Edge-based finite elements and vector ABCs applied to 3D scattering

An edge based finite element formulation with vector absorbing boundary conditions is presented for scattering by composite structures having boundaries satisfying impedance and/or transition conditions. Remarkably accurate results are obtained by placing the mesh a small fraction of a wavelength away from the scatterer

The scattering of a cylindrical invisibility cloak: reduced parameters and optimization

We investigate the scattering of 2D cylindrical invisibility cloaks with simplified constitutive parameters with the assistance of scattering coefficients. We show that the scattering of the cloaks originates not only from the boundary conditions but also from the spatial variation of the component of permittivity/permeability. According to our formulation, we propose some restrictions to the invisibility cloak in order to minimize its scattering after the simplification has taken place. With our theoretical analysis, it is possible to design a simplified cloak by using some peculiar composites like photonic crystals (PCs) which mimic an effective refractive index landscape rather than offering effective constitutives, meanwhile canceling the scattering from the inner and outer boundaries.Comment: Accepted for J. Phys.

Modelling Of Periodic Perfectly Conducting Rough Surfaces In Terms Of Higher Order Impedances

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2008Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2008Bu çalışmada, mükemmel iletken periyodik engebeli yüzeylerin, yüksek mertebeden periyodik empedans sınır koşuluna sahip düzlemsel bir yüzey ile eşdeğer olarak modellenmesini sağlayan bir metod sunulmaktadır. Önerilen metod, periyodik engebeli yüzeyin yüksek mertebeden periyodik empedans koşuluna sahip düzlemsel bir yüzey ile gösterilimine dayanmaktadır. Sadelik amacıyla, analiz tek boyutlu yüzeyler için gerçekleştirilmektedir. Yüksek mertebeden periyodik empedans sınır koşulunda bulunan homojen olmayan yüzey empedansları ile mükemmel iletken periyodik yüzeyin değişimi arasındaki açık ilişki, toplam elektrik alanın Taylor serisi açılımı ile elde edilirler. Yüzey empedansları ile periyodik yüzeyin değişimi arasındaki direkt ilişki, empedansların da periyodik olmasını sağlamaktadır. Yüzey empedansları aydınlatma açısından bağımsızdırlar ve mükemmel iletken periodik engebeli yüzeyler için evrensel bir eşdeğer sınır koşulu oluşturmaktadırlar. Dolayısıyla, mükemmel iletken engebeli yüzeyleri yukarıda anlatıldığı gibi homojen olmayan empedans sınır koşuluna sahip bir düzlemsel yüzey ile modelleyerek, söz konusu saçılma problemi için daha basit bir formülasyon elde etmek mümkündür. Öte yandan, göz önünde bulundurulmalıdır ki; burada önerile metod ile, orjinal yüzeyden saçılan alanı sadece yüzeyin maksimum noktasına teğet olan farazi bir düzlemin üzerinde kalan bölgede hesaplamak mümkündür. Elde edilen, yüksek mertebeden sınır koşuluna sahip düzlemsel yüzeyle ilişkili eşdeğer saçılma problemi; problemi, lineer bir denklem sisteminin çözümüne indirgeyen Floquet teoremi ile çözülmüştür. Önerilen metodun sayısal sonuçları, Sonlu Farklar Zaman Domeni(FDTD) Metodu ile elde edilen referans sonuçlar ile karşılaştırılmaktadırlar. Sayısal simülasyonlar göstermektedir ki, önerilen metod ile tatmin edici sonuçlar alınmaktadır ve metod hesaplama süresi açısından etkindir. Uygulamalardan görüldüğü üzere, referans yöntem ve önerilen yöntemin sonuçları arasındaki ortalama kare hata (MSE), %1 in altında kalmaktadır.In this study, a method is presented for the equivalent representation of a perfectly conducting (PEC) periodic rough surface in terms of planar boundary with higher order periodic impedance boundary condition (HPIBC). In the proposed approach, the periodic rough surface is replaced by a flat one having higher order periodic impedance boundary condition (HPIBC). For the sake of simplicity the analysis is carried out for one-dimensional (1-D) surfaces. The explicit relations between the inhomogeneous surface impedances appearing in HPIBC and the surface variation of the periodic PEC surface are derived through the Taylor expansion of the total field. This direct relation between the surface impedances and the variation of the periodic surface also causes the impedances to be periodic. The surface impedances are independent of the incidence angle, and yield to an universal equivalent boundary condition for perfectly conducting periodic rough surfaces. Therefore, by representing the perfectly conducting(PEC) rough surface in terms of a plane one characterized by the above mentioned inhomogeneous impedance boundary condition, one can achieve a simpler formulation of the corresponding scattering problem. On the other hand, it has to be remarked that the method allows to computing the field scattered by the original surface only in the region exterior to the fictitious plane. The resulting scattering problem related to a planar surface with HPIBC is solved by using the Floquet mode expansion, which reduces the problem to the solution of a linear system of equations. The results of the proposed method are compared with those of Finite Difference Time Domain (FDTD) Method for a number of different surfaces. Numerical simulations show that the method yields accurate results and it is computationally effective. From the numerical implementations it is observed that the MSE error between two methods is always less than 1%.Yüksek LisansM.Sc

Novel Analytical Continuation Based Shape Reconstruction Methods For Perfect Electric Conducting Targets

Tez (Doktora) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2009Thesis (PhD) -- İstanbul Technical University, Institute of Science and Technology, 2009Bu çalışmanın amacı yanına erişilemeyen, elektriksel açıdan mükemmel iletken cisimlerin şekillerinin elektromagnetik dalgalar kullanılarak belirlenebilmesi için yeni ve etkin yöntemlerin geliştirilmesidir. Bu çerçevede, problemin kötü-kurulmuş ve doğrusal olmayan kısımlarını ayrı ayrı ele alan iki farklı yöntem sunulmuştur. Her iki yöntemin de ilk adımında, gürültülü uzak alan verisinden bilinmeyen cismin yakınındaki saçılan alan belirlenmeye çalışılır. Bu amaçla her iki yöntem için de tek katman potansiyel yaklaşımından faydalanılmış ve uzak alan verisi dairesel bir bölge üzerine bir tek katman potansiyel yoğunluğu anlamında devam ettirilmiştir. İlk yöntemde, bu dairesel bölgenin bilinmeyen cismi minimum yarıçapla kapsadığı varsayılmaktadır. Minimum dairenin içerisinde alan, bilinmeyen cismin yüzeyine kadar, daha önce bulunan potansiyel yoğunluğu aracılığıyla hesaplanmış saçılan alanın Taylor serisi açılımı kullanılarak analitik olarak devam ettirilir. İkinci yöntem içinse, dairesel bölge cismin içerisine yerleştirilmiş ve bilinmeyen cismi, bilinmeyen yüzeyin dışındaki bölgede cisimle aynı alan dağılımını yaratacak homojen olmayan bir yüzey empedansına sahip bir empedans silindiri olarak modellemek için kullanılmıştır. Her iki yöntemin de son aşamasında, bilinmeyen cismin üzerinde toplam alanın sıfıra gitmesi biçimindeki sınır koşulu kullanılarak şekil bulma problemi Gauss-Newton algoritmasıyla yinelemeli olarak çözülen doğrusal olmayan bir eşitliğin köklerinin bulunmasına indirgenir.The aim of this study is to develop new and efficient methods to reconstruct the shape of inaccessible, perfect electric conducting targets through the use of electromagnetic waves. Within this framework, two different shape reconstruction methods, which handle the ill-posedness and the nonlinearity of the underlying inverse problem separately, are presented. At the initial step, both methods deal with the reconstruction of the scattered field in the vicinity of the unknown target from the noise corrupted far field pattern. To this aim, the single layer potential approach is utilized for both methods and the far field pattern is backpropagated to a circular domain in terms of a single layer potential density. In the first method, the circular domain is assumed to cover the unknown target with minimum radius. Inside the minimum circle, the Taylor series expansion of the scattered field calculated from the potential density is exploited to analytically continue the field to the unknown boundary. For the second method, the circular domain is located inside the target and it is used to model the unknown target as an impedance cylinder having inhomogeneous surface impedance which generates the same field distribution with the target outside of the unknown boundary. At the final step of both methods, by exploiting the boundary condition that the total electric field on the unknown boundary vanishes, the shape reconstruction problem is reduced to finding the roots of a non-linear equation which is solved iteratively via Gauss-Newton Method.DoktoraPh

Scattering from an Impedance Object at the Edge of a Perfectly Conducting Wedge

Cataloged from PDF version of article.In this study, scattering from impedance bodies positioned at the edge of a perfectly electrically conducting wedge is investigated. In the treatment of the problem, eigenfunction expansion in terms of spherical vector wave functions is employed. A complete dyadic Green's function for the spherical impedance boss at the edge is developed. It is observed that the scattering is highly enhanced by the edge guided waves. Additionally, using T-matrix method, the solution is extended to the general case of irregularly shaped scatterers. The T-matrix solution is verified by applying it to the case of a spherical scatterer and results are compared with the dyadic Green's function solution. © 2013 IEEE

Production of herbal shampoo from Madagascar periwinkle

Recently, the society interest in using herbal shampoos has increased significantly. Big companies have started to produce herbal shampoo to meet the market demands. However, most of these shampoos contain chemicals that could be harmful to human health. The main purpose of this study are to produce homemade herbal shampoo from Madagascar Periwinkle plant without harmful chemicals and have additional benefits to consumers. By using stem and leaf extract from the plant and cold press machine, we analysed the quantitative aspect of the extracts and use it as a main ingredient to produce homemade herbal shampoo. The extracts are then used as ingredients in production of the shampoo. Several tests were conducted to determine the performance of the shampoos. The Fourier Transform Infrared Spectroscopy (FTIR) analysis shows presence oh -OH groups and C=O bond to indicate the presence of vinca alkaloids. The performance tests result inconsistent to one extract only. This study provided an information to future researchers regarding the study of Madagascar Periwinkle and effectiveness of this plant in herbal shampoos production

Invisibility and Inverse Problems

This survey of recent developments in cloaking and transformation optics is an expanded version of the lecture by Gunther Uhlmann at the 2008 Annual Meeting of the American Mathematical Society.Comment: 68 pages, 12 figures. To appear in the Bulletin of the AM

Acoustic modelling of bat pinnae utilising the TLM method

This thesis describes the numerical modelling of bioacoustic structures, the focus being the outer ear or pinnae of the Rufous Horseshoe bat (Rhinolophus rouxii). There have been several novel developments derived from this work including: • A method of calculating directionality based on the sphere with a distribution of measuring points such that each lies in an equal area segment. • Performance estimation of the pinna by considering the directionality of an equivalent radiating aperture. • A simple synthetic geometry that appears to give similar performance to a bat pinna. The outcome of applying the methods have yielded results that agree with measurements, indeed, this work is the first time TLM has been applied to a structure of this kind. It paves the way towards a greater understanding of bioacoustics and ultimately towards generating synthetic structures that can perform as well as those found in the natural world