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1
Barua, Suchi. "Modelling and analysis of semiconductor optical amplifiers for high-speed communication systems using finite-difference beam propagation method". Thesis, Curtin University, 2014. http://hdl.handle.net/20.500.11937/1406.
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Semiconductor optical amplifiers (SOAs) have attracted lots of interests because of their application potential in the field of optical communications. The output pulse of SOA can be changed due to the variation of input parameters such as input pulse shape, input pulse width, input pulse energy. For this reason, in this research pulse shape dependent propagation characteristics and gain saturation characteristics depending on different parameters have been analyzed and compared for faster communication systems.
2
Yao, Lan. "Experimental and numerical study of dynamic crack propagation in ice under impact loading". Thesis, Lyon, 2016. http://www.theses.fr/2016LYSEI043/document.
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Les phénomènes liés au comportement à la rupture de la glace sous impact sont fréquents dans le génie civil, pour les structures offshore, et les processus de dégivrage. Pour réduire les dommages causés par l'impact de la glace et optimiser la conception des structures ou des machines, l'étude sur le comportement à la rupture dynamique de la glace sous impact est nécessaire. Ces travaux de thèse portent donc sur la propagation dynamique des fissures dans la glace sous impact. Une série d'expériences d'impact est réalisée avec un dispositif de barres de Hopkinson. La température est contrôlée par une chambre de refroidissement. Le processus dynamique de la rupture de la glace est enregistré avec une caméra à grande vitesse et ensuite analysé par des méthodes d'analyse d'images. La méthode des éléments finis étendus complète cette analyse pour évaluer la ténacité dynamique. Au premier abord, le comportement dynamique de la glace sous impact est étudié avec des échantillons cylindriques afin d'établir la relation contrainte-déformation dynamique qui sera utilisée dans les simulations numériques plus tard. Nous avons observé de multi-fissuration dans les expériences sur les échantillons cylindriques mais son étude est trop difficile à mener. Pour mieux comprendre la propagation des fissures dans la glace, des échantillons rectangulaires avec une pré-fissure sont employés. En ajustant la vitesse d'impact on aboutit à la rupture des spécimens avec une fissure principale à partir de la pré-fissure. L'histoire de la propagation de fissure et de sa vitesse sont évaluées par analyse d'images basée sur les niveaux de gris et par corrélation d'images. La vitesse de propagation de la fissure principale est identifiée dans la plage de 450 à 610 m/s ce qui confirme les résultats précédents. Elle varie légèrement au cours de la propagation, dans un premier temps elle augmente et se maintient constante ensuite et diminue à la fin. Les paramètres obtenus expérimentalement, tels que la vitesse d'impact et la vitesse de propagation de fissure, sont utilisés pour la simulation avec la méthode des éléments finis étendus. La ténacité d'initiation dynamique et la ténacité dynamique en propagation de fissure sont déterminées lorsque la simulation correspond aux expériences. Les résultats indiquent que la ténacité dynamique en propagation de fissure est linéaire vis à vis de la vitesse de propagation et semble indépendante de la température dans l'intervalle -15 à -1 degrésThe phenomena relating to the fracture behaviour of ice under impact loading are common in civil engineering, for offshore structures, and de-ice processes. To reduce the damage caused by ice impact and to optimize the design of structures or machines, the investigation on the dynamic fracture behaviour of ice under impact loading is needed. This work focuses on the dynamic crack propagation in ice under impact loading. A series of impact experiments is conducted with the Split Hopkinson Pressure Bar. The temperature is controlled by a cooling chamber. The dynamic process of the ice fracture is recorded with a high speed camera and then analysed by image methods. The extended finite element method is complementary to evaluate dynamic fracture toughness at the onset and during the propagation. The dynamic behaviour of ice under impact loading is firstly investigated with cylindrical specimen in order to obtain the dynamic stress-strain relation which will be used in later simulation. We observed multiple cracks in the experiments on the cylindrical specimens but their study is too complicated. To better understand the crack propagation in ice, a rectangular specimen with a pre-crack is employed. By controlling the impact velocity, the specimen fractures with a main crack starting from the pre-crack. The crack propagation history and velocity are evaluated by image analysis based on grey-scale and digital image correlation. The main crack propagation velocity is identified in the range of 450 to 610 m/s which confirms the previous results. It slightly varies during the propagation, first increases and keeps constant and then decreases. The experimentally obtained parameters, such as impact velocity and crack propagation velocity, are used for simulations with the extended finite element method. The dynamic crack initiation toughness and dynamic crack growth toughness are determined when the simulation fits the experiments. The results indicate that the dynamic crack growth toughness is linearly associated with crack propagation velocity and seems temperature independent in the range -15 to -1 degrees
3
Li, Liangpan. "Local spectral asymptotics and heat kernel bounds for Dirac and Laplace operators". Thesis, Loughborough University, 2016. https://dspace.lboro.ac.uk/2134/23004.
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In this dissertation we study non-negative self-adjoint Laplace type operators acting on smooth sections of a vector bundle. First, we assume base manifolds are compact, boundaryless, and Riemannian. We start from the Fourier integral operator representation of half-wave operators, continue with spectral zeta functions, heat and resolvent trace asymptotic expansions, and end with the quantitative Wodzicki residue method. In particular, all of the asymptotic coefficients of the microlocalized spectral counting function can be explicitly given and clearly interpreted. With the auxiliary pseudo-differential operators ranging all smooth endomorphisms of the given bundle, we obtain certain asymptotic estimates about the integral kernel of heat operators. As applications, we study spectral asymptotics of Dirac type operators such as characterizing those for which the second coefficient vanishes. Next, we assume vector bundles are trivial and base manifolds are Euclidean domains, and study non-negative self-adjoint extensions of the Laplace operator which acts component-wise on compactly supported smooth functions. Using finite propagation speed estimates for wave equations and explicit Fourier Tauberian theorems obtained by Yuri Safarov, we establish the principle of not feeling the boundary estimates for the heat kernel of these operators. In particular, the implied constants are independent of self-adjoint extensions. As a by-product, we affirmatively answer a question about upper estimate for the Neumann heat kernel. Finally, we study some specific values of the spectral zeta function of two-dimensional Dirichlet Laplacians such as spectral determinant and Casimir energy. For numerical purposes we substantially improve the short-time Dirichlet heat trace asymptotics for polygons. This could be used to measure the spectral determinant and Casimir energy of polygons whenever the first several hundred or one thousand Dirichlet eigenvalues are known with high precision by other means.
4
Bacon, David R. "Finite amplitude propagation in acoustic beams". Thesis, University of Bath, 1986. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.483000.
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Meyer, Arnd, Frank Rabold y Matthias Scherzer. "Efficient finite element simulation of crack propagation". Universitätsbibliothek Chemnitz, 2006. http://nbn-resolving.de/urn:nbn:de:swb:ch1-200601402.
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The preprint delivers an efficient solution technique for the numerical simulation of crack propagation of 2D linear elastic formulations based on finite elements together with the conjugate gradient method in order to solve the corresponding linear equation systems. The developed iterative numerical approach using hierarchical preconditioners comprehends the interesting feature that the hierarchical data structure will not be destroyed during crack propagation. Thus, one gets the possibility to simulate crack advance in a very effective numerical manner including adaptive mesh refinement and mesh coarsening. Test examples are presented to illustrate the efficiency of the given approach. Numerical simulations of crack propagation are compared with experimental data.
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Chao, Jenny C. 1976. "The propagation mechanism of high speed turbulent deflagrations /". Thesis, McGill University, 2002. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=33961.
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The propagation regimes of combustion waves in a 30 cm by 30 cm square cross-sectioned tube with an obstacle array of staggered vertical cylindrical rods (with BR = 0.41 and BR = 0.19) are investigated. Mixtures of hydrogen, ethylene, propane, and methane with air at ambient conditions over a range of equivalence ratios are used. In contrast to the previous results obtained in circular cross-sectioned tubes, it is found that only the quasi-detonation regime and the slow turbulent deflagration regimes are observed for ethylene-air and for propane-air. The transition from the quasi-detonation regime to the slow turbulent deflagration regime occurs at D/lambda ≈ 1 (where D is the tube "diameter" and lambda is the detonation cell size). When D/lambda >> 1, the quasi-detonation velocities that are observed are similar to those in unobstructed smooth tubes. For hydrogen-air mixtures, it is found that there is a gradual transition from the quasi-detonation regime to the high speed turbulent deflagration regime. The high speed turbulent deflagration regime is also observed for methane-air mixtures near stoichiometric composition. (Abstract shortened by UMI.)
7
Ordovas, Miquel Roland. "Covariant projection finite elements for transient wave propagation". Thesis, Imperial College London, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.342285.
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Ritchie, Stephen John Kerr. "The high speed double torsion test". Thesis, Imperial College London, 1996. http://hdl.handle.net/10044/1/11437.
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Jurgens, Henry Martin. "High-accuracy finite-difference schemes for linear wave propagation". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ27970.pdf.
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Lilla, Antonio de. "Finite difference seismic wave propagation using variable grid sizes". Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/54427.
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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1997.Includes bibliographical references (leaves 115-118).
by Antonio De Lilla.
M.S.
11
Arshad, Kamran. "Modelling of radio wave propagation using Finite Element Analysis". Thesis, Middlesex University, 2007. http://eprints.mdx.ac.uk/9768/.
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Fourth generation (4G) wireless communication systems are intended to support high data rates which requires careful and accurate modelling of the radio environment. In this thesis, for the first time finite clement based accurate and computationally efficient models of wave propagation in different outdoor and indoor environments has been developed. Three different environments were considered: the troposphere, vegetation and tunnels and wave propagation in these environments were modelled using finite element analysis. Use of finite elements in wave propagation modelling is a novel idea although many propagation models and approaches were used in past. Coverage diagrams, path loss contours and power levels were calculated using developed models in the troposphere, vegetation and tunnels. Results obtained were compared with commercially available software Advanced Refractive Effects Prediction Software (AREPS) to validate the accuracy of the developed approach and it is shown that results were accurate with an accuracy of 3dB. The developed models were very flexible in handling complex geometries and similar analysis can be easily extended to other environments. A fully vectored finite element base propagation model was developed for straight and curved tunnels. An optimum range of values of different electrical parameters for tunnels of different shapes has been derived. The thesis delivered a novel approach to modelling radio channels that provided a fast and accurate solution of radio wave propagation in realistic environments. The results of this thesis will have a great impact in modelling and characterisation of future wireless communication systems.
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Hamiche, Karim. "A high-order finite element model for acoustic propagation". Thesis, University of Southampton, 2016. https://eprints.soton.ac.uk/400677/.
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Sound propagation in complex non-uniform mean flows is an important research area for transport, building and power generation industries. Unsteady flows are responsible for noise generation in rotating and pulsating machines. Sound propagates in ducts and radiates through their openings. Duct discontinuities and complex flow effects on acoustic propagation need to be investigated. Although it provides accurate results, the most commonly used Computational AeroAcoustics propagation method, the full potential theory, does not describe the whole physics. Turbofan exhaust noise radiation involves strong refraction of the sound field occurring through jet shear layer, as well as interaction between the acoustic field and the vorticity/entropy waves. The Linearised Euler Equations are able to represent these effects. Solving these equations with time-domain solvers presents shortcomings such as linear instabilities and impedance modelling, which can be avoided by solving in the frequency domain. Nevertheless the classical Finite Element Method in frequency domain suffers from dispersion error and high memory requirements. These drawbacks are particularly critical at high frequencies and with the Linearised Euler Equations, which involve up to five unknowns. To circumvent these obstacles a novel approach is developed in this thesis, using a high-order Finite Element Method to solve the Linearised Euler Equations in the frequency domain. The model involves high-order polynomial shape functions with unstructured triangular meshes, numerical stabilisation and Perfectly Matched Layers. The computational effort is further optimised by coupling the Linearised Euler Equations in the regions of complex sheared mean flow with the Linearised Potential Equation in the regions of irrotational mean flow. The numerical model is applied to aeroengine acoustic propagation either by an intake or by an exhaust. Comparisons with analytic solutions demonstrate the method accuracy which properly represents the acoustic and vorticity waves, as well as the refraction of the sound field across the jet shear layer. The benefits in terms of memory requirements and computation time are significant in comparison to the standard low-order Finite Element Method, even more so with the coupling technique.
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Mossberg, Eva. "Higher order finite difference methods for wave propagation problems". Licentiate thesis, Uppsala universitet, Avdelningen för teknisk databehandling, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-86003.
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Wave propagation is described by the wave equation, or in the time-periodic case, by the Helmholtz equation. For problems with small wavelengths, high order discretizations must be used to resolve the solution. Two different techniques for finding compact finite difference schemes of high order are studied and compared. The first approach is Numerov's idea of using the equation to transfer higher derivatives to lower order ones for the Helmholtz equation, or, for the wave equation, from time to space. The second principle is the method of deferred correction, where a lower order approximation is used for error correction. For the time-independent Helmholtz problem, sharp estimates for the error are derived, in order to compare the arithmetic complexity for both approaches with a non-compact scheme. The characteristics of the errors for fourth order as well as sixth order accuracy are demonstrated and the advantages and disadvantages of the methods are discussed. A time compact, Numerov-type, fourth order method and a fourth order method using deferred correction in time are studied for the wave equation. Schemes are derived for both the second order formulation of the equation, and for the system in first order form. Stability properties are analyzed and numerical experiments have been performed, for both constant and variable coefficients in the equations. For the first order formulation, a staggered grid is used.
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Baker, Andrew C. "Finite amplitude propagation of focused ultrasonic waves in water". Thesis, University of Bath, 1989. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329055.
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Fang, Wen. "Studies of propagation delay for high-speed bipolar logic circuits". Thesis, University of Southampton, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.280890.
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Príncipe, Ricardo Javier. "Subgrid scale stabilized finite elements for low speed flows". Doctoral thesis, Universitat Politècnica de Catalunya, 2008. http://hdl.handle.net/10803/6870.
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La descripción del flujo de fluidos involucra la solución de las ecuaciones de Navier-Stokes compresible, un problema muy complejo cuya estructura matemática no es del todo comprendida. Por lo tanto, mediante análisis asintótico, se pueden derivar modelos simplificados bajo ciertas hipótesis sobre el problema hechas en términos de parámetros adimensionales que miden la importancia relativa de los diferentes procesos físicos. Los flujos a baja velocidad se pueden describir por diferentes modelos que incluyen las ecuaciones de Navier Stokes incompresible cuya matemática es mucho mas conocida. Sin embargo, algunos flujos importantes no se pueden considerar incompresibles debido a la presencia de efectos térmicos. En esta clase de problemas se pueden derivar otra clase de ecuaciones simplificadas: las ecuaciones de Boussinesq y las ecuaciones de bajo numero de Mach.La complejidad de estos problemas matemáticos hace que su solución numérica sea muy difícil. En estos problemas el método de los elementos finitos es inestable, lo que en la práctica implica soluciones numéricas que presentan oscilaciones nodo a nodo de naturaleza no física. En las ecuaciones de Navier Stokes incompresible, dos fuentes bien conocidas de inestabilidad son la condición de incompresibilidad y la presencia del término convectivo. Muchas técnicas de estabilización utilizadas hoy en día se basan en la separación de escalas, descomponiendo la incógnita en una parte gruesa inducida por la discretización del domino y una parte fina de subescala. Modelar la subescala y su influencia conduce a un problema modificado para la escala gruesa que resulta estable.
Aunque las técnicas de estabilización son ampliamente utilizadas hoy en día, importantes problemas permanecen abiertos. Contribuyendo a su comprensión, en este trabajo se analizan varios aspectos del modelado de las subescalas. Para problemas escalares de segundo orden, se encuentra la dependencia de la subescala con el tamaño de la malla en el caso general de mallas anisótropas. Estas ideas son extendidas a sistemas de ecuaciones para considerar el problema de Oseen. También se analiza el modelado de las subescalas en problemas transitorios, obteniendo un mejor esquema de integración temporal para el problema de escala gruesa. Para considerar flujos a baja velocidad, se presenta la extensión de estas técnicas a problemas no lineales acoplados, lo que esta íntimamente relacionado con el problema del modelado de la turbulencia, que es un tema en si mismo.
Los flujos acoplados térmicamente, aparte del interés intrínseco que merecen, son importantes desde un punto de vista ingenieril. Una solución precisa del problema de flujo es necesaria para definir las cargas térmicas sobre las estructuras, que en muchos casos responden fuertemente, haciendo el problema acoplado. Esta clase de problemas, que motivaron este trabajo, incluyen la respuesta estructural en el caso de un incendio.
A general description of a fluid flow involves the solution of the compressible Navier-Stokes equations, a very complex problem whose mathematical structure is not well understood. Therefore, simplified models can be derived by asymptotic analysis under some assumptions on the problem, made in terms of dimensionless parameters that measure the relative importance of different physical processes. Low speed flows can be described by several models including the incompressible Navier Stokes equations whose mathematical structure is much better understood. However many important flows cannot be considered as incompressible, even at low speed, due to the presence of thermal effects. In such kind of problems another class of simplified equations can be derived: the Boussinesq equations and the Low Mach number equations.
The complexity of these mathematical problems makes their numerical solution very difficult. For these problems the standard finite element method is unstable, what in practice means that node to node oscillations of non physical nature may appear in the numerical solution. In the incompressible Navier Stokes equations, two well known sources of numerical instabilities are the incompressibility constraint and the presence of the convective terms. Many stabilization techniques used nowadays are based on scale separation, splitting the unknown into a coarse part induced by the discretization of the domain and a fine subgrid part. The modelling of the subgrid scale and its influence leads to a modified coarse scale problem that now can be shown to be stable.
Although stabilization techniques are nowadays widely used, important problems remain open. Contributing to their understanding, several aspects of the subgrid scale modelling are analyzed in this work. For second order scalar problems, the dependence of the subgrid scale on the mesh size, in the general anisotropic case, is clarified. These ideas are extended to systems of equations to consider the Oseen problem. The modelling of the subgrid scales in transient problems is also analyzed, leading to an improved time discretization scheme for the coarse scale problem. To consider low speed flow models, the extension of these techniques to nonlinear and coupled problems is presented, something that is intimately related to the problem of turbulence modelling, which a entire subject on its own right.
Thermally coupled flow problems, despite the intrinsic interest they deserve, are important from an engineering point of view. An accurate solution of a flow problem is needed to define thermal loads on structures which, in many cases have a strong response, making the problem coupled. This kind of problems, that motivated this work, include the problem of a structural response in the case of fires.
17
Li, Kuo-Hui. "RF beamformers for high-speed wireless communications". Diss., Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/14768.
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Erdem, Birsen. "Finite Volume Solutions Of 1d Euler Equations For High Speed Flows With Finite-rate Chemistry". Master's thesis, METU, 2003. http://etd.lib.metu.edu.tr/upload/2/703868/index.pdf.
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In this thesis, chemically reacting flows are studied mainly for detonation problems under 1D, cylindrical and spherical symmetry conditions. The mathematical formulation of chemically reacting, inviscid, unsteady flows with species conservation equations and finite-rate chemistry is described. The Euler equations with finite-rate chemistry are discretized by Finite-Volume method and solved implicitly by using a time-spliting method. Inviscid fluxes are computed using Roe Flux Difference Splitting Model. The numerical solution is implemented in parallel using domain decomposition and PVM library routines for inter-process communication. The solution algorithm is validated first against the numerical and experimental data for a shock tube problem with and without chemical reactions and for a cylindrical and spherical propagation of a shock wave. 1D, cylindrically and spherically symmetric detonations of H2:O2:Ar mixture are studied next.
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Slawinski, Raphael. "Finite-difference modeling of seismic wave propagation in fractured media". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2000. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape4/PQDD_0027/NQ49538.pdf.
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Lidgate, Simon. "Advanced finite difference - beam propagation : method analysis of complex components". Thesis, University of Nottingham, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.408596.
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Nešpůrek, Lukáš. "STOCHASTIC CRACK PROPAGATION MODELLING USING THE EXTENDED FINITE ELEMENT METHOD". Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2010. http://www.nusl.cz/ntk/nusl-233900.
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Tato disertační práce vychází z výzkumu v rámci francouzsko-českého programu doktorátu pod dvojím vedením na pracovišti Institut français de mécanique avancée v Clermont-Ferrand a na Ústavu fyziky materiálu AV v Brně. Úvodní výzkumný úkol na brněnském pracovišti se zabýval numerickou analýzou pole napětí v okolí čela trhliny v tenké kovové fólii. Zvláštní pozornost byla zaměřena na vliv speciálního typu singularity v průsečíku čela trhliny s volným povrchem. Těžiště disertační práce spočívá v numerickém modelování a stochastické analýze problémů šíření trhlin se složitou geometrií v dvojrozměrném prostoru. Při analýze těchto problémů se dříve zřídka používaly numerické metody, a to z důvodu vysoké náročnosti na výpočtový čas. V této disertaci je ukázáno, že aplikací moderních metod numerické mechaniky a vhodných technik v analýze spolehlivosti lze tyto problémy řešit s pomocí numerických metod i na PC. Ve spolehlivostní analýze byla využita lineární aproximační metoda FORM. Pro rychlost šíření trhlin se vycházelo z Parisova-Erdoganova vztahu. Pro parametry tohoto vztahu byl použit dvourozměrný statistický model, který postihuje vysokou citlivost na korelaci obou parametrů. Mechanická odezva byla počítána rozšířenou metodou konečných prvků (XFEM), která eliminuje výpočetní náročnost a numerický šum související se změnou sítě v klasické metodě konečných prvků. Prostřednictvím přímé diferenciace bylo odvozeno několik vztahů pro derivace funkce odezvy, čímž se dosáhlo lepší numerické stability a konvergence spolehlivostní analýzy a výrazného zkrácení doby výpočtu. Problém zatížení s proměnou amplitudou byl řešen aplikací transformace zatížení metodou PREFFAS. Využití distribuce výpočtů v síti PC umožnilo další zrychlení analýzy.
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Abdallah, Riyadh A. "Finite element based beam propagation analysis of optical semiconductor devices". Thesis, City, University of London, 2007. http://openaccess.city.ac.uk/20121/.
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Compact and low-cost semiconductor laser sources have significant potential for use in applications that are currently dominated by expensive solid-state lasers. The direct application of high-power semiconductor lasers for free-space and satellite communications, visual displays, biomedical applications and remote sensing, optical recording, spectroscopy, optical data storage, laser printers, laser radar and also for materials processing is becoming increasingly attractive due to the remarkable improvement in performance of high-power laser diodes. In addition, high-power spatially and spectrally coherent sources are required for the efficient pumping of solid-state and fiber lasers and efficient nonlinear frequency conversion to the short-wavelength part of the visible spectrum, which is not readily available with semiconductor sources directly. The early development of the semiconductor amplifier had initially been assisted by the use of the semi-analytical and numerical approaches, which has been extended to include segmented sections to allow for lateral variations of the optical and electronic parameters. In this work, a vectorial finite element beam propagation method (FEB PM), which is numerically efficient and has incorporated a wide-angle approach to tackle rapid axial variations and the perfectly matched boundary condition, to avoid reflections from the orthodox computational window, has been employed to study and design the guided-wave photonic devices. The evolution of the optical beam profile along a high power tapered semiconductor amplifier has been demonstrated by employing this method. Numerically simulated results indicate the generation of many higher order modes, and their interference with the fundamental mode causes a variation of the optical beam, both along the transverse and the axial directions, which could significantly modify the output beam quality, which also leads to beam filamentation. In this thesis, the FEBPM approach has also been utilized to study rigorously the complex refractive index profiles, which provide modal gain in the semiconductor structures. The power gain in an active photonic device, such as a laser or an amplifier is due to the presence of the imaginary part of the complex refractive index in the core. The injected current generates carrier density and when the density is above the transparent carrier density then the optical field can be amplified. In case of a highpower tapered semiconductor optical amplifier (SOA), the width of the SOA changes continuously, which reduces the power density to improve the total gain. The modal gain properties and field expansion have therefore been examined in this work. The effect of gain reduction along the transverse directions due to non-uniform transverse field profile is also demonstrated. Furthermore, the effect of gain saturation on the total optical gain of the amplifier is studied by considering both the transverse and axial variation of the local gain coefficient. Finally, the study of the far field profiles and birefringence for various tapered waveguide structures, with particular interest to the very wide width SOA structures is carried out.
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Lee, Sun Ung. "Dynamics of high-speed rotating machines". Thesis, Imperial College London, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.326078.
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Owadally, A. S. "Propagation models for an improved trade-off between speed and accuracy". Thesis, University of Surrey, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.402874.
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Ghazal, Ammar. "Propagation channel characterisation and modelling for high-speed train communication systems". Thesis, Heriot-Watt University, 2015. http://hdl.handle.net/10399/3079.
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High-mobility scenarios, e.g., High-Speed Train (HST) scenarios, are expected to be typical scenarios for the Fifth Generation (5G) communication systems. With the rapid development of HSTs, an increasing volume of wireless communication data is required to be transferred to train passengers. HST users demand high network capacity and reliable communication services regardless of their locations or speeds, which are beyond the capability of current HST communication systems. The features of HST channels are significantly different from those of low-mobility cellular communication systems. For a proper design and evaluation of future HST wireless communication systems, we need accurate channel models that can mimic the underlying channel characteristics, especially the non-stationarity for different HST scenarios. Inspired by the lack of such accurate HST channel models in the literature, this PhD project is devoted to the modelling and simulation of non-stationary Multiple-Input Multiple-Output (MIMO) channels for HST communication systems. In this thesis, we first give a comprehensive review of the measurement campaigns conducted in different HST scenarios and address the recent advances in HST channel models. We also highlight the key challenges of HST channel measurements and models. Then, we study the characterisation of non-stationary channels and propose a theoretical framework for deriving the statistical properties of these channels. HST wireless communication systems encounter different channel conditions due to the difference of surrounding geographical environments or scenarios. HST channel models in the literature have either considered large-scale parameters only and/or neglected the non-stationarity of HST channels and/or only consider one of the HST scenarios. Therefore, we propose a novel generic non-stationary Geometry-Based Stochastic Model (GBSM) for wideband MIMO HST channels in different HST scenarios, i.e., open space, viaduct, and cutting. The corresponding simulation model is then developed with angular parameters calculated by the Modified Method of Equal Area (MMEA). The system functions and statistical properties of the proposed channel models are thoroughly studied. The proposed generic non-stationary HST channel models are verified by measurements in terms of stationary time for the open space scenario and the Autocorrelation Function (ACF), Level Crossing Rate (LCR), and stationary distance for the viaduct and cutting scenarios. Transmission techniques which are capable of utilising Three-Dimensional (3D) spatial dimensions are significant for the development of future communication systems. Consequently, 3D MIMO channel models are critical for the development and evaluation of these techniques. Therefore, we propose a novel 3D generic non-stationary GBSM for wideband MIMO HST channels in the most common HST scenarios. The corresponding simulation model is then developed with angular parameters calculated by the Method of Equal Volume (MEV). The proposed models considers several timevarying channel parameters, such as the angular parameters, the number of taps, the Ricean K-factor, and the actual distance between the Transmitter (Tx) and Receiver (Rx). Based on the proposed generic models, we investigate the impact of the elevation angle on some of the channel statistical properties. The proposed 3D generic models are verified using relevant measurement data. Most standard channel models in the literature, like Universal Mobile Telecommunications System (UMTS), COST 2100, and IMT-2000 failed to introduce any of the HST scenarios. Even for the standard channel models which introduced a HST scenario, like IMT-Advanced (IMT-A) and WINNER II channel models, they offer stationary intervals that are noticeably longer than those in measured HST channels. This has inspired us to propose a non-stationary IMT-A channel model with time-varying parameters including the number of clusters, powers, delays of the clusters, and angular parameters. Based on the proposed non-stationary IMT-A channel model, important statistical properties, i.e., the time-variant spatial Cross-correlation Function (CCF) and time-variant ACF, are derived and analysed. Simulation results demonstrate that the stationary interval of the developed non-stationary IMT-A channel model can match that of relevant HST measurement data. In summary, the proposed theoretical and simulation models are indispensable for the design, testing, and performance evaluation of 5G high-mobility wireless communication systems in general and HST ones in specific.
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Wheel, Marcus A. "High speed double torsion testing of pipe grade polyethylenes". Thesis, Imperial College London, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.318493.
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Ozenc, Kaan. "Finite Element Simulation Of Crack Propagation For Steel Fiber Reinforced Concrete". Master's thesis, METU, 2009. http://etd.lib.metu.edu.tr/upload/3/12610837/index.pdf.
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Steel fibers or fibers in general are utilized in concrete to control the tensile cracking and to increase its toughness. In literature, the effects of fiber geometry, mechanical properties, and volume on the properties of fiber reinforced concrete have often been experimentally investigated by numerous studies. Those experiments have shown that useful improvements in the mechanical behavior of brittle concrete are achieved by incorporating steel fibers. This study proposes a simulation platform to determine the influence of fibers on crack propagation and fracture behavior of fiber reinforced concrete. For this purpose, a finite element (FE) simulation tool is developed for the fracture process of fiber reinforced concrete beam specimens subjected to flexural bending test.
Within this context, the objective of this study is twofold. The first one is to investigate the effects of finite element mesh size and element type on stress intensity factor (SIF) calculation through finite element analysis. The second objective is to develop a simulation of the fracture process of fiber reinforced concrete beam specimens.
The properties of the materials, obtained from literature, and the numerical simulation procedure, will be explained. The effect of fibers on SIF is included by unidirectional elements with nonlinear generalized force-deflection capability. Distributions and orientation of fibers and possibility of anchorage failure are also added to simulation.
As a result of this study it was observed that with the adopted simulation tool, the load-deflection relation obtained by experimental studies is predicted reasonably.
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George, David L. "Finite volume methods and adaptive refinement for tsunami propagation and inundation /". Thesis, Connect to this title online; UW restricted, 2006. http://hdl.handle.net/1773/6752.
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Zheng, Hui. "Application of the hybrid finite element procedure to crack band propagation". Ohio : Ohio University, 1987. http://www.ohiolink.edu/etd/view.cgi?ohiou1183125160.
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Dalo, Dominic N. "A finite element solution of thermal wave propagation in elastic media /". Online version of thesis, 1987. http://hdl.handle.net/1850/8821.
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Kim, Hyun Sil. "Instability of finite amplitude wave propagation in harmonically heterogeneous elastic solids". Diss., Georgia Institute of Technology, 1989. http://hdl.handle.net/1853/16437.
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O'Rourke, Carl. "Finite-difference time-domain simulation of femtosecond pulse propagation in semiconductor". Thesis, University of Surrey, 2006. http://epubs.surrey.ac.uk/843730/.
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A new model is developed for simulating the injection of short-duration optical pulses into semiconductor lasers. The optical gain of the semiconductor is described by an empirical model which fixes all parameters following a fit to gain curves at two different densities of population inversion. The two-curve fit enforces the correct behaviour with respect to changes in carrier density and is intermediate between a rate equation model and a full microscopic approach. The electromagnetic field is integrated by the finite-difference time-domain method. The model incorporates an accurate complex susceptibility, material and gain dispersion and full phase information, and may be integrated on a desktop computer to simulate time evolution over a period of nanoseconds in a matter of hours. An appropriate choice of the grid resolution may be used to incorporate a realistic frequency dependent refractive index and simultaneously reduce the time for computation. The model is applied to the experimentally observed phenomena of dark pulse formation following injection of optical pulses into semiconductor lasers and furnishes an explanation for their formation, subsequent evolution and stability. The stability of dark pulses is explained as a re-distribution of the steady-state electric field. These investigations led to a hypothesis, and subsequent demonstration by numerical simulation, for generating stable streams of customisable pulse trains by coherent control of the phase of the injected pulse.
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Holt-Phoenix, Marianne S. (Marianne Shue). "Wave propagation in finite element and mass-spring-dashpot lattice models". Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/35683.
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Thesis (Nav. E. and S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (p. 42).
Numerical efficiency comparisons of a four-node finite element model (FEM), a mass-spring lattice model (MSLM), and a mass-spring-dashpot lattice model (MSDLM) are investigated. Specifically, the error in the ultrasonic phase speed with variations in Poisson's ratio and angle of incidence is evaluated in each model of an isotropic elastic solid. With regard to phase speed, materials with constant N grid spaces per P-wavelength having Poisson's ratios between 0.0 and 0.25 are modeled more accurately with the MSLM. Materials with Poisson's ratios between 0.35 and 0.5 and N grid spaces per P-wavelength are more accurately modeled with the FEM. Materials whose Poisson's ratio is between 0.25 and 0.35 are modeled equally accurately. With regard to phase speed, viscoelastic materials modeled with FEM and MSDLM show good agreement with known analytical solutions. The computational expense of all three models is also examined. The number of floating point operations (FLOPS) needed to achieve a specified phase speed accuracy is calculated for each different model. While the FEM and MSLM have nearly the same computation cost, the MSDLM is 5 times more costly than either the FEM or MSLM.
by Marianne S. Holt-Phoenix.
Nav.E.and S.M.
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Burrows, Richard. "Numerical finite element modelling of the high speed resistance welding process". Thesis, Swansea University, 2008. https://cronfa.swan.ac.uk/Record/cronfa42942.
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Hurrell, Andrew M. "Finite difference modelling of acoustic propagation and its applications in underwater acoustics". Thesis, University of Bath, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.250842.
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Hayner, Mark A. (Mark Andrew). "Optimized finite difference schemes for wave propagation in high loss viscoelastic material". Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/36488.
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Arthurs, Christopher J. "Efficient simulation of cardiac electrical propagation using adaptive high-order finite elements". Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:ad31f06f-c4ed-4c48-b978-1ef3b12fe7a1.
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This thesis investigates the high-order hierarchical finite element method, also known as the finite element p-version, as a computationally-efficient technique for generating numerical solutions to the cardiac monodomain equation. We first present it as a uniform-order method, and through an a priori error bound we explain why the associated cardiac cell model must be thought of as a PDE and approximated to high-order in order to obtain the accuracy that the p-version is capable of. We perform simulations demonstrating that the achieved error agrees very well with the a priori error bound. Further, in terms of solution accuracy for time taken to solve the linear system that arises in the finite element discretisation, it is more efficient that the state-of-the-art piecewise linear finite element method. We show that piecewise linear FEM actually introduces quite significant amounts of error into the numerical approximations, particularly in the direction perpendicular to the cardiac fibres with physiological conductivity values, and that without resorting to extremely fine meshes with elements considerably smaller than 70 micrometres, we can not use it to obtain high-accuracy solutions. In contrast, the p-version can produce extremely high accuracy solutions on meshes with elements around 300 micrometres in diameter with these conductivities. Noting that most of the numerical error is due to under-resolving the wave-front in the transmembrane potential, we also construct an adaptive high-order scheme which controls the error locally in each element by adjusting the finite element polynomial basis degree using an analytically-derived a posteriori error estimation procedure. This naturally tracks the location of the wave-front, concentrating computational effort where it is needed most and increasing computational efficiency. The scheme can be controlled by a user-defined error tolerance parameter, which sets the target error within each element as a proportion of the local magnitude of the solution as measured in the H^1 norm. This numerical scheme is tested on a variety of problems in one, two and three dimensions, and is shown to provide excellent error control properties and to be likely capable of boosting efficiency in cardiac simulation by an order of magnitude. The thesis amounts to a proof-of-concept of the increased efficiency in solving the linear system using adaptive high-order finite elements when performing single-thread cardiac simulation, and indicates that the performance of the method should be investigated in parallel, where it can also be expected to provide considerable improvement. In general, the selection of a suitable preconditioner is key to ensuring efficiency; we make use of a variety of different possibilities, including one which can be expected to scale very well in parallel, meaning that this is an excellent candidate method for increasing the efficiency of cardiac simulation using high-performance computing facilities.
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Kundu, Abhishek. "Efficient uncertainty propagation schemes for dynamical systems with stochastic finite element analysis". Thesis, Swansea University, 2014. https://cronfa.swan.ac.uk/Record/cronfa42292.
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Efficient uncertainty propagation schemes for dynamical systems are investigated here within the framework of stochastic finite element analysis. Uncertainty in the mathematical models arises from the incomplete knowledge or inherent variability of the various parametric and geometric properties of the physical system. These input uncertainties necessitate the use of stochastic mathematical models to accurately capture their behavior. The resolution of such stochastic models is computationally quite expensive. This work is concerned with development of model order reduction techniques for obtaining the dynamical response statistics of stochastic finite element systems. Efficient numerical methods have been proposed to propagate the input uncertainty of dynamical systems to the response variables. Response statistics of randomly parametrized structural dynamic systems have been investigated with a reduced spectral function approach. The frequency domain response and the transient evolution of the response of randomly parametrized structural dynamic systems have been studied with this approach. An efficient discrete representation of the input random field in a finite dimensional stochastic space is proposed here which has been integrated into the generic framework of the stochastic finite element weak formulation. This framework has been utilized to study the problem of random perturbation of the boundary surface of physical domains. Truncated reduced order representation of the complex mathematical quantities which are associated with the stochastic isoparametric mapping of the random domain to a deterministic master domain within the stochastic Galerkin framework have been provided. Lastly, an a-priori model reduction scheme for the resolution of the response statistics of stochastic dynamical systems has also been studied here which is based on the concept of balanced truncation. The performance and numerical accuracy of the methods proposed in this work have been exemplified with numerical simulations of stochastic dynamical systems and the convergence behavior of various error indicators.
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Gültop, Tekin. "A finite strain theory of elastoplasticity and its application to wave propagation". Doctoral thesis, University of Cape Town, 1993. http://hdl.handle.net/11427/17310.
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Bibliography: pages 154-164.A constitutive theory of finite strain plasticity is developed by using the methods of convex analysis. The theory abstracts and extends the classical assumptions of a convex region of admissible stresses, and the normality law. The overall effects of plastic behaviour are contained in the theory through the presence of one or more internal variables. The thermodynamic restrictions of the second law together with the use of results of convex analysis lead in a natural way to the evolution equation or flow law. Non-smooth yield surfaces are included in the theory; nevertheless, the form of this theory makes a study of propagation of singular surfaces awkward. With a view to carrying out such a study, an alternative means of treating non-smooth convex yield surfaces is developed. This alternative theory is essentially a synthesis of the theory of Sewell, and that presented earlier in the thesis. The theory of singular surfaces is reviewed in the context of finite strain elastoplasticity, and necessary conditions for the propagation of acceleration waves are derived. A comparison of elastic and plastic wave speeds is made, and inequalities similar to those of Mandel for the small-strain case are derived. The propagation conditions for principal waves in both longitudinal and transverse directions, and the corresponding wave speeds, are found and compared for solids obeying a neo-Hookean elastic law, and with either the von Mises or Tresca yield criteria.
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Senarath, Aditha Srikantha. "Finite Different Time-Domain Simulation of Terahertz Waves Propagation Through Unmagnetized Plasma". Wright State University / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=wright1629431383655508.
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Land, J. George. "An axisymmetric finite element solution for elastic wave propagation through threaded connections". Thesis, This resource online, 1996. http://scholar.lib.vt.edu/theses/available/etd-11072008-063025/.
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Kursungecmez, Hatice. "Numerical simulation of shock propagation in one and two dimensional domains". Thesis, University of Edinburgh, 2015. http://hdl.handle.net/1842/14174.
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The objective of this dissertation is to develop robust and accurate numerical methods for solving the compressible, non-linear Euler equations of gas dynamics in one and two space dimensions. In theory, solutions of the Euler equations can display various characteristics including shock waves, rarefaction waves and contact discontinuities. To capture these features correctly, highly accurate numerical schemes are designed. In this thesis, two different projects have been studied to show the accuracy and utility of these numerical schemes. Firstly, the compressible, non-linear Euler equations of gas dynamics in one space dimension are considered. Since the non-linear partial differential equations (PDEs) can develop discontinuities (shock waves), the numerical code is designed to obtain stable numerical solutions of the Euler equations in the presence of shocks. Discontinuous solutions are defined in a weak sense, which means that there are many different solutions of the initial value problems of PDEs. To choose the physically relevant solution among the others, the entropy condition was applied to the problem. This condition is then used to derive a bound on the solution in order to satisfy L2-stability. Also, it provides information on how to add an adequate amount of diffusion to smooth the numerical shock waves. Furthermore, numerical solutions are obtained using far-field and no penetration (wall) boundary conditions. Grid interfaces were also included in these numerical computations. Secondly, the two dimensional compressible, non-linear Euler equations are considered. These equations are used to obtain numerical solutions for compressible ow in a shock tube with a 90° circular bend for two channels of different curvatures. The cell centered finite volume numerical scheme is employed to achieve these numerical solutions. The accuracy of this numerical scheme is tested using two different methods. In the first method, manufactured solutions are used to the test the convergence rate of the code. Then, Sod's shock tube test case is implemented into the numerical code to show the correctness of the code in both ow directions. The numerical method is then used to obtain numerical solutions which are compared with experimental data available in the literature. It is found that the numerical solutions are in a good agreement with these experimental results.
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Reid, Robert. "Propagation and period-doubling of coherent structures in coupled lattice maps". Thesis, University of Warwick, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.369417.
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Hussain, Tanweer. "Modelling and controlling variation propagation in mechanical assembly of high speed rotating machines". Thesis, University of Nottingham, 2012. http://eprints.nottingham.ac.uk/28465/.
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Assembly plays a vital role in the quality of a final product and has a great impact on the manufacturing cost. The mechanical assemblies consist of parts that inevitably have variations from their ideal dimensions. These variations propagate and accumulate as parts are assembled together. Excessive amount of variations in an assembly may cause improper functionality of the product being assembled. Improving assembly quality and reducing the assembly time and cost are the main objectives of this thesis. The quality of an assembly is determined in terms of variations in critical assembly dimensions, also known as Key Characteristics (KCs). Key Characteristics are designated to indicate where excess variation will affect product quality and what product features and tolerances require special attention. In order to improve assembly quality and reduce assembly time and cost, it is necessary to: (1) model non-ideal parts based on tolerances defined in design standards or current industrial practice of component inspection, (2) model assemblies and their associated assembly processes to analyse tolerance stack-up in the assembly, (3) develop probabilistic model to predict assembly variation after product assembly, and (4) implement control strategies for minimising assembly variation propagations to find optimum configuration of the assembly. Two assembly models have been developed, a linear model and a fully non-linear model for calculating assembly variation propagations. The assembly models presented in this thesis also allows for inclusion of geometric feature variation of each assembly component. Methods of incorporating geometric feature variations into an assembly variation model are described and analysis techniques are explained. The assembly variation model and the geometric variation models have been developed for 20 and 3D assemblies. Modelling techniques for incorporating process and measurement noise are also developed and described for the nonlinear assembly model and results are given to demonstrate the calculation of assembly variations while considering part, process and measurement errors. Two assembly case studies originating in sub-assemblies of aero-engines have been studied: Case Study 1, representing the rotating part (rotor) of an aero-engine, and Case Study 2, representing non-rotating part (stator) of an aero-engine. A probabilistic method based on the linear model is presented as a general analytical method for analysis of 3D mechanical assemblies. Probability density functions are derived for assembly position errors to analyse a general mechanical assembly, and separate probability functions are derived for the Key Characteristics (KCs) for assembly in Case Studies 1 and 2. The derived probability functions are validated by using the Monte Carlo simulation method based on the exact (full non-linear) model. Results showed that the proposed probabilistic method of estimating tolerance accumulation in mechanical assemblies is very efficient and accurate when compared to the Monte Carlo simulation method, particularly if large variations at the tails of the distributions are considered. Separate control strategies have been implemented for each case study. Four methods are proposed to minimise assembly variations for Case Study 1, and one error minimisation method is suggested for assemblies of Case Study 2. Based on the developed methods to optimise assembly quality, the two case studies were investigated, and it was found that the proposed optimisation methods can significantly improve assembly quality. The developed optimisation methods do not require any special tooling (such as fixtures) and can easily be implemented in practice.
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Haxha, Shyqyri. "Optimization of ultra-high speed electrooptic modulators using the finite element method". Thesis, City University London, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.397673.
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Tong, Tsz Kin Jimmy. "A finite element approach to the planing problem of high speed craft". Thesis, University of Southampton, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.278920.
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Tilbrook, Matthew Thomas Materials Science & Engineering Faculty of Science UNSW. "Fatigue crack propagation in functionally graded materials". Awarded by:University of New South Wales. Materials Science & Engineering, 2005. http://handle.unsw.edu.au/1959.4/21885.
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Propagation of cracks in functionally graded materials (FGMs) under cyclic loading was investigated via experiments and finite element (FE) analysis. Alumina-epoxy composites with an interpenetrating-network structure and tailored spatial variation in composition were produced via a multi-step infiltration technique. Compressed polyurethane foam was infiltrated with alumina slip. After foam burn-out and sintering, epoxy was infiltrated into the porous alumina body. Non-graded specimens with a range of compositions were produced, and elastic properties and fatigue behaviour were characterised. An increase in crack propagation resistance under cyclic loading was quantified via a novel analytical approach. A simulation platform was developed with the commercial FE package ANSYS. Material gradient was applied via nodal temperature definitions. Stress intensity factors were calculated from nodal displacements near the crack-tip. Deflection criteria were compared and the local symmetry criterion provided the most accurate and efficient predictions. An automated mesh-redefinition algorithm enabled incremental simulation of crack propagation. Effects of gradient and crack-geometry parameters on crack-tip stresses were investigated, along with influences of crack-shape, crack-bridging, residual stresses and plasticity. The model provided predictions and data analysis for experimental specimens. Fatigue cracks in graded specimens deflected due to elastic property mismatch, concordant with FE predictions. In other FGMs, thermal or plastic properties may dominate deflection behaviour. Weaker step-interfaces influenced crack paths in some specimens; otherwise effects of toughness variation and gradient steps on crack path were negligible. Crack shape has an influence, but this is secondary to that of elastic gradient. Cracks in FGM specimens initially experienced increase in fatigue resistance with crack-extension followed by sudden decreases at step-interfaces. Bridging had a notable effect on crack propagation resistance but not on crack path. Similarly, crack paths did not differ between monotonic and cyclic loading, although crack-extension effects did. Recommendations for analysis and optimisation strategies for other FGM systems are given. Experimental characterization of FGMs is important, rather than relying on theoretical models. Opportunities for optimization of graded structures are limited by the properties of the constituent materials and resultant general crack deflection behaviour.
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Jahnke, Gunnar. "Methods for Seismic Wave Propagation on Local and Global Scales with Finite Differences". Diss., lmu, 2009. http://nbn-resolving.de/urn:nbn:de:bvb:19-112352.
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Liow, J. (Jeih-San). "A two dimensional finite-difference simulation of seismic wave propagation in elastic media". Diss., Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/25781.
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Gonzalez, Csaszar Eduardo. "Analysis of optical propagation in isotropic nonlinear devices by the finite element method". Thesis, University College London (University of London), 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.244741.
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