Geometrical Theory Of Diffraction Research Articles

Using the Correlation Model of Random Quadrupoles of Sources to Calculate the Efficiency of Turbulent Jet Noise Screening with Geometric Diffraction Theory

The paper presents the results of a computational–experimental study of jet noise reduction using the screening effect. According to correlation theory, the noise sources of a single circular jet are represented by a set of uncorrelated quadrupoles. Using the basic relations of geometric diffraction theory (GDT), expressions are obtained for the sound field emitted by a point quadrupole source located near an acoustically rigid infinite half-plane. Using the expressions obtained in the framework of the GDT, the correlation model of the jet noise sources was adapted to the calculation of the sound field in the presence of a flat rectangular screen. A comparison of the calculated and experimentally measured spectra of sound pressure levels, performed at a given jet velocity and various screen positions, showed good qualitative, and for certain observation angles, quantitative agreement.

Traffic Noise Mitigation Using Single and Double Barrier Caps of Different Shapes for an Extended Frequency Range

The primary function of noise barriers is to shield inhabitants of affected areas from excessive noise generated by road traffic. To enhance the performance of noise barriers while simultaneously adhering to height restrictions, the attachment of structures (caps) of different shapes to the tops of conventional screens can be considered. These caps can significantly impact the diffracted sound energy, thereby increasing the desired global acoustic losses. This work presents a comprehensive study of the acoustic performance of noise barriers with single and double attached caps of different shapes through a calculation of their insertion losses (IL). This study comprehensively addresses and compares different types, sizes, combinations, and numbers of noise barrier caps for different scenarios (including sloping and absorbent grounds) and sources (“car” and “ambulance”) for an extended frequency band up to 10 kHz. To the best of the authors’ knowledge, this is a range that has not previously been analyzed. A variety of different cap shapes were considered including cylinders, rectangles, trapezoids, and Y/T-shaped forms. To calculate the IL, an innovative and fast uniform theory of diffraction (UTD)-based method developed by the authors was applied in all simulations. The results showed that the Y-shaped single and double barrier caps were, in general, the most effective at increasing IL without raising the height of the barrier, thereby successfully managing the aesthetic impact. The results also showed how the consideration of sloping and absorbent floors could also contribute to improved noise abatement.

Sub-structure characteristic mode computation utilising field-based MM/GTD hybrid methods

It is pointed out how the (sub-structure) characteristic modes of a perfect electrically conducting (PEC) object, in the presence of an electrically very large second PEC object, can be computed using a field-based hybrid method that combines the method of moments (MM) and the geometrical theory of diffraction (GTD). An example is provided to demonstrate this. This possibility has not yet been explicitly stated elsewhere (and so perhaps not realised), nor example computations provided. We believe this realisation might widen the scope of application of characteristic mode analysis.

A Heuristic UTD Solution for Scattering by a Thin Lossless Anisotropic Slab

A heuristic uniform theory of diffraction (UTD) corner and wedge diffraction coefficient for predicting electromagnetic scattering by a thin lossless anisotropic slab is presented. An approach based on modifying UTD corner and wedge diffraction formulations for a half-plane with the reflection and transmission coefficients of an anisotropic medium is proposed. The formulation works for both uniaxial and biaxial media and the final expressions are in closed form and easy to implement in ray-tracing software. Numerical results for various scattering problems showing excellent agreement with reference solutions are provided.

Electromagnetic Scattering by Artificial Plasma Clouds in the Ionosphere

Vaporized samarium (Sm) has been released in the ionosphere for artificially creating plasma clouds, which may be tailored as radio wave reflectors for over-the-horizon communications. The model of samarium releases needs to be further refined for filling the gap between experimental observations and theoretical simulations. A sophisticated simulation model, so far to date, for samarium release in the ionosphere has been developed, and the redundancy reaction, inversion, and the photochemical reactions are all taken into account. The model results are consistent with experimental observations. Due to the geomagnetic field and neutral wind, the shape of artificial plasma clouds significantly changes. The error of the spherically symmetric reflection model is large. According to the density distribution of plasma clouds, the geometric theory of diffraction (GTD) is introduced here to deal with the electromagnetic scattering. The GTD-based scattering model has been established and used to study the time–space evolution of the ground-received power patterns in this article.

Joint Scattering Center Enumeration and Parameter Estimation in GTD Model

In this article, a novel multistage CLEAN-based method based on generalized likelihood ratio test (GLRT) is proposed for scattering centers’ enumeration and their parameter estimation. To analyze the performance of the proposed scattering center enumeration algorithm, the probability density function (pdf) of the GLRT is derived for both the known noise variance case and the unknown noise variance case. In addition, we derived a closed form for the probability of correct enumeration of scattering centers, obtained in the case of the known model parameter. A method to reduce the computational complexity of the proposed algorithm has also been introduced. Despite the conventional methods which are based on the damped exponential models, the proposed method uses the geometrical theory of diffraction (GTD) model whose performance does not degrade as the bandwidth increases. The simulation results show an appropriate performance of this method at low signal-to-noise ratio (SNR) situations.

The electromagnetic scattering modelization of metallic object in rectangular waveguide by hybrid method (MOM–PO–GTD)

ABSTRACT This article deals with a new hybrid approach combining a Moment Method (MOM), with the Geometrical Theory of Diffraction (GTD), this approach evaluates the currents surface on the microstrip antenna placed in the rectangular Electric Magnetic Electric Magnetic(EMEM) Waveguides. The incident field is taken as a TEM polarized plane wave in the principal plane. The new hybrid current function based on the Geometrical Theory of Diffraction (GTD), consists in approximating the total current on a homogenous metallic pattern on two steps; in the inside of the metal is dominated by the Physical Optic (PO) method, and the edges, tips effects are described by the GTD method. A set of test functions (sinusoidal or triangular or others) used in the MoM method is replaced by one and unique test function, to get a rapid convergence and less computation time. The new hybrid method can be applied to different structures to calculate scattering for validation proposes. The result of this new method electromagnetic variables current and filed distributions are in agreement with those given by the MoM method. An accurate gain in precision and time, memory resources are obtained.

Design of an Anechoic Chamber for W-Band and mmWave

In this paper, we describe the design of an electrically large anechoic chamber for usage on millimetre-wave bands. Ansys Savant sotware was used to perform a simulation of the chamber, using physical optics coupled with uniform theory of diffraction (PO/UTD). Moreover, a method based on an open waveguide probe is described in this paper to obtain the electrical properties of the RF absorbers at millimetre-wave frequencies. Two different source antennas were simulated in this work and the corresponding quiet zones predicted. The largest quiet zone was 30 m m × 30 m m × 50 m m , for a chamber size of 1.2 m m × 0.6 m m × 0.6 m .

Gaussian source beam diffraction by a perfect electromagnetic half-plane.

In the present work, diffraction of a Gaussian source beam by a perfect electromagnetic conductor (PEMC) semi-screen is investigated. Due to the special property of the PEMC sheet, which is a combination of perfect electric conductor and perfect magnetic conductor surfaces, the reflected wave from the PEMC surface has a cross-polarized component in addition to the co-polarized component. For an electric line source illumination, the diffracted fields are derived by considering the analogy between the transition boundaries and scattered geometric optics fields. Later, the complex point source technique is applied for evaluation of Gaussian beam diffraction. The finite magnitude values of fields are derived with the aid of an improved version of the well-known uniform theory of diffraction for evanescent plane waves. Also, the resultant waves are plotted and discussed for different groups of parameters.

A Fast UTD-Based Method for the Analysis of Multiple Acoustic Diffraction over a Series of Obstacles with Arbitrary Modeling, Height and Spacing

A uniform theory of diffraction (UTD)-based method for analysis of the multiple diffraction of acoustic waves when considering a series of symmetric obstacles with arbitrary modeling, height and spacing is hereby presented. The method, which makes use of graph theory, funicular polygons and Fresnel ellipsoids, proposes a novel approach by which only the relevant obstacles and paths of the scenario under study are considered, therefore simultaneously providing fast and accurate prediction of sound attenuation. The obstacles can be modeled either as knife edges, wedges, wide barriers or cylinders, with some other polygonal diffracting elements, such as doubly inclined, T- or Y-shaped barriers, also considered. In view of the obtained results, this method shows good agreement with previously published formulations and measurements whilst offering better computational efficiency, thus allowing for the consideration of a large number of obstacles.

Deterministic Modeling of Indoor Stairwells Propagation Channel

A deterministic approach is presented for modeling a wireless propagation channel in a stairwell environment. The ray-tracing method is used to find the reflected and diffracted rays with the highest contribution to the total received power at a given location. Most significant reflected rays occur at the surfaces of specific steps of the stairwell and the surrounding lateral walls, while most significant diffracted beams emanate from the edges of steps located between transmitter and receiver antennas. The uniform theory of diffraction is used to calculate the diffracted contributions from the edges, whereas the multiple diffractions are calculated using heuristic diffraction coefficients. Unlike the reported models, the proposed model is deterministic and focuses on the radio propagation within stairwells at the X-band frequencies range. Experimental validation is performed using two types of antennas, horns and patches, for an operating frequency of 10 GHz. Simulated results of narrowband and wideband radio channels showed a good agreement with the experimental ones. The correlation coefficient between measured and simulation results is around 80% on average, and the residual errors between measured and simulated total received power results are less than 13%, which means that the proposed model has fair accuracy for such complex environments.

PARDOS: An Educational Software Tool for the Analysis of Sound Propagation

The educational software tool PARDOS is presented for the analysis of sound propagation, being applicable for supplementing and enhancing the understanding and learning of the principles of acoustic engineering thanks to the features it offers. With this tool, students (high school or undergraduate as well as Ph.D.) will be able to create environments - multi-modeled obstacles - and settings based on real scenarios, load acoustic sources, simulate the expected insertion losses (IL) and acoustic attenuation at any receiver (listener) location, or use the auralization module to test the acoustic quality of the design carried out. Moreover, the learning of the fundamentals of acoustic propagation is supported by a set of four guided practices embedded in the software application, which can also be very useful for teachers. The main advantage of the presented tool is that, unlike most of the computationally demanding alternatives for acoustic propagation - which usually imply complex CAD files, and highly dense raytracing approaches - PARDOS is a user-friendly teaching-oriented software developed with MATLAB that successfully combines the use of the Uniform Theory of Diffraction (UTD), Image Source (IS) method, graph theory, funicular polygons, and Fresnel ellipsoids to obtain high accuracy performances while significantly lowering computational requirements. Moreover, the design of any obstacle can easily be achieved using the proper combination of wedges, rectangles, cylinders, and T/Y-shaped structures.

Modeling Equation for Multiple Knife-Edge Diffraction

This article is devoted to the canonical problem of the radio-wave propagation in the presence of multiple knife-edges. The system of integral equations in a spectral domain for the scattered field is derived. On the basis of its solution diffraction and reflection effects spawned by the multiple knife-edges may be accurately modeled. The electromagnetic field is generated by a source of the monochromatic radio waves with an arbitrary beam pattern. The efficient numerical solution to the proposed equation is given. The comparison with the two-way parabolic equation (PE) method, the uniform theory of diffraction (UTD) method, and the finite element method (FEM) is presented.

Analytical methods for perfect wedge diffraction: A review

The subject of diffraction of waves by sharp boundaries has been studied intensively for well over a century, initiated by groundbreaking mathematicians and physicists including Sommerfeld, Macdonald and Poincaré. The significance of such canonical diffraction models, and their analytical solutions, was recognised much more broadly thanks to Keller, who introduced a geometrical theory of diffraction (GTD) in the middle of the last century, and other important mathematicians such as Fock and Babich. This has led to a very wide variety of approaches to be developed in order to tackle such two and three dimensional diffraction problems, with the purpose of obtaining elegant and compact analytic solutions capable of easy numerical evaluation.The purpose of this review article is to showcase the disparate mathematical techniques that have been proposed. For ease of exposition, mathematical brevity, and for the broadest interest to the reader, all approaches are aimed at one canonical model, namely diffraction of a monochromatic scalar plane wave by a two-dimensional wedge with perfect Dirichlet or Neumann boundaries. The first three approaches offered are those most commonly used today in diffraction theory, although not necessarily in the context of wedge diffraction. These are the Sommerfeld–Malyuzhinets method, the Wiener–Hopf technique, and the Kontorovich–Lebedev transform approach. Then follows three less well-known and somewhat novel methods, which would be of interest even to specialists in the field, namely the embedding method, a random walk approach, and the technique of functionally-invariant solutions.Having offered the exact solution of this problem in a variety of forms, a numerical comparison between the exact solution and several powerful approximations such as GTD is performed and critically assessed.

Hybrid Computational Techniques: Electromagnetic Propagation Analysis in Complex Indoor Environments

In this article, we compare deterministic methodologies for characterizing channel behavior in heterogeneous and composite scenarios. These techniques include one that combines a 3D ray launching (RL) approach based on geometrical optics (GO), a second based on GO and the uniform theory of diffraction (UTD), and another that includes a diffusion equation (DE) method based on the equation of transfer. A new methodology based on the GO and DE is presented and shown to achieve accurate results when compared with real measurements. The proposed technique provides a computational time reduction of up to 90% compared to the conventional approach using GO with the UTD and DE.

Radiation and Scattering of an Arbitrarily Flanged Dielectric-Loaded Waveguide

In this article, we present a new methodology in spectral domain to study a novel, complex canonical electromagnetic problem constituted of perfectly electrically conducting (PEC) wedges immersed in complex environments. In particular, we present an arbitrarily flanged dielectric-loaded waveguide that resembles practical structures in scattering analysis, radar applications, antenna’s design, and electromagnetic compatibility. The proposed method is based on the recently developed semianalytical method known as the generalized Wiener–Hopf technique that extends the applicability of classical Wiener–Hopf method to a new variety of problems constituted of different geometries and materials. In this article, the method is further extended and it is now capable of handling piecewise constant inhomogeneous dielectric layers by resorting to the application of characteristic Green’s function procedure starting from the wave equation. The method has the benefit to be a comprehensive mathematical model and to be quasi-analytical, thus allowing us to investigate the true physics of the problem in terms of field’s components. The proposed solution is also of interest in computational electromagnetics to benchmark numerical codes. Validation through numerical results is reported in terms of engineering quantities such as geometrical theory of diffraction (GTD)/uniform theory of diffraction (UTD) coefficients, total far fields, and modal fields.

A Channel Model for Polarized Off-Body Communications With Dynamic Users

This paper presents an off-body channel model for polarized communications with dynamic users. The model is based on Geometrical Optics and Uniform Theory of Diffraction and accounts for free space propagation, reflections, and diffractions. It allows for arbitrary antennas’ polarizations and gain patterns and supports a number of on-body antenna placements. In order to take the influence of users’ motion into account, a mobility model for wearable antennas on dynamic users is used. Signal depolarization mechanisms are identified, and simulations are performed to analyze the influence of user dynamics on the channel. The results show that significant polarization mismatch losses occur due to wearable antenna rotations, resulting in received power variations up to 37.5 dB for the line-of-sight component and 41.4 dB for the scattered one. The importance of taking signal polarization into account is demonstrated by comparing the simulation results between polarized and nonpolarized channel models in a free space propagation scenario, where a difference up to 53 dB in between the two is observed.

Scattering by a perfectly conducting convex hyperbolic reflector

ABSTRACTThe scattered fields of a line source by a perfectly conducting convex hyperbolic reflector are obtained. The scattering integral is constructed using the method of modified theory of physical optics. The fields of reflected geometrical optics and nonuniform edge diffracted are evaluated using the asymptotic methods of stationary phase and edge point technique, respectively. The uniform diffracted field expressions are obtained by the method of uniform theory of diffraction. The field expressions are plotted and analysed numerically.

X-Ray Transmission through Infinite Dielectric Wedge-Shaped Objects

The problem of transmission and reflection of X-rays (0.1 A ≤ λ ≤ 10 A) from an infinite amorphous homogeneous wedge-shaped dielectric plate has been analytically solved. The obtained solution is a “zero” step in solving the problem of X-ray diffraction on a semi-infinite amorphous homogeneous dielectric wedge within the heuristic geometric diffraction theory.

Aerodynamic and Stealth Integrated Design of DSI Inlet by Surrogate-Based Multi-Objective Optimization with EHVI Infill Criterion

To deal with the problem of aerodynamic and stealth integrated optimization of DSI inlet, a multi-objective optimization study on aerodynamic and stealth of the DSI inlet is carry out which based on the deformation of the three-dimensional compression bump surface. The FFD parametric method is used to parameterize the bump surface; CFD calculation based on RANS equations is used to analyze the aerodynamic performance of the DSI inlet, large element physical optical method and uniform theory of diffraction are used to calculate RCS of the DSI inlet; And ASMOPSO algorithm with the Kriging surrogate model which based on the expect hyper-volume improvement infill criterion is adopted for integrated optimization design. The results of DSI inlet aerodynamic and stealth integrated optimization exhibit considerable improvement.