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This dissertation presents procedures for implementing high order boundary conditions in time and frequency domains for solving exterior problems governed by the Helmholtz equation and the wave equation exterior to a perfectly conducting scatterer. Solving problems governed by two and three-dimensional wave equations in exterior domains is a complex task. There are techniques to reduce the computational complexities; one technique is On-Surface Radiation Boundary Conditions (OSRBC). There has been a recent interest in revisiting this technique for two and three-dimensional problems. In this research, we explore the implementation of a new high order OSRBC based on the high order local boundary conditions introduced in for two and three dimensions to solve the wave equation in exterior domains. As will be seen later, the OSRBC implementations require normal derivatives of the scattered field on the surface of the scatterer. For the two-dimensional case, we develop a Fourier spectral method based on discrete Fourier transform to find the normal derivative of the electromagnetic field on the surface of the scatterer. The method involves transforming the high order time dependent local boundary conditions to frequency-domain and implementing it on the surface of the scatterer. The normal derivative of the scattered field is needed for applications such as the calculation of the radar cross-section and the surface current. We consider the two-dimensional problem for a perfectly conducting scatterer with arbitrary cross section. The numerical implementations and their performances for a wide range of frequencies are demonstrated and compared to the frequency-domain integral equation for the scattered field. The advantage of the new method is that the On-Surface Radiation Boundary Conditions (OSRBCs) is applicable to a wide range of frequencies. A series of numerical tests demonstrate the accuracy and efficiency of these conditions to a wide range of frequencies. Both the exact solutions, as well as the high order local boundary conditions solutions, are compared. For the two and three-dimensional time dependent wave equations cases, we simulate exact solutions in a large exterior domain because explicit solutions are not available. The implementation involves a new novel approach based on bilinear transformation, which simplified the implementation process and lead to higher accuracy compared to the different types of finite difference schemes used to approximate the first and second order partial derivative in the new high order OSRBC and the auxiliary functions that define the high order boundary conditions. A series of numerical tests demonstrate the accuracy and efficiency of the new high order OSRBC for two and three-dimensional problems. Both the long domain solutions, as well as the OSRBC solutions, are compared.
Surface Impedance Boundary Conditions is perhaps the first effort to formalize the concept of SIBC or to extend it to higher orders by providing a comprehensive, consistent, and thorough approach to the subject. The product of nearly 12 years of research on surface impedance, this book takes the mystery out of the largely overlooked SIBC. It provides an understanding that will help practitioners select, use, and develop these efficient modeling tools for their own applications. Use of SIBC has often been viewed as an esoteric issue, and they have been applied in a very limited way, incorporated in computation as an ad hoc means of simplifying the treatment for specific problems. Apply a Surface Impedance "Toolbox" to Develop SIBCs for Any Application The book not only outlines the need for SIBC but also offers a simple, systematic method for constructing SIBC of any order based on a perturbation approach. The formulation of the SIBC within common numerical techniques—such as the boundary integral equations method, the finite element method, and the finite difference method—is discussed in detail and elucidated with specific examples. Since SIBC are often shunned because their implementation usually requires extensive modification of existing software, the authors have mitigated this problem by developing SIBCs, which can be incorporated within existing software without system modification. The authors also present: Conditions of applicability, and errors to be expected from SIBC inclusion Analysis of theoretical arguments and mathematical relationships Well-known numerical techniques and formulations of SIBC A practical set of guidelines for evaluating SIBC feasibility and maximum errors their use will produce A careful mix of theory and practical aspects, this is an excellent tool to help anyone acquire a solid grasp of SIBC and maximize their implementation potential.
When the temperature of a gas is not too high and the density of a gas is not too low, the transfer of heat by radiation is usually negligibly small in comparison with that by conduction and convection. However, in the hypersonic flow of space flight, particularly in the re-entry of a space vehicle, and in the flow problem involving nuclear reaction such as in the blast wave of nuclear bomb or in the peaceful use of the controlled fusion reaction, the temperature of the gas may be very high and the density of the gas may be very low. As a result, thermal radiation becomes a very important mode of heat transfer. A complete analysis of such high temperature flow fields should be based upon a study of the gasdynamic field and the radiation field simultaneously. Hence during the last few years, considerable efforts have been made to study such interaction problems between gasdynamic field and radiation field and a new title, Radiation Gasdynamics, has been suggested for this subject. Even though radiative transfer has been studied for a long time by astro physicists, the interaction between the radiation field and the gadsynamic field has been only extensively studied recently.
The work presented in this thesis deals with the behavior of the scattered electromagnetic field on the surface of the scatterer. Namely, our goal is to find the normal derivative of the field numerically for applications such as the calculation of the radar cross-section or surface current. The phenomenon is addressed through the resolution of a wave problem with boundary conditions specified using a sequence of local operators. We consider the two dimensional problem with an arbitrary-shaped scatterer made in a material assumed perfectly conducting. At first, we introduce the main theoretical ideas and concepts that will serve our work and that have inspired it; this prceeds from the mathematical formulation of the solutions, to the solving procedures and through the expression of the boundary conditions. In a second part, a method to find exact solutions will be presented for comparison purpose. The solutions thus found will be later compared to the results obtained from our numerical procedures. These procedures are presented in detail in the third and fourth chapters, they include a brute-force method and a Laplace-transform based method. Finally, the results are presented, discussed and a statement of the method performance will be set. Figures are included to illustrate our views.
Theoretical and Applied Mechanics covers the proceedings of the 16th International Congress of Theoretical and Applied Mechanics, held at the Technical University of Denmark, Lyngby, Denmark on August 19-25, 1984. The contributors consider the significant advances in the thriving field of mechanics. This book is composed of 21 chapters, and begins with an overview of space research contributions in understanding fluid media mechanics. This topic is followed by discussions on some aspects and fundamentals of mechanics, such as chaos, computer application, resonant phenomena, adiabaticity, and nonlinear acoustics. The following chapters explore the various applications of theoretical and applied mechanics, including in marine structures, oil recovery, and ice and snow mechanics. This book also deals with nonlinear wave motion, hydrodynamic systems, ocean wave spectra, and Helmholtz concept. The remaining chapters look into the issues of steady water bifurcation, concept of anisotropic soils, and flow visualization. This book is of great value to physicists and research workers who wish to expand their knowledge in mechanics.
"This volume presents in eleven chapters key computational methods for acoustics and vibro-acoustics problems. Each chapter, written by different authors, presents a state of the art of well-established or innovative methods, techniques or algorithms. A bibliography is included at the end of each chapter."--BOOK JACKET.