Dissertations / Theses on the topic 'Dynamic meshing'

Computational Fluid Dynamics (CFD) is a field that is constantly advancing. Its advances in terms of capabilities are a result of new theories, faster computers, and new numerical methods. In this thesis, advances in the computational fluid dynamic modeling of moving bodies and combustive flows are investigated. Thus, the basic theory behind CFD is being extended to solve a new class of problems that are generally more complex. The first chapter that investigates some of the results, chapter IV, discusses a technique developed to model unsteady aerodynamics with moving boundaries such as flapping winged flight. This will include mesh deformation and fluid dynamics theory needed to solve such a complex system. Chapter V will examine the numerical modeling of a combustive flow. A three dimensional single vane burner combustion chamber is numerically modeled. Species balance equations along with rates of reactions are introduced when modeling combustive flows and these expressions are discussed. A reaction mechanism is validated for use with in situ reheat simulations. Chapter VI compares numerical results with a laminar methane flame experiment to further investigate the capabilities of CFD to simulate a combustive flow. A new method of examining a combustive flow is introduced by looking at the solutions ability to satisfy the second law of thermodynamics. All laminar flame simulations are found to be in violation of the entropy inequality.

For the numerical simulation of many problems of engineering interest, it is desirable to have an automated mesh adaption tool. This is important especially for problems characterized by anisotropic features and require mesh clustering in the direction of high gradients. Another significant issue in meshing emerges in unsteady simulations with moving boundaries, where the boundary motion has to be accommodated by deforming the computational grid. Similarly, there exist problems where current mesh needs to be adapted to get more accurate solutions. To solve these problems, we propose three novel procedures. In the first part of this work, we present an optimization procedure for three-dimensional anisotropic tetrahedral grids based on metric-driven h-adaptation. Through the use of topological and geometrical operators, the mesh is iteratively adapted until the final mesh minimizes a given objective function. We propose an optimization process based on an ad-hoc application of the simulated annealing technique, which improves the likelihood of removing poor elements from the grid. Moreover, a local implementation of the simulated annealing is proposed to reduce the computational cost. Many challenging unsteady multi-physics problems are characterized by moving boundaries and/or interfaces. When the boundary displacements are large, degenerate elements are easily formed in the grid such that frequent remeshing is required. We propose a new r-adaptation technique that is valid for all types of elements (e.g., triangle, tet, quad, hex, hybrid) and deforms grids that undergo large imposed displacements at their boundaries. A grid is deformed using a network of linear springs composed of edge springs and a set of virtual springs. The virtual springs are constructed in such a way as to oppose element collapsing. Both frequent remeshing, and exact-pinpointing of clustering locations are great challenges of numerical simulations, which can be overcome by adaptive meshing algorithms. Therefore, we conclude this work by defining a novel mesh adaptation technique where the entire mesh is adapted upon application of a force field in order to comply with the target mesh or to get more accurate solutions. The method has been tested for two-dimensional problems of a-priori metric definitions as well as for oblique shock clusterings.

Orientador: Aparecido Carlos Gonçalves
Resumo: Pares engrenados são elementos de transmissão de potência amplamente utilizados em máquinas e equipamentos, todavia as falhas catastróficas desses componentes são comuns e dispendiosas. A análise de vibrações está entre as técnicas de diagnóstico de defeitos incipientes utilizadas em manutenção preditiva, posto que a presença de uma falha altera o comportamento dinâmico do sistema e o estado de degradação pode ser detectado pelo monitoramento dos sinais de vibração. Na indústria atual, onde as aquisições de dados, tanto para controle de processos, quanto para o monitoramento das condições de integridade de equipamentos, são realizadas em tempo real, faz-se necessário o desenvolvimento de métodos que aumentem a confiabilidade das tomadas de decisões em relação à identificação, localização e prognóstico de falhas. O objetivo deste trabalho é desenvolver um modelo matemático de par de engrenagens que auxilie no monitoramento da condição e validar o modelo dinâmico com dados de vibração de um multiplicador de velocidades obtidos experimentalmente. Para tanto, foi elaborada uma metodologia baseada no modelo dinâmico de par engrenado com 6 graus de liberdade para simulação de sinais de vibração; nesse modelo, inclui-se erros geométricos no perfil do dente; de maneira analítica, simula-se uma a trinca do dente de uma das engrenagens que ocasiona a queda de rigidez em função do tempo; desenvolveu-se também um experimento com um multiplicador de velocidades; e, por fim, algumas técnic... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: Spur gears are transmission power elements widely used in machinery, however catastrophic failures of this components are just as common and onerous. Vibration analysis is a technique, in among of others, that can be used in diagnostics of incipient damages, common in predictive maintenance, because they change the dynamic behavior of the mechanical system, and the degradation state can be detected by vibration signal or noise. In the current industry production, in which real-time data acquisition - whether for processes control, or for health condition monitoring of equipment - is the reality, it is necessary to develop auxiliary methods that provide high reliability to identification, localization and failure prognostics. In this work, the main objective is to provide a spur gears’ model-based methodology for condition-monitoring and to validate a dynamic model with experimental vibration data of a gearbox. Hence, a dynamic model of spur meshing gears was developed considering a 6 degrees of freedom and time-varying meshing stiffness to simulate vibrations signals; a tooth profile error was also included; in this analytical model, a straight crack was simulated by reducing the meshing stiffness in a tooth; experiments with a gearbox experimental set were run; and, some signal processing was apllied in the vibration data. The results allowed the model validation with the comparison between simulate and experimental signals, in time-domain and frequency-domain
Mestre

This thesis is centered on the construction of a cardiac atlas to serve as a common reference frame in the Virtual Physiological Human (VPH). The construction covers the entire construction pipeline, starting from a set of 3D+t multislice computed tomography images, then performing a spatial normalization of these images, segmentation of the synthesized mean image, multi-structure meshing, and finally mapping of the mesh back to the population of images. In addition, two applications are presented in this thesis. First, the atlas is used to frame a spatio-temporal model of cardiac morphology which models the variability along both 'axes' simultaneously. Such a unified approach should be preferable over existing methods, which decouple the two sources of variation and then model them separately, in isolation. Second, the proposed atlas is applied to develop an acceleration technique for performing personalized simulation of cardiac electrophysiology (EP). The prior knowledge encapsulated in our atlas is used, in conjunction with a numerical solver of cardiac EP, to build a statistical model linking cardiac morphology with the steady states of myocardial cell models that pre condition detailed cardiac EP simulations. This application puts the proposed dynamic cardiac atlas in the context of VPH-related simulations, of which the computational costs are currently greatly in excess of what is acceptable for their adoption in current clinical practice.
Esta tesis está centrada en la construcción de un atlas cardiaco, para servir como marco común de referencia en el Virtual Physiological Human (VPH). La construcción consiste en la trayectoria completa, empezando con un conjunto de imágenes 3D+t de tomografía computacional multi-corte, y entonces hacer una normalización espacial de las imágenes, segmentación de la imagen promedio sintetizada, un mallado multi-estructura, y finalmente la transformación de la malla a la población de imágenes. Adicionalmente, la tesis presenta dos aplicaciones del atlas. Primero, el atlas se usa para enmarcar un modelo espacio-temporal de la morfología cardiaca que modela la variación a lo largo de ambos 'ejes' simultáneamente. Tal propuesta debe ser preferible sobre otros m\'etodos existentes, los cuales desacoplan las dos fuentes de variación para modelarlas separadamente, en isolación. Segundo, el atlas está aplicado al desarrollo de una técnica de aceleración para simulaciones personalizadas de electrofisiología (EF) cardiaca. El conocimiento previo encapsulado en nuestro atlas se usa, en conjunto con un solver de EF cardiaca, para construir un modelo estadístico conectando morfología cardiaca con los steady states de modelos celulares del miocardio que precondicionan a simulaciones detalladas de EF cardiaca. Esta aplicación posiciona el propuesto atlas dinámico cardiaco en el contexto de simulaciones relacionadas al VPH, cuyo costo computacional actual está en gran exceso de lo aceptable para su adopción en la práctica clínica de hoy en día.

La boîte de transmission principale (BTP) est une des principales sources dubruit perçu dans les cabines d’hélicoptères et pénalise fortement le confort acoustique deséquipages et passagers. Afin de réduire l’impact de cette source, les phénomènes acoustiqueset vibratoires mis en jeu par les boîtes de transmission à engrenages doivent être compris etsimulés durant les phases de développement. De cette façon, le comportementvibroacoustique des BTP pourra être amélioré dès la conception, réduisant ainsi le coût, lamasse et les difficultés d’intégration des solutions d’insonorisation. Ce travail présente lesBTP d’hélicoptères ainsi que le bruit qu’elles génèrent. Il présente également nosdéveloppements concernant la modélisation du comportement dynamique des BTP afin d’encalculer le bruit. Nous avons développé un code éléments finis permettant d’effectuer desétudes paramétriques afin d’ajuster le design des boîtes de transmission lors des phases dedéveloppement. Notre modèle est capable de calculer les efforts dynamiques aux paliers detransmissions composées de plusieurs engrenages cylindriques et spiro-coniques. Enfin, nousanalysons des mesures acoustiques et vibratoires effectuées autour de deux BTP pourplusieurs conditions de couple et vitesse. Ces mesures nous permettent de mieux comprendrele comportement vibroacoustique des BTP et de confirmer certaines tendances observées avecnotre modèle
Main gearbox (MGB) is one of the main noise sources in helicopter cabinsand it strongly penalizes acoustic comfort of crews and passengers. In order to reduce theimpact of this source, acoustic and vibration mechanisms of gearboxes have to be understoodand simulated during the development phases. By this way, MGB vibroacoustic behaviourcould be improved by design, thus reducing cost, additional weight and integration difficultiesof sound-proofing solutions. This work presents helicopters MGB and the noise they generate.It also presents our developments regarding the modelling of MGB dynamic behaviour fornoise computation. We have developed a finite elements code allowing to conduct parametricstudies to tune the gearboxes design in early development phases. Our model is able tocompute dynamic loads on bearings of any transmission composed of several cylindrical andspiral bevel gears. At last, we analyse acoustic and vibration measurements done around twoMGB for several conditions of torque and speed. These measurements allow to betterunderstand MGB vibroacoustic behaviour and to confirm some trends observed with ourmodel

A diffuse-interface finite-element method has been applied to simulate the flow of two-component rheologically complex fluids. It treats the interfaces as having a finite thickness with a phase-field parameter varying continuously from one phase to the other. Adaptive meshing is applied to produce fine grid near the interface and coarse mesh in the bulk. It leads to accurate resolution of the interface at modest computational costs. An advantage of this method is that topological changes such as interfacial rupture and coalescence happen naturally under a short-range force resembling the van der Waals force. There is no need for manual intervention as in sharp-interface model to effect such event. Moreover, this energy-based formulation easily incorporates complex rheology as long as the free energy of the microstructures is known. The complex fluids considered in this thesis include viscoelastic fluids and nematic liquid crystals. Viscoelasticity is represented by the Oldroyd-B model, derived for a dilute polymer solution as linear elastic dumbbells suspended in a Newtonian solvent. The Leslie-Ericksen model is used for nematic liquid crystals，which features distortional elasticity and viscous anisotropy. The interfacial dynamics of such complex fluids are of both scientific and practical significance. The thesis describes seven computational studies of physically interesting problems. The numerical simulations of monodisperse drop formation in microfluidic devices have reproduced scenarios of jet breakup and drop formation observed in experiments. Parametric studies have shown dripping and jetting regimes for increasing flow rates, and elucidated the effects of flow and rheological parameters on the drop formation process and the final drop size. A simple liquid drop model is used to study the neutrophil, the most common type of white blood cell, transit in pulmonary capillaries. The cell size, viscosity and rheological properties are found to determine the transit time. A compound drop model is also employed to account for the cell nucleus. The other four cases concern drop and bubble dynamics in nematic liquid crystals, as determined by the coupling among interfacial anchoring, bulk elasticity and anisotropic viscosity. In particular, the simulations reproduce unusual bubble shapes seen in experiments, and predict self-assembly of microdroplets in nematic media.

The author developed a computational framework for the study of the correlation between airway morphology and aerosol deposition based on a population of human subjects. The major improvement on the previous framework, which consists of a geometric airway model, a computational fluid dynamics (CFD) model, and a particle tracking algorithm, lies in automatic geometry construction and mesh generation of airways, which is essential for a population-based study. The new geometric model overcomes the shortcomings of both centerline (CL)-based cylindrical models, which are based on the skeleton and average branch diameters of airways called one-dimensional (1-D) trees, and computed tomography (CT)-based models. CL-based models are efficient in terms of pre- and post-processing, but fail to represent trifurcations and local morphology. In contrast, in spite of the accuracy of CT-based models, it is time-consuming to build these models manually, and non-trivial to match 1-D trees and three-dimensional (3-D) geometry. The new model, also known as a hybrid CL-CT-based model, is able to construct a physiologically-consistent laryngeal geometry, represent trifurcations, fit cylindrical branches to CT data, and create the optimal CFD mesh in an automatic fashion. The hybrid airway geometries constructed for 8 healthy and 16 severe asthmatic (SA) subjects agreed well with their CT-based counterparts. Furthermore, the prediction of aerosol deposition in a healthy subject by the hybrid model agreed well with that by the CT-based model. To demonstrate the potential application of the hybrid model to investigating the correlation between skeleton structure and aerosol deposition, the author applied the large eddy simulation (LES)-based CFD model that accounts for the turbulent laryngeal jet to three hybrid models of SA subjects. The correlation between diseased branch and aerosol deposition was significant in one of the three SA subjects. However, whether skeleton structure contributes to airway abnormality requires further investigation.

The client-server architecture of STAR-CCM+ allows multiple users to collaborate on a simulation set-up. The effectiveness of collaboration with this architecture is tested and evaluated on five models. The testing of these models is a start to finish set-up of an entire simulation excluding computational time for generating mesh and solving the solution. The different models have distinct differences which test every operation that would be used in a general CFD simulation. These tests focus on reducing the time spent preparing the geometry to be meshed, including setting up for a conformal mesh between multiple regions in conjugate heat transfer models. Results from these five tests show a maximum speed up of 36%.

Transmissions are an essential part of a vehicle powertrain. An optimally designed powertrain can result in energy savings, reduced environmental impact and increased comfort and reliability. Along with other components of the powertrain, efficiency is also a major concern in the design of transmissions. The churning power losses associated with the motion of gears through the oil represent a significant portion of the total power losses in a transmission and therefore need to be estimated. A lack of reliable empirical models for the prediction of these losses has led to the emergence of CFD (Computational Fluid Dynamics) as a means to (i) predict these losses and (ii) promote a deeper understanding of the physical phenomena responsible for theselosses in order to improve existing models. The commercial CFD solver STAR-CCM+ is used to investigate the oil distribution and the churning power losses inside two gearbox configurations namely an FZG (Technical Institute for the Study of Gears and Drive Mechanisms) gearbox and a planetary gearbox. A comparison of two motion handling techniques in STARCCM+ namely MRF (Moving Reference Frame) and RBM (Rigid Body Motion) models is made in terms of the accuracy of results and the computational requirements using the FZG gearbox. A sensitivity analysis on how the size of gap between the meshing gear teeth affects the flow and the computational requirements is also done using the FZG gearbox. Different modelling alternatives are investigated for the planetary gearbox and the best choices have been determined. The numerical simulations are solved in an unsteady framework where the VOF (Volume Of Fluid) multiphase model is used to track the interface between the immiscible phases. The overset meshing technique has been used to reconfigure the mesh at each time step. The results from the CFD simulations are presented and discussed in terms of the modelling choices made and their effect on the accuracy of the results. The MRF method is a cheaper alternative compared to the RBM model however, the former model does not accurately simulate the transient start-up and instead provides just a regime solution of the unsteady problem. As expected, the accuracy of the results suffers from having a large gap between the meshing gear teeth. The use of compressible ideal gas model for the air phase with a pressure boundary condition gives the optimum performance for the planetary gearbox. The outcomes can be used toeffectively study transmission flows using CFD and thereby improve the design of future transmissions for improved efficiency.

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

碩士
佛光人文社會學院
資訊學研究所
91
The cloth is a common material in our daily life, it could be used for many applications, such as clothing and decorating. In the past, because of the limitation of computing and measuring instruments, people cannot analytically get the characteristic of the cloth. For the recent two decades, the technology has made great progress in both computing and computer graphics, the simulation of cloth’s behavior becomes practical. The cloth is a kind of deformable object that would interact with the surrounding environment, and make change to the appearance. Comparing with the rigid objects that have the static appearance, the cloth is much more difficult to simulate. To adjust the appearance of the cloth manually is unpractical, and the effect is unnatural, either. So constructing a model that the cloth can automatically interact with the surroundings is the best solution. For the recent years, the research of cloth simulation focuses on promoting the efficiency of collision detection and finding solutions to numerical instability. In most studies, using a static set of grid particles to represent the cloth structure is a popular method since it’s simplicity. But it needs more mesh resolution to represent the surface near the wrinkles. Increasing the particle resolution will make computation load grow exponentially, and so as the mesh quantity. For a real-time simulation system, decreasing the computation load is the primary task under certain realism requirement, so increasing particle resolution to improve the appearance of wrinkle is not a practical way. For the purpose of improving the cloth’s appearance but not increasing much computation, we propose a method that could dynamically increase the mesh resolution and correct the regular mesh triangulation of the cloth. By combining with ‘Approximate Implicit Integration’ proposed by Kang et al. and the technique to accelerate collision detection, we demonstrate how to construct a real-time cloth simulation system, and show our improvement by experiments.

Dottorato di Ricerca in Ingegneria Civile ed Industriale. Ciclo XXXI
In questo lavoro di tesi si è analizzato il comportamento dinamico di ruote dentate alleggerite. Tale tipologia di ingranaggi prevede l’utilizzo di corpi ruota dalla topologia ottimizzata al fine di ridurre la massa complessiva. Le modalità con cui è ottenuta la riduzione di peso hanno un grosso impatto sulla risposta dinamica dell’intera trasmissione. In applicazioni dove le performances in termini di NVH sono rilevanti, quali ad esempio quelle in ambito automotive, è fondamentale prevedere tale comportamento già nella fase di design iniziale, attarverso tecniche di simulazione numerica. Da parte dell’industria vi è quindi la richiesta di strumenti di calcolo sufficientemente dettagliati da poter replicare gli effetti dovuti agli alleggerimenti del corpo ruota. Al tempo stesso la richiesta è per strumenti di simulazione che siano il più efficienti possibile, in modo da poter essere utilizzati in simulazioni a livello di sistema. In questo lavoro di tesi due differenti approcci sono stati considerati. Un primo approccio prevede l’utilizzo di formulazioni analitiche, dove la rigidezza di contatto degli ingranaggi è stata calcolata in una fase di pre-processing attraverso l’uso di codici agli elementi finiti. Tale approcccio ha dimostrato di riuscire a modellare gli effetti dovuti alla variazione di rigidezza introdotta dai fori di alleggerimento nel corpo ruota, riuscendo altresì a mantenere un ottimo livello di efficienza computazionale. Il secondo approccio considerato in questo lavoro di tesi è basato su un codice MB commerciale, oppurtunamente esteso per considerare gli effetti dovuti agli alleggerimenti. Tale metodologia è stata validata sperimentalmente in termini di errore di trasmissione e di deformazione al piede del dente. Tale approccio, seppur più oneroso dal punto di vista computazionale, garantisce un ottimo livello di accuratezza. La formuazione ibrida MB-FEM consente di analizzare eventi in time-domain a livello di sistema, come mostrato nel caso di studio esaminato nell’ultimo capitolo di questa dissertazione, dove sono state confrontate le performances acustiche di due differenti layout. In tal modo si è potuto apprezzare come la topologia del corpo ruota abbia un effetto non trascurabile sull’emissione sonora della trasmissione. Entrambe le formulazioni sono state analizzate nel dettaglio, riportando altresì i punti di forza e di debolezza
Università della Calabria

碩士
國立中正大學
機械工程學系暨研究所
101
The planetary gear system has several advantages: compact structure, high loading capacity and power/volume ratio, and easily achieve concentric design for input and output axes, so it’s widely used for various type of mechanical drive. The requirement of operating vibration, noise, and mechanical efficiency of planetary gear system become higher and higher as technology evolved; thus, the detailed dynamic analysis of the planetary gearing system have become crucial than before. This paper is trying to calculate the time-varying contact line of the 2K-H planetary gearing system model by 3D gear geometry, and using finite element analysis software, ANSYS, to simulate gear’s meshing stiffness. And the ANSYS stiffness result is taken as the spring stiffness to the simplified rotor-spring dynamic model of planetary gear. The dynamic response of this simplified model is compared with the result obtained by the ANSYS Workbench and the result obtained by the LS-DYNA. The numerical result showed that the dynamic response of gear pair simulated by the proposed simplified dynamic model is similar to the rotational vibration obtained by LS-DYNA and ANSYS. The proposed simplified model can be used to improve the dynamic quality of planetary gear system.

This dissertation is concerned with the development of a general computational framework for mesh adaption such as is required in the three-dimensional lagrangian finite element simulation of strongly nonlinear, possibly dynamic, problems. It is shown that, for a very general constitutive framework, the solutions of the incremental boundary value problem obey a minimum principle, provided that the constitutive updates are formulated appropriately. This minimum principle is taken as a basis for asymptotic error estimation. In particular, we chose to monitor the error of a lower-order projection of the finite element solution. The optimal mesh size distribution then follows from a posteriori error indicators which are purely local, i. e., can be computed element-by-element. A sine qua non condition for the successful accomplishment of the kind of analysis envisioned in this work is the possibility to mesh the deforming domains of analysis. In the first section of this thesis a method is presented for mesh generation in complex geometries and general--possibly non-manifold--topologies. The robustness and versatility of the computational framework is demonstrated with the aid of convergence studies and selected examples of application and the results contrasted with previous approaches

碩士
國立中央大學
機械工程學系
102
The target of this research is to study the effects of a tooth crack to the dynamic characteristics of a spur gear pair. A finite element software, ANSYS, is used for the simulation. The meshing stiffness of the gear pair is calculated by analyzing the contact force and deformation. The stiffness is dependent of the crack position and the contact condition. For reducing the calculation time, each gear can also be divided into two smaller parts which is analogical to springs. Then, the gear dynamic model is developed by the lumped-parameter method for the vibration response, and the equations of motion are derived. The dynamic responses of the gear pair are solved by using the numerical method. The features of dynamic signals are found for spur gear pair with the tooth crack and compared with those for gears with no crack.

In recent years, the growing concern about pollution from fossil fuels has resulted in an increased effort for utilization of low-grade heat sources such as industrial waste heat, solar energy and other renewable sources. There also exits the problem of supplying electricity to a sizeable proportion of India’s population, which does not have access to the electricity grid. Distributed power generation using solar energy can help solve this energy problem without deteriorating the environment. For this purpose, various thermodynamic cycles such as the organic Rankine cycle (ORC), supercritical steam Rankine cycle, Kalina cycle and organic Brayton cycle have been proposed and studied for conversion of low-grade heat sources into electricity. Among these options, ORCs are reported to be more efficient at a small scale and with medium temperature heat source, and have been commercially implemented in several projects. However, with moderate temperatures, thermal efficiencies are generally low, and hence it becomes critical to choose appropriate process parameters for optimum cycle performance. Also, at the distributed scales (~10 kW), the choice of expander (or turbine) becomes extremely critical, as conventional turbines used in (Steam) Rankine cycle at these scales become incompatible. Positive displacement expanders such as scroll expanders have emerged as attractive options for micro-solar ORC based applications. This PhD work is based on a micro-solar ORC based application, with focus towards designing an optimized scroll expander for such application. Scroll turbomachine, though being used as compressor for quite some time, is not yet available commercially as expander. After a literature review of research work done in the field of distributed solar power generation and scroll turbomachine, a thermodynamic analysis is performed for the ORC to study the effects of various thermodynamic parameters. In the context of optimizing the cycle for a distributed power generation, a basic ORC as well as a regenerative ORC are studied. The thermodynamic analysis is tailored towards selection of appropriate working fluid to further optimize the ORC cycle for such applications, and these candidate fluids also aid in design of an optimized scroll expander. The remaining part of the thesis is focused on analysis and design of scroll expander. Scroll devices have traditionally been used as compressors. In the absence of any commercially available scroll expanders, several researchers have modified scroll compressors to operate in the expander mode to generate electricity. In the present thesis, a numerical model for scroll compressor has been modified and adapted for the study of scroll expander. This scroll expander model is a semi-empirical (or semi-numerical) model formulated using the conservation of mass, first law of thermodynamics and empirical formulation based on experiments conducted on scroll turbomachines. Such semi-empirical methods have been traditionally used and proven to be effective in rapid prototyping of scroll compressors. The presents work has also used the scroll model to analyze the scroll expander with the aid of Ns-Ds diagram, which is a more conventional way of analyzing a turbomachine through non-dimensionalization. It is found that the scroll expander occupies a unique domain in the Ns-Ds space, thus suggesting the scope of novel and unexplored domain of its application. The semi-empirical approach, though fairly robust, is not capable of giving any spatial distribution of thermal-fluid properties inside the scroll expander. Hence, to obtain a better insight into the physics of expansion inside a scroll, the scroll expander is analyzed using computational fluid dynamic (CFD) simulation. The CFD analysis, though requiring much more computational resources compared to the semi-empirical method, is capable of predicting more details such as the distribution of various thermal-fluid property inside the scroll at different locations. The present commercially available CFD software cannot be easily adapted to simulate the thermal-fluid phenomena inside the scroll expander, as the continuously changing geometry of a rotating scroll prohibits the use of standard turbomachinery modelling techniques. A comparative study of semi-empirical method and CFD simulation is performed. It is observed that CFD can very well be used for qualitative studies and complementing the semi-empirical method for study of novel and unconventional scroll expanders. Finally, the thesis is concluded by summarizing the work in the context of designing an optimized scroll expander and micro-solar application and highlighting certain unexplored domain to take this study further.