Dissertations / Theses on the topic 'Time domain spectral element'
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Davies, Richard Wyn. "A hybrid spectral element method for the time domain solution of wave scattering problems." Thesis, Swansea University, 2007. https://cronfa.swan.ac.uk/Record/cronfa42272.
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This thesis considers the development and analysis of a hybrid spectral element method for the solution of two-dimensional wave scattering problems in the time domain. The components, namely a quadrilateral formulation of the diagonal mass matrix spectral element method and a triangular formulation of the spectral discontinuous Galerkin finite element method, are introduced and tested separately before being coupled to form the final hybrid procedure. Subsequently, a simple circular scattering problem is analysed to validate the computational model and various methods of curved boundary representation are tested to assess their impact on solution accuracy. Finally, a range of two-dimensional wave scattering problems are modelled, showing the computational efficiency of the higher order approximation in comparison with low order linear models.
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Bottero, Alexis. "Simulation numérique en forme d'onde complète d'ondes T et de sources acoustiques en mouvement." Thesis, Aix-Marseille, 2018. http://www.theses.fr/2018AIXM0325/document.
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Cette thèse mêle observations, simulations et développement d'outils numériques haute performance dans le domaine de l’acoustique sous-marine, et notamment pour l’étude des ondes T. Après une revue de la littérature sur les ondes T, nous avons analysé des données réelles enregistrées en Italie. Afin de modéliser le phénomène nous avons développé un solveur éléments spectraux axisymétriques dans le domaine temporel, que nous présentons et validons. Nous présentons également une étude paramétrique de l'influence de la pente du plancher océanique dans un scénario typique de génération/conversion d'une onde T. L'énergie et la durée de ces ondes s’avère être particulièrement sensible à l'environnement. En particulier nous avons vu que les pentes et les caractéristiques du fond marin jouaient un rôle capital. Nos études confirment qu’aux distances régionales le profil de vitesse dans l'océan s'avère n'être qu'un paramètre de deuxième ordre. Pour en évaluer l’impact nous avons développé une procédure pour le calcul de cartes de perte de transmission et de dispersion à partir de simulations numériques en forme d'onde complète dans le domaine temporel. Dans un second temps nous montrons qu'un bateau commercial de taille moyenne peut créer par diffraction des ondes T d'une d'amplitude conséquente et de faible dispersion. Ce mode de génération d'onde T, encore non documenté, doit être particulièrement fréquent dans les zones où le trafic maritime est important et pourrait expliquer certaines ondes T abyssales encore incomprises. Pour finir, nous présentons des outils numériques pour calculer le champ acoustique créé par une source en mouvementThis thesis combines observations, simulations and development of high performance numerical tools in the field of underwater acoustics, and in particular for the study of T-waves. After a literature review on T-waves, we analysed real data recorded in Italy. In order to model the phenomenon we have developed an axisymmetric spectral element solver in the time domain, which we present and validate. We also present a parametric study of the influence of seafloor slope in a typical scenario of generation / conversion of a T-wave. The energy and duration of these waves is particularly sensitive to the environment. In particular, we have seen that the slopes and characteristics of the seabed are of crucial importance. Our studies confirm that at regional distances the ocean speed profile is only a second order parameter. To evaluate its impact we have developed a procedure for the calculation of transmission and dispersion loss maps from full waveform numerical simulations in the time domain. In a second step we show that a medium-sized commercial boat can create T-waves of a significant amplitude and of low dispersion by diffraction. This T-wave generation mode, still undocumented, must be particularly frequent in areas where maritime traffic is dense and could explain some abyssal T-waves still misunderstood. Finally, we present numerical tools for calculating the acoustic field created by a moving source
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朱展強 and Chin-keung Chu. "Parallel computation for time domain boundary element method." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1999. http://hub.hku.hk/bib/B31220678.
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雷哲翔 and Zhexiang Lei. "Time domain boundary element method & its applications." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1993. http://hub.hku.hk/bib/B31233703.
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Hargreaves, J. A. "Time domain boundary element method for room acoustics." Thesis, University of Salford, 2007. http://usir.salford.ac.uk/16604/.
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This thesis is about improving the suitability of the time domain Boundary Element Method (BEM) for predicting the scattering from surface treatments used to improve the acoustics of rooms. The discretised integral equations are typically solved by marching on in time from initial silence; however, this being iterative has potential for divergence. Such instability and high computational cost have prohibited the time domain BEM from widespread use. The underlying integral equation is known to not possess unique solutions at certain frequencies, physically interpreted as cavity resonances, and these manifest as resonant poles, all excited and potentially divergent due to numerical error. This has been addressed by others using the combined field integral equation; an approach built upon in this thesis. Accuracy and stability may also be compromised by poor discretisation and integration accuracy. The latter is investigated on real-world surfaces, demonstrating that the popular Gaussian integration schemes are not suitable in some circumstances. Instead a contour integration scheme capable of resolving the integrands‟ singular nature is developed. Schroeder diffusers are Room Acoustic treatments which comprise wells separated by thin fins. The algorithm is extended to model such surfaces, applying the combined field integral equation to the body and an open surface model to the fins. It is shown that this improves stability over an all open surface model. A new model for compliant surfaces is developed, comparable to the surface impedance model used in the frequency domain. This is implemented for surfaces with welled and absorbing sections, permitting modelling of a Schroeder diffuser as a box with surface impedances that simulate the delayed reflections caused by the wells. A Binary Amplitude Diffuser - a partially absorbing diffuser - is also modelled. These new models achieve good accuracy but not universal stability and avenues of future research are proposed to address the latter issue.
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Chu, Chin-keung. "Parallel computation for time domain boundary element method /." Hong Kong : University of Hong Kong, 1999. http://sunzi.lib.hku.hk/hkuto/record.jsp?B20565574.
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Lei, Zhexiang. "Time domain boundary element method & its applications /." [Hong Kong : University of Hong Kong], 1993. http://sunzi.lib.hku.hk/hkuto/record.jsp?B13570365.
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Ali, Hassan O. "Finite-element time-domain analysis of axisymmetrical radiating structures." Thesis, University of Ottawa (Canada), 1992. http://hdl.handle.net/10393/7897.
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A method of computation based on the finite element method was used to solve axisymmetrical electromagnetic wave propagation problems directly in the time-domain. The finite element method, employing first order triangular elements, was used to generate a system of second order linear differential equations. The system of differential equations was solved for the magnetic field using a suitable differential equation solving algorithm written in the course of this work. The method was used to model several situations involving axisymmetrical radiating structures directly in the time domain and the results compared well with the published data. Situations involving pulses, which are of particular interest to EMI/EMC field, were successfully studied. Conclusions were drawn on the suitability of the method in modeling radiated emissions from printed circuit board configurations, under the influence of transient exciting fields.
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Allgaier, Markus [Verfasser]. "Ultrafast nonlinear optics: from spectral to time domain applications / Markus Allgaier." Paderborn : Universitätsbibliothek, 2019. http://d-nb.info/1177138034/34.
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Hissen, Huzifa Zain Alabdeen Abdarahman. "Spectral characterization of materials using terahertz time domain spectroscopy (THz-TDS)." Thesis, Stellenbosch : Stellenbosch University, 2014. http://hdl.handle.net/10019.1/96030.
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Thesis (MSc)--Stellenbosch University, 2014.ENGLISH ABSTRACT: Terahertz (THz) radiation is often used in many promising applications such as information and communication technology and airport security. Optimized and adapted terahertz fields hold huge promise for leading this technology further. This study is focused on terahertz time domain spectroscopy (THz-TDS). In THz-TDS the electric field is measured, therefore both amplitude and phase information of the THz pulse can be obtained. We used the pump-probe technique in order to measure a THz pulse from the photoconductive antenna. A pulsed fiber laser with FWHM of ' 100 fs was used for this. The frequency spectrum of the measured THz pulse was obtained via a fast Fourier transform. We studied the principles of the THz pulse generation as well as detection, with a photoconductive antenna as emitter and detector. In this study terahertz spectroscopy has been used to investigate the refractive index and absorption coefficient of different types of materials in the terahertz region. The last part of this study deals with a simple process for material parameter extraction of a polymer sample using commercial software called Teramat1.0. It uses the sample thickness, the reference THz pulse and the transmitted THz pulse to retrieve the complex refractive index of the sample.
AFRIKAANSE OPSOMMING: Terahertz (THz) straling word gereeld gebruik vir belowende toepassings soos inligting en kommunikasie tegnologie en lughawe sekuriteit. Geoptimeerde en aangepaste terahertz velde dra by tot die bevordering van die tegnologie. Hierdie studie fokus op terahertz tyd domein spektroskopie (THz-TDS). In THz-TDS word die elektriese veld gemeet en dus word beide amplitude en fase inligting van die THz puls verkry. Ons gebruik ’n pomp en toets tegniek om die THz puls deur ’n fotogeleidende antenna te bepaal. ’n Gepulseerde vesel laser met FWHM van 100 fs word hiervoor benut. Die frekwensie spektrum van die gemete THz puls word bereken deur ’n vinnige Fourier transvorm te bereken. Die beginsels van die generering en deteksie van THz pulse is bestudeer met ’n fotogeleidende antenna as sender en ontvanger. In die studie is terahertz spektroskopie gebruik om die brekingsindekse en die absorpsie koeffisiënte van verskillende materiale in die terahertz gebied te bepaal. Die laaste gedeelte van die studie handel oor ’n eenvoudige proses om die materiaal parameters van ’n polimeer te bepaal deur gebruik te maak van kommersiële sagteware Teramat 1.0. Die monster dikte, die THz verwysingspuls en die deurgelate puls word gebruik om die komplekse brekingsindeks van die materiaal te bereken.
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Marais, Neilen. "Efficient high-order time domain finite element methods in electromagnetics." Thesis, Stellenbosch : University of Stellenbosch, 2009. http://hdl.handle.net/10019.1/1499.
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Thesis (DEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009.The Finite Element Method (FEM) as applied to Computational Electromagnetics (CEM), can beused to solve a large class of Electromagnetics problems with high accuracy and good computational efficiency. For solving wide-band problems time domain solutions are often preferred; while time domain FEM methods are feasible, the Finite Difference Time Domain (FDTD) method is more commonly applied. The FDTD is popular both for its efficiency and its simplicity. The efficiency of the FDTD stems from the fact that it is both explicit (i.e. no matrices need to be solved) and second order accurate in both time and space. The FDTD has limitations when dealing with certain geometrical shapes and when electrically large structures are analysed. The former limitation is caused by stair-casing in the geometrical modelling, the latter by accumulated dispersion error throughout the mesh. The FEM can be seen as a general mathematical framework describing families of concrete numerical method implementations; in fact the FDTD can be described as a particular FETD (Finite Element Time Domain) method. To date the most commonly described FETD CEM methods make use of unstructured, conforming meshes and implicit time stepping schemes. Such meshes deal well with complex geometries while implicit time stepping is required for practical numerical stability. Compared to the FDTD, these methods have the advantages of computational efficiency when dealing with complex geometries and the conceptually straight forward extension to higher orders of accuracy. On the downside, they are much more complicated to implement and less computationally efficient when dealing with regular geometries. The FDTD and implicit FETD have been combined in an implicit/explicit hybrid. By using the implicit FETD in regions of complex geometry and the FDTD elsewhere the advantages of both are combined. However, previous work only addressed mixed first order (i.e. second order accurate) methods. For electrically large problems or when very accurate solutions are required, higher order methods are attractive. In this thesis a novel higher order implicit/explicit FETD method of arbitrary order in space is presented. A higher order explicit FETD method is implemented using Gauss-Lobatto lumping on regular Cartesian hexahedra with central differencing in time applied to a coupled Maxwell’s equation FEM formulation. This can be seen as a spatially higher order generalisation of the FDTD. A convolution-free perfectly matched layer (PML) method is adapted from the FDTD literature to provide mesh termination. A curl conforming hybrid mesh allowing the interconnection of arbitrary order tetrahedra and hexahedra without using intermediate pyramidal or prismatic elements is presented. An unconditionally stable implicit FETD method is implemented using Newmark-Beta time integration and the standard curl-curl FEM formulation. The implicit/explicit hybrid is constructed on the hybrid hexahedral/tetrahedral mesh using the equivalence between the coupled Maxwell’s formulation with central differences and the Newmark-Beta method with Beta = 0 and the element-wise implicitness method. The accuracy and efficiency of this hybrid is numerically demonstrated using several test-problems.
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Dinevik, Vilhelm. "Comparative Analysis of Adaptive Domain Decomposition Algorithms for a Time-Spectral Method." Thesis, KTH, Skolan för elektroteknik och datavetenskap (EECS), 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-289366.
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Time-spectral solvers for partial differential equations (PDE) have been explored in various forms during the last few decades. The generalized weighted residual method (GWRM) is one such method with a high accuracy and efficiency. The GWRM has so far been implemented almost exclusively with a uniform grid of subdomains in the spatial domain. Recent research has indicated that an adaptive grid can yield a significant improvement in accuracy and efficiency of the GWRM. In this thesis a comparison is performed between a uniform grid and three different adaptive grid decomposition methods. Three initial- value PDEs are used to benchmark these methods; the one-dimensional Burger’s equation, the 4th order Fisher-Kolmogorov equation and the non-linear Schrödinger equation. It was found that the average adaptive algorithm is the most efficient out of the algorithms evaluated in this thesis. The average adaptive algorithms solution time was up to 1.6 times faster than the uniform algorithm when solving the Fisher-Kolmogorov equation and with an error up to a factor of 22.5 smaller than the uniform algorithm when solving the one- dimensional Burger’s equation. The uniform algorithm needed 25 spatial subdomains to get errors of the same order of magnitude as the average adaptive algorithm got using only 12 spatial subdomains. The average subdomain decomposition algorithm is a fast, robust and efficient method, which can be applied to a variety of different problems to further increase the efficiency of the GWRM.Tidsspektrala lösningar av partiella differential ekvationer (PDE) har utforskats på många olika sätt under de senaste årtiondena. Den generaliserade viktade residual metoden (GWRM) är en sådan metod som har uppnått hög noggrannhet och effektivitet. Metoden har hittills, nästan enbart, implementerats med en likformig subdomänsuppdelning i rumsdomänen. Nyligen utförd forskning indikerar att GWRM kan ge signifikant förbättrad precision och effektivitet om man implementerar adaptiva rums- och tidsdomäner. I detta examensarbete utförs en jämförelse mellan en likformig subdomänsuppdelning i rummet och tre olika adaptiva algoritmer för subdomänsuppdelning. Dessa algoritmer testas på tre olika PDE, endimensionella Burgers ekvation, fjärde ordningens Fisher-Kolmogorovs ekvation och den icke-linjära Schrödingerekvationen. Slutsatsen var att den medelvärdesbildande adaptiva algoritmen var den mest effektiva metoden. Den löste ekvationerna upp till 2.7 gånger snabbare än den likformiga algoritmen, med ett fel som var upp till 22.5 gånger mindre än den likformiga metodens fel. Den likformiga metoden behövde 25 rumsdomäner för att få en precision av samma potens som de adaptiva algoritmerna åstadkom med enbart 12 rumsdomäner. Den medelvärdesbildande algoritmens subdomänsuppdelning är snabb, robust och effektiv. Den kan appliceras på en mängd olika problem för att öka effektiviteten av GWRM.
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Bolis, Alessandro. "Fourier spectral/hp element method : investigation of time-stepping and parallelisation strategies." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/25140.
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As computer hardware has evolved, the time required to perform numerical simulations has reduced, allowing investigations of a wide range of new problems. This thesis focuses on algorithm optimization, to minimize run-time, when solving the incompressible Navier-Stokes equations. Aspects affecting performance related to the discretisation and algorithm parallelization are investigated in the context of high-order methods. The roles played by numerical approximations and computational strategies are highlighted and it is recognized that a versatile implementation provides additional benefits, allowing an ad-hoc selection of techniques to fit the needs of heterogeneous computing environments. We initially describe the building blocks of a spectral/hp element and pure spectral method and how they can be encapsulated and combined to create a 3D discretisation, the Fourier spectral/hp element method. Time-stepping strategies are also described and encapsulated in a flexible framework based on the General Linear Method. After implementing and validating an incompressible Navier-Stokes solver, two canonical turbulent flows are analyzed. Afterward a 2D hyperbolic equation is considered to investigate the efficiency of low- and high-order methods when discretising the spatial and temporal derivatives. We perform parametric studies, monitoring accuracy and CPU-time for different numerical approximations. We identify optimal discretisations, demonstrating that high-order methods are the computationally fastest approach to attain a desired accuracy for this problem. Following the same philosophy, we investigate the benefits of using a hybrid parallel implementation. The message passing model is introduced to parallelize different kernels of an incompressible Navier-Stokes solver. Monitoring the parallel performance of these strategies the most efficient approach is highlighted. We also demonstrate that hybrid parallel solutions can be used to significantly extend the strong scalability limit and support greater parallelism.
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Wang, Duan. "Element level time domain system identification techniques with unknown input information." Diss., The University of Arizona, 1995. http://hdl.handle.net/10150/187220.
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A finite element based linear time domain system identification algorithm is proposed to estimate the stiffness and damping coefficients of structures at the element level using response data alone without using information of excitation measurements. The unknown input excitation could be applied at any location of the structure including at the ground level representing the seismic excitation. The proposed method is an Iterative Least-Squares with Unknown Input (ILS-UI) procedure. The element-level structural parameters can be identified directly by using proposed ILS-UI procedure. No information of the modal properties is required. The efficiency and robustness of the proposed algorithm is illustrated by numerical examples. For verification purposes, both noise-free and noise-included output responses are considered in numerical examples. The applications of three types of structures, i.e., shear-type buildings, trusses, and frames, are considered in this dissertation. In all examples, the identified results indicate that the proposed ILS-UI method identified the structural parameters very well. For the successful implementation of the proposed method, only a small number of sampling time points are required, and a long time duration of responses is not necessary. For a large system, since it is practically impossible to measure responses at every dynamic degree of freedom, the absence of some observation points of responses and its effect on the proposed system identification technique must be studied. Based on the above ILS-UI procedure, a new technique combined with the Kalman filter technique is developed to identify all element-level structural parameters using measuring responses at several optimal locations only. The optimal numbers and locations of measurement points required to identify uniquely the system using this proposed ILS-EKF-UI technique are determined. Again, numerical examples with two special cases are used to illustrate the applications of this new technique. The results of numerical examples indicate that this new system identification technique is very economical, simple, and robust, since the input is not required to be measured and only several observations are required.
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Li, Ran. "Load profiling on time and spectral domain : from big data to smart data." Thesis, University of Bath, 2015. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.665434.
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With the promotion of demand side responses (DSRs) and low carbon technologies (LCTs), there is a growing interest in visualising the demand information at individual consumer and low voltage (LV) network level, where demands are less aggregated and highly volatile. Yet, traditional load profiling techniques, which are carried out on small data, are struggling to meet the requirements on accuracy and granularity. This thesis contributes to this area by extending traditional load profiling to a big-data context, where refined load profiles (smart data) can be extracted by two novel load profiling techniques for LV networks and individual consumers. The refined load profiles aim to: i) economically visualise LV networks with limited smart-grid monitoring data; ii) transform the smart metering data into a high-detail granular representation of the customers’ daily demand. For the LV networks, this thesis develops a novel concept, LV network templates, which aim to visualise the LV networks in a cost-effective manner. A novel three-stage load profiling method is proposed as: clustering, classification and scaling. By using statistical time-series analysis, three steps are undertaken: i) cluster a vast amount of load data according to their load shapes; ii) classify un-monitored substations to the most similar cluster without sample metering; iii) and also scale them to the right magnitude without sample metering. Through this method, limited representative monitoring data can be used to develop a library of typical load profiles for un-monitored networks, thus saving the cost of extensive monitoring for every single substation. In addition, it is the first load profiling method that can accurately express both load shapes and magnitudes for LV networks. Regarding the customer’s demand representation, the developed time-series analysis needs to be updated due to the volatile and uncertain nature of smart metering data, including inter-related factors such as overall load shapes, sudden spikes and magnitudes. Therefore, an innovative spectral load profiling is proposed to decompose these factors into different spectral levels, characterised by spectral features. By analysing the extracted features on each spectral level separately through multi-resolution analysis, the interference among different factors can effectively be prevented. The proposed method, for the first time, is able to fully capture the energy characteristics at the household level. The developed LV network load templates provide an economical but straightforward way to quantify the available headroom of unmonitored substations over time, providing quantitative information for distribution network operators to integrate LTCs at the minimal costs. The spectral load profiling gives an insight into customer’s energy behaviours with high granularity and accuracy. It can support the customer-specified DSR, tariff design, smart metering validation and load forecasting.
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Park, Sungho. "Development and Applications of Finite Elements in Time Domain." Diss., Virginia Tech, 1996. http://hdl.handle.net/10919/30693.
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A bilinear formulation is used for developing the time finite element method (TFM) to obtain transient responses of both linear, nonlinear, damped and undamped systems. Also the formulation, used in the h-, p- and hp-versions, is extended and found to be readily amenable to multi-degree-of-freedom systems. The resulting linear and nonlinear algebraic equations for the transient response are differentiated to obtain the sensitivity of the response with respect to various design parameters. The present developments were tested on a series of linear and nonlinear examples and were found to yield, when compared with other methods, excellent results for both the transient response and its sensitivity to system parameters. Mostly, the results were obtained using the Legendre polynomials as basis functions, though, in some cases other orthogonal polynomials namely, Hermite, Chebyshev, and integrated Legendre polynomials were also employed (but to no great advantage). A key advantage of TFM, and the one often overlooked in its past applications, is the ease in which the sensitivity of the transient response with respect to various design parameters can be obtained. Since a considerable effort is spent in determining the sensitivity of the response with respect to system parameters in many algorithms for parametric identification, an identification procedure based on the TFM is developed and tested for a number of nonlinear single-and two-degree-of-freedom system problems. An advantage of the TFM is the easy calculation of the sensitivity of the transient response with respect to various design parameters, a key requirement for gradient-based parameter identification schemes. The method is simple, since one obtains the sensitivity of the response to system parameters by differentiating the algebraic equations, not original differential equations. These sensitivities are used in Levenberg-Marquardt iterative direct method to identify parameters for nonlinear single- and two-degree-of-freedom systems. The measured response was simulated by integrating the example nonlinear systems using the given values of the system parameters. To study the influence of the measurement noise on parameter identification, random noise is added to the simulated response. The accuracy and the efficiency of the present method is compared to a previously available approach that employs a multistep method to integrate nonlinear differential equations. It is seen, for the same accuracy, the present approach requires fewer data points. Finally, the TFM for optimal control problems based on Hamiltonian weak formulation is proposed by adopting the p- and hp-versions as a finite element discretization process. The p-version can be used to improve the accuracy of the solution by adding more unknowns to each element without refining the mesh. The usage of hierarchical type of shape functions can lead to a significant saving in computational effort for a given accuracy. A set of Legendre polynomials are chosen as higher order shape functions and applied to two simple minimization problems for optimal control. The proposed formulation provides very accurate results for these problems.Ph. D.
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Lee, Anyu 1963. "Transient analysis of interconnections using spectral method." Thesis, The University of Arizona, 1988. http://hdl.handle.net/10150/276750.
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The present paper introduces one very efficient and flexible time-domain analysis technique to predict the kinds of reflections and crosstalk. Numerical results show that this technique is indeed efficient and accurate in the transient analysis of a general multiple line system. Furthermore, this algorithm can be eventually coded in a form of a subroutine compatible with any standard CAD program, such as SPICE.
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Kim, Joonshik. "Finite Element Time Domain Techniques for Maxwell's Equations Based on Differential Forms." The Ohio State University, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=osu1293588301.
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Eller, David. "On an Efficient Method fo Time-Domain Computational Aeroelasticity." Doctoral thesis, KTH, Farkost och flyg, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-584.
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The present thesis summarizes work on developing a method for unsteady aerodynamic analysis primarily for aeroelastic simulations. In contrast to widely used prediction tools based on frequency-domain representations, the current approach aims to provide a time-domain simulation capability which can be readily integrated with possibly nonlinear structural and control system models. Further, due to the potential flow model underlying the computational method, and the solution algorithm based on an efficient boundary element formulation, the computational effort for the solution is moderate, allowing time-dependent simulations of complex configurations. The computational method is applied to simulate a number of wind-tunnel experiments involving highly flexible models. Two of the experiments are utilized to verify the method and to ascertain the validity of the unsteady flow model. In the third study, simulations are used for the numerical optimization of a configuration with multiple control surfaces. Here, the flexibility of the model is exploited in order to achieve a reduction of induced drag. Comparison with experimental results shows that the numerical method attains adequate accuracy within the inherent limits of the potential flow model. Finally, rather extensive aeroelastic simulations are performed for the ASK 21 sailplane. Time-domain simulations of a pull-up maneuver and comparisons with flight test data demonstrate that, considering modeling and computational effort, excellent agreement is obtained. Furthermore, a flutter analysis is performed for the same aircraft using identified frequency-domain loads. Results are found to deviate only slightly from critical speed and frequency obtained using an industry-standard aeroelastic analysis code. Nevertheless, erratic results for control surface hinge moments indicate that the accuracy of the present method would benefit from improved control surface modeling and coupled boundary layer analysis.QC 20100531
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Nezhi, Zouhair [Verfasser]. "Adaptive time domain boundary element method for sound radiation of tyres / Zouhair Nezhi." Hannover : Technische Informationsbibliothek und Universitätsbibliothek Hannover (TIB), 2014. http://d-nb.info/1058237659/34.
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Dosopoulos, Stylianos. "Interior Penalty Discontinuous Galerkin Finite Element Method for the Time-Domain Maxwell's Equations." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1337787922.
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Wang, Shumin. "Improved-accuracy algorithms for time-domain finite methods in electromagnetics." The Ohio State University, 2003. http://rave.ohiolink.edu/etdc/view?acc_num=osu1061225243.
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Edelvik, Fredrik. "Hybrid Solvers for the Maxwell Equations in Time-Domain." Doctoral thesis, Uppsala universitet, Avdelningen för teknisk databehandling, 2002. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-2156.
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The most commonly used method for the time-domain Maxwell equations is the Finite-Difference Time-Domain method (FDTD). This is an explicit, second-order accurate method, which is used on a staggered Cartesian grid. The main drawback with the FDTD method is its inability to accurately model curved objects and small geometrical features. This is due to the Cartesian grid, which leads to a staircase approximation of the geometry and small details are not resolved at all. This thesis presents different ways to circumvent this drawback, but still take advantage of the benefits of the FDTD method. An approach to avoid staircasing errors but still retain the efficiency of the FDTD method is to use a hybrid grid. A few layers of unstructured cells are used close to curved objects and a Cartesian grid is used for the rest of the domain. For the choice of solver on the unstructured grid two different alternatives are compared: an explicit Finite-Volume Time-Domain (FVTD) solver and an implicit Finite-Element Time-Domain (FETD) solver. The hybrid solvers calculate the scattering from complex objects much more efficiently compared to using FDTD on highly resolved Cartesian grids. For the same accuracy in the solution roughly a factor of 10 in memory requirements and a factor of 20 in execution time are gained. The ability to model features that are small relative to the cell size is often important in electromagnetic simulations. In this thesis a technique to generalize a well-known subcell model for thin wires, in order to take arbitrarily oriented wires in FETD and FDTD into account, is proposed. The method gives considerable modeling flexibility compared to earlier methods and is proven stable. The results show excellent consistency and very good accuracy on different antenna configurations. The recursive convolution method is often used to model frequency dispersive materials in FDTD. This method is used to enable modeling of such materials in the unstructured FVTD and FETD solvers. The stability of both solvers is analyzed and their accuracy is demonstrated by computing the radar cross section for homogeneous as well as layered spheres with frequency dependent permittivity.
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Cai, Yong. "Finite-element time- domain modelling of periodic structures with floquet modal absorbing boundry condition." Thesis, University of Warwick, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.487809.
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Rawat, Vineet. "Finite Element Domain Decomposition with Second Order Transmission Conditions for Time-Harmonic Electromagnetic Problems." The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1243360543.
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Ledfelt, Gunnar. "Hybrid Time-Domain Methods and Wire Models for Computational Electromagnetics." Doctoral thesis, Stockholm : Tekniska högsk, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3115.
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Donderici, Burkay. "Time-Domain Solvers for Complex-Media Electrodynamics and Plasma Physics." The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1216744283.
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Szumski, Ricard Gerard. "A finite element formulation for the time domain vibration analysis of an elastic-viscoelastic structure." Diss., Georgia Institute of Technology, 1993. http://hdl.handle.net/1853/17053.
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Bavelis, Konstantinos. "Finite-element time-domain modelling of cylindrical structures with a modal non-reflecting boundary condition." Thesis, University of Warwick, 2010. http://wrap.warwick.ac.uk/36763/.
Full textAbstract:
This dissertation presents Galerkin weighted residual Finite-Element Time-Domain (FETD) formulations using a 2D cylindrical modal non-reflecting boundary condition (MNRBC) for the modelling of plane wave scattering from cylindrical structures of arbitrary cross-section surrounded by free space. Chapter 1 begins by presenting the motivation for this work. Key concepts regarding cylindrical geometries are introduced at this stage. The Galerkin weighted residual Finite-Element method is briefly outlined. Chapter 2 presents a novel scattered field FETD-MNRBC formulation for the transverse electric polarisation of a modal non-reflecting boundary condition for plane wave scattering from perfectly electrically conductive (PEC) cylindrical structures of arbitrary cross-section. The boundary condition is based on a Vector-Fitting (VF) approximation of the boundary kernel appearing in the time-domain formulation. The convolution integral appearing in the time-domain formulation of the boundary condition is calculated recursively using the Vector-Fitting coefficients. Accurate numerical results are shown for the bistatic scattering width (BSW) that validate the approach. Chapter 3 focuses on the VF approximation of the cylindrical boundary kernel. Two approaches are investigated; the so called Vector-Fitting G function approximation (VFG) and the Vector-Fitting U function approximation (VFU). Both approaches produce satisfactory finite-element results with the VFU being more versatile. Chapter 4 presents, for the first time, the total field FETD-MNRBC formulation for both transverse electric and transverse magnetic polarisations. The VFU approach is employed. The structures considered in this chapter are not only PEC cylinders but also dielectric ones of various cross-sections and various values of relative permittivity and permeability. The numerical results demonstrate the good accuracy of this formulation. Chapter 5 combines the cylindrical modal non-reflecting boundary condition with the Floquet theorem and extends this formulation, for the first time, to azimuthally periodic cylinders using scattered and total field time-domain formulations. The advantages and disadvantages of the periodic modal non-reflecting boundary condition approach are discussed and numerical results for the BSW are shown. Chapter 6 presents a novel sparse-matrix scattered field FETD-MNRBC formulation in which the fully dense submatrices associated with the boundary integral are avoided. Through numerical results the accuracy of the proposed formulation is investigated. Chapter 7 concludes the work by summarizing the main achievements and discussing its impact in electromagnetics.
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Loh, Tian Hong. "An exact port boundary condition for the finite-element time-domain modelling of microwave devices." Thesis, University of Warwick, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.412895.
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Kabir, S. M. Raiyan. "Finite element time domain method with a unique coupled mesh system for electromagnetics and photonics." Thesis, City University London, 2015. http://openaccess.city.ac.uk/14523/.
Full textAbstract:
The finite difference time domain (FDTD) method is a popular technique, being used successfully to analyse the electromagnetic properties of many structures, including a range of optical or photonic devices. This method offers several major advantages such as, a minimum level of calculation is required for each of the cells into which the structure is divided, as well as data parallelism and explicit and easy implementation. However, due to the use of the Finite Difference grid, this method suffers from higher numerical dispersion and inaccurate discretisation due to staircasing at slanted and curve edges. The rectangular computational domain in 2D and cuboid computational domain in 3D sometimes makes the method very resource intensive especially for large simulations. Although the finite element (FE) approach is superior for the discretisation of both 2D and 3D structures, most of the FE-based time domain approaches reported so far suffer from limitations due to the implicit or iterative form or the mass matrix formulation, for example. Therefore, the speed of the simulation is much slower than the FDTD method. Time domain analysis of electromagnetic is a very resource intensive numerical technique. Due to the slow performance the FE based techniques are not as popular as the FDTD method. In this research work a new FE based time domain technique has been proposed for both 2D and 3D problems which is similar to the FDTD method explicit and data parallel in nature. The method proposed does not requires any matrix formulation or iteration. It uses minimum possible CPU cycles among any FE-based techniques. The method also utilises a unique meshing scheme to reduce the number of calculation to at least half for 2D and one fifth for 3D compared to any full mesh FE based technique. The method also shows very low numerical dispersion when used with equilateral elements in both 2D and 3D. Thus the proposed method effectively produces results with less numerical dispersion error with lower density mesh compared to the FDTD method. When the advantage in resolution is taken into consideration, calculation of each time-step using the proposed method is significantly faster than the FDTD method.
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Bramas, Bérenger. "Optimization and parallelization of the boundary element method for the wave equation in time domain." Thesis, Bordeaux, 2016. http://www.theses.fr/2016BORD0022/document.
Full textAbstract:
La méthode des éléments frontières pour l’équation des ondes (BEM) est utilisée en acoustique eten électromagnétisme pour simuler la propagation d’une onde avec une discrétisation en temps(TD). Elle permet d’obtenir un résultat pour plusieurs fréquences à partir d’une seule résolution.Dans cette thèse, nous nous intéressons à l’implémentation efficace d’un simulateur TD-BEM sousdifférents angles. Nous décrivons le contexte de notre étude et la formulation utilisée qui s’exprimesous la forme d’un système linéaire composé de plusieurs matrices d’interactions/convolutions.Ce système est naturellement calculé en utilisant l’opérateur matrice/vecteur creux (SpMV). Nousavons travaillé sur la limite du SpMV en étudiant la permutation des matrices et le comportementde notre implémentation aidé par la vectorisation sur CPU et avec une approche par bloc surGPU. Nous montrons que cet opérateur n’est pas approprié pour notre problème et nous proposonsde changer l’ordre de calcul afin d’obtenir une matrice avec une structure particulière.Cette nouvelle structure est appelée une matrice tranche et se calcule à l’aide d’un opérateur spécifique.Nous décrivons des implémentations optimisées sur architectures modernes du calculhaute-performance. Le simulateur résultant est parallélisé avec une approche hybride (mémoirespartagées/distribuées) sur des noeuds hétérogènes, et se base sur une nouvelle heuristique pouréquilibrer le travail entre les processeurs. Cette approche matricielle a une complexité quadratiquesi bien que nous avons étudié son accélération par la méthode des multipoles rapides (FMM). Nousavons tout d’abord travaillé sur la parallélisation de l’algorithme de la FMM en utilisant différentsparadigmes et nous montrons comment les moteurs d’exécution sont adaptés pour relâcher le potentielde la FMM. Enfin, nous présentons des résultats préliminaires d’un simulateur TD-BEMaccéléré par FMMThe time-domain BEM for the wave equation in acoustics and electromagnetism is used to simulatethe propagation of a wave with a discretization in time. It allows to obtain several frequencydomainresults with one solve. In this thesis, we investigate the implementation of an efficientTD-BEM solver using different approaches. We describe the context of our study and the TD-BEMformulation expressed as a sparse linear system composed of multiple interaction/convolutionmatrices. This system is naturally computed using the sparse matrix-vector product (SpMV). Wework on the limits of the SpMV kernel by looking at the matrix reordering and the behavior of ourSpMV kernels using vectorization (SIMD) on CPUs and an advanced blocking-layout on NvidiaGPUs. We show that this operator is not appropriate for our problem, and we then propose toreorder the original computation to get a special matrix structure. This new structure is called aslice matrix and is computed with a custom matrix/vector product operator. We present an optimizedimplementation of this operator on CPUs and Nvidia GPUs for which we describe advancedblocking schemes. The resulting solver is parallelized with a hybrid strategy above heterogeneousnodes and relies on a new heuristic to balance the work among the processing units. Due tothe quadratic complexity of this matrix approach, we study the use of the fast multipole method(FMM) for our time-domain BEM solver. We investigate the parallelization of the general FMMalgorithm using several paradigms in both shared and distributed memory, and we explain howmodern runtime systems are well-suited to express the FMM computation. Finally, we investigatethe implementation and the parametrization of an FMM kernel specific to our TD-BEM, and weprovide preliminary results
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Jiang, Wenwei. "Spectral-based Substructure Transfer Path Analysis of Steady-state and Transient Vibrations." University of Cincinnati / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1273168454.
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Gruber, Michael E. [Verfasser]. "Spectral Domain and FFT Accelerated Cavity Green's Function Boundary Element Methods for Numerical Modeling of Reverberation Chambers / Michael E. Gruber." München : Verlag Dr. Hut, 2016. http://d-nb.info/1106593758/34.
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Yoo, Hyungsuk. "Quality of the Volterra transfer function estimation /." Digital version accessible at:, 1998. http://wwwlib.umi.com/cr/utexas/main.
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Rieben, Robert N. "A novel high order time domain vector finite element method for the simulation of electromagnetic devices /." For electronic version search Digital dissertations database. Restricted to UC campuses. Access is free to UC campus dissertations, 2004. http://uclibs.org/PID/11984.
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Ling, Xiaolin. "Linear and nonlinear time domain system identification at element level for structural systems with unknown excitation." Diss., The University of Arizona, 2000. http://hdl.handle.net/10150/284163.
Full textAbstract:
Three time domain system identification (SI) approaches, i.e., Modified Iterative Least Square with Unknown Input (ILS-UI), Localized Structural Identification, and Modified Iterative Least Square--Extended Kalman Filter with Unknown Input (ILS-EKF-UI), are proposed to identify defects at the element level of structures. In all these methods, structures are modeled using the finite element method (FEM) and the structural parameters (stiffness and damping) are identified using only output response measurements without using any information on input excitation. Excitations are identified as a byproduct of the SI procedures. If damping is considered to be proportional or Rayleigh-type, the time domain SI technique becomes nonlinear even though the dynamic system remains linear. The Modified ILS-UI approach is essentially a nonlinear SI algorithm. The Localized Structural Identification combines a time domain SI technique and FEM formulation representing a part of the structure. The time domain responses at each time instance represent an equilibrium status of the system which is reflected in the nodal equilibrium in the FEM. Using the Localized Structural Model, only dynamic responses at the local region closely connected to the part of the structure to be identified are required. This dramatically reduces the measurement requirements, and makes it possible to identify the parameters of the whole structure by identifying only part of it. This study discusses how to select elements of the local structure and how to determine the locations and number of the output measurements. The Modified ILS-EKF-UI approach was developed by combining the Modified ILS-UI and the Localized Structural Identification. Using the Modified ILS-EKF-UI approach, the system can be identified using responses at a reduced number of dynamic degrees of freedom. This method allows the finite element mesh to be refined further for more localized parameter identification without additional response information. All three methods are verified using numerical examples. They identify the structures very well. They are found to be more accurate than other methods currently reported in the literature even when input excitation information is used to identify structures. Various types of structures are examined, including shear buildings, plane frames, and plane trusses. The proposed methods are found to be robust even when the responses are contaminated with noise.
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Abenius, Erik. "Direct and Inverse Methods for Waveguides and Scattering Problems in the Time Domain." Doctoral thesis, Uppsala : Acta Universitatis Upsaliensis : Univ.-bibl. [distributör], 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-6013.
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Bourgeois, Jacqueline M. "A complete three-dimensional electromagnetic simulation of ground-penetrating radars using the finite-difference time-domain method." Diss., Georgia Institute of Technology, 1997. http://hdl.handle.net/1853/14845.
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Kachanovska, Maryna. "Fast, Parallel Techniques for Time-Domain Boundary Integral Equations." Doctoral thesis, Universitätsbibliothek Leipzig, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-132183.
Full textAbstract:
This work addresses the question of the efficient numerical solution of time-domain boundary integral equations with retarded potentials arising in the problems of acoustic and electromagnetic scattering. The convolutional form of the time-domain boundary operators allows to discretize them with the help of Runge-Kutta convolution quadrature. This method combines Laplace-transform and time-stepping approaches and requires the explicit form of the fundamental solution only in the Laplace domain to be known. Recent numerical and analytical studies revealed excellent properties of Runge-Kutta convolution quadrature, e.g. high convergence order, stability, low dissipation and dispersion.
As a model problem, we consider the wave scattering in three dimensions. The convolution quadrature discretization of the indirect formulation for the three-dimensional wave equation leads to the lower triangular Toeplitz system of equations. Each entry of this system is a boundary integral operator with a kernel defined by convolution quadrature. In this work we develop an efficient method of almost linear complexity for the solution of this system based on the existing recursive algorithm. The latter requires the construction of many discretizations of the Helmholtz boundary single layer operator for a wide range of complex wavenumbers. This leads to two main problems:
the need to construct many dense matrices and to evaluate many singular and near-singular integrals.
The first problem is overcome by the use of data-sparse techniques, namely, the high-frequency fast multipole method (HF FMM) and H-matrices. The applicability of both techniques for the discretization of the Helmholtz boundary single-layer operators with complex wavenumbers is analyzed. It is shown that the presence of decay can favorably affect the length of the fast multipole expansions and thus reduce the matrix-vector multiplication times. The performance of H-matrices and the HF FMM is compared for a range of complex wavenumbers, and the strategy to choose between two techniques is suggested.
The second problem, namely, the assembly of many singular and nearly-singular integrals, is solved by the use of the Huygens principle. In this work we prove that kernels of the boundary integral operators $w_n^h(d)$ ($h$ is the time step and $t_n=nh$ is the time) exhibit exponential decay outside of the neighborhood of $d=nh$ (this is the consequence of the Huygens principle). The size of the support of these kernels for fixed $h$ increases with $n$ as $n^a,a<1$, where $a$ depends on the order of the Runge-Kutta method and is (typically) smaller for Runge-Kutta methods of higher order. Numerical experiments demonstrate that theoretically predicted values of $a$ are quite close to optimal.
In the work it is shown how this property can be used in the recursive algorithm to construct only a few matrices with the near-field, while for the rest of the matrices the far-field only is assembled. The resulting method allows to solve the three-dimensional wave scattering problem with asymptotically almost linear complexity. The efficiency of the approach is confirmed by extensive numerical experiments.
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Benavides, Iglesias Alfonso. "Experimental time-domain controlled source electromagnetic induction for highly conductive targets detection and discrimination." Texas A&M University, 2003. http://hdl.handle.net/1969.1/5810.
Full textAbstract:
The response of geological materials at the scale of meters and the response
of buried targets of different shapes and sizes using controlled-source electromagnetic
induction (CSEM) is investigated. This dissertation focuses on three topics; i) frac-
tal properties on electric conductivity data from near-surface geology and processing
techniques for enhancing man-made target responses, ii) non-linear inversion of spa-
tiotemporal data using continuation method, and iii) classification of CSEM transient
and spatiotemporal data.
In the first topic, apparent conductivity profiles and maps were studied to de-
termine self-affine properties of the geological noise and the effects of man-made con-
ductive metal targets. 2-D Fourier transform and omnidirectional variograms showed
that variations in apparent conductivity exhibit self-affinity, corresponding to frac-
tional Brownian motion. Self-affinity no longer holds when targets are buried in the
near-surface, making feasible the use of spectral methods to determine their pres-
ence. The difference between the geology and target responses can be exploited using
wavelet decomposition. A series of experiments showed that wavelet filtering is able
to separate target responses from the geological background.
In the second topic, a continuation-based inversion method approach is adopted,
based on path-tracking in model space, to solve the non-linear least squares prob-
lem for unexploded ordnance (UXO) data. The model corresponds to a stretched-
exponential decay of eddy currents induced in a magnetic spheroid. The fast inversion of actual field multi-receiver CSEM responses of inert, buried ordnance is also shown.
Software based on the continuation method could be installed within a multi-receiver
CSEM sensor and used for near-real-time UXO decision.
In the third topic, unsupervised self-organizing maps (SOM) were adapted for
data clustering and classification. The use of self-organizing maps (SOM) for central-
loop CSEM transients shows potential capability to perform classification, discrimi-
nating background and non-dangerous items (clutter) data from, for instance, unex-
ploded ordnance. Implementation of a merge SOM algorithm showed that clustering
and classification of spatiotemporal CSEM data is possible. The ability to extract tar-
get signals from a background-contaminated pattern is desired to avoid dealing with
forward models containing subsurface response or to implement processing algorithm
to remove, to some degree, the effects of background response and the target-host
interactions.
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Srisukh, Yudhapoom. "Development of hybrid explicit/implicit and adaptive h and p refinement for the finite element time domain method." Columbus, Ohio : Ohio State University, 2006. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1135879014.
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Kachanovska, Maryna. "Fast, Parallel Techniques for Time-Domain Boundary Integral Equations." Doctoral thesis, Max-Planck-Institut für Mathematik in den Naturwissenschaften, 2013. https://ul.qucosa.de/id/qucosa%3A12278.
Full textAbstract:
This work addresses the question of the efficient numerical solution of time-domain boundary integral equations with retarded potentials arising in the problems of acoustic and electromagnetic scattering. The convolutional form of the time-domain boundary operators allows to discretize them with the help of Runge-Kutta convolution quadrature. This method combines Laplace-transform and time-stepping approaches and requires the explicit form of the fundamental solution only in the Laplace domain to be known. Recent numerical and analytical studies revealed excellent properties of Runge-Kutta convolution quadrature, e.g. high convergence order, stability, low dissipation and dispersion.
As a model problem, we consider the wave scattering in three dimensions. The convolution quadrature discretization of the indirect formulation for the three-dimensional wave equation leads to the lower triangular Toeplitz system of equations. Each entry of this system is a boundary integral operator with a kernel defined by convolution quadrature. In this work we develop an efficient method of almost linear complexity for the solution of this system based on the existing recursive algorithm. The latter requires the construction of many discretizations of the Helmholtz boundary single layer operator for a wide range of complex wavenumbers. This leads to two main problems:
the need to construct many dense matrices and to evaluate many singular and near-singular integrals.
The first problem is overcome by the use of data-sparse techniques, namely, the high-frequency fast multipole method (HF FMM) and H-matrices. The applicability of both techniques for the discretization of the Helmholtz boundary single-layer operators with complex wavenumbers is analyzed. It is shown that the presence of decay can favorably affect the length of the fast multipole expansions and thus reduce the matrix-vector multiplication times. The performance of H-matrices and the HF FMM is compared for a range of complex wavenumbers, and the strategy to choose between two techniques is suggested.
The second problem, namely, the assembly of many singular and nearly-singular integrals, is solved by the use of the Huygens principle. In this work we prove that kernels of the boundary integral operators $w_n^h(d)$ ($h$ is the time step and $t_n=nh$ is the time) exhibit exponential decay outside of the neighborhood of $d=nh$ (this is the consequence of the Huygens principle). The size of the support of these kernels for fixed $h$ increases with $n$ as $n^a,a<1$, where $a$ depends on the order of the Runge-Kutta method and is (typically) smaller for Runge-Kutta methods of higher order. Numerical experiments demonstrate that theoretically predicted values of $a$ are quite close to optimal.
In the work it is shown how this property can be used in the recursive algorithm to construct only a few matrices with the near-field, while for the rest of the matrices the far-field only is assembled. The resulting method allows to solve the three-dimensional wave scattering problem with asymptotically almost linear complexity. The efficiency of the approach is confirmed by extensive numerical experiments.
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Houghton, Andrew Warren. "Time domain filtered cross spectral density detection and direction finding of spread spectrum signals, and implementation using acousto-optic correlation." Thesis, University of Plymouth, 1996. http://hdl.handle.net/10026.1/2003.
Full textAbstract:
This thesis presents a technique for the detection of spread spectrum signals, of arbitrary form, even when the signal power spectral density (PSD) is well below the surveillance receiver noise spectral density, using a pair of antennas with broadband (1 GHz or more) receivers. Cross correlating the outputs of two receivers, spatially separated by a distance of the order of one metre or more, produces a cross correlation function (ccf) in which the noise components are spread uniformly over the whole width while the signal component, the narrow autocorrelation function (acf) of the spread spectrum signal, is concentrated near to the centre. The acf is displaced from the centre of the ccf by a small time shift equal to the time difference of arrival of the signal at the two antennas. A simple time domain filter can select a narrow centre portion of the ccf, rejecting the remainder which contains only noise. Taking the Fourier transform of this windowed ccf produces the "time domain filtered cross spectral density" (TDFCSD), in which the signal to noise ratio is independent of receiver bandwidth. Spread spectrum signals can then be both detected and characterised in an extremely sensitive broadband system by threshold detection applied to the magnitude of this TDFCSD. High resolution direction fmding can then be achieved by estimating the time difference of arrival at the two antennas from the phase slope of the appropriate part of the TDFCSD. An analysis of the performance of this dual receiver system is presented. A computer simulation illustrates the signal processing involved and shows excellent agreement with the analysis. An analysis of the detection performance of this system acting in an electronic support measure (ESM) role and comparison with other systems shows that, in addition to being able to obtain more information, this system can offer significantly greater sensitivity than a crystal video receiver. Acousto-optic correlation may be used to perform the cross correlation and time domain filtering of wideband signals in real time, with final processing of the much reduced data set to obtain and analyse the TDFCSD being carried out digitally. A novel non-heterodyning space integrating architecture capable of forming the true correlation function using the zeroth diffraction orders from acousto-optic cells was invented, the operation of which is not explained by the commonly used methods of analysis. By looking again at the acousto-optic interaction, it is shown that there is considerable information in the zeroth diffraction order and a unified theory of one dimensional space integrating correlators is developed, in which many known architectures can be treated as special cases of a general all order correlator. Because of practical difficulties in using a space integrating correlator to obtain the TDFCSD for continuous inputs, later work concentrated on time integrating correlation. Theoretical analysis and practical results are presented for a time integrating acousto-optic correlator, demonstrating that it gives itself naturally to the signal processing operations required and could be used in a real surveillance system making use of the TDFCSD for detection and direction finding.
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Mordini, Nicola. "Multicentre study for a robust protocol in single-voxel spectroscopy: quantification of MRS signals by time-domain fitting algorithms." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2014. http://amslaurea.unibo.it/7579/.
Full textAbstract:
Magnetic Resonance Spectroscopy (MRS) is an advanced clinical and research application which guarantees a specific biochemical and metabolic characterization of tissues by the detection and quantification of key metabolites for diagnosis and disease staging.
The "Associazione Italiana di Fisica Medica (AIFM)" has promoted the activity of the "Interconfronto di spettroscopia in RM" working group. The purpose of the study is to compare and analyze results obtained by perfoming MRS on scanners of different manufacturing in order to compile a robust protocol for spectroscopic examinations in clinical routines. This thesis takes part into this project by using the GE Signa HDxt 1.5 T at the Pavillion no. 11 of the S.Orsola-Malpighi hospital in Bologna.
The spectral analyses have been performed with the jMRUI package, which includes a wide range of preprocessing and quantification algorithms for signal analysis in the time domain. After the quality assurance on the scanner with standard and innovative methods, both spectra with and without suppression of the water peak have been acquired on the GE test phantom. The comparison of the ratios of the metabolite amplitudes over Creatine computed by the workstation software, which works on the frequencies, and jMRUI shows good agreement, suggesting that quantifications in both domains may lead to consistent results.
The characterization of an in-house phantom provided by the working group has achieved its goal of assessing the solution content and the metabolite concentrations with good accuracy. The goodness of the experimental procedure and data analysis has been demonstrated by the correct estimation of the T2 of water, the observed biexponential relaxation curve of Creatine and the correct TE value at which the modulation by J coupling causes the Lactate doublet to be inverted in the spectrum.
The work of this thesis has demonstrated that it is possible to perform measurements and establish protocols for data analysis, based on the physical principles of NMR, which are able to provide robust values for the spectral parameters of clinical use.
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Tan, Ke. "Convolutional and recurrent neural networks for real-time speech separation in the complex domain." The Ohio State University, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=osu1626983471600193.
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Kung, Christopher W. "Development of a time domain hybrid finite difference/finite element method for solutions to Maxwell's equations in anisotropic media." Columbus, Ohio : Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1238024768.
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Wassef, Karim N. "Nonlinear transient finite element analysis of conductive and ferromagnetic regions using a surface admittance boundary condition." Diss., Georgia Institute of Technology, 1999. http://hdl.handle.net/1853/13318.
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Potvin, Marie-Josee. "Comparison of time-domain finite element modelling of viscoelastic structures using an efficient fractional Voigt-Kelvin model or prony series." Thesis, McGill University, 2001. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=37814.
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The thesis centers on time domain modelling of viscoelastic materials. Classical models are compared to models involving fractional derivatives, which are derivatives of an order between 0 and 1. Parameters for classical and fractional order models are found for two materials, polymethylmethacrylate and 3M ISD 112, an acrylic based material sold as a viscoelastic layer by 3M. In both cases, only Prony series with several parameters achieve a good representation over a large frequency range. In the case of 3M ISD 112, a fractional model with only two parameters gives a good representation over a frequency range of three decades, which is often sufficient.An algorithm based on an approximated definition of the fractional derivative and a trapezoidal rule is described to solve constitutive equations with fractional derivatives. The algorithm is implemented in C and tested against a numerical Laplace inverse for the case of a material submitted to sinusoidal strains. The algorithm gives accurate results and does not require very small steps, which is usually the case for algorithms based on finite differences or Grunwald series.
The algorithm is adapted to the structure of a user subroutine of a commercial finite element package, Samcef, for a six component isotropic tensor. The model assumes a constant bulk modulus and has one fractional derivative of the deviatoric strain. The Jacobian of the constitutive equation with a fractional derivative is derived and implemented. The results from the subroutine are compared satisfactorily to results from the numerical Laplace inverse for a cubic element submitted to sinusoidal strains.
Finally, the different models are tested to represent the experimental behaviour of slewing beams made either of polymethylmethacrylate or steel covered by constrained viscoelastic layers. The classical models give generally a poor representation of the experimental behaviour, except for the Prony series. The fractional model give a representation as satisfactory as the ones obtained with the Prony series, but for a much higher CPU times due to the hereditary nature of the fractional derivative. It is therefore recommended to use Prony series models, unless the data to perform the parameter identification is limited. In that case, the fractional order model becomes interesting despite the higher demands on the CPU time.
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Chilton, Ryan Austin. "H-, P- and T-Refinement Strategies for the Finite-Difference-Time-Domain (FDTD) Method Developed via Finite-Element (FE) Principles." The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1219064270.
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