The scientific literature on airflow in the respiratory system is usually associated with rigid ducts. Many studies have been conducted in the frequency domain to assess respiratory system mechanics. Time-domain analyses appear more independent from the hypotheses of periodicity, required by frequency analysis, providing data that are simpler to interpret since features can be easily associated to time. However, the complexity of the bronchial tree makes 3-D simulations too expensive computationally, limiting the analysis to few generations. 1-D modelling in space-time variables has been extensively applied to simulate blood pressure and flow waveforms in arteries, providing a good compromise between accuracy and computational cost. This work represents the first attempt to apply this formulation to study pulse waveforms in the human bronchial tree. Experiments have been carried out, in this work, to validate the model capabilities in modelling pressure and velocity waveforms when air pulses propagate in flexible tubes with different mechanical and geometrical properties. The experiments have shown that the arrival of reflected air waves occurs in correspondence of the theoretical timing once the wave speed is known. Reflected backward compression waves have generated an increase of pressure (P) and decrease of velocity (U) while expansion backward waves have produced a decrease of P and increase of U according to the linear analysis of wave reflections. The experiments have demonstrated also the capabilities of Wave intensity analysis (WIA), an analytical technique used to study wave propagation in cardiovascular system, in separating forward and backward components of pressure and velocity also for the air case. After validating the 1-D modelling in space and time variables, several models for human airways have been considered starting from simplified versions (bifurcation trachea- main bronchi, series of tubes) to more complex systems up to seven generations of bifurcations according to both symmetrical and asymmetrical models. Calculated pressures waveforms in trachea are shown to change accordingly to both peripheral resistance and compliance variations, suggesting a possible non-invasive assessment of peripheral conditions. A favourable comparison with typical pressure and flow waveforms from impulse oscillometry system, which has recently been introduced as a clinical diagnostic technique, is also shown. The results suggested that a deeper investigation of the mechanisms underlying air wave propagation in lungs could be a useful tool to better understand the differences between normal and pathologic conditions and how pathologies may affect the pattern of pressure and velocity waveforms.

Neste trabalho, estudamos a evolução temporal de pacotes de ondas gaussiano, propagando-se ao longo de hetero-estruturas clássicas. Estas hetero-estruturas apresentam características e propriedades semelhantes a diversas hetero-estruturas quânticas, as quais são exaustivamente estudadas. Estes sistemas têm recebido muito interesse nestes últimos anos, possibilitando uma melhor compreensão dos próprios sistemas quânticos, bem como o aparecimento de novos transdutores e sonares. Os pacotes de ondas propagam-se em meios que apenas permitem ondas longitudinais. Dentre a gama de sistemas unidimensionais abordados neste trabalho, destacamos o estudo de um emissor de pulsos acústicos, do problema do isolamento acústico de ambientes, enfatizando o problema da transmissão de um pacote de ondas incidindo normalmente em um sistema de dupla parede idêntica, com um meio absorvedor situado entre as paredes, e dos cristais acústicos unidimensionais lineares e não-lineares. Nos sistemas bidimensionais, existe o aparecimento de muitos efeitos que acontecem simultaneamente, como espalhamentos, difração, refração, reflexão e transmissão de ondas acústicas planas e esféricas. Destacamos o estudo de três problemas: 1) Espalhamento de uma onda por um disco não-rigido, com impedância acústica característica menor do que a do meio externo; 2) Propagação de um pulso em um sistema periódico de cilindros perfeitamente rígidos, formando um cristal clássico; 3) Propagação do pacote onda por uma guia perfeitamente rígida, de secção reta constante, a qual é cortada por uma outra guia retangular, de características semelhantes à primeira. A evolução temporal do pacote de ondas foi obtido através do desenvolvimento um novo algoritmo numérico. Este algoritimo é baseado na técnica do Split-Operator (SO), que é uma técnica de separação de operadores diferentes, situados no argumento de uma exponencial. Através deste novo algoritmo, foi ) possível estudar as propriedades dinâmicas de ondas de pressão em sistemas acústicos uni e bidimencionais, calculando a estrutura de bandas onde o método do PWE (do inglês plane-wave-expansion) apresenta fraca convergência ou falha. Comparando o nosso novo algoritimo com o método FDTD (fine difference time-domain method), usualmente utilizado em problemas de acústica, o método possui uma série de vantagens: 1) conserva o espaço de fase; 2) o SO é temporalmente inversível enquanto o FDTD não tem esta propriedade; 3) tem uma estabilidade numérica maior. Um método alternativo de evolução foi desenvolvido a partir do SO, quando consideramos aplicações consecutivas. Este método foi denominado de método Split-Operator Modificado (MSO). Este método tem as mesmas propriedades, porém dispende cerca de 20% menos de tempo computacional. Posteriormente, o método foi generalizado para incluir termos de dissipação e termos não-lineares / In this work, we studied the time evolution of a Gaussian wave packet, moving in classical heterostructures. These heterostructures have similar properties and behavior as quantum artificial structures, which are extensively studied in the literature. These classical heterostructures have been explored in these last years, to understand the quantum systems with more accuracy, as well as, to develop new kind of transducers and sonars. All the considered systems just support longitudinal acoustic waves. Here we studied several unidimensional systems, and the most relevant ones are the periodic pulse emitter, the problem of two identical walls containing an dissipative media placed between them, and the linear and non-linear unidimensional acoustic crystals. For two dimensional systems, we have several phenomena occurring at the same time: scattering, diffraction, refraction, reflection and transmission of plane and circular waves. We analyzed three distinct systems: i) Scattering of a wave due to a non-rigid disc; ii) Pulse propagation through a periodic array of perfectly rigid cylinders; iii) pulse propagation in a rectangular wave guide, crossed by another rectangular wave guide. We present here, a new computational algorithm, based on the Split-Operator (SO) technique, which allow us to study the dynamic properties of pressure waves in one and two-dimensional acoustical systems. With this new algorithm, we studied classical artificial structures, with high mismatch differences, where the PWE (plane-wave-expansion) method converges very slowly or fails. Our new algorithm also allow us to study the dynamitic properties of the system, while the PWE just give us the band structure of the system. Comparing this new time evolution algorithm for acoustic waves in classical systems with the FDTD method (finite difference time-domain method), usually used in problems of acoustics, the method has a series of advantages: i) the phase space is conserved; ii) the SO has time reversal symmetry, while FDTD doesn\'t have this property; iii) the SO has a larger numeric stability. An alternative method for time evolution was developed starting from the SO, when we considered consecutive applications. This method was denominated modified Split-Operator method (MSO). This method has the same properties, however MSO needs about 20% less computational time. Alternatively, the method was extended to systems that present dissipative and non-linear terms

The time-dependent elastic wave equation is the foundation of seismology. It is very useful in studying the wave propagation in elastic solids. Simulation of Rayleigh waves, which is governed by the equation, requires high accuracy solutions. Finite difference method have been widely used for the simulation of Rayleigh waves. However, it is not obvious how to effectively impose the free surface boundary condition on a curved surface. On the other hand, discontinuous Galerkin methods are more flexible in handling complex geometries. / In this thesis, we develop a class of discontinuous Galerkin methods for time-dependentelastic wave equation in isotropic homogeneous medium. This method is explicit, locally and globally energy conserving. Also, the L² convergence of this method is optimal and the convergence in energy norm is independent of Poisson's ratio. / Besides, we apply our method to simulate Rayleigh waves on curved free surfaces. We also impose a perfectly matched layer to absorb the outward waves. Numerical examples show that our method can accurately capture features of Rayleigh waves even in a domain with high Poisson's ratio. / 時間依賴型彈性波動方程」是地震學的基礎。這組方程對於波在彈性固體中傳播的研究非常有用。雷利波是由這個方程所控制。模擬雷利波須要有非常準確的解。有限差分法廣泛地應用在雷利波的模擬上，可是如何有效地施加自由表面邊界條件於曲面上的方法並不明顯。另一方面，間斷伽遼金方法能更靈活地處理複雜的幾何形狀。 / 在這篇論文中，我們發展了一類間斷伽遼金方法去求「在均勻各向同性的介質上的時間依賴型彈性波動方程」的解。我們將表明，這種方法是顯式的，局部及全域能量守恆的，而它的收斂是最優的和獨立於泊松比的。 / 除此之外，我們運用這個方法來模擬雷利波在具有起伏的自由表面的半空間模型的傳播。我們會使用完美匹配層去吸收朝外的波動。數值算例反映，即使在高柏松比的介質中，這個方法也可以準確地捕捉雷利波的特徵。 / Lam, Chi Yeung. / Thesis M.Phil. Chinese University of Hong Kong 2014. / Includes bibliographical references (leaves 44-47). / Abstracts also in Chinese. / Title from PDF title page (viewed on 06, October, 2016). / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only.

M.L.V. Pitteway. / "November 8, 1960." / Bibliography: p. 26. / Army Signal Corps Contract DA36-039-sc-78108. Dept. of the Army Task 3-99-20-001 and Project 3-99-00-000.

The present work explores the possibilities of adapting poro-elastic lightweight acoustic materials to specific applications. More explicitly, a design approach is presented where finite element based numerical simulations are combined with optimization techniques to improve the dynamic and acoustic properties of lightweight multilayered panels containing poro-elastic acoustic materials. The numerical models are based on Biot theory which uses equivalent fluid/solid models with macroscopic space averaged material properties to describe the physical behaviour of poro-elastic materials. To systematically identify and compare specific beneficial or unfavourable material properties, the numerical model is connected to a gradient based optimizer. As the macroscopic material parameters used in Biot theory are interrelated, they are not suitable to be used as independent design variables. Instead scaling laws are applied to connect macroscopic material properties to the underlying microscopic geometrical properties that may be altered independently. The design approach is also combined with a structural sandwich panel mass optimization, to examine possible ways to handle the, sometimes contradicting, structural and acoustic demands. By carefully balancing structural and acoustic components, synergetic rather than contradictive effects could be achieved, resulting in multifunctional panels; hopefully making additional acoustic treatment, which may otherwise undo major parts of the weight reduction, redundant. The results indicate a significant potential to improve the dynamic and acoustic properties of multilayered panels with a minimum of added weight and volume. The developed modelling techniques could also be implemented in future computer based design tools for lightweight vehicle panels. This would possibly enable efficient mass reduction while limiting or, perhaps, totally avoiding the negative impact on sound and vibration properties that is, otherwise, a common side effect of reducing weight, thus helping to achieve lighter and more energy efficient vehicles in the future. / <p>QC 20120815</p>

This thesis focuses on the analytical modeling of periodic structures which contain bands with multiple modes of propagation. The work is motivated by several structures which exhibit dual-mode propagation bands. Initially, transmission line models are focused on. Transmission line models of periodic structures have been used extensively in a wide variety of applications due to their simplicity and the ease with which one can physically interpret the resulting wave propagation effects. These models, however, are fundamentally limited, as they are only capable of capturing a single mode of propagation. In this work multiconductor transmission line theory, which is the multi-mode generalization of transmission line theory, is shown to be an effective and accurate technique for the analytical modeling of periodically loaded structures which support multiple modes of propagation. Many results from standard periodic transmission line analysis are extended and generalized in the multiconductor line analysis, providing a familiar intuitive model of the propagation phenomena. The shielded Sievenpiper structure, a periodic multilayered geometry, is analyzed in depth, and provides a canonical example of the developed analytical method. The shielded Sievenpiper structure exhibits several interesting properties which the multiconductor transmission line analysis accurately captures. It is shown that under a continuous change of geometrical parameters, the dispersion curves for the shielded structure are transformed from dual-mode to single-mode. The structure supports a stop-band characterized by complex modes, which appear as pairs of frequency varying complex conjugate propagation constants. These modes are shown to arise even though the structure is modeled as lossless. In addition to the periodic analysis, the scattering properties of finite cascades of such structures are analyzed and related to the dispersion curves generated from the periodic analysis. Excellent correspondence with full wave finite element method simulations is demonstrated. In conclusion, a physical application is presented: a compact unidirectional ring-slot antenna utilizing the shielded Sievenpiper structure is constructed and tested.

This thesis focuses on the analytical modeling of periodic structures which contain bands with multiple modes of propagation. The work is motivated by several structures which exhibit dual-mode propagation bands. Initially, transmission line models are focused on. Transmission line models of periodic structures have been used extensively in a wide variety of applications due to their simplicity and the ease with which one can physically interpret the resulting wave propagation effects. These models, however, are fundamentally limited, as they are only capable of capturing a single mode of propagation. In this work multiconductor transmission line theory, which is the multi-mode generalization of transmission line theory, is shown to be an effective and accurate technique for the analytical modeling of periodically loaded structures which support multiple modes of propagation. Many results from standard periodic transmission line analysis are extended and generalized in the multiconductor line analysis, providing a familiar intuitive model of the propagation phenomena. The shielded Sievenpiper structure, a periodic multilayered geometry, is analyzed in depth, and provides a canonical example of the developed analytical method. The shielded Sievenpiper structure exhibits several interesting properties which the multiconductor transmission line analysis accurately captures. It is shown that under a continuous change of geometrical parameters, the dispersion curves for the shielded structure are transformed from dual-mode to single-mode. The structure supports a stop-band characterized by complex modes, which appear as pairs of frequency varying complex conjugate propagation constants. These modes are shown to arise even though the structure is modeled as lossless. In addition to the periodic analysis, the scattering properties of finite cascades of such structures are analyzed and related to the dispersion curves generated from the periodic analysis. Excellent correspondence with full wave finite element method simulations is demonstrated. In conclusion, a physical application is presented: a compact unidirectional ring-slot antenna utilizing the shielded Sievenpiper structure is constructed and tested.

The topic of antenna pattern synthesis, in the context of beam shaping, is considered. One approach to this problem is to use the method of point matching. This method can be used to approximate antenna patterns with a set of uniformly spaced sources with suitable directivities. One specifies a desired antenna pattern and approximates it with a combination of beams. This approach results in a linear system of equations that can be solved for a set of beam coefficients. With suitable shifts between the matching points and between the source points, a good agreement between the assumed and the reproduced antenna patterns can be obtained along an observation line. This antenna modelling could be used in the program NERO to compute the field at the receiver antenna for a realistic 2D communication link. It is verified that the final result is not affected by the details of the antenna modelling.

Guided wave techniques have great potential for the structural health monitoring of plate-like components. Previous research has demonstrated the effectiveness of combining laser-ultrasonic techniques with time-frequency representations to experimentally develop the dispersion relationship of a plate; the high fidelity, broad bandwidth and point-like nature of laser ultrasonics are critical for the success of these results. Unfortunately, laser ultrasonic techniques are time and cost intensive, and are impractical for many in-service applications. Therefore this research develops a complementary digital signal processing methodology that uses mounted piezoelectric elements instead of optical devices. This study first characterizes the spatial and temporal effects of oil coupled and glued piezoelectric sources, and then develops a procedure to interpret and model the distortion caused by their limited bandwidth and finite size. Furthermore, it outlines any inherent difficulties for time and frequency domain considerations. The deconvolution theory for source function extraction in the time - and frequency domain under the presence of noise is provided and applied to measured data. These considerations give the background for further studies to develop a dispersion relationship of a plate with the fidelity and bandwidth similar to results possible with laser ultrasonics, but made using mounted piezoelectric sources.

Geotechnical properties vary in space. Statistical parameters such as mean, deviation, and correlation length are characteristics for each sediment and formation history. The effects of spatial variability on the macro-scale mechanical properties of soils are investigated using Monte Carlo non-linear finite element simulations. Boundary conditions include 1) isotropic loading, 2) zero-lateral strain loading, 3) drained and undrained deviatoric loading, and 4) small-strain wave propagation. Emphasis is placed on identifying the effects of spatial variability on the stiffness and strength of soils, recognizing emergent phenomena, and creating the background for new geotechnical design methods that take into consideration spatial variability. The arithmetic mean of soil properties cannot be used to estimate the stiffness or strength of heterogeneous soils. Greater deviation and longer relative correlation length in the spatial distribution of soil properties yield a softer and weaker mechanical response. Load transfer concentrates along stiffer zones, leading to stress-focusing and lower K0 values. Drained loading promotes internal homogenization. Undrained deviatoric loading can cause percolation of internal weakness and shear strain localization. Spatial heterogeneity adds complexity to elastic wave propagation. Heterogeneous soil mixtures can be engineered to attain unique macroscale behavior