Modal Reflection Coefficients Research Articles

Feature detection in guided wave ultrasound measurements using simulated spectrograms and generative machine learning

Guided wave ultrasound field measurements capture reflections from aluminothermic welds in the rail track at unknown distances from a transducer. The measured guided wave signals are complex and challenging to interpret due to multiple dispersive modes propagating in the rail, changing environmental conditions, and noise. Data-driven machine learning techniques have been applied to complex signal-processing problems and have shown significant potential in learning and resolving complex problems. This study aims to understand the implications of using various simulated data sets for application within an appropriate learning framework to capture the underlying features of the field measurements and maximise the performance of these techniques. The use of simulated spectrograms, including variations of signal attributes (attenuation, positions of welds, noise, and mode reflection coefficients) generally observed in experimental measurements, allows the reflections of individual modes to be isolated or combined for training. This allows us to present the training data in three distinct forms. The first dataset has the highest mode reflection information density per sample and consists of simulated spectrogram data with multiple reflections of modes from multiple welds, like experimentally obtained spectrograms. The second training dataset consists of spectrograms with multiple mode reflections; however, only for a single weld reflection per spectrogram. The third training set contains a reflection of a single mode from a single weld in each spectrogram. The data-driven models applied are principal component autoregression and variational auto-encoders. The reconstruction error and latent space interpretability were considered as metrics for the algorithms’ ability to learn using test sets, i.e., unseen data. The results show that datasets with sufficient feature variation and higher mode reflection information density better construct the test set of simulated and experimental spectrograms. However, training using the third dataset shows more interpretable latent variables for an artificial growing defect attached to the rail. Furthermore, data-driven machine learning methods trained using simulated spectrogram data are useful for reconstructing and learning features from experimental measurements, provided that the training data have representative mode feature variation and noise.

Theoretical model of azimuthal combustion instability subject to non-trivial boundary conditions

The problem of evaluating the sensitivity of non-trivial boundary conditions to the onset of azimuthal combustion instability is a longstanding challenge in the development process of modern gas turbines. The difficulty lies in how to describe three-dimensional in- and outlet boundary conditions in an artificial computational domain. To date, the existing analytical models have still failed to quantitatively explain why the features of the azimuthal combustion instability of a combustor in laboratory environment are quite different from that in a real gas turbine, making the stability control devices developed in laboratory generally lose the effectiveness in practical applications. To overcome this limitation, we provide a novel theoretical framework to directly include the effect of non-trivial boundary conditions on the azimuthal combustion instability. A key step is to take the non-trivial boundary conditions as equivalent distributed sources so as to uniformly describe the physical characteristics of the inner surface in an annular enclosure along with different in- and outlet configurations. Meanwhile, a dispersion relation equation is established by the application of three-dimensional Green’s function approach and generalized impedance concept. Results show that the effects of the generalized modal reflection coefficients on azimuthal unstable modes are extremely prominent, and even prompt the transition from stable to unstable mode, thus reasonably explaining why the thermoacoustic instability phenomena in a real gas turbine are difficult to observe in an isolated combustion chamber. Overall, this work provides an effective tool for analysis of the azimuthal combustion instability including various complicated boundary conditions.

유도파 모드분리 및 반사계수에 의한 배관 결함검출

유도파 모드분리 및 반사계수에 의한 배관 결함검출

Broad-Angle Multichannel Metagrating Diffusers

We present a semianalytical scheme for the design of broad-angle multichannel metagratings (MGs), sparse periodic arrangements of loaded conducting strips (meta-atoms), embedded in a multilayer printed circuit board (PCB) configuration. By judicious choice of periodicity and angles of incidence, scattering off such an MG can be described via a multiport network, where the input and output ports correspond to different illumination and reflection directions associated with the same set of propagating Floquet–Bloch (FB) modes. Since each of these possible scattering scenarios can be modeled analytically, constraints can be conveniently applied on the modal reflection coefficients (scattering matrix entries) to yield a diffusive response, which, when resolved, produce the required MG geometry. We show that by demanding a symmetric MG configuration, the number of independent S parameters can be dramatically reduced, enabling satisfaction of multiple such constraints using a single sparse MG. Without any full-wave optimization, this procedure results in a fabrication-ready layout of a multichannel MG, enabling retroreflection suppression and diffusive scattering from numerous angles of incidence simultaneously. This concept, verified experimentally via a five-channel MG prototype, offers an innovative solution to both monostatic and bistatic radar cross Section (RCS) reduction, avoiding design and implementation challenges associated with dense metasurfaces used for this purpose.

Approximate reflection and transmission coefficients at an interface between two contrasting viscoelastic anisotropic half-spaces

SUMMARY We present and compare two new methods to calculate the reflection and transmission coefficients of qP, qSV and qSH waves at a flat interface between two contrasting viscoelastic anisotropic half-spaces. The first method is a new version of the real ray tracing method that extends the traditional real ray tracing method to tackle the triplications of the qSV wave. This new version is valid for computing the reflection and transmission coefficients of the qP and qSV waves for all pre- and post-critical angles of incidence, which cannot be handled by the previous approach. The second method employs the real slowness direction and Snell's law to provide approximate reflection and transmission coefficients for such wave modes. This approximate method has attractive advantages which are not only the avoidance of the searching process inherent in the real ray tracing method but also offers competitive economical solutions to the first method. Many viscoelastic VTI models are utilized to validate the two methods. The numerical results show that (1) the ray angles, ray velocities, ray Q-factors, and the magnitudes of the reflection and transmission coefficients obtained by the two methods closely agree with each other, except in the vicinity of the critical angles and (2) the phase angles of the reflected and transmitted waves exhibit complexities due to the existence of some reverse phenomena between the approximate and the real ray tracing solutions. However, such phase discrepancies do not affect the magnitudes of the reflection and transmission coefficients. Moreover, the energy ratios of the different wave modes satisfy the energy conservation criterion at the interface.

Interferometry of black holes with Hawking radiation

We investigate the wave optical imaging of black holes with Hawking radiation. The spatial correlation function of Hawking radiation is expressed in terms of transmission and reflection coefficients for scalar wave modes and evaluated by numerically summing over angular quantum numbers for the Unruh-Hawking state of the Kerr-de Sitter black hole. Then, wave optical images of an evaporating black hole are obtained by the Fourier transformation of the spatial correlation function. For short wavelength, the image of the black hole with the outgoing mode of the Unruh-Hawking state has the appearance of a star with its surface given by the photon sphere. It is found that interference between incoming modes from the cosmological horizon and reflected modes due to the scattering of the black hole can enhance brightness of images in the vicinity of the photon sphere. For a long wavelength, the entire field of view is bright, and the emission region of Hawking radiation cannot be identified.

Novel Double-Ridged Waveguide Orthomode Transducer for mm-Wave Application

This letter presents our study on a millimeter wave (mm-wave) orthomode transducer (OMT) based on novel double-ridged waveguide junction. By reducing the length of ridges in the square waveguide, the ridge width of proposed junction is about three times wider than that of conventional junction. This benefits the fabrication and mechanical performance of double-ridged waveguide OMT in the higher mm-waveband or terahertz band. The simulated reflection coefficients for both orthogonal modes are less than −20 dB over 37% bandwidth. As practical application, an E-band OMT based on the novel junction is fabricated. A smooth-horn is integrated with the OMT as a dual-polarized antenna. The measured results show that reflection coefficients less than −15 dB, isolation better than 39 dB, and cross-polarization discrimination (XPD) higher than 23 dB are achieved from 71 to 86 GHz.

Characteristic mode analysis of resistor-loaded frequency selective surfaces: Theoretical research and experimental verification

In this paper, the modal behaviors and the frequency properties of resistor-loaded frequency selective surfaces have been systematically studied based on the periodic moment method and characteristic mode analysis. Through the observation of the phase curves of the reflection coefficients, we found that as the resistance value increases, the resonant frequency gradually moves toward higher frequencies. By calculating and analyzing the modal currents and the modal reflection coefficients of each mode, we know that the phenomenon is caused by higher-order modes. In order to reduce the influence of higher-order modes on the resonant frequency, a method of using more load resistors is proposed. To verify the proposed method, an absorber with high absorptivity is designed and measured. The measurement shows that the absorptivity of the designed absorber is more than −18dB in the range of 1.92–3.99GHz. The experiment proves that the proposed loading method can be used to flexibly regularize the equivalent impedance of the lossy frequency selective surface without worrying about the negative effects of load resistors in the design of a circuit analog absorber.

Metagratings for Perfect Mode Conversion in Rectangular Waveguides: Theory and Experiment

We present a complete design scheme, from theoretical formulation to experimental validation, exploiting the versatility of metagratings (MGs) for designing a rectangular waveguide (RWG) $\mbox{TE}_{10}$ - $\mbox{TE}_{20}$ mode converter (MC). MG devices, formed by sparse periodically positioned polarizable particles (meta-atoms), were mostly used to date for beam manipulation applications. In this paper, we show that the appealing diffraction engineering features of the MGs in such typical free-space periodic scenarios can be utilized to efficiently mould fields inside waveguides (WGs). In particular, we derive an analytical model allowing harnessing of the MG concept for realization of perfect mode conversion in RWGs. Conveniently, the formalism considers a printed-circuit-board (PCB) MG terminating the RWG, operating as a reflect-mode MC. Following the typical MG synthesis approach, the model directly ties the meta-atom position and geometry with the modal reflection coefficients, enabling resolution of the detailed fabrication-ready design by enforcement of the functionality constraints: elimination of the fundamental $\mbox{TE}_{10}$ reflection and power conservation (passive lossless MG). This reliable semianaltyical scheme, verified via full-wave simulations and laboratory measurements, establishes a simple and efficient alternative to common RWG MCs, typically requiring challenging deformation of the WG designed through time-consuming full-wave optimization. In addition, it highlights the immense potential MGs encompass for a wide variety of applications beyond beam manipulation.

Open-geometry modal method based on transverse electric and transverse magnetic mode expansion for orthogonal curvilinear coordinates.

A vectorial modal method is presented based on transverse magnetic and transverse electric mode expansion, which significantly simplifies the evaluation of the operator matrix. The method, which features a true open boundary condition, is introduced for an orthogonal curvilinear coordinate system with the specific examples of circular and elliptical geometries presented. We validate the method by considering challenging problems, such as the calculation of spontaneous emission rates, of modal reflection coefficients and of the effect of the emitter spatial misalignment on the spontaneous emission β factor. Results are compared with literature.

Giant Reflection Coefficient on Sc0.26Al0.74N Polycrystalline Diamond Surface Acoustic Wave Resonators

Since the commercialization of surface acoustic wave (SAW) devices, the technology is steadily increasing the device performances without compromising their power handling, size and price. Herein, one‐port SAW resonators are fabricated on scandium aluminum nitride (Sc0.26Al0.74N)/polycrystalline diamond heterostructures. SAW propagation properties are studied using three different piezoelectric thin‐film thicknesses within the heterostructure. The Rayleigh and Sezawa resonance frequencies are above 1.5 and 2.5 GHz, respectively, achieving Sezawa mode reflection coefficients below −50 dB. The polycrystalline diamond substrate is synthesized by microwave plasma chemical vapor deposition (MPCVD) on top of a 500 μm‐thick Si (001) substrate. The Sc0.26Al0.74N thin films are synthesized by reactive sputtering at nominally room temperature. The thin film's composition is analyzed by Rutherford backscattering spectrometry (RBS). The full width at half maximum (FWHM) of the X‐ray diffraction (XRD) ω scans below 3° indicates that the synthesized Sc0.26Al0.74N thin films are highly c‐axis oriented. The electromechanical coupling coefficient, quality factor, and dielectric loss parameters are computed by curve fitting the device electrical measurements to the simulation results of a modified Butterworth Van Dyke (mBVD) model implemented in the advance design system (ADS) tool.

Fiber bragg grating based detection of part-thickness cracks in bent composite laminates using feature-guided waves

Composite structures with bends are widely used in aerospace and industrial sectors. However health monitoring of such structures is challenging due to their complex topographical features. Recent literature shows that bends in composite laminates can confine and guide ultrasonic energy along their length, known as feature-guided waves (FGW). This article demonstrates a fiber Bragg grating based technique using FGW modes for defect detection and identification in bent composite laminates. In addition, the effects of defect depth and excitation frequency on the FGW mode reflection coefficient are reported using 3D finite element simulations. Physical insight into the reflection behavior is discussed based on an analysis of mode interaction with part-thickness cracks.

Scattering of surface modes propagating along metamaterial interface

The problem of surface mode scattering from an iris diaphragm is studied by the integral equation technique. The theory is illustrated by the example of scattering of mode, propagating along metamaterial interface. The characteristics of radiation modes excited by the diaphragm are considered. The surface mode reflection and transmission coefficients, the near-field distribution and far-field radiation pattern are calculated.

Hydro-elastic wave proliferation over an impermeable seabed with bottom deformation

ABSTRACTA hydro-elastic frame has been considered to investigate the proliferation of waves over small base deformation on an infinitely extended flexible seabed. The flexible base surface is assumed as a thin elastic plate of very small thickness and it depends on the Euler–Bernoulli beam equation. For any particular frequency, there are two different modes of time-harmonic propagating wave exists rather than one mode of propagating wave along the positive horizontal direction. The waves with smaller wavenumber spread along the free-surface of the sea (say, free-surface mode) and the waves with higher wavenumber spread along the flexible base surface (say, flexural mode). A simplified perturbation approach is utilised to bring down the entire equations which govern the original boundary value problem (bvp) to a less complex bvp for the first-order velocity potential function. The first-order potential function along with the first-order reflection and transmission coefficients for both modes are calculated by a procedure based upon Fourier transform approach. A shape of sinusoidal swells flexible base surface is taken as an example to approve the scientific results. It is observed that when the train of normal incident propagating wave spreads over base distortion because of either the free-surface unsettling influence or the flexural wave movement in the sea, the reflected and transmitted energy are always feasible to be exchanged from one particular wave mode to another wave mode. Furthermore, we notice that the realistic changes in the flexural rigidity behaviour on the flexible base surface of the sea have a significant effect on the problem of water wave proliferation over small base deformation. Moreover, the energy conservation equation is derived with the help of the Green's integral theorem. The results for the values of reflection and transmission coefficients obtained for both the free-surface unsettling influence as well as flexural wave movement in the fluid are found to satisfy the energy conservation equation almost accurately.

A Wideband Planar Differential Antenna Loaded with Metasurface

This paper reports a novel wideband differential antenna based on metasurface. It consists of a differential slot coupled antenna and a planar metasurface. Planar metasurface is composed of 4 × 4 elements, and a rectangular slot is embedded on the ground plane to couple energy between the differential feeding lines and metasurface. The feeding line employs the three-line coupled lines which could provide wideband filter-integrated feature. The lower and upper resonant frequencies of the proposed antenna can be adjusted independently by tuning the distance of the metasurface units and the length of the rectangular slot in metallic ground, respectively. Moreover, a differential dielectric resonator antenna is simulated for performance comparison. It is concluded that the proposed antenna has many advantages, such as low profile (0.08λ0 at 5 GHz), frequency independent, high-gain, and symmetrical radiation pattern, comparing with the differential dielectric resonator antenna. Furthermore, a differential metasurface-based antenna prototype has been fabricated to experimentally validate the principle and design approach. The measured impedance bandwidth with odd mode reflection coefficient below − 10 dB is 19.35% from 4.76 to 5.78 GHz. Meanwhile, the measured boresight gain at 5 GHz is 6.4 dB, and low cross-polarization level could be achieved.

Effects of vertical porous barrier on progressive waves in a two layered fluid

Two dimensional scattering of linear water wave by thin vertical permeable plate in a two-layer fluid with free surface is considered. The permeable barrier is completely submerged in the upper layer of finite depth over a layer whose depth is either infinite or finite. For discontinuity in the potential across the plate, we use Green's integral theorem to formulate the problem in terms of a hypersingular integral equation. A collocation method using a finite series of Chebyshev polynomials of second kind have been introduced to get the unknown difference potential numerically. The reflection and transmission coefficients for surface and internal modes are computed as an integral involving difference potentials. The proportion of reflected and transmitted energies of both the wavenumbers are calculated. Moreover, amount of energy dissipated due to the presence of permeable plate in the upper layer are obtained. Numerical results are calculated and depicted graphically against the wavenumber for various non-dimensionalized parameters.

A modal collocation algorithm for high-frequency propagation in discontinuous waveguides

A procedure for analyzing high-frequency propagation in adjoining constant width waveguides was presented previously [Ginsberg, JASA 134, 4217 (2013)]. The parallel walls were taken to be rigid, and no results were provided. This presentation uses that development as the foundation for the analysis of waveguides having an arbitrary number of sections, each of whose walls have arbitrary local impedance. The analytical basis is description of the pressure field in terms of transverse pressure modes, each of which is associated with upstream and downstream propagation at wavenumbers that may be complex, depending on the wall properties. Pressure and particle velocity conditions at junctions of two segments are enforced exactly by collocation along the common cross section. Initiating the procedure at a passive termination leads to recursion relations for modal reflection and transmission coefficients at each junction, which are independent of the nature of the excitation. Satisfaction of the conditions at th...

Arbitrary Antenna Arrays Without Feed Networks Based on Cavity-Excited Omega-Bianisotropic Metasurfaces

We present a design procedure for low-profile single-feed cavity-excited omega-bianisotropic metasurface antennas (CX-OBMAs), capable of generating an arbitrary desirable field distribution on their aperture. The procedure relies on a greedy algorithm that optimizes the source position inside the cavity and the magnitude and phase of the metasurface modal reflection coefficients, such that the power profile generated by the cavity fields matches the one corresponding to the specified aperture distribution. Once this local power conservation is established, we invoke previously derived analytical formulas to evaluate the (passive lossless) metasurface constituents that facilitate the required field transformation. We demonstrate this scheme by designing CX-OBMAs that generate radiation patterns with prescribed directivity and side-lobe level, for which the aperture fields can be readily stipulated using established antenna array theory. The good agreement between full-wave simulations and semianalytical predictions points out the immense potential of these novel radiators, capable of controlling near- and far-field distributions almost at will. In particular, they can emulate antenna array performance, without requiring design and implementation of complicated, expensive, and lossy feed networks.

Scattering of torsional flexural guided waves from circular holes and crack-like defects in hollow cylinders

A study on the scattering of torsional flexural guided waves from circular holes and crack-like defects in hollow cylinders is carried out through finite element simulation and laboratory experiment. Two types of defects: circular holes and crack-like defects with varying orientation angles are studied. Results show that the reflection coefficients for flexural modes with different helix angles of propagation detecting a crack-like defect vary greatly, while for a circular hole the reflection coefficients for different flexural modes are similar. It is observed that there exists a specific flexural mode that shows the maximum reflection coefficient for a crack-like defect. The scattering properties can be utilized to classify defects as circular hole and crack-like, the range of the orientation angle of a crack-like defect can also be predicted. The detection sensitivity for oblique crack-like defects is significantly improved by using flexural modes that propagate at a helix angle with respect to the pipe axis. A flexural mode sweeping technique is proposed for classifying and characterizing defects in hollow cylinders. The proposed classification and characterization method is validated by laboratory experiments. Pipeline inspection using flexural guided waves can be supplemental to current inspection using only axisymmetric waves in order to improve the probability of an accurate and reliable inspection process.

Attenuation of Electromagnetic Wave Propagating Through Roofing Sheet: Aluminum, Zinc and Asbestos

In this work, we study the attenuation characteristics of electromagnetic waves propagating through selected roofing sheets with lossy dielectric constant property. Wave equation relating the electromagnetic wave propagating through the materials was derived from Maxwell’s equations considering all the parameters enshrined in the propagation constant such as the permeability, permittivity and dielectric constant of the Material. The wave equation was solved using method of separation of variable in 1-D and 2-D. An expression for Fresnel formula that was used in analysis of the relative amplitude for both reflection and transmission coefficients for parallel and perpendicular modes of polarization behaviour of the propagated waves was derive by considering small change in the refractive index of the materials. From the results of analysis, it was observed that the relative amplitudes which represent attenuation characteristics of the propagated wave in the materials for different incident and transmitted angles varied according to polarization modes and materials.