378 research outputs found
A TWO-DIAMETER HELICAL ENDMILL BEAM MODEL FOR TOOL TIP DYNAMICS PREDICTION WITH APPLICATION TO MILLING
The aim of this dissertation is to describe the dynamic response of helical endmill geometries to enable the use of receptance coupling substructure analysis (RCSA) to predict the tool tip vibration response of arbitrary tool-holder-spindle-machine combinations. The tool tip vibration response, or receptance, is a key input for milling stability prediction. Currently, a measurement is required to determine the tool tip receptance for each tool-holder-spindle-machine combination, which may not be possible in production environments. In the RCSA approach, the spindle receptances are measured once and archived, while the tool and holder are modeled. Tool tip receptances are predicted by analytically coupling the tool and holder models, described as equivalent diameter Timoshenko beams, to the archived spindle receptances. In this study, a two-diameter Timoshenko beam model approach is derived and applied to predict the dynamic response of the fluted portion of the endmill geometry
XVoxel-Based Parametric Design Optimization of Feature Models
Parametric optimization is an important product design technique, especially
in the context of the modern parametric feature-based CAD paradigm. Realizing
its full potential, however, requires a closed loop between CAD and CAE (i.e.,
CAD/CAE integration) with automatic design modifications and simulation
updates. Conventionally the approach of model conversion is often employed to
form the loop, but this way of working is hard to automate and requires manual
inputs. As a result, the overall optimization process is too laborious to be
acceptable. To address this issue, a new method for parametric optimization is
introduced in this paper, based on a unified model representation scheme called
eXtended Voxels (XVoxels). This scheme hybridizes feature models and voxel
models into a new concept of semantic voxels, where the voxel part is
responsible for FEM solving, and the semantic part is responsible for
high-level information to capture both design and simulation intents. As such,
it can establish a direct mapping between design models and analysis models,
which in turn enables automatic updates on simulation results for design
modifications, and vice versa -- effectively a closed loop between CAD and CAE.
In addition, robust and efficient geometric algorithms for manipulating XVoxel
models and efficient numerical methods (based on the recent finite cell method)
for simulating XVoxel models are provided. The presented method has been
validated by a series of case studies of increasing complexity to demonstrate
its effectiveness. In particular, a computational efficiency improvement of up
to 55.8 times the existing FCM method has been seen.Comment: 22 page
Extension de la méthode des Différences Spectrales à la combustion
L'amélioration des outils d'ingénierie utilisés dans le design des dispositifs industriels de combustion est indispensable afin de respecter les demandes de plus en plus restrictives pour réduire les émissions de gaz à effets de serre. Parmi eux, la mécanique des fluides numériques (CFD) est devenue essentielle pour étudier et optimiser les chambres de combustion au cours des dernières décennies. Elle se complète parfaitement aux expériences réelles qui peuvent être très couteuses et avec lesquelles il est impossible d'obtenir des informations sur n'importe quelle quantité d'intérêt en tout point de la chambre de combustion. En utilisant les simulations aux grandes échelles (LES), la CFD décrit directement l'interaction entre les flammes et les structures turbulentes avec une faible modélisation. La qualité des résultats LES est ainsi très dépendante de la discrétisation utilisée incluant à la fois le maillage et également les propriétés de dissipation et de dispersion des méthodes numériques utilisées. Cependant, la plupart des codes LES employés de nos jours dans l'industrie utilisent des schémas de discrétisation spatiale de basordre (LO) à cause de leur faible coût de calcul et leur facilité d'implémentation sur des maillages complexes. Pourtant, les méthodes numériques d'ordres élevés (HO) pour la LES sont développées depuis deux décennies et ont été appliquées sur des écoulements non-réactifs amenant à des résultats plus précis que les méthodes LO avec un plus faible coût de calcul. Bien que les méthodes HO semblent très prometteuses en combustion, en particulier pour mieux décrire le front de flamme, leur utilisation pour des écoulements réactifs restent encore à être démontrée. Au cours de ces travaux, les avantages et les bénéfices des méthodes HO en combustion sont évalués en utilisant la méthode des Différences Spectrales (SD) avec du raffinement . Premièrement, il est démontré que la formulation originelle des SD est instable pour des écoulements multi-espèces avec des propriétés thermodynamiques variant avec la température et la composition. Il a été constaté que calculer les variables primitives aux points solutions puis de les extrapoler aux points flux, au lieu de faire l'inverse en extrapolant d'abord les variables conservatives, rend stable la méthode SD dans ce cas-ci. De plus, une nouvelle méthodologie, également plus stable pour calculer les flux diffusifs aux interfaces des cellules est détaillée. Enfin, les conditions aux limites caractéristiques et de murs ont été étendues aux écoulements multiespèces dans le formalisme SD. Avec ces développements, des flammes laminaires pré-mélangées 1D et 2D ont été simulées avec des mécanismes réduits à 2 réactions ou des mécanismes réduits analytiquement. Les résultats sont très proches de ceux obtenus avec des solveurs de référence bien établis en combustion. Il est montré que pour un même niveau d'erreur, il est plus efficace d'utiliser des maillages grossiers avec des grandes valeurs de et non l'inverse. Par conséquent, le raffinement local en , qui applique des grandes valeurs de dans les régions d'intérêts seulement, permet de garder une bonne précision à un coût de calcul plus faible. Ceci est particulièrement intéressant pour des simulations de combustion où le front de flamme est très localisé et requiert une plus grande précision que le reste de l'écoulement. Il est également observé sur ces cas simples 1D et 2D que la méthode SD est moins sensible à la discrétisation du front de flamme que les solveurs volumes finis comme AVBP. Pour terminer, deux différentes configurations de flammes 3D turbulentes ont été simulées avec l'algorithme des SD étendu aux écoulements réactifs
Studies of hybrid pixel detectors for use in Transmission Electron Microscopy
Hybrid pixel detectors (HPDs) are a class of direct electron detectors that have been adopted for use in a wide variety of experimental modalities across all branches of electron microscopy. Nevertheless, this does not preclude the possibility of further improvement and optimisation of their performance for specific applications and increasing the range of experiments for which they are suitable. The aims of this thesis are two-fold. Firstly, to develop a more comprehensive understanding of the current generation HPDs using Si sensors, with a view to optimising their design. Secondly, to determine the advantages of alternative sensor materials that, in principle, should improve the performance of HPDs in transmission electron microscopy (TEM) due to their increased stopping power.
The three chapters review the relevant theoretical background. This includes the physics underpinning the performance of semiconductor-based sensors in electron microscopy as well as the operation of detectors more generally and the theory underlying the metrics used to evaluate detector performance in Chapter 1. In Chapter 2, TEM as a key tool in the study of nano- and atomic scale systems is also introduced, along with an overview of the detector technologies used in TEM. Also presented as part of the background material in Chapter 3 is a description of the experimental methods and software packages used to acquire the results presented in the latter half of the thesis.
Chapter 4, the first results chapter, presents a comparison of the performance of Medipix3 detectors with Si sensors with various combination of pixel pitch and sensor thickness for 60 keV and 200 keV electrons. In Chapter 5, simulations of the interactions of electrons with energies ranging from 30-300 keV with GaAs:Cr and CdTe/CZT, two of the most viable alternatives to Si for use in the sensors of HPDs, are compared with simulations of the interactions of electrons with Si. A comparative study of the performance of a Medipix3 device with GaAs:Cr sensor with that of a Si sensor of the same thickness and pixel pitch for electrons with energies ranging from 60-300 keV is presented in Chapter 6. Also included in this Chapter are the results of investigations into the defects present in the CaAs:Cr sensor material and how these affect device performance. These consist of confocal scanning transmission electron microscopy scans used to estimate the size and shape of individual pixels and how these relate to the linearity of pixels’ response, as well as studies of how the efficacy of a standard flat field depends on the incident electron flux. In the final results chapter, the focus shifts to preliminary measurements of the response of an integrating detector with GaAs:Cr sensor to electrons. These initial experimental measurements prompted further simulations investigating how the backside contact of GaAs:Cr sensors can be improved when using electrons
A digital twin development framework for fatigue failure prognosis of a vertical oil well drill string
This thesis presents a novel methodology for fatigue life prognosis of vertical oil well drill strings through the development of a digital twin frame work. A technique is proposed to classify vibration types with their severities and estimate the remaining useful life time of the drill string based on various indirect measurements made at the surface level. The classification was done using a machine learning algorithm developed based on a Hidden Markov Model HMM). Training data for the algorithm were generated using a bond graph simulation of a vertical drill string. A three-dimensional lumped segment bond graph element and an interface element available in the literature were used to develop the simulation. The bond graph elements are
developed based on a Newton-Eular formulation and body-fixed coordinates. The
simulation was upgraded by introducing a fluid drag model and refining it with accurate element compliance values. Non linear fluid drag force statistical models were
developed through the design of experiments(DoE) approach considering the non-linear geometry of the drill pipes,the drilling fluid rheology, and fluid velocity. A series of fluid-structure interaction(FSI) simulations were employed to develop the statistical models for the lateral vibration damping and the axial drag force dueto
the drilling fluid flow through the pipe and the annular space. An apparatus was designed and fabricated to verify the FSI simulation. Further, a method was introduced to accurately determine the axial, shear, bending, and torsional compliances of geometrically-complex drill string segments represented by the bond graph elements. The trained HMM-based classifier using bond graph-generated training data selects the appropriate parameter set for the same bond graph to generate stress history for fatigue life prognosis. A generalized fatigue life estimation method was developed using SalomeMecaᵀᴹ, an open-source finite element analysis code. A detailed workflow for multi-axial, non-proportional, and variable amplitude (MNV) fatigue analysisis
also provided. Three case studies are presented to demonstrate the significance of the nonlinear fluid drag models, the fatigue prognosis framework, and the digital twin development framework. In the first case study, the bond graph with the developed drag models showed higher stress fluctuations at the drill pipe threaded connection than the one with a static model. The second case study demonstrated the function of the proposed fatigue life prognosis framework as an optimization tool. In the case study, the optimum placement of the stabilizers reduced the drill collar damage by 66% compared to the worst-case scenario. The third case study used a laboratory-scale vertical drill string vibration simulator apparatus designed and fabricated to
implement the framework as a proof of concept. It demonstrated the potential to use surface measurements to classify the vibration type and its severity for fatigue life prognosis
An unstructured Finite-Volume Level Set / Front Tracking method for capillary flows
In this thesis the unstructured Finite-Volume hybrid Level Set / Front Tracking method (LENT) for immiscible two-phase flows is extended to enable the simulation of capillary flows. The major contributions are a more accurate interface curvature approximation, an accuracy driven pressure velocity coupling algorithm, an approximation technique for consistent mass fluxes for momentum convection and two novel approaches for the computation of volume fractions from triangulated surfaces. All proposed techniques and algorithms are devised for unstructured Finite-Volume meshes. The improved curvature approximation uses a signed distance field as input and utilizes surface-mesh/volume-mesh mappings to reduce curvature variation in interface normal direction. A novel, local correction approach is introduced to further reduce the curvature error in cells intersected by the interface. To ensure a prescribed solution accuracy, an iterative, accuracy driven pressure velocity coupling algorithm is presented that builds on the established segregated solution algorithms. The necessity of consistent mass fluxes for momentum convection in the presence of differing fluid densities is analyzed. For interface advection methods that do not utilize phase-specific volumetric fluxes, a method to obtain approximate, consistent mass fluxes is proposed. The resulting improvements for capillary flows are demonstrated using canonical verification and validation test cases. Two novel algorithms to compute volume fractions on unstructured volume meshes from oriented triangle surfaces meshes are introduced, one based on geometric intersections and one based on approximation and adaptive refinement. Intended for the phase indicator calculation in the context of Level Set / Front Tracking methods, both algorithms are shown to be sufficiently accurate to initialize volume fractions also for the Volume-of-Fluid method. In fact, test cases demonstrate that both approaches’ accuracy is only limited by the resolution of the surface mesh
A Computational Study of Cooling Via Ultrasound Enhanced Heat Transfer of Acoustically Driven Flows
Previous work has shown that acoustic streaming improves heat transfer via enhanced fluid mixing. A fluid undergoing ultrasonic propagation is seen to exhibit distinctive circulatory flow structures that encourage thermal boundary layer dispersion of any given heat source. The work is found to be essential for enhanced cooling in applications such as microelectronics where the use of thermoacoustic engines can be used to continually provide a cooled environment though enhanced heat transfer. Computational Fluid Dynamics can provide an in-depth understanding of the flow structures and resulting behaviours for a fluid undergoing acoustic streaming. A computational procedure has been developed to simulate the acoustic effects of a given fluid from a 2nd-order solution. The model was found to produce results that formed a good approximation to the experimentation, built upon a base convection model whose mesh was validated using a previous experiment to observe thermal boundary layer behavior. A detailed frequency study on the effects of fluid structure and development was modeled in a given geometry. A study was conducted on the mechanisms behind the flow development between each incremental rise in frequency. It was found that a change was induced between the number of circulatory loops in each configuration along with a cut-off point in frequency magnitude beyond which the circulation was found to concentrate around the acoustic source. Insight into the degree of dynamic stability was gained through modelling the fluid through one wave period where it was found that the model was able to resolve the expansive and contractive behavior of a boundary layer through one wave period. The results were also able to visualise the cooling mechanisms by resolving the temperature distribution across the entire period. The effects of geometry on heat transfer were studied to ascertain an optimum configuration where enhanced heat transfer was seen to be at its greatest for the given domain. A detailed study of geometry height was conducted to investigate the changes in loop formulation and the consequential effect on heat transfer. Heat transfer was found to correlate with flow structure stability in which two distinctive counter rotating loops resulted in an increase disruption to the thermal boundary layer. This was found to happen in the medium position of the height range
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