nombre de pales del swirler, l'angle de les pales, l'amplada de la càmera de combustió i la posició axial de l'orifici de l'injector) en els patrons del camp fluid, la distribució de la mida de gotes del combustible líquid i l'aparició d'inestabilitats en la càmera de combustió mitjançant una matriu ortogonal L9 de Taguchi. Aquest estudi estadístic és un bon punt de partida per a futurs estudis de injecció, atomització i combustió en cremadors LDI. / [EN] Aeronautical gas turbine engines present the main challenge of increasing the efficiency of the cycle while keeping the pollutant emissions below stringent restrictions. This has led to the design of new injection-combustion strategies working on more risky and problematic operating points such as those close to the lean extinction limit. In this context, the Lean Direct Injection (LDI) concept has emerged as a promising technology to reduce oxides of nitrogen (NOx) for next-generation aircraft power plants In this context, this thesis aims at contributing to the knowledge of the governing physical mechanisms within an LDI burner and to provide analysis tools for a deep characterisation of such complex flows. In order to do so, a numerical CFD methodology capable of reliably modelling the 2-phase nonreacting flow in an academic LDI burner has been developed in an Eulerian-Lagrangian framework, using the U-RANS and LES turbulence approaches. The LDI combustor taken as a reference to carry out the investigation is the laboratory-scale swirled-stabilised CORIA Spray Burner. The multi-scale problem is addressed by solving the complete inlet flow path through the swirl vanes and the combustor through two different CFD codes involving two different meshing strategies: an automatic mesh generation with adaptive mesh refinement (AMR) algorithm through CONVERGE and a more traditional static meshing technique in OpenFOAM. On the one hand, a methodology to obtain an optimal mesh strategy using AMR has been defined, and its benefits against traditional fixed mesh approaches have been exploited. In this way, the applicability of grid control tools available in CONVERGE such as fixed embedding and AMR has been demonstrated to be an interesting option to face this type of multi-scale problem. The results highlight an optimisation of the use of the computational resources and better accuracy in the simulations carried out with the presented methodology. On the other hand, the use of CFD tools has been combined with the application of systematic advanced modal decomposition techniques (i.e., Proper Orthogonal Decomposition and Dynamic Mode Decomposition). The numerical identification of the main acoustic modes in the chamber have proved their potential when studying the characteristics of the most powerful coherent flow structures of strongly swirled jets in a LDI burner undergoing vortex breakdown (VBB). Besides, the implementation of these mathematical procedures has allowed both retrieving information about the flow dynamics features and providing a systematic approach to identify the main mechanisms that sustain instabilities in the combustor. Last, this analysis has also allowed identifying some key features of swirl spray systems such as the complex pulsating, intermittent and cyclical spatial patterns related to the Precessing Vortex Core (PVC). Finally, the validated methodology is exploited through a Design of Experiments (DoE) to quantify the influence of critical design factors on the non-reacting flow. In this way, the individual contribution of some functional parameters (namely the number of swirler vanes, the swirler vane angle, the combustion chamber width and the axial position of the nozzle tip) into both the flow field pattern, the spray size distribution and the occurrence of instabilities in the combustion chamber are evaluated throughout a Taguchi's orthogonal array L9. Such a statistical study has supposed a good starting point for subsequent studies of injection, atomisation and combustion on LDI burners. / Belmar Gil, M. (2020). Computational study on the non-reacting flow in Lean Direct Injection gas turbine combustors through Eulerian-Lagrangian Large-Eddy Simulations [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/159882