Dissertationen zum Thema „CFD numerical simulation“

When designing an armour piercing kinetic energy projectile with a discarding sabot, it is important to know how the projectile is affected by the sabot during the discard. If the projectile has no active guiding systems, small disturbances to the initial flight path of the projectile can result in a significant deviance from its intended target. To investigate whether the discard process and its effect on a projectile could be simulated a CFD model was built based on a generic design for an APFSDS projectile for the Carl Gustaf M4 system built by Saab Dynamics AB. The model had to replicate the course of a sabot discard as accurately as possible and be able to track how it affects the projectile. ANSYS Fluent was used to build a model of a quarter of the design incorporating one of the four sabot petals and the part of the projectile in between two of its four fins, utilizing symmetry at zero angle of attack. To achieve a realistic discard process, fluents 6DOF solver was used allowing the sabot to discard due to aerodynamic loads rather than by following a pre-determined path. To allow the components to move inside the domain an overset interface was implemented around the sabot to allow the mesh to update itself during the simulation, keeping its quality consistent. The models initial stage includes a small gap between the sabot and projectile which is necessary to avoid contact which causes issues with mesh creation and divergence. The projectile geometry was based on rough estimates of how a fin stabilized projectile for the M4 would look whereas the sabot design was based on earlier sabot designs and iterated to achieve a discard phase with no contact. The iterative process which produced the sabot design proved that its geometry greatly affects how it discards from the projectile. To track how the projectile is affected during the discard, the forces and moments applied to the projectile from the flow including its interaction with the sabot were tracked. To compare there forces and moments for different discard courses, the sabots centre of gravity was changed to provoke it to discard differently. The model showed a clear difference in how the projectile was affected by the flow during the different discards. This shows the importance of designing the sabot to discard cleanly and quickly so as to not alter the trajectory of the projectile in flight. Also investigated was the effect of the initial gap on the course of the discard. Adding the gap is a simplification which should affect the discard so that it no longer matches a physical test case and so will have to be tuned to find what gap provides the most realistic discard. Although this model could not be validated given the lack of a physical counterpart, it could be determined that the gap greatly affects the discard and that this model is very sensitive to the initial load put on the sabot. Before the model can be used for concept development, a few things need to be worked out. These include a contact definition, inner ballistic effects and building a model that can be test fired to generate validationdata.

This thesis demonstrates the use of Computational Fluid Dynamics (CFD) for the simulation of manoeuvring helicopters. Results are presented for the problem of shipborne operations, for which a literature survey showed that little work has been carried out. The CFD solver HMB2 was first validated using available experimental data for isolated ship wakes and helicopter loads at low advance ratios. A rotorcraft flight mechanics model was then developed and integrated into HMB2. The model includes a trimming method and a linearisation routine based on finite differences. The linear model of the aircraft can be used to estimate the controls applied by the pilot during a prescribed manoeuvre via the use of the SYCOS inverse-simulation method or via an LQR auto-pilot. The methods developed in the framework of this thesis include a general multi-body grid motion and an alternative formulation for earth-fixed frame of reference in the CFD. A study of the ship/rotor wake interaction was carried out using the actuator disc method that approximated the effect of the rotor, in a steady fashion and without resolving the flow around the blades. Various positions and thrust of the rotor were tested and the flowfield obtained via coupled simulations were compared with those obtained by super-imposing isolated rotor and ship flowfields. The results show that the superposition principle is not valid and leads to flowfields that have little to do with the real flow that is dominated by the interaction of helicopter and ship airwakes. The case of a rotor hovering in close proximity to a frigate deck was reproduced with fullyresolved blades, and the results shows a significant reduction of thrust due to the flow topology behind the hangar. The Helicopter Flight Mechanics (HFM) method was tested by simulating the aircraft response to a collective pilot input, using simplified models and coupled with CFD. Then, the coupled HFM/HMB2 method was used, in conjunction with the LQR auto-pilot, to simulate the phase of landing of a Sea King helicopter. Simulations were carried-out in free-air and above the frigate deck and the specified trajectories were followed adequately. Results for the ship landing show that the wake of the ship alters the obtained landing trajectory and that the current method captures some of the effects of the wake interaction.

A numerical investigation of the performance of two candidate designs for injection into supersonic flow, including a comparison of two renormalized group theory (RNG) based k-epsilon turbulence models with a more conventional k-epsilon model. The chosen designs were an unswept ramp injector with four injection ports and a novel nine-hole injector array. The objectives of the investigation were to provide reliable computational solutions to the flowfields in question using both RNG and standard k-epsilon turbulence models and to compare the solutions to experiment, thereby to judge the relative performance of the turbulence models. A second objective of the investigation was to use the computed data to provide design insights for the nine-hole injector array. This investigation made use of GASP(tm) version 2.2, a commercial computational fluid dynamics code that was augmented by the addition of one RNG-based k-epsilon turbulence model derived by Zhou, et. al. and one variant of Zhou's model, which was derived by the author. Mesh sequencing studies were performed to measure solution quality, with the fine mesh for the injector array containing roughly one million grid nodes and the fine mesh for the ramp injector containing more than six million grid nodes. Results of these studies indicated that the injector-array solution was significantly under-resolved in the farfield, though the quality was better in the vicinity of the injector itself. The ramp-injector solution, while not perfectly grid-resolved, showed much better grid convergence in both the nearfield and farfield. Accordingly, comparison with experiment was better for the ramp injector than for the injector array. For both injectors, the differences between solutions generated with RNG-based k-epsilon and standard k-epsilon turbulence models were negligibly small." Despite inadequate grid resolution in the farfield, the computational investigation of the nine-hole injector array did yield several important design insights. Particularly, the significance to mixing and losses of the placement of the outer injectors of the second and third rows was determined.
Ph. D.

Ambulance hall is a significant station during emergency treatment. Patients need to be transferred from ambulance cars to the hospital’s building in the hall. Eligible performance of ventilation system to supply satisfied thermal comfort and healthy indoor air quality is very important. Computational fluid dynamic (CFD) simulation as a broadly applied technology for predicting fluid flow distribution has been implemented in this project. There has two objectives for the project. The first objective is to make comparison between the three CFD software which consists of ANSYS Fluent, Star-CCM+ and IESVE Mcroflo according to CFD modeling of the baseline model. And the second objective is to build CFD modeling for cases with difference boundary conditions to verify the designed ventilation system performance of the ambulance hall. In terms of simulation results from the three baseline models, ANSYS Fluent is conclusively recommended for CFD modeling of complicated indoor fluid environment compared with Star-CCM+ and IESVE Microflo. Regarding to the second objective, simulation results of case 2 and case 3 have shown the designed ventilation system for the ambulance hall satisfied thermal comfort level which regulated by ASHRAE standard with closed gates. Nevertheless, threshold limit value of the contaminants concentration which regulated by ASHRAE IAQ Standard cannot be achieved. From simulation results of case 4.1 to 4.3 shown that the designed ventilation system cannot satisfy indoor thermal comfort level when the gates of the ambulance hall opened in winter. In conclusion, measures for decreasing contaminants concentration and increasing indoor air temperature demanded to be considered in further design.

Crude oils are complex mixtures of hydrocarbons. Among these components, high molecular weight paraffin (waxes) and asphaltenes may cause severe flow assurance issues in production and transportation subsea pipeline systems. Wax molecules are dissolved in the crude oil, when the temperature decreases, the solubility of high molecular weight paraffins decreases rapidly, and this results in the formation of gels of complex morphology, that deposit on the cold walls of the subsea pipelines during the flow of waxy crude oils, and this is a big problem for the subsea pipelines. The cost of wax management is enormous and rapidly increasing because of increased oil production in deep-sea areas. In this work, is presented a new numerical model for predicting wax deposition in oil pipes. While the almost part of the models existing in the literature are based on a separated two-fluid description of the oil/wax-oil flow, the model proposed is based on a mixture description, where wax is considered as a suspended phase surrounded by crude oil. When wax separates in the crude oil, the mixture becomes a gel-like fluid, and this mechanism is incorporated in the flow model by a specific rheological model where the mixture viscosity depends on both the local wax concentration and its “age”. In order to simultaneously model moving regions of different “ages”, our model solves a transport equation for the ageing time. Deposition is modelled by solving wax mass balance, dissolved asphaltenes mass balance, and energy. Wax generation and asphaltenes removal are modelled as a first-order reaction coupling mass balance between the species, where the local equilibrium saturation is given as a function of the mixture temperature. In this work, is present the implementation of the model in the open-source C++ library OpenFOAM and validation of the model between some similar works. Besides a discussion of the results for wax deposition and a parametric study about the critical parameters.

Current research in experimental and computational fluid dynamics is focused in the area of flow control. Flow control devices are usually classified as either passive or active. Plasma actuators are active flow control devices that require input from an external power source. Current efforts have modeled the effects of plasma actuators as a body force near the electrode. The research presented herein focuses on modeling the fluid-plasma interaction seen in dielectric barrier discharge plasma actuators as a body force vector in the region above the embedded electrode using computational fluid dynamics (CFD). This body force is modeled as the product of the gradient of the potential due to the electric field and the net charge density. In a passive flow control study, two-dimensional simulations using CFD are done with a smooth and bumpy Eppler 398 airfoil with laminar, transition, and turbulent models in an effort to improve the understanding of the flow over bumpy airfoils and to quantify the advantages or disadvantages of the bumps.

Extreme hydrological events associated with global warming are likely to produce an increasing number of flooding scenarios resulting in significant bridge inundation and associated damages. During large floods, the presence of a bridge in an open channel triggers a highly turbulent flow field including 3D complex coherent structures around bridge structures. These turbulence structures are highly energetic and possess high sediment entrainment capacity which increases scouring around the bridge foundation and consequently lead to structural stability problems or even failure of the structure. Hence, understanding the complex turbulent flow field for these extreme flow conditions is crucial to estimate the failure risks for existing bridges and better design of future bridges. This research employs the method Large Eddy Simulation (LES) to predict accurately the 3D turbulent flow around bridge structures. The LES code is refined with a novel free surface algorithm based on the Level Set Method (LSM) to determine the complex water surface profiles. The code is used to analyze the hydrodynamics of compound channel flow with deep and shallow overbanks, free flow around a bridge abutment, pressure flow with a partially submerged bridge deck and bridge overtopping flow. All simulations are validated with data from complementary physical model tests under analogous geometrical and flow conditions. Primary velocity, bed shear stress, turbulence characteristics and 3D coherent flow structures are examined thoroughly for each of the flow cases to explain the hydrodynamics of these complex turbulent flows.

Work here presented is the result of basic research in key aspects of the currently available engineering tools and methodologies for the design, optimisation and development of thermal systems and equipment: turbulence modelling, high performance computing and quality tests and procedures so as to assess credibility to the numerical solutions (verification and validation).

The thesis comprises six main chapters written in a paper format. Two of them have already been published in international journals, one in the proceedings of a Spanish conference and two in proceedings of international conferences on Computational Fluid Dynamics and heat transfer. The last chapter has recently been submitted for publication to an international journal. Therefore, all the chapters are written so as to be self-contained, complete and concise. As a consequence, some contents of the chapters such those describing the governing equations, or the verification procedure used to assess the credibility of the numerical solutions, are repeated in several of them. Furthermore, as only minor changes have been introduced in the chapters respect to the original papers, each of them reflects the know-how of the CTTC (Heat and Mass Transfer Technological Centre were the research has been carried out) when they were published.

Papers presented in chapters 1 and 2 deal with turbulence modelling. A general overview is given on the formulation and numerical techniques of the different levels of turbulence modelling: Direct Numerical Simulation (DNS), Large Eddy Simulation (LES) and Reynolds Averaged Navier-Stokes Simulation (RANS). Main attention is focussed on the eddy viscosity two-equation RANS models. Their formulation is presented in more detail, and numerical solutions of the most extended. Benchmark problems on turbulence modelling are given compared to the available experimental data.

Chapters 3 and 4 focus on the use of the multiblock method (domain decomposition method), as a numerical technique that combined with the parallel computing may allow reducing the demanding computational time and memory (high performance computing). The multiblock approach used is based on the conservation of all the physical quantities (fully conservative method) and on an explicit information exchange between the different blocks of the domain. The goal of the work presented in these two chapters is to verify that such a multiblock approach does not introduce additional uncertainty in the numerical solutions.

Chapter 5 presents a tool that has been developed at the CTTC for the verification of finite volume computations. In fact, this tool is also partially used and described in the results presented in the previous chapters. Here, it is described and discussed in detail and it is applied to a set of different CFD and heat transfer problems in two and three dimensions, with free and forced convection, with reactive and non-reactive flows and with laminar and turbulent flows.

The last chapter shows a complete study for the development of a credible heat transfer relation for the heat evacuated from a ventilation channel. Such study comprises all the different steps that have to be accomplished so as to develop credible and applicable results in mechanical engineering. It comprises a description of the mathematical model to represent the physical phenomena in the channel, the numerical model to solve the set of coupled differential equations of the mathematical model, the construction and testing of an ad-hoc experimental set-up, and a verification and validation (V&V) test that guarantees that the numerical solution is an accurate enough approximation of the mathematical model (verification), and that it properly predicts the reality (validation).

Phase-contrast magnetic resonance imaging (PC-MRI) is a powerful tool for measuring blood flow and has a wide range of cardiovascular applications. Simulation of PC-MRI from numerical flow data would be useful for addressing the data quality of PC-MRI measurements and to study and understand different artifacts. It would also make it possible to optimize imaging parameters prior to the PC-MRI measurements and to evaluate different methods for measuring wall shear stress.

Based on previous studies a PC-MRI simulation tool was developed. An Eulerian-Lagrangian approach was used to solve the problem. Computational fluid dynamics (CFD) data calculated on a fix structured mesh (Eulerian point of view) were used as input. From the CFD data spin particle trajectories were computed. The magnetization of the spin particle is then evaluated as the particle travels along its trajectory (Lagrangian point of view).

The simulated PC-MRI data were evaluated by comparison with PC-MRI measurements on an in vitro phantom. Results indicate that the PC-MRI simulation tool functions well. However, further development is required to include some of the artifacts. Decreasing the computation time will make more accurate and powerful simulations possible. Several suggestions for improvements are presented in this report.

Numerical simulation of noise production and propagation is a very complex problem. A methodology fitting for one particular problem can fail for another one. So there are no general guidelines on how to deal with such phenomena. In the present work, noise propagated in non-uniform mean-flow is considered. For most cases, in the propagation field, there is a rather significant region where the mean flow is not uniform, but the sound production is negligible compared to the noise emitted by the source region. In this

nearfield, a linear set of propagation equations may be considered (LEE). For such problems, the following simulation methodology is proposed:

1. Incompressible/compressible LES simulation in the source region.

2. Linearized Euler Equations to propagate the noise through the nonlinear mean flow.

3. Kirchhoff method in the farfield, if necessary.

This thesis deals with the second item of this system (LEE), including interfacing with the other two steps.
Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

This thesis presents the results from the computational fluid dynamics simulations of surface effect ship model tests. The model tests being simulated are of a generic T-Craft model running in calm seas through a range of Froude numbers and in two head seas cases with regular waves. Simulations were created using CD-adapcoâ s STAR-CCM+ and feature incompressible water, compressible air, pitch and heave degrees of freedom, and the volume of fluid interface-capturing scheme. The seals are represented with rigid approximations and the air cushion fans are modeled using constant momentum sources. Drag data, cushion pressure data, and free surface elevation contours are presented for the calm seas cases while drag, pressure, heave, and roll data are presented for the head seas cases. The calm seas cases are modeled both with no viscosity and with viscosity and turbulence. All simulations returned rather accurate estimations of the free surface response, ship motions, and body forces. The largest source of error is believed to be due to the rigid seal approximations. While the wakeâ s amplitude is smaller when viscosity is neglected, both viscous and inviscid simulationsâ estimations of the free surface qualitatively match video footage from the model tests. It was found that shear drag accounts for about a quarter of the total drag in the model test simulations with viscosity, which is a large source of error in inviscid simulations. Adding the shear drag calculated using the ITTC-1957 friction coefficient line to the total drag from the inviscid simulation gives the total drag from the viscous simulations within a 6% difference.
Master of Science

Le celle a combustibile ad ossido solido (Solid Oxide Fuel Cells - SOFCs) sono una tecnologia promettente in grado di produrre potenza elettrica e termica con un’efficienza eccezionale. Tuttavia, è necessaria una comprensione più approfondita dei processi fisico-chimici che si verificano all’interno della cella per risolvere la maggior parte dei problemi di degradazione che attualmente impediscono la maturità della tecnologia. Un approccio sistematico e sinergico tra misure sperimentali, l’uso di strumenti di analisi e techniche innovative – con particolare attenzione alla deconvoluzione degli spettri di impedenza elettrochimica (Electrochemical Impedance Spectroscopy - EIS) mediante il metodo della distribuzione dei tempi di rilassamento (Distributed Relaxation Times - DRT) – e teoria modellistica ha dimostrato di essere importante per la stima dei parametri che descrivono le caratteristiche microstrutturali ed elettrochimiche di due tipi di SOFC planari anodo-supportate, una progettata per funzionare ad una temperatura intermedia (750ºC) e l’altra per farlo a bassa temperatura (650ºC). Un macro-modello CFD (Computational Fluid Dynamics) dei campioni testati, che incorpora i parametri ottenuti dalla procedura menzionata, è stato convalidato confrontando le curve di polarizzazione simulate con quelle sperimentali. Questo modello ha dimostrato di essere un valido strumento per ottimizzare la microstruttura delle celle e per stabilire le basi per analizzare gli effetti di potenziali fenomeni di degrado nella cella e, infine, prevedere la generazione di elettricità a lungo termine in condizioni di funzionamento predeterminate. Inoltre, un modello CFD di una cella di tipo tubolare all’interno di un generatore di potenza (cioè, stack SOFC) di 500 Wel ha permesso di apprezzare come un singolo elemento dello stack si comporta in condizioni operative quasi realistiche.
Solid oxide fuel cell (SOFC) is a promising electrochemical technology that can produce electrical and thermal power with outstanding efficiencies, however, a more profound understanding of the physicochemical processes occurring within the cell is necessary to overcome most of the degradation issues currently impeding the maturity of the technology. A systematic synergetic approach between experimental measurements, the use of novel analysis tools and techniques – with special attention to the deconvolution of electrochemical impedance spectroscopy (EIS) spectra by means of the distribution of relaxation times (DRT) method – and modelling theory has proved to be instrumental for the estimation of parameters describing the microstructural and electrochemical properties of two types of planar anode-supported SOFCs, one designed to operate at intermediate temperatures (750ºC) and the other at low temperatures (650ºC). A comprehensive macro-scale computational fluid dynamics (CFD) model of the tested samples incorporating the aforementioned parameters has been validated by confronting the simulated polarization curves with the experimental ones. This model has demonstrated to be a compelling tool to optimize the microstructure of the cells whilst establishing the bases to monitor and analyse the effects of potential degradation phenomena in the cell and predict the electrical output of the cell in the long run under pre-determined operating conditions. Additionally, a CFD model of a tubular-type cell comprised in the power module (i.e. SOFC stack) of a characterised 500Wel power generator has enabled to appreciate how a singular element of the stack behaves under nearly realistic operating conditions.

Laser metal deposition is one of the most versatile methods in the expanding field of additive manufacturing. Its outstanding advantage is its capability to process a variety of metallic materials for the freeform fabrication of objects having sound mechanical properties. The process is used in applications of rapid manufacturing, components repair and surface coating. During recent years, modelling has been increasingly used to study and improve the laser metal deposition process. However, most models have focused on analysing individual stages of the deposition process and thus have not thoroughly dealt with the occurrence of mutually-influencing phenomena. This work presents a new numerical model that, starting from the simulation of powder particles in the deposition head, integrates the important phenomena and interactions that govern the dynamics of a powder stream and a deposition melt pool, within a single model for the first time.The resulting model is comprehensive enough to allow the prediction of the morphology of deposited tracks and structures and the heat flows during their creation; as well as the flexibility to simulate, in principle, any deposition shape. The model has been demonstrated using the settings of an actual laser metal deposition system, and has been applied to study clad formation in the deposition of single tracks, layers, walls and simple three-dimensional structures. Moreover, the model has been used to study the formation of irregularities and excessive mass deposition. A new sensor-less deposition control technique based on the simulation and testing of different deposition strategies prior to actual deposition, is proposed. As a demonstration of this control technique, the model has also been used to study the case where excessive deposition develops at intersecting or cornered tracks. Improved deposition strategies have been tested using the model and applied to real deposits. A two fold improvement in layer height control has been achieved in the case of cornered layers.The outcome of the work presented in this thesis can be applied in further studies and prediction of laser deposited shapes for real applications. Furthermore, it can be potentially used for improvement of the laser metal deposition technology through the simulation of deposition strategies prior to actual processing.

[ES] La comprensión de los fenómenos físicos que acontecen en la región densa (también conocida como campo cercano) durante la atomización de los sprays ha sido una de las mayores incógnitas a la hora de estudiar sus aplicaciones. En el sector industrial, el rango de interés abarca desde toberas en aplicaciones propulsivas a sprays en aplicaciones médicas, agrícolas o culinarias. Esta evidente falta de conocimiento obliga a realizar simplificaciones en la modelización, provocando resultados poco precisos y la necesidad de grandes caracterizaciones experimentales en la fase de diseño. De esta manera, los procesos de rotura del spray y atomización primaria se consideran problemas físicos fundamentales, cuya complejidad viene dada como resultado de un flujo multifásico en un régimen altamente turbulento, originando escenarios caóticos. El análisis de este problema es extremadamente complejo debido a la ausencia sustancial de teorías validadas referentes a los fenómenos físicos involucrados como son la turbulencia y la atomización. Además, la combinación de la naturaleza multifásica del flujo y su comportamiento turbulento resultan en una gran dificultad para afrontar el problema. Durante los últimos 10 años, las técnicas experimentales han sido finalmente capaces de visualizar la región densa, pero la confianza, análisis y efectividad de dichos experimentos en esta región del spray todavía requiere de mejoras sustanciales. En este contexto, esta tesis trata de contribuir al entendimiento de estos procesos físicos y de proporcionar herramientas de análisis para estos flujos tan complejos. Para ello, mediante Direct Numerical Simulations se ha afrontado el problema resolviendo las escalas de movimiento más pequeñas, y capturando todas las escalas de turbulencia y eventos de rotura. Uno de los objetivos de la tesis ha sido evaluar la influencia de las condiciones de contorno del flujo entrante en la atomización primaria y en el comportamiento turbulento del spray. Para ello, se han empleado dos condiciones de contorno diferentes. En primer lugar se ha empleado una condición de contorno sintética para producir turbulencia homogenea a la entrada, simulando el comporamiento de la tobera. Una de las características más interesantes de este método es la posibilidad de retocar los parámetros dentro del algoritmo. En particular, la escala de longitud integral se ha variado para evaluar la influencia de las estructuras mas grandes de la tobera en la atomización primaria. El análisis de la condición de contorno sintética también ha permitido el diseño óptimo de simulaciones de las cuales se han derivado estadísticas turbulentas significativas. En este escenario, se han llevado a cabo estudios más profundos sobre la influencia de propiedades de las estructuras turbulentas como la homogeneidad y la anisotropía tanto en el espectro de los flujos como en las estadísticas de las gotas. Para tal fin, se han desarrollado metodologías novedosas para computar el análisis espectral y la estadística de las gotas Entre los resultados de este análisis destaca la independencia de la condición de contorno de entrada en las estadísticas de las gotas, mientras que por otra parte, recalca que las características turbulentas desarrolladas en el interior de la tobera afectan a la cantidad total de masa atomizada. Estas consideraciones se encuentran respaldadas por el análisis espectral realizado, mediante el cuál se concluye que la turbulencia multifásica comparte el comportamiento universal descrito por las teorías de Kolmogorov.
[CAT] La comprensió dels fenòmens físics que succeïxen en la regió densa (també coneguda com a camp pròxim) durant l'atomització dels sprays ha sigut una de les majors incògnites a l'hora d'estudiar les seues aplicacions. En el sector industrial, el rang d'interés comprén des de toveres en aplicacions propulsives a sprays en aplicacions mèdiques, agrícoles o culinàries. Esta evident falta de coneixement obliga a realitzar simplificacions en la modelització, provocant resultats poc precisos i la necessitat de grans caracteritzacions experimentals en la fase de disseny. D'esta manera, els processos de ruptura del spray i atomització primària es consideren problemes físics fonamentals, la complexitat dels quals ve donada com resultat d'un flux multifàsic en un règim altament turbulent, originant escenaris caòtics. L'anàlisi d'este problema és extremadament complex a causa de l'absència substancial de teories validades dels fenòmens físics involucrats com són la turbulència i l'atomització. A més, la combinació de la naturalesa multifàsica del flux i el seu comportament turbulent resulten en una gran dificultat per a afrontar el problema. Durant els últims 10 anys les tècniques experimentals han sigut finalment capaces de visualitzar la regió densa, però la confiança, anàlisi i efectivitat dels experiments en esta regió del spray encara requerix de millores substancials. En este context, esta tesi tracta de contribuir en l'enteniment d'estos processos físics i de proporcionar ferramentes d'anàlisi per a estos fluxos tan complexos. Per a això, per mitjà de Direct Numerical Simulations s'ha afrontat el problema resolent les escales de moviment més menudes, al mateix temps que es capturen totes les escales de turbulència i esdeveniments de ruptura. Un dels objectius de la tesi ha sigut avaluar la influència que les condicions de contorn del flux entrant tenen en l'atomització primària i en el comportament turbulent del spray. Per a això, s'han empleat dos condicions de contorn diferents. En primer lloc s'ha empleat una condició de contorn sintètica per a produir turbulència homogènia a l'entrada, simulant el comportament de la tovera. Una de les característiques més interessants d'este mètod és la possibilitat de retocar els paràmetres dins de l'algoritme. En particular, l'escala de longitud integral s'ha variat per a avaluar la influència de les estructures mes grans de la tovera en l'atomització primària. L'anàlisi de la condició de contorn sintètica també ha permés el disseny òptim de simulacions de les quals s'han derivat estadístiques turbulentes significatives. En este escenari, s'han dut a terme estudis més profunds sobre la influència de propietats de les estructures turbulentes com l'homogeneïtat i l'anisotropia tant en l'espectre dels fluxos com en les estadístiques de les gotes. Per a tal fi, s'han desenrotllat metodologies noves per a computar l'anàlisi espectral i l'estadística de les gotes. Entre els resultats d'esta anàlisi destaca la independència de la condició de contorn d'entrada en les estadístiques de les gotes, mentres que d'altra banda, es recalca que les característiques turbulentes desenrotllades en l'interior de la tovera afecten a la quantitat total de massa atomitzada. Estes consideracions es troben recolzades per l'anàlisi espectral realitzat, per mitjà del qual es conclou que la turbulència multifásica compartix el comportament universal descrit per les teories de Kolmogorov.
[EN] The understanding of the physical phenomena occurring in the dense region (also known as near field) of atomizing sprays has been long seen as one of the biggest unknown when studying sprays applications. The industrial range of interest goes from nozzles in combustion and propulsion applications to medical sprays, agricultural and food process applications. This substantial lack of knowledge is responsible for some important simplification in modeling, that often result to be inaccurate or simply partial, leading to the evident need of large experimental characterization during the design phase. In fact, the spray breakup and primary atomization processes are indeed fundamental problems of physics, which complexity results from the combination of a multiphase flow in a highly turbulent regime that leads to chaotic scenarios. The analysis of this problem is extremely problematic, due to a substantial lack of definitive theories about the physical phenomena involved, namely turbulence and atomization. Furthermore, the combination of the multiphase nature of the flow and its turbulent behavior makes substantially difficult to address the problem. Only within the last 10 years, experimental techniques have been capable of visualizing the dense region, but the experiments reliability, analysis and effectiveness in this region still requires vast improvements. In this scenario, this thesis aims to contribute in the understanding of these physical process and to provide analysis tools for these complex flows. In order to do so, Direct Numerical Simulations have been used for addressing the problem at its smallest scale of motion, while reliably capturing all turbulence scales and breakup events. The multiphase nature of the flow is accounted for by using the Volume of Fluid method. One of the goal of the thesis was to assess the influence of the inflow boundary conditions on the primary atomization and on the spray's turbulence behavior. In order to do so, two different boundary conditions were used. In a first place, a synthetic inflow boundary condition was used in order to produce a homogeneous turbulence inflow, simulating the nozzle behavior. One of the interesting features of this method was the possibility of tweaking the parameters within the algorithm. In particular, the integral length scale was varied in order to assess the influence of nozzle larger turbulent structures on the primary atomization. The analysis on the synthetic boundary condition also allowed to optimally design simulations from which derive meaningful turbulence statistics. On this framework, further studies were carried over on the influence of turbulent structures properties, namely homogeneity and anisotropy, on both the flows spectra and droplets statistics. In order to achieve this goal, novel procedures for both computing the flow spectra and analyzing droplets were developed and are carefully addressed in the thesis. The results of the analysis highlight the independence of droplets statistics from the inflow boundary condition, while, on the other hand, remarking how the total quantity of atomized mass is significantly affected by the turbulence features developed within the nozzle. This considerations are supported by the spectrum analysis performed, which also highlighted how multiphase turbulence shares the universal features described in Kolmogorov theories.
Crialesi Esposito, M. (2019). Analysis of primary atomization in sprays using Direct Numerical Simulation [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/133975
TESIS

Ce travail de thèse présente la modélisation numérique CFD du comportement thermique cutané humain en cryothérapie corps entier (CCE), un traitement par le froid qui consiste à exposer l'ensemble du corps à des températures très basses (-110°C) pendant 3 à 4 minutes. L'objectif est d'optimiser les protocoles de cryothérapie spécifiques et propres à chaque personne. Des approches expérimentales (cartographies cutanées de températures par thermographies infrarouges) et des mesures de températures réelles dans la chambre principale de cryothérapie sont réalisées pour alimenter lesconditions aux limites d’entrée des simulations d’une part et de valider les résultats numériques d’autre part. Un modèle mathématique a été établi permettant de prédire le refroidissement de la température cutanée dans différentes zones du corps au cours d'une séance de cryothérapie corps entier CCE pour différentes populations et morphotypes.Des simulations numériques ont été réalisées pour étudier le comportement thermo-aéraulique de la chambre de cryothérapie vide et occupée. Ces simulations ont montré que la présence d'une personne perturbe les champs thermiques et aérauliques dans la chambre pendant la séance de CCE, entraînant une augmentation de la température de la chambre due à la dissipation de chaleur par la personne. Ce phénomène devient plus remarquable lorsque le nombre de sujets augmente. D'autres simulations ont été réalisées pour étudier la cinétique de refroidissement cutané pour différents morphotypes pendant une séance de CCE. Les modèles numériques ont été validés par les résultats expérimentaux, démontrant ainsi une bonne concordance entre les deux résultats
This thesis presents the CFD modelling of human cutaneous thermal behaviour in whole-body cryotherapy (WBC), a cold treatment that involves exposing the entire body to very low temperatures (-110°C) for 3 to 4 minutes. The objective is to optimize cryotherapy protocols that are specific and tailored to everyone. Experimental approaches (cutaneoustemperature mapping by infrared thermography) and measurements of actual temperatures in the main cryotherapy chamber are carried out to provide input boundary conditions for simulations, on the one hand, and to validate numerical results on the other hand. A mathematical model has been developed to predict the cooling of skin temperature in different body areas during a whole-body cryotherapy (WBC) session for various populations and morphotypes.Numerical simulations were conducted to determine the thermo-aerodynamic behaviour of both the empty and occupied cryotherapy chamber. These simulations revealed that the presence of a person disrupts the thermal and airflow fields within the chamber during the WBC session, leading to an increase in chamber temperature due to heat dissipation bythe person. This phenomenon becomes more pronounced as the number of subjects increases. Further simulations were carried out to investigate the cutaneous cooling kinetics for different morphotypes during WBC sessions. The numericalmodels were validated against experimental results, demonstrating a good agreement between the two results

Thesis (Ph. D.)--Aerospace Engineering, Georgia Institute of Technology, 2006.
Yedidia Neumeier, Committee Member ; Jerry Seitzman, Committee Member ; Fotis Sotiropoulos, Committee Member ; Tim Lieuwen, Committee Member ; suresh menon, Committee Chair.

Le contrôle précis des modèles géométriques joue un rôle important dans de nombreux domaines. Pour l’optimisation de forme en CFD, le choix des paramètres de contrôle et la technique de déformation de forme est critique. Nous proposons un modeleur paramétrique avec une nouvelle méthode de déformation d’objets, ayant pour objectif d’être intégré dans une boucle d’optimisation automatique de forme avec un solveur CFD. Notre méthodologie est basée sur une double paramétrisation des objets : géométrique et architecturale. L’approche géométrique consiste à décrire les formes par un squelette, composé d’une famille de courbes B-Splines, appelées courbes génératrice et courbes de section. Le squelette est paramétré avec une approche architecturale. Au lieu d’utiliser les points de contrôle de la représentation classique par courbes B-Splines, la géométrie est contrôlée par ces paramètres architecturaux. Cela permet de réduire considérablement le nombre de degrés de liberté utilisés dans le problème d’optimisation de forme, et permet de maintenir une description haut niveau des objets. Notre technique intègre un contrôle de forme et un contrôle de régularité, permettant d’assurer la génération de nouvelles formes valides et réalistes. Les déformations de la géométrie sont réalisées en posant un problème inverse : déterminer une géométrie correspondant à un jeu de paramètres cibles. Enfin, une technique de reconstruction de surface est proposée. Nous illustrons le modeleur paramétrique développé et intégré dans une boucle d’optimisation automatique de forme sur trois cas : un profil d’aile d’avion, un foil AC45 d’un voilier de course et un bulbe de chalutier de pêche
The precise control of geometric models plays an important role in many domains. For shape optimisation in CFD, the choice of control parameters and the way to deform a shape are critical. In this thesis, we propose a new approach to shape deformation for parametric modellers with the purpose of being integrated into an automatic shape optimisation loop with a CFD solver. Our methodology is based on a twofold parameterisation: geometrical and architectural. The geometrical approach consist of a skeleton-based representation of object. The skeleton is made of a family of B-Spline curves, called generating curve and section curves. The skeleton is parametrised with an architectural approach: meaningful design parameters are chosen on the studied object. Thus, instead of using the control points of a classical B-spline representation, we control the geometry in terms of architectural parameters. This reduce the number of degrees of freedom and maintain a high level description of shapes. We ensure to generate valid shapes with a strong shape consistency control based on architectural considerations. Deformations of the geometry are performed by solving optimisation problems on the skeleton. Finally, a surface reconstruction method is proposed to evaluate the shape’s performances with CFD solvers. We illustrate the parametric modeller capabilities on three problems, performed with an automatic shape optimisation loop: the wind section of an plane (airfoil), the foil of an AC45 racing sail boat and the bulbous bow of a fishing trawler

Performance of a refrigerator can be characterized with its ability to maintain a preset low temperature by spending the least amount of electricity. It is important to understand natural convection inside a refrigerator for optimizing its design for performance. Computational Fluid Dynamics (CFD) together with experiments is a very powerful tool for visualizing flow and temperature fields that are essential for understanding a phenomenon that involves both fluid and heat flow. In this aspect, simulations are performed for compartment and total refrigerator models using the package program Fluent which is based on finite volume method. An experimental study is performed to determine the constant wall temperature boundary conditions for the numerical models. Effect of radiation is also investigated by comparing the numerical study of a different full refrigerator model with a similar one in literature. While evaluating the radiation effect, convection boundary condition is selected by defining overall heat transfer coefficient between the ambient room air at a constant temperature and the inner surfaces of the walls. Based on assumptions, related heat transfer analyses are done using compartment and total refrigerator model analyses. Performing CFD simulations of a refrigerator cabinet for visualizing the flow and temperature fields which is the aim of the study is achieved and some observations that can be useful in design optimization are made.

Desalination has been increasingly adopted over the last decades as an option, and sometimes as a necessity to overcome water shortages in many areas around the world. Today, several thermal and physical separation technologies are well established in large scale production for domestic and industrial purposes.  Membrane distillation is a novel thermally-driven process that can be adapted effectively for water desalination or water treatment in industrial applications, due to its potential lower energy consumption and simplicity. The general objective of this thesis is to contribute to the technical understanding of membrane distillation as a new technology in water treatment for both industrial and drinking water purposes, as a starting point for further improvement. The thesis includes experimental and numerical investigations that highlight some aspects of the technology application and fundamental aspects. In the field of industrial application, an experimental and numerical assessment has been carried out on an Air Gap Membrane Distillation (AGMD) prototype to assess the utilization of the technology in thermal cogeneration plants; in particular, demineralization of water boiler feed water and treating flue gas condensate. The main assessment parameters were water quality and energy consumption. The results from full-scale simulations of a system of 10 m3/hr production capacity,  connected to the district heating network were as follows: 5 to 12 kWh/m3 specific thermal energy consumption, and  0,6 to 1,5 kWh/m3 specific electricity consumption, depending upon the heat source (district heat supply line or low-grade steam). For desalination applications, experimental and simulation work was conducted on an AGMD semi-commercial system as part of the EU MEDESOL project. The aim was to evaluate AGMD performance with saline water of 35 g/l NaCl in order to establish an operation data base for simulation of a three-stage AGMD desalination system. Specific thermal energy consumption was calculated as 950 kWht/m3 for a layout without heat recovery, and 850 kWht/m3 for a layout with one stage heat recovery.  The lack of internal heat recovery in the current MD module means that most of the heat supplied to MD system was not utilized efficiently, so the thermal energy consumption is high. This would mean that a large solar field is needed. In order to analyze the flow conditions in feed flow and cooling channels, CFD was used as tool to analyze a spacer-obstructed flow channel for different types of spacer geometrical characteristics: flow of attack angle, spacer to channel thickness ratio, and void ratio. Velocity profiles, shear stress, and pressure drop were the main assessment criteria. Results show the flow of attack angle has a very minimum effect on the performance of spacers. The effect of spacer to channel thickness ratio was significant in all assessment parameters. Higher void ratios were found advantageous in promoting flow mixing, but resulted in lower sheer stress and hence reduced heat transfer. Physical modifications were implemented on a semi-commercial AGMD prototype to assess experimentally any improvement in its performance. These modifications were mainly focused on reducing the conductive heat transfer losses by modifying the physical support in the air gap that separates the membrane from the condensation surface. In addition, several feed channel spacers were tested and assessed based on their effect in increasing the mass transfer while maintaining or reducing pressure drop. The modifications yielded a two-fold augmentation: slight increase in the distillate mass flow rate (9-11%), and increased thermal efficiency (6%). The pressure drop in the module was reduced by 50% through selecting the appropriate spacer that would achieve the above mass flow rate increase.
QC 20111021

La cavitation est l'un des phénomènes physiques les plus contraignants influençant les performances des machines hydrauliques. Il est donc primordial de savoir prédire son apparition et son développement, et de quantifier les pertes de performances qui lui sont associées. L'objectif de ce travail est de développer un algorithme 3D instationnaire pour la simulation numérique de la cavitation dans le code industriel « Code_Saturne ». Il est basé sur la méthode à pas fractionnaires et préserve le principe du minimum/maximum sur le taux de vide. Un solveur implicite, basé sur l'équation de transport du taux de vide couplée avec les équations Navier-Stokes est proposé. Un traitement numérique spécifique des termes sources de cavitation permet d'obtenir des valeurs physiques du taux de vide (entre 0 et 1) sans aucune limitation artificielle. L'influence des modèles de turbulence RANS sur la simulation de la cavitation est étudiée sur deux types de géométries 2D (Venturi et Hydrofoil). Cela confirme que la modification de Reboud et al. (1998) appliquée aux modèles à viscosité turbulente à deux équations, k-epsilon et k-omega-SST, permet de reproduire les principales caractéristiques du comportement instationnaire de la poche de cavitation. Le modèle du second ordre RSM-SSG, basé sur le transport des contraintes de Reynolds, se révèle capable de reproduire le comportement instationnaire de l'écoulement sans aucune modification arbitraire. Les effets tridimensionnels intervenant dans les mécanismes d'instabilité de la poche sont également analysés. Ce travail nous permet d'aboutir à un outil numérique, validé sur des configurations d'écoulements cavitants complexes, afin d'améliorer la compréhension des mécanismes physiques qui contrôlent les effets instationnaires tridimensionnels intervenants dans les mécanismes d'instabilité
Cavitation is one of the most demanding physical phenomena influencing the performance of hydraulic machines. It is therefore important to predict correctly its inception and development, in order to quantify the performance drop it induces, and also to characterize the resulting flow instabilities. The aim of this work is to develop an unsteady 3D algorithm for the numerical simulation of cavitation in an industrial CFD solver « Code_saturne ». It is based on a fractional step method and preserves the minimum/maximum principle of the void fraction. An implicit solver, based on a transport equation of the void fraction coupled with the Navier-Stokes equations is proposed. A specific numerical treatment of the cavitation source terms provides physical values of the void fraction (between 0 and 1) without including any artificial numerical limitation. The influence of RANS turbulence models on the simulation of cavitation on 2D geometries (Venturi and Hydrofoil) is then studied. It confirms the capability of the two-equation eddy viscosity models, k-epsilon and k-omega-SST, with the modification proposed by Reboud et al. (1998) to reproduce the main features of the unsteady sheet cavity behavior. The second order model RSM-SSG, based on the Reynolds stress transport, appears able to reproduce the highly unsteady flow behavior without including any arbitrary modification. The three-dimensional effects involved in the instability mechanisms are also analyzed. This work allows us to achieve a numerical tool, validated on complex configurations of cavitating flows, to improve the understanding of the physical mechanisms that control the three-dimensional unsteady effects involved in the mechanisms of instability

In der vorliegenden Arbeit werden verschiedene Simulationsansätze angewandt um die Hydrodynamik in einem neu entwickelten Wirbelschichtvergaser zu untersuchen. Die Ansätze umfassen a) entdimensionalisierter Ähnlichkeitskennzahlen und empirischer Gleichungen, b) 1D Simulationen mittels ASPEN Plus®, c) 3D CFD Simulationen mittels Ansys Fluent® zur detaillierten Abbildung der zu erwartenden Hydrodynamik. Vor- und Nachteile der jeweiligen Ansätze sowie Klassen von ermittelbaren Simulationsdaten werden diskutiert. Ein Schwerpunkt der Arbeit liegt in der Identifizierung geeigneter Experimente aus der Literatur, auf Basis von Ähnlichkeitskennzahlen, um die Simulationen zu validieren. Die Vergasersimulationen zeigen, dass sich erwartungsgemäß ein aus hydrodynamischer Sicht gestufter Prozess ausbildet. Die entstehenden Zonen lassen sich als Festbett, blasenbildende Wirbelschicht, Jet-Wirbelschicht mit Rezirkulationszelle und strähnenbildende, zirkulierende Wirbelschicht identifizieren und entsprechen demnach dem Verfahrensanspruch.

Cette thèse présente les résultats de deux études : la première concerne la convection naturelle turbulente dans une cavité rectangulaire chauffée uniformément par le bas et remplie d’un nanofluide et la seconde concerne l’investigation du transfert de chaleur conjugué dans un échangeur de chaleur à tubes ailetés.L’enceinte de la première étude a un faible rapport d’aspect. Ses parois gauche, droite et supérieure sont maintenues à une température relativement basse. Le fluide de travail est un nanofluide constitué d’eau et de nanoparticules, soient d’alumine (Al2O3), ou de cuivre (Cu) ou d’oxyde de cuivre (CuO). L’influence des paramètres tels que le nombre de Rayleigh (basé sur la hauteur H de la cavité et la densité de flux de chaleur), le type de nanofluide et la fraction volumique des nanoparticules sur la performance de refroidissement est présentée. Les équations de Navier-Stokes et les équations de conservation de la masse et de l'énergie sont résolues pour une géométrie bidimensionnelle par la méthode numérique des volumes finis. L'algorithme SIMPLE est utilisé pour le couplage pression-vitesse. La discrétisation des termes convectifs est faite avec le schéma QUICK. Le modèle de turbulence k-epsilon standard est utilisé. Le maillage du domaine simulé est généré par le code Gambit. Les résultats montrent que pour toutes les valeurs de Ra, le nombre de Nusselt moyen augmente d’une façon linéaire et monotone avec l’augmentation de la concentration des nanoparticules dans le fluide de base. Le flux de chaleur moyen prend des valeurs qui diminuent en fonction de l’ordre suivant : Cu, CuO et Al2O3.La deuxième étude est une investigation numérique des caractéristiques dynamique et thermique d'un échangeur de chaleur. Les calculs supposent un transfert de chaleur et un écoulement en régime permanent. Le nombre de Nusselt et le coefficient de frottement qui caractérisent l'échangeur de chaleur sont déterminés pour différentes valeurs du nombre de Reynolds. L’équation de conservation de l'énergie dans le fluide et l’équation de conduction de la chaleur dans le solide en trois dimensions ont été résolues avec les équations de la conservation de la masse et de la quantité de mouvement afin de déterminer ces caractéristiques. Les deux régimes d'écoulement laminaire et turbulent sont considérés. L'effet de la modélisation de la turbulence a été étudié en utilisant trois modèles différents (Spalart-Allmaras modèle de turbulence à une équation, le modèle k-epsilon ; standard et le modèle RSM). La validation du modèle a été effectuée en comparant les facteurs de frottement, f, et le facteur, j, de Colburn avec les données expérimentales trouvées dans la littérature. Les résultats tracés ont montré un bon accord qualitatif entre les résultats numériques et les données expérimentales. Les résultats montrent également que le plus simple des trois modèles de turbulence testés (à savoir. Spalart-Allmaras) donne les valeurs les plus proches des données expérimentales
This thesis presents the results of two studies: the first concerns natural turbulent convection in a rectangular cavity heated from the bottom wall and filled with a nanofluid and the second relates to the investigation of conjugate heat transfer in a fin-and-tube heat exchanger.The cavity of the first study is tall and has a heat source embedded on its bottom wall, while its left, right and top walls are maintained at a relatively low temperature. The working fluid is a water based nanofluid having three nanoparticle types: alumina, copper and copper oxide. The influence of pertinent parameters such as the Rayleigh number, the type of nanofluid and solid volume fraction of nanoparticles on the cooling performance is studied. Steady forms of twodimensional Reynolds-Averaged-Navier-Stokes equations and conservation equations of mass and energy, coupled with the Boussinesq approximation, are solved by the volume control based on the discretisation method employing the SIMPLE algorithm for pressure-velocity coupling. Turbulence is modeled by using the standard k-epsilon model. The Rayleigh number, Ra, is varied from 2.49xE09 to 2.49xE11. The volume fractions of nanoparticles where varied too. Stream lines, isotherms, velocity profiles and temperature profiles are presented for various combinations of Ra, the type of nanofluid and solid volume fraction of nanoparticles. The results are reported in the form of average Nusselt number on the heated wall. It is shown that for all values of Ra, the average heat transfer rate from the heat source increases almost linearly and monotonically as the solid volume fraction increases. Finally the average heat transfer rate takes on values that decrease according to the ordering Cu, CuO and Al2O3.In the second study We determined the heat transfer and friction characteristics of a realistic fin-and-tube heat exchanger. The computations assume steady-state heat transfer and fluid flow. Nusselt number and friction factor characteristics of the heat exchanger are presented for various values of Reynolds numbers. The energy conservation and the heat conduction equations in 3 dimensions have been solved in the fluid and the solid respectivelyalong with the mass and momentum conservation equations in order to determine these characteristics. Both laminar and turbulent flow regimes are considered. The effect of turbulence modeling was investigated using three different models (the one equation Spalart-Allmaras turbulence model, the standard k-epsilon; model and the RSM model). The computations allowed the determination of the dynamic and thermal fields. Model validation was carried out by comparing the calculated friction factor f and Colburn j-factor to experimental results found in the literature. The plotted results showed a qualitatively good agreement between numerical results and experimental data. The results obtained also showed that the simplest of the three turbulence models tested(i.e. Spalart-Allmaras) gives the closest values to the experimental data

This research presents the results of numerical study of heat transfer and pressure drop in a heat exchanger that is designed with different shape pin fins. The heat exchanger used for this research consists of a rectangular duct fitted with different shape pin fins, and is heated from the lower plate. The pin shape and the compact heat exchanger (CHE) configuration were numerically studied to maximize the heat transfer and minimize the pressure drop across the heat exchanger. A three dimensional finite volume based numerical model using FLUENT© was used to analyze the heat transfer characteristics of various pin fin heat exchangers. The simulation applied to estimate the heat transfer coefficient and pressure drop for a wide range of Reynolds numbers with different pin fins. Circular pin configuration variations included changes in pin spacing, axial pitch and pin height ratio. Rectangular and drop-shaped pin variations also included changes in length and aspect ratio. Correlations for Nusselt number and friction factor were developed. The optimum drop shaped pin array was shown to match the heat transfer rates obtained by the optimum circular pin configuration while incurring less than one third the specific fluid friction power losses. The data and conclusions of this study can be applied to the optimization of different heat exchangers which are used in industry, especially oil cooler in power transformers which are currently working with low cooling efficiency. It can also be used in the design of electronic components, turbine blade cooling or in other high heat flux dissipation applications requiring a low-profile, high area-density based micro-heat exchanger design. This study also shows that numerical models backed with experimental analysis can reduce both the time and money required to create and evaluate engineering concepts, especially those that deal with fluid flow and heat transfer. In the following chapters, first the problems which are encountered by power transformer suppliers are described. Then pin fin technology is studied with more details as a novel solution to the oil cooling problem. Some studies on behavior of power transformer coolers are also conducted to make their problems more clear. Available experimental data in the Iran Transfo company have been used for validation of these studies. They are presented as separated papers at the end of thesis. Finally the results of pin fin studies are presented and horizontal continuous casting (HCC) is explained as a manufacturing method for pin fins production. A separate paper which is based on experimental study on HCC is also included at the end of thesis.

Forskningen presenterad är ett resultat av en numerisk studie av värmeöverföring och tryckfall i en värmeväxlare designad med olika former av Kylflänsar. Värmeväxlaren består av ett rektangulärt kanal utrustat med olika former av Kylflänsar och är uppvärmd underifrån. Kylflänsar forma och den kompakta värmeväxlarens utformning är studerade numeriskt för att maximera värmeöverföringen och minimera tryckfallet över värmeväxlaren.En tredimensionell finit volym baserad på en numerisk modell i FLUENT© användes för att analysera värmeöverföringsegenskaper för olika Kylflänsar konfigurationer. Genom simuleringar uppskattades värmegenomgångstalet och tryckfallet för olika Reynolds tal och Kylflänsar konfigureringar. Cirkulära Kylflänsar konfigurationer inkluderar variation av avstånd mellan Kylflänsar, och förhållandet mellan axiellt avstånd och höjd. Rektangulära och droppformade Kylflänsar inkluderade även variation för längd och aspekt förhållande. Korrelation mellan Nusselts tal och friktionsfaktor utvecklades. Optimal matris för hur droppformade Kylflänsar placerades visades överensstämma med optimal överföring för cirkulära Kylflänsar men bara med en tredjedel av friktionsförlusterna för fluiden. Data och slutsatser från studien kan användas inom för optimering av värmeväxlare använda i industrin, speciellt oljekylda högspänningstransformatorer som har låg effektivitet i kylningen. Resultaten kan även användas inom design av elektronikkomponenter, kylning av turbinblad eller andra komponenter med högt värmeflöde där låg profil, och stor ytdensitet behövs. Studien visar att kombinationen av numeriska modeller som valideras genom experiment kan reducera både tid och kostnad vid utveckling och utvärdering av ingenjörsverktyg, speciellt inom fluidmekanik och värmeöverföring. I följande kapitel beskrivs först problem som identifierats av tillverkare av högspänningstransformatorer. Kylflänsar studeras i detalj som en ny lösning till de identifierade problemen med oljekylning. Några studier har genomförts för att ytterligare belysa problemen kring högspänningstransformatorers kylning. Tillgängliga data från Iran Transfo company har använts för validering av resultat från studierna. Studierna presenteras som separata artiklar i slutet av avhandlingen. Avslutningsvis presenteras resultat från studierna av Kylflänsar och en horisontell kontinuerlig gjutprocess (HCC) för tillverkning av Kylflänsar. HCC-studien presenteras som en separat artikel inkluderad sist i avhandlingen.

Silicon Carbide (SiC) has the potential to significantly improve electronics. As a material, it can conduct heat better, carry larger currents and can give faster responses compared to today’s technologies. One way to produce SiC for use in electronics is by growing a thin layer in a CVD-reactor (chemical vapour deposition). A CVD-reactor leads a carrier gas with small parts of active gas into a heated chamber (susceptor). The gas is then rapidly heated to high temperatures and chemical reactions occur. These new chemical substances can then deposit on the substrate surface and grow a SiC layer. This thesis investigates the effect of different opening angles on a susceptor inlet in a SiC horizontal hot-walled CVD-reactor at Linköping University. The susceptor inlet affects both the flow and heat transfer and therefore has an impact on the conditions over the substrate. A fast temperature rise in the gas as close to the substrate as possible is desired. Even temperaturegradients vertically over the substrate and laminar flow is desired. The CVD-reactor is modeled with conjugate heat transfer using CFD simulations for three different angles of the inlet. The results show that the opening angle mainly affects the temperature gradient over the substrate and that a wider opening angle will cause a greater gradient. The opening angle will have little effect on the temperature of the satellite and substrate.

The premise of the work presented here is to use a common analytical tool, Computational Fluid dynamics (CFD), along with a difference turbulence models. Eddy viscosity models as well as state-of-the-art Large Eddy Simulation (LES) were used to study the flow past bluff bodies. A suitable CFD code (CFX5.6b) was selected and implemented. Simulation of turbulent transport for the gas through the gaps of the randomly distributed spherical fuel elements (pebbles) was performed. Although there are a number of numerical studies () on flows around spherical bodies, none of them use the necessary turbulence models that are required to simulate flow where strong separation exists. With the development of high performance computers built for applications that require high CPU time and memory; numerical simulation becomes one of the more effective approaches for such investigations and LES type of turbulence models can be used more effectively. Since there are objects that are touching each other in the present study, a special approach was applied at the stage of building computational domain. This is supposed to be a considerable improvement for CFD applications. Zero thickness was achieved between the pebbles in which fission reaction takes place. Since there is a strong pressure gradient as a result of high Reynolds Number on the computational domain, which strongly affects the boundary layer behavior, heat transfer in both laminar and turbulent flows varies noticeably. Therefore, noncircular curved flows as in the pebble-bed situatio n, in detailed local sense, is interesting to be investigated. Since a compromise is needed between accuracy of results and time/cost of effort in acquiring the results numerically, selection of turbulence model should be done carefully. Resolving all the scales of a turbulent flow is too costly, while employing highly empirical turbulence models to complex problems could give inaccurate simulation results. The Large Eddy Simulation (LES) method would achieve the requirements to obtain a reasonable result. In LES, the large scales in the flow are solved and the small scales are modeled. Eddy viscosity and Reynolds stress models were also be used to investigate the applicability of these models for this kind of flow past bluff bodies at high Re numbers.

Wakes are present in many engineering flows. These flows include internal flows such as mixing chambers and turbomachinery as well as external flows like flow over high-lift or multi-element airfoils. Many times these wakes are exposed to flow conditions such as adverse pressure gradients and streamline curvature that alter the mean flow and turbulent structure of the wake. The ability to understand how pressure gradients and streamline curvature affects the structure of the wake is essential to predicting how the wake will affect the performance of the application in which it is found. The effects of pressure gradients and curvature of low-speed wakes has been extensively documented. As the transonic flow regime is becoming of more interest as gas speeds in turbomachinery increase this work fills a void in the body of wake knowledge pertaining to curved wakes in high speed flows. An under-resolved direct numerical simulation of transonic wake flow being shed by a cambered airfoil in the presence of adverse pressure gradients and streamline curvature is therefore presented here. It was observed that the turbulence characteristics arising from the cambered airfoil that generates the wake dominate the evolution of the wake for different distances downstream depending on the component of the Reynolds stresses that is being considered. These characteristics dissipated the most quickly in the shear stresses and endured the longest in the tangential normal stresses. Previous work in low-speed wakes has indicated that curvature creates new production terms that translate into asymmetry in the profiles of the wake. Curvature was observed to have limited influence on the evolution of the streamwise normal stresses and an extensive impact on the tangential normal stresses. The transport of the Reynolds shear stresses indicate that the asymmetry in this stress is caused indeed by curvature but through turbulent diffusion and not production. The k-ε turbulence model overpredicted the effect of curvature on the turbulence stresses in the wake. This led to accelerated wake decay and spread compared to the UDNS data.

Les émissions polluantes à l’échappement des véhicules automobiles sont l'une des principales sources de pollution de l'air dans le monde d'aujourd'hui. Par conséquent, la législation a évolué afin de limiter ces émissions. L'un des aspects clés pour répondre consiste à bien maîtriser les échanges gazeux au sein du moteur à combustion interne. Cette amélioration est possible par l'optimisation de répartiteurs d'admission d'air. Dans ces répartiteurs d'admission d'air, la maitrise de l’écoulement de type tumble est une piste de progrès. Des volets sont installés à la sortie du répartiteur afin d'améliorer le rapport de tumble et donc le mélange air-carburant (VTS-Variable Tumble System). Une autre caractéristique de l'écoulement à l'intérieur des répartiteurs est l'effet des écoulements pulsés qui engendrent des fluctuations de pression assez importante. Par conséquent, le but de cette étude consiste à simuler le flux d'air pulsé à l'intérieur des répartiteurs d'admission et à identifier l'effet des pulsations de pression sur les composants actifs tels que les volets. Le travail de simulation dans la présente thèse a été effectué à partir du code open source CFD OpenFOAM. Dans un premier temps, l'effet des pulsations de pression est simulé à l'intérieur d'un tube d'acier et une méthodologie de simulation est développée. Les résultats de la simulation sont validés à partir de résultats expérimentaux obtenus sur un dispositif spécifique, le banc dynamique. Ensuite, des simulations ont été effectuées sur le répartiteur d'admission principal avec des volets. Tout d’abord, les simulations sont effectuées en régime permanent avec cinq positions d'ouverture différentes du clapet. Les forces et les moments agissant sur le volet en régime permanent sont obtenus et analysés. Puis, des simulations en régime transitoire avec des effets de pulsation de pression sont effectuées. Les résultats de la simulation instationnaire sont comparés aux résultats expérimentaux en termes de fluctuations de pression relative. Les effets des pulsations de pression sur les forces aérodynamiques et les moments agissant sur les volets sont analysés et commentés
The exhaust emissions from automobiles are one of the major sources of air pollution in today’s world. Thence,research and development is the key feature of the modern automotive industries to meet strict emission legislation. One of the key aspects to meet these requirements is to improve the gas exchange process within internal combustion engines. It is possible by the design optimization of the air intake manifolds for internal combustion engines. One of such advancement in air intake manifolds is variable tumble systems (VTS). In VTS system, tumble flaps are installed at the exit of the manifold runner in order to improve tumble ratio and hence air-fuel mixing. Another feature of the flow inside the intake manifolds is pressure pulsation effect. Therefore, the aim of the Ph.D. work is to simulate the pulsating air flow inside the air intake manifolds and to identify the effect of the pressure pulsations on the active components like tumble flaps. The simulation work in the present thesis has been carried out on open source CFD code OpenFOAM. In a first step, the effect of pressure pulsations is simulated inside a steel tube and a simulation methodology is developed. The results of the simulation are validated on a specific experimental device, the dynamic flow bench. Then,simulations have been carried out on the main intake manifold with tumble flaps. Firstly, the simulations are performed with five different opening positions of the tumble flap in a steady state configuration. The forces and moments acting on the flap in steady state are obtained and analyzed. Then, unsteady simulations with pressure pulsation effects are performed. The results of obtained from unsteady simulation are compared with the experimental results in terms of relative pressure fluctuations. The effect of the pressure pulsation on the aerodynamic forces and moments acting on the tumble flaps are analyzed and explained

L’Oxydation HydroThermale (OHT) est l’une des technologies mises en œuvre pour le traitement des déchets liquides organiques radio-contaminés. Grâce aux propriétés de l’eau supercritique, l’OHT permet d’obtenir une minéralisation complète des composés organiques, avec des temps de séjour très courts, dans des réacteurs continus et compacts. Un modèle thermohydraulique couplé à un modèle de cinétique de combustion a été précédemment développé par le CEA et implémenté sur la plateforme de mécanique des fluides numérique ANSYS Fluent. Dans l’objectif d’améliorer la description des écoulements dans ces réacteurs, des mesures de masse volumique de mélanges ont permis d’ajuster puis d’implémenter un modèle en accord avec les données expérimentales. La problématique de la précipitation des sels minéraux a également été traitée au cours de cette étude, en mettant en œuvre une méthodologie expérimentale permettant d’acquérir des données de solubilité de composés modèles, et aussi de collecter et de caractériser les dépôts obtenus lors de la précipitation de ces composés dans un dispositif continu ad hoc. Ce travail expérimental a été poursuivi par l’implémentation de ces phénomènes de précipitation et de dépôt au sein du modèle numérique
HydroThermal Oxidation (HTO) is one of the technologies applied for radio-contaminated organic liquid waste treatment. HTO uses the peculiar properties of supercritical water to achieve a complete destruction of organic compounds within very short residence times in continuous compact reactors. A thermohydraulic model, coupled with combustion kinetics, was previously developed by the CEA, and implemented using CFD software ANSYS Fluent. In order to improve the description of the flow in continuous HTO reactors, mixtures density measurements allowed to find a model fitting these experimental data. This model was consequently incorporated into the previous numerical simulation. The issue of mineral salts precipitation was also taken into account, by an experimental methodology allowing to acquire solubility data of model compounds, and also to collect and characterise the deposit obtained after precipitation of these compounds in an ad hoc continuous device. This experimental work was followed by the implementation of these precipitation and deposition phenomena in the numerical model

The main objective of this PhD thesis is the study of Diesel sprays under evaporative conditions by means of Large Eddy Simulations (LES) techniques. This study has been performed implementing a precise, low-demanding LES model in the free, full-purpose Computational Fluid Dynamics (CFD) code OpenFOAM. The starting point was a careful and exhaustive review of the physical processes involved in sprays. An emphasis in CFD methodology, particularly for LES methods, was essential for the thesis, as we were able to find the possible problems and limitations of our approximation. Moreover, as the most widely used techniques for the industrial simulation of sprays are based on the Reynolds-Averaged Navier-Stokes models, we have highlighted the many advantages of LES modeling. As the latter are, by definition, more computationally expensive than RANS, we made an optimal configuration that, while it is able to recover accurately the experimental results, its characteristic time is in the same order of magnitude that RANS ones. As applicability is a must in this thesis, we use the surname ¿Engineering¿ LES. One of the key points of the thesis has been the correct configuration of the flow turbulent conditions on the inlet. In order to get accurate results, the turbulent structures coming from this inlet need to be time- and spacecoherent. An adequate calibration of this conditions is needed to perform any spray simulation. Last but not least, all the simulations performed where validated against experiments, obtaining a very good agreement even close to the nozzle
Mompó Laborda, JM. (2014). Engineering Large Eddy Simulation of Diesel Sprays [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/37345
TESIS

The transportation of gas-liquid mixtures in horizontal pipes as two-phase stratified flow is examined using computational fluid dynamics (CFD) method. The design of these pipelines requires accurate prediction of flow parameters such as pressure drop and liquid holdup. Many empirical correlations have been developed in the last 70 years and are well documented in the literature to obtain these parameters using experimental, analytical and numerical methods. In this investigation, the numerical method based on CFD code-FLUENT is used as an alternative to the experimental method to obtained numerical data such as gas wall shear stress, liquid holdup and pressure drop in order to calculate interfacial shear stress using semi-mechanistic flow model for stratified-smooth and stratified-wavy flow based on the 3D CFD models developed in FLUENT DesignModeler. The Volume of Fluid (VOF) model and k-ω SST turbulence model were used to obtain numerical data from the CFD models for validations. In the 3D CFD model for gas flow over stationary liquid surface, the average gas velocities and corresponding liquid heights from the experimental data were validated in the two-phase flow domain. The interfacial friction factor correlation proposed was in good agreement against the existing two-phase friction factors using conventional two-phase flow calculation method, while the mathematical formulations involving hydrostatic force for the interfacial and gas wall shear stresses were poorly correlated against existing correlations. In the co-current gas-liquid flow 3D CFD model, the pressure drop, gas wall shear stress, interfacial shear stress and liquid holdups were in excellent agreement and the interfacial friction factor correlations proposed were in good agreement with the published correlations. The flow patterns were correctly predicted as stratified-smooth and wavy flow on the flow map. A design procedure involving both 3D CFD models was proposed and presented in Appendix D.

Deep knowledge of combustion phenomena is of great scientific and technological interest. In fact, better design of combustion equipments (furnaces, boilers, engines, etc) can contribute both in the energy efficiency and in the reduction of pollutant formation.

One of the limitations to design combustion equipments, or even predict simple flames, is the resolution of the mathematical formulation. Analytical solutions are not feasible, and recently numerical techniques have received enormous interest. Even though the ever-increasing computational capacity, the numerical resolution requires large computational resources due to the inherent complexity of the phenomenon (viz. multidimensional flames, finite rate kinetics, radiation in participating media, turbulence, etc). Thus, development of capable mathematical models reducing the complexity and the stiffness as well as efficient numerical techniques are of great interest.

The main contribution of the thesis is the analysis and application of the laminar flamelet concept to the numerical simulation of both laminar and turbulent non-premixed flames. Assuming a one-dimensional behavior of combustion phenomena in the normal direction to the flame front, and considering an appropriate coordinates transformation, flamelet approaches reduce the complexity of the problem.

The numerical methodology employed is based on the finite volume technique and a parallel multiblock algorithm is used obtaining an excellent parallel efficiency. A post-processing verification tool is applied to assess the quality of the numerical solutions.

Before dealing with flamelet approaches, a co-flow partially premixed methane/air laminar flame is studied for different levels of partial premixing. A comprehensive study is performed considering different mathematical formulations based on the full resolution of the governing equations and their validation against experimental data from the literature. Special attention is paid to the prediction of pollutant formation.

After the full resolution of the governing equations, the mathematical formulation of the flamelet equations and a deep study of the hypothesis assumed are presented. The non-premixed methane/air laminar flame is considered to apply the flamelet modelling approach, comparing the results with the simulations obtained with the full resolution of the governing equations. Steady flamelets show a proper performance to predict the main flame features when differential diffusion and radiation are neglected, while unsteady flamelets are more suitable to account for these effects as well as pollutant formation. Assumptions of the flamelet equations, the scalar dissipation rate modelling, and the evaluation of the Lagrangian flamelet time for unsteady flamelets are specially analysed.

For the numerical simulation of turbulent flames, the mathematical formulation based on mass-weighted time-averaging techniques, using RANS EVM two-equation models is considered. The laminar flamelet concept with a presumed PDF is taken into account. An extended Eddy Dissipation Concept model is also applied for comparison purposes. A piloted non-premixed methane/air turbulent flame is studied comparing the numerical results with experimental data from the literature. A clear improvement in the prediction of slow processes is shown when the transient term in the flamelet equations is retained. Radiation is a key aspect to properly define the thermal field and, consequently, species such as nitrogen oxides. Finally, the consideration of the round-jet anomaly is of significant importance to estimate the flame front position.

In conclusion, flamelet modelling simulations are revealed to be an accurate approach for the numerical simulation of laminar and turbulent non-premixed flames. Detailed chemistry can be taken into account and the stiffness of the chemistry term is solved in a pre-processing task. Pollutant formation can be predicted considering unsteady flamelets.

L’objectif de ce travail fut d’améliorer la modélisation et la simulation de la polymérisation radicalaire dans des réacteurs continus et discontinus. Une solution analytique explicite généralisée (AS) fut obtenue dans le cas de la polymérisation en masse/solution, homogène et isotherme menée dans un réacteur fermé de volume variable. Les différentes étapes considérées furent l'initiation, la propagation, le transfert au monomère, au solvant, à un agent de transfert de chaîne, la terminaison par combinaison et dismutation. Différents modèles rendant compte des effets de gel, de vitrification et de cage ont également été considérés. AS a été validée avec succès par comparaison avec des solutions numériques et des données expérimentales de la littérature. Par ailleurs, AS a été étendue à des conditions pour lesquelles elle ne fut pas originellement développée comme par exemple des conditions non isothermes. La polyvalence et la flexibilité de AS sur l’ensemble de l’échelle de conversion du monomère furent ainsi démontrées. Ensuite, pour élargir encore plus son champ d'application, AS fut utilisée dans des simulations numériques (CFD). Une nouvelle transformation très simple a été proposée afin d’adimensionnaliser les constantes cinétiques en terme de concentration. Cela a permis de rentrer dans les simulations les données chimiques sous leur forme originale en mole et de faciliter ainsi le codage et le débogage du code de calcul. Cette transformation a ensuite été utilisée pour évaluer trois géométries tubulaires de microréacteur, un réacteur tubulaire droit (STR), à géométrie hélicoïdale (CTR) et à inversion de flux (CFIR), dans des conditions d'alimentation différentes (fluides d’entrée non ou parfaitement mélangés) et à de très faibles nombres de Reynolds (<1). La modélisation a été réalisée avec des paramètres constants ou variables des propriétés physiques du fluide sous écoulement (densité, viscosité et conductivité thermique) ainsi qu’en variant de manière discrète les coefficients de diffusion. Leurs effets sur les résultats de simulation ont été observés et comparés avec les données expérimentales publiées pour 4 monomères différents et furent en très bon accord. Les résultats pour le cas d’un mélange parfait furent indépendants de la géométrie des microréacteurs. Le CFIR semble être le réacteur le plus prometteur puisque, dans les conditions de microréaction étudiées, il a permis le meilleur contrôle des caractéristiques du polymère synthétisé
This thesis aimed at improving the modeling and simulation of free radical polymerization (FRP) in batch as well as in flow reactors. A generalized explicit analytical solution (AS) was obtained in case of variable volume, bulk/solution polymerization, homogeneous and isothermal batch reactor. The reaction steps included initiation, propagation, transfer to monomer, transfer to solvent, transfer to chain transfer agent (CTA), termination by combination and disproportionation. Different models of gel, glass and cage effects were also implemented explicitly. AS was validated against numerical solutions as well as published experimental data and was found in good agreement. Furthermore, its applicability was extended to conditions for which AS was not derived, i.e. non-isothermal conditions. The versatility and flexibility of AS over the complete range of monomer conversion were thus demonstrated. Then, to broaden its applications range even more, AS was used in CFD simulations. A new and simple transformation was proposed to make kinetic rate coefficients dimensionless in terms of concentration. This enabled chemical data to be fed in molar form to CFD modeling. It also enabled easy coding and debugging by keeping the original form of generation terms intact. The results were found to be improved after validation against experimental data. This transformation was then used for evaluating three tubula microreactor geometries, namely straight tube reactor (STR), coiled tube reactor (CTR) and coil flow inverter reactor (CFIR), under different feed conditions (unmixed or perfectly mixed) at very low Reynolds numbers (<1). The modeling for FRP was performed with constant or variable fluid physical parameters (density, viscosity and thermal conductivity) along with discrete variation of diffusion coefficients. Their effects on simulation results were observed and compared with published experimental data for 4 different monomers and were found to match perfectly. Results for mixed feed condition were found to be independent of microreactor geometry. CFIR seems to be the most promising reactor design under microreaction investigated conditions as it allowed the best control over polymer characteristics

L'aérodynamique interne est un élément fondamental pour améliorer la combustion dans les moteurs à allumage commandé. Une meilleure maitrise des écoulements internes est permise grâce aux outils de simulation CFD qui sont de plus en plus utilisés dans le processus de développement des moteurs à allumage commandé. Cette thèse avait pour objectif d’étendre l'approche hybride RANS/LES-temporelle dite HTLES, initialement dédiée pour des écoulements statistiquement stationnaires, aux écoulements moteurs avec des parois mobiles et des modes opératoires cycliques, puis de la valider dans des configurations représentatives des écoulements moteurs. Cette approche vise à modéliser les régions proches parois par approche statistique RANS et tend continûment vers la LES temporelle loin des parois si la discrétisation spatiale et temporelle est suffisamment résolue. Le formalisme temporel permet une hybridation RANS/LES consistante dans un écoulement statistiquement stationnaire, les deux méthodes se basant sur des opérateurs temporels (respectivement la moyenne temporelle et le filtrage temporel). Une première amélioration de l’approche HTLES a été proposée en ajoutant une fonction de protection qui impose le mode RANS dans la région proche paroi, indépendamment de la discrétisation locale (spatiale et temporelle). Dans les écoulements cycliques, l’approche HTLES modélise les échelles turbulentes non-résolues en se basant sur des moyennes de phase des grandeurs résolues qui sont inconnues lors de la simulation. La moyenne glissante exponentielle (EWA) a été utilisée afin d’approximer ces moyennes de phase. Une formule pour définir la largeur de la moyenne glissante a été proposée de sorte que les fluctuations turbulentes (hautes fréquences) soient filtrées des quantités résolues, tout en conservant les composantes cycliques (basses fréquences). Cette approche a été implémentée dans le code de calcul industriel CONVERGE CFD. Elle a d'abord été validée dans deux configurations stationnaires : un canal plan infini et un banc volute. A cet effet, les résultats ont été comparés aux données de référence et aux résultats RANS et LES. Dans les régions proches parois où le maillage est sous résolu pour la LES, EWA-HTLES a mieux prédit l’écoulement grâce à l'utilisation du mode RANS, permettant une meilleure prédiction des pertes de charge. La résolution des grandes échelles dans la région centrale a permis d'obtenir des prédictions aussi précises qu’une simulation LES en termes de vitesses moyennes et des fluctuations. La validation de l'EWA-HTLES a également été effectuée dans deux configurations moteurs : le tumble compressé et le moteur Darmstadt, tous deux présentant des caractéristiques aérodynamiques typiques aux moteurs à allumage commandé telles que la génération et la compression du mouvement de tumble et la variabilité cyclique. Pour chaque configuration, un nombre total de 40 cycles consécutifs simulés à l'aide de EWA-HTLES a été utilisé pour calculer les deux premiers moments statistiques. Les résultats ont été comparés aux données de la PIV, et aux résultats donnés par les simulations RANS et LES. Les résultats ont montré que le modèle développé arrive à contrôler correctement la transition RANS-LES dans des configurations complexes avec des conditions d'écoulement non stationnaires et des déformations géométriques importantes, assurant le mode RANS aux parois et la LES au centre du cylindre. La résolution des grandes échelles a permis une bonne prédiction des phénomènes instationnaires, particulièrement l'évolution des caractéristiques du mouvement de tumble et des phénomènes associés aux variabilités cycliques, tels que l'augmentation locale de vitesses fluctuantes. Les résultats de l'EWA-HTLES sont similaires à ceux prédits par la LES et meilleurs que ceux donnés par les simulations RANS. Ces résultats montrent des perspectives encourageantes pour l'application de cette méthode dans de nombreuses configurations industrielles
Internal aerodynamics is a key element for improving the combustion efficiency in Spark-Ignition (SI) engines. Within this context, CFD tools are increasingly used to investigate in-cylinder flows and to support the design of fuel-efficient engines. The present research aimed at extending and validating a non-zonal hybrid Reynolds-Averaged Navier-Stokes / Temporal Large-Eddy Simulation (HTLES) approach, initially formulated for stationary flows, to cyclic SI engine flows with moving walls. The aim was to model the near-wall regions and coarse mesh regions in RANS, while solving the turbulent scales in core regions with sufficient mesh resolution using temporal LES, in a seamless approach with no a priori user input. HTLES was retained as it proposed a consistent hybridization combining time-averaging in RANS regions with temporal filtering in TLES.A first development consisted in implementing a smooth shielding function that enforces the RANS mode in near-wall regions, regardless of the local temporal and spatial resolution. The extension of HTLES to cyclic flows was then achieved via the formulation of a method allowing approximating the phase averages of resolved flow quantities based on an Exponentially Weighted Average (EWA). A dynamic expression for the width of the weighted average was proposed, in order to ensure that the high frequency turbulent fluctuations be filtered out from the resolved quantities, while keeping the low frequency cyclic components of the flow variables. The resulting EWA-HTLES model was implemented in the commercial CONVERGE CFD code. The developed EWA-HTLES model was first applied to the simulation of two steady flow configurations: a minimal turbulent channel and a steady flow rig. Predictions were confronted with reference data, as well as with those from RANS and LES. All simulations relied on the use of standard wall laws and coarse grids at walls. Imposing the RANS mode at walls yielded EWA-HTLES predictions of pressure losses much closer to DNS and experimental findings than with LES. At the same time, it allowed yielding results in terms of mean and RMS velocities s in the core regions of the same quality than LES, and superior to RANS.Finally, EWA-HTLES was applied to the simulation of two cyclic flows representative of SI engines: the compressed tumble and the Darmstadt single-cylinder pentroof 4valve engine. For each configuration, a total number of 40 consecutive cycles were simulated. The results were confronted to PIV data, and to RANS and LES predictions obtained using the same numerical set-up. It was shown that EWA-HTLES successfully drives the RANS-to-LES transition in such complex configurations exhibiting unsteady flow features and important cyclic geometrical deformations. It switched from the RANS mode at the walls to LES in the core region of the cylinder, allowing a better prediction of unsteady phenomena including the evolution of the overall tumble characteristics and phenomena associated to cyclic variability. The EWA-HTLES results were shown to be comparable to those predicted by LES, and superior to RANS.The performed developments and obtained results open encouraging perspectives for the application of this hybrid RANS/LES method in industrial configurations involving non-stationary conditions and in particular moving boundaries

Se realiza un modelo CFD-3D de un compresor centrífugo. Se determina la configuración numérica adecuada para simular puntos de funcionamiento cercanos a bombeo. Se validan los resultados frente a ensayos experimentales realizados en cámara anecoica. Se analizan los resultados, justificando las características del ruido emitido por el compresor a partir de los patrones de flujo encontrados. En particular, se analiza el flujo en la holgura entre los álabes y la carcasa del compresor y en la parte posterior del rodete. Por último, se analiza la influencia de las geometrías aguas arriba del compresor en el ruido producido por éste.
Navarro García, R. (2014). A numerical approach for predicting flow-induced acoustics at near-stall conditions in an automotive turbocharger compressor [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/44114
TESIS
Premiado

Afin de prédire l’évolution de la composition chimique du laitier dans différents procédés sidérurgiques, un modèle CFD a été développé. Les coefficients de transfert de masse sont estimés à partir des modèles basés sur les paramètres physico-chimiques et hydrodynamiques, comme par exemple la diffusivité des espèces chimiques et la divergence de l’interface. Ces modèles ont été développé pour la prédiction du transfert gaz-liquide où le les nombres de Schmidt (Sc=ν⁄D) sont relativement faible (Sc≈200). Par contre, les procédés industriels ont un nombre de Sc considérablement plus importante, de l’ordre de 103 à 104. Pour évaluer la pertinence de ces modèles, l’hydrodynamique au voisinage d’une interface liquide-liquide a été étudiée. Un modèle CFD et des mesures par l’anémométrie laser (LDA) ont été utilisés pour calculer et valider les champs de vitesse d’une maquette à eau d’une lingotière de coulée continue (CC).Le modèle de transfert de masse d’une lingotière de coulée continu industriel nous a montré que les coefficients de transfert de masse ne sont pas distribués de manière homogène, et les propriétés physiques du laitier ne doivent pas y être non plus. Cette distribution non-homogène a été confirmée par des essais physiques. Les écoulements calculés numériquement ont été utilisé pour prédire les coefficients de transfert de masse entre les deux phases liquide. Ces paramètres seront utilisés comme donnée d’entré pour un modèle de thermodynamique afin de prédire l’évolution de la composition chimique du laitier
In order to characterize the mass transfer coefficients (MTC) of different species across liquid steel/slag interface, a multiphase Computational Fluid Dynamic (CFD) model was developed. MTC’s are estimated from models based on physicochemical and hydrodynamic parameters, such as mass diffusivity, interface shear and divergence strength. These models were developed for gas-liquid interactions with relative low Schmidt (Sc=ν⁄D) numbers (Sc≈200). However, the industrial processes involve mass transfer of chemical species with Sc number ranging from 103 to 104. To evaluate the applicability of these existing models, the fluid flow in the vicinity of a liquid/liquid interface is investigated. Computational Fluid Dynamic (CFD) and Laser Doppler Anemometry (LDA) were used to calculate and measure the velocity field on a continuous casting (CC) water model configuration. The work provides new insights and original measures to understand the fluid flow near liquid-liquid interfaces.The mass transfer model of an industrial continuous casting mold showed that the mass transfer coefficients are not homogeneously distributed, and slag properties should follow this trend. This non-homogeneity was confirmed by physical experiments performed with a water model of a CC configuration and its CFD representation. The calculated flow was used to predict the MTC and the interface area between phases, since the interface is constantly moving. These parameters will be the input of thermodynamic models to predict slag composition and viscosity. This methodology is currently under validation, and it will also be applied to improve steel plant performance in the desulphurization process

Together with the world economic growth is the increasing of energy generation demand. However, the upgrade of world power production capability could affect the environment negatively. Even the clean and renewable sources, such as hydroelectricity and wind powers have socio-economic and environmental disadvantages. For example, the required flooded area for a hydro power plant construction could devastate entire forests, and the installation of a wind farm power plant could affect migratory rotes of birds and generate high levels of noise. Hence, for the balancing of advantages and disadvantages of each power generation source, it is necessary to diversify, which requires investments in new power sources. In this context, the energy generation in the ocean is highlighted. The first point concerning the ocean energy is that there is no need of population removal from the installation area, such as the onshore based methods and the second point is that most of the population is concentrated in coastal areas. Therefore the production occurs near to the demand, decreasing the costs with energy distribution. The two main methodologies for harassing energy from oceans are based on gravity waves and in tides. And since the tidal cycles are governed mainly by the gravitational interaction between oceans, Moon and Sun, they are easily predictable, which increases the reliability of such systems. These works explores methodologies to analyse the power generation from a single axial tidal current turbine through a Steady State RANS methodology. Are discussed the effects of flow directionality, inlet velocity profile and turbulence levels and the results are compared with an experimental scheme. It is proposed an alternative methodology for free surface modelling in the CFD analysis. The usual methodology, VOF, it is based on a homogeneous, biphasic approach which requires an additional mesh refinement and is computationally expensive. This new methodology introduces an elastic wall approach in the free surface region in which the stiffness is calculated to provide the same restoring effect as gravity. In general, the results for open domain matched with the experimental results, validating the numerical model and the confined domain has shown a higher power and thrust coefficients if compared with the open domain, which is in accordance with the actuator disk theory approach. The elastic free surface presented convergence problems related to high Froude numbers and therefore to high deformations. However, a simulation with 10% of the original inlet velocity was performed, achieving reasonable results for both power and thrust coefficients evaluation.
O crescimento econômico mundial e o aumento na demanda pela geração de energia andam juntos. No entanto, uma maior capacidade de produção de energia poderia afetar negativamente o meio ambiente. Mesmo as fontes limpas e renováveis, como a hidrelétrica e a eólica acarretam em impactos socioeconômicos e ambientais. Por exemplo, a construção de uma usina hidrelétrica demanda uma imensa área alagada que pode devastar florestas inteiras e a instalação de uma usina eólica pode afetar a migração de certas espécies de pássaros e produzir altos níveis de barulho. Portanto, para equilibrar as vantagens e desvantagens devidas a cada meio de produção de energia, é necessária a diversificação, que demanda de investimentos em novas fontes. Neste contexto, a geração de energia nos oceanos é destacada. O primeiro ponto a respeito desta fonte é de que não há a necessidade de remoção da população na área de instalação, tal como os métodos de geração dentro do continente. O segundo principal ponto é a respeito da distribuição de energia. A maior parte da população mundial vive em regiões costeiras, diminuindo, portanto, a distância entre a produção e demanda, reduzindo assim, seus custos. As duas principais metodologias para se explorar a energia proveniente dos oceanos são: Energia de Ondas e Energia de Marés. E considerando que os ciclos de mare são governados principalmente pela interação gravitacional entre os oceanos, lua e sol, eles são facilmente previsíveis, o que aumenta a confiabilidade dos sistemas de geração de energia baseados em marés. Este trabalho explora as metodologias para analisar a geração de energia a partir de uma única turbina axial de corrente de maré através de uma metodologia baseada nas equações de Navier-Stokes com a média de Reynolds, analisadas em regime permanente. São discutidos efeitos da direção do escoamento, perfil de velocidades na entrada e nos níveis de turbulência. Os resultados são comparados com experimentos. É proposta uma metodologia alternativa para a modelagem da superfície livre com CFD uma vez que a metodologia atual é baseada em um escoamento bifásico que demanda de um refinamento adicional da malha e é computacionalmente caro. A nova metodologia usa uma parede elástica na região da superfície livre com a rigidez ajustada para se obter o mesmo efeito de restauração que a gravidade. De maneira geral, os resultados para o domínio aberto se aproximaram dos resultados experimentais, validando o modelo numérico e além disso, o modelo considerando confinamento da turbine mostrou maiores valores para os coeficientes de potência e empuxo, estando portanto, de acordo com a teoria do disco atuador. O modelo com a superfície livre elástica apresentou problemas de convergência, relacionados com números de Froude elevados, uma vez que isto se relaciona com maiores deformações na região da superfície livre. Uma simulação com 10% da velocidade original foi realizada, obtendo-se resultados coerentes para ambos coeficientes de potência e empuxo.

Submitted by Edson Rodrigo Del Rio Vieira null (rodrigodelrio@uol.com.br) on 2016-12-04T15:37:23Z No. of bitstreams: 1 Vieira_ERDR_ME_ILHA.pdf: 8354186 bytes, checksum: 2d0cc3e0eee52b2d654d403b5f472d54 (MD5)
Approved for entry into archive by Felipe Augusto Arakaki (arakaki@reitoria.unesp.br) on 2016-12-06T13:47:18Z (GMT) No. of bitstreams: 1 vieira_erdr_me_ilha.pdf: 8354186 bytes, checksum: 2d0cc3e0eee52b2d654d403b5f472d54 (MD5)
Made available in DSpace on 2016-12-06T13:47:18Z (GMT). No. of bitstreams: 1 vieira_erdr_me_ilha.pdf: 8354186 bytes, checksum: 2d0cc3e0eee52b2d654d403b5f472d54 (MD5) Previous issue date: 2016-08-26
Item merged in doublecheck by Juliano Benedito Ferreira (julianoferreira@reitoria.unesp.br) on 2017-01-20T16:08:54Z Item was identical to item(s): 145094, 144451 at handle(s): http://hdl.handle.net/11449/145008, http://hdl.handle.net/11449/144479
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)
Segundo a Sociedade de Cirurgia Vascular da América do Norte e Sociedade Internacional de Cirurgia Cardiovascular, aneurismas são dilatações superiores ou iguais a 1,5 vezes o diâmetro original da artéria. Estima-se que aproximadamente 2% a 5% da população adulta tem algum tipo de aneurisma cerebral. Grande parte dos casos não chegam a causar sintomas ou gerar problemas graves, entretanto o risco de sua ruptura gera consequências clínicas desafiadoras, visto que tem grande taxa de morbimortalidade, tornando difícil a tarefa de tomar a decisão de intervenção ou não no tratamento do aneurisma. Neste trabalho, propõe-se o estudo numérico do escoamento em aneurismas do tipo basilar empregando-se a técnica da dinâmica de fluidos computacional. Comparando diferentes casos de aneurismas rompidos e não rompidos, identificamos características no escoamento para auxiliar a tomada de decisões no tratamento de pacientes. Os casos de aneurismas são oriundos de tomografias computadorizadas de pacientes reais, os quais foram utilizados para a criação dos modelos digitais. As simulações computacionais foram realizadas com oprogramaopen sourceOpenFOAM® . Outros programasopen source também foram utilizados na criação dos modelos, assim como para o tratamento dos resultados. Identificamos uma correlação entre os dados de tensão cisalhante na parede e a probabilidade de ruptura. Para os oito casos estudados, os resultados de previsão de ruptura e não ruptura apresentaram grande compatibilidade. Dentre os quatrocasos de aneurismas previamente reconhecidos como rompidos, trêsforam identificados pelas simulações comoaneurismas com alto risco de ruptura e umfoi identificado com risco intermediário de ruptura. Já para os quatro casos de aneurismas previamente reconhecidos como rompidos, doisforam identificados pelas simulações como aneurismas de baixo risco de ruptura e doiscom risco intermediário de ruptura.
According to the Society for Vascular Surgery of North America and the International Society of Cardiovascular Surgery, aneurysms are dilations greater than or equal to 1.5 times the original artery diameter.It is estimated that approximately 2% to 5% of adult population has some type of brain aneurysm. In most cases aneurysms are small enough not to cause symptoms or serious problems, however the risk of rupture generates challenging clinical consequences, since they have high rate of morbidity and mortality, making difficult the decision whether or not to treat the aneurysm. This work presents the study of the flow through the basilar type aneurysms using the technique of computational fluid dynamics. Comparing different cases of ruptured and nonruptured aneurysms, we identified some flow characteristics that can help in the decision making process for the treatment of patients. The cases studied were taken from CT scans of real patients, which were used for creation of digital models of aneurysm used in the simulations. The simulations were performed using open source software OpenFOAM® . Others open source softwares were also used in the creation of models as well as for post processing. We investigated the correlation between wall shear stress and the likelihood of rupture. For the eight cases, the rupture prediction results showed good correlation. For the four ruptured cases simulated, three were identified with high risk of rupture and one with intermediary risk of rupture. As for the four not ruptured cases, two were identified with low risk of rupture and two with intermediary risk of rupture.
CNPq: 134375/2015-7

A numerical study of axisymmetric synthetic jet flow was conducted. The synthetic jet cavity was modeled as a rigid chamber with a piston-like moving diaphragm at its bottom. The Shear-Stress-Transportation (SST) k-omega and #61559; turbulence model was employed to simulate turbulence. Based on time-mean analysis, three flow regimes were identified for typical synthetic jet flows. Typical vortex dynamics and flow patterns were analyzed. The effects of changes of working frequency, cavity geometry (aspect ratio), and nozzle geometry were investigated. A control-volume model of synthetic jet cavity was proposed based on the numerical study, which consists of two first-order ODEs. With appropriately selected parameters, the model was able to predict the cavity pressure and average velocity through the nozzle within 10% errors compared with full simulations. The cavity model can be used to generate the boundary conditions for synthetic jet simulations and the agreement to the full simulation results was good. The saving of computational cost is significant. It was found that synthetic jet impingement heat transfer outperforms conventional jet impingement heat transfer with equivalent average jet velocity. Normal jet impingement heat transfer using synthetic jet was investigated numerically too. The effects of changes of design and working parameters on local heat transfer on the impingement plate were investigated. Key flow structures and heat transfer characteristics were identified. At last, a parametric study of an active heat sink employing synthetic jet technology was conducted using Large Eddy Simulation (LES). Optimal design parameters were recommended base on the parametric study.

Hollow fibre and spiral wound membrane (SWM) modules are the most common commercially available membrane modules. The latter dominate especially for RO, NF and UF and are the focus of this study. The main difficulty these types of modules face is concentration polarisation. In SWM modules, the spacer meshes that keep the membrane leaves apart also help reduce the effects of concentration polarisation. The spacer filaments act as flow obstructions, and thus encourage flow destabilisation and increase mass transfer enhancement. One of the detrimental aspects of the use of spacers is an increase of pressure losses in SWM modules. This study analyses the mechanisms that give rise to mass transfer enhancement in narrow spacer-filled channels, and investigates the relationship between flow destabilisation, energy losses and mass transfer. It shows that the regions of high mass transfer on the membrane surface correlate mainly with those regions where the fluid flow is towards the membrane. Based on the insights gained from this analysis, a series of multi-layer spacer designs are proposed and evaluated. In this thesis, a Computational Fluid Dynamics (CFD) model was used to simulate steady and unsteady flows with mass transfer in two- and three-dimensional narrow channels containing spacers. A solute with a Schmidt number of 600 dissolving from the wall and channel Reynolds numbers up to 1683 were considered. A fully-developed concentration profile boundary condition was utilised in order to reduce the computational costs of the simulations. Time averaging and Fourier analysis were performed to gain insight into the dynamics of the different flow regimes encountered, ranging from steady flow to vortex shedding behind the spacer filaments. The relationships between 3D flow effects, vortical flow, pressure drop and mass transfer enhancement were explored. Greater mass transfer enhancement was found for the 3D geometries modelled, when compared with 2D geometries, due to wall shear perpendicular to the bulk flow and streamwise vortices. Form drag was identified as the main component of energy loss for the flow conditions analysed. Implications for the design of improved spacer meshes, such as extra layers of spacer filaments to direct the bulk flow towards the membrane walls, and filament profiles to reduce form drag are discussed.