Circular Wave Research Articles

Imager observation of concentric mesospheric gravity waves over Srinagar, Jammu and Kashmir, India

Using the NARL airglow imager in the Indian subtropical station of Srinagar, Kashmir, India, we report the characteristics of the concentric deformed circular pattern of gravity waves detected at ∼85 km height. OH molecular emission intensities from the height of ∼85 km as observed by the imager in the wide band optical filter of centre wavelength of 840 nm show concentric deformed circular gravity waves drifting northwestward. Using Atmospheric Infrared Sounder (AIRS) image at the centre wavelength of 8.1 µm, deep convective activity around the imager site in the lower atmosphere is recognized as the source mechanism of the observed concentric gravity waves in the mesosphere. The determined phase speed, direction and period of these concentric gravity waves (CGWs) is ∼47 m/s, 150° north of east and ∼10 min (intrinsic period ∼8 min) respectively. At heights of ∼97 km [OI (1S)] and ∼250 km [OI (1D)], these concentric gravity waves are sporadically visible depending on the possible vertical movement of airglow layer height. The dissipated portions of these concentric gravity waves get converted into turbulence at these heights. Using anelastic gravity wave dispersion relation, decreasing horizontal wavelength from the centre of concentric gravity waves is explained. This result is novel in the sense that it disagrees with most of the earlier interpretations of such observations.

Dual‐band double circularly polarized wide‐angle beam‐scanning antenna using planar phase gradient transmitarrays

In this article, a dual-band double circularly polarized wide-angle beam scanning transmitarrays antenna is presented. The transmitarrays antenna can convert the spherical wave into plane circular polarized wave at 20 and 30 GHz bands at the same time and same direction with wide-angle beam scanning characteristic. The unit cell consists of two kinds of Jerusalem Cross to achieve phase shift ranging from 0° to 300° at 20 GHz, from 0° to 312° at 30 GHz with acceptable loss. The fabricated antenna has a wave beam-scanning angle from 1.5° to 52.5° at 20 GHz and from 1.3° to 51.6° at 30 GHz. The maximum gains are 22.1 dBic at 20 GHz and 25.0 dBic at 30 GHz.

Novel circularly polarized slot array antennas with a wide 3-dB axial ratio bandwidth based on a spindle-shaped radiating cavity

AbstractA novel wide 3-dB axial ratio (AR) circularly polarized 2 × 2 array antenna is proposed in this paper. The spindle-shaped coupling cavity with tilted waveguide is capable of generating circular polarization waves from incident linear waves, which improves the AR bandwidth (ARBW) of the antenna. With this structure, a similar amplitude of the two orthogonal transmitted wave components and a stable phase difference of nearly 90° can be generated. The circularly polarized antenna proposed herein has been fabricated. According to the measurement results, the operating bandwidth from 5.32 to 6.13 GHz is <−10 dB. In addition, the measured ARBW, which is below 3 dB, can cover the range of 5.41–6.02 GHz. The maximum gain of the antenna can attain 15.65 dBi, and the efficiency is better than 80%.

Lightweight switchable bifunctional metasurface based on VO2: High-efficiency absorption and ultra-wideband circular polarization conversion

Switchable multi-function metasurfaces have received much attention due to their excellent performance of manipulating electromagnetic waves. In this paper, we present a novel design that can realize a high-efficiency absorber and an ultra-wideband circular polarization converter based on an anisotropic metasurface through the phase transition of vanadium dioxide (VO2). At room temperature, the metasurface acts an ultra-wideband circular polarization converter with the VO2 film maintains an insulating state, the theoretical analysis and numerical simulation show that the polarization conversion ratio reaches more than 90% in an ultra-wide band (6.3–15.0 THz) and even reaches almost 100% at two peaks, the relative bandwidth is 82%. When the temperature increases to 68 ℃, the metasurface becomes a high-efficiency absorber with VO2 film in metallic state. Numerical calculations show that the absorber has over 90% high-efficiency absorption of X-linearly polarized wave in the 10.8–13.2 THz frequency range and over 90% high-efficiency absorption of left-handed circular polarization wave in the 10.8–14.4 THz frequency range, both are angle insensitivity for small incident angles from 0° to 20°. The physical mechanisms of the two functions based on our structure are discussed. Our novel design has potential significance in future terahertz medical imaging, non-destructive detectors, polarization converters, and THz communication system.

Wave breaking and jet formation on axisymmetric surface gravity waves

Axisymmetric standing waves occur across a wide range of free surface flows. When these waves reach a critical height (steepness), wave breaking and jet formation occur. For travelling surface gravity waves, wave breaking is generally considered to limit wave height and reversible wave motion. In the ocean, the behaviour of directionally spread waves lies between the limits of purely travelling (two dimensions) and axisymmetric (three dimensions). Hence, understanding wave breaking and jet formation on axisymmetric surface gravity waves is an important step in understanding extreme and breaking waves in the ocean. We examine an example of axisymmetric wave breaking and jet formation colloquially known as the ‘spike wave’, created in the FloWave circular wave tank at the University of Edinburgh, UK. We generate this spike wave with maximum crest amplitudes of 0.15–6.0 m (0.024–0.98 when made non-dimensional by characteristic radius), with wave breaking occurring for crest amplitudes greater than 1.0 m (0.16 non-dimensionalised). Unlike two-dimensional travelling waves, wave breaking does not limit maximum crest amplitude, and our measurements approximately follow the jet height scaling proposed by Ghabache et al. (J. Fluid Mech., vol. 761, 2014, pp. 206–219) for cavity collapse. The spike wave is predominantly created by linear dispersive focusing. A trough forms, then collapses producing a jet, which is sensitive to the trough's shape. The evolution of the jets that form in our experiments is predicted well by the hyperbolic jet model proposed by Longuet–Higgins (J. Fluid Mech., vol. 127, 1983, pp. 103–121), previously applied to jets forming on bubbles.

Spectral characterisation of inertial particle clustering in turbulence

Clustering of inertial particles is important for many types of astrophysical and geophysical turbulence, but it has been studied predominately for incompressible flows. Here, we study compressible flows and compare clustering in both compressively (irrotationally) and vortically (solenoidally) forced turbulence. Vortically and compressively forced flows are driven stochastically either by solenoidal waves or by circular expansion waves, respectively. For compressively forced flows, the power spectrum of the density of inertial particles is a useful tool for displaying particle clustering relative to the fluid density enhancement. Power spectra are shown to be particularly sensitive for studying large-scale particle clustering, while conventional tools such as radial distribution functions are more suitable for studying small-scale clustering. Our primary finding is that particle clustering through shock interaction is particularly prominent in turbulence driven by spherical expansion waves. It manifests itself through a double-peaked distribution of spectral power as a function of Stokes number. The two peaks are associated with two distinct clustering mechanisms; shock interaction for smaller Stokes numbers and the centrifugal sling effect for larger values. The clustering of inertial particles is associated with the formation of caustics. Such caustics can only be captured in the Lagrangian description, which allows us to assess the relative importance of caustics in vortically and compressively forced turbulence. We show that the statistical noise resulting from the limited number of particles in the Lagrangian description can be removed from the particle power spectra, allowing us a more detailed comparison of the residual spectra. We focus on the Epstein drag law relevant for rarefied gases, but show that our findings apply also to the usual Stokes drag.

A novel device to improve robustness of end plate beam–column connections: Analytical model development

A novel device to increase rotational capacity of steel end plate beam–column connections is proposed. By inserting a steel sleeve with a designated length, thickness and wall curvature between the end plate and the washer, the load path between the end plate and the bolts can be interrupted, promoting a more ductile response. For a specific sleeve material and dimensions, the capacity of sleeve is controlled by the amplitude value. The sleeve develops severe plastic deformations and is crushed between the end plate and the washer at the plastic amplitude. This study proposes an analytical solution to predict the plastic amplitude for the sleeve with a circular wave form. The sleeve is mathematically represented using shell of revolution theories under axisymmetric load. To simplify the analysis, the sleeve material is analysed using the two-moment limited interaction yield condition and follows an idealised rigid-perfectly plastic material response. The comparison between the proposed analytical solution with validated FE models show that the plastic amplitude is accurately captured. However, the predicted rotational capacity is very conservative.

Wave energy extraction from a floating flexible circular plate

We present a theoretical model to investigate the hydrodynamics of a floating flexible circular wave energy converter (WEC). Decomposition in rigid and bending elastic modes of the plate allows us to investigate power extraction efficiency in monochromatic incident waves. We show that plate elasticity increases the number of eigenfrequencies, which has a positive beneficial effect on power output. We also show how plate radius and power take-off (PTO) distribution affect the response of the system and the consequent absorbed energy. This work highlights the need to extend theoretical studies and experimental investigations on flexible devices, currently seen as the future of WEC technology.

Design and analysis of multiband composite antenna with dual – sense circular polarization characteristics

In the current era of wireless communication, the antenna researchers are widely focused on two major areas such as bandwidth enhancement and generation circular polarization characteristics. Bandwidth enhancement provides higher data rate without any increment in power level. In the same way, orientation independency between transmitter and receiver antennas can be provided by circular polarization characteristic. In this project, a dual-sense Circularly Polarized (CP) hybrid antenna is reported. The antenna that is proposed works over the range of frequency 3.28 GHz to 5.78 GHz with a fractional bandwidth of 55.18%. Dual Circular Polarization wave with dual-sense characteristics for which Axial Ratio is less than 3 dB are created in the range of frequency 3.81 to 4.28 GHz and 4.98 to 5.28 GHz respectively. Based on the above parameters the antenna has applications like WLAN and WIMAX.

Tunable polarization encoder capable of polarization conversion and separation based on a layered photonic structure

In this paper, a polarization encoder with a polarization conversion function is investigated using a layered photonic structure comprising nonlinear materials and indium antimonide. The electric field of the incident linear polarization waves considered are parallel to the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x-y</i> plane at an angle of 45° to the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> -axis, and can be decomposed into components directed along the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> - and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</i> -axes, generating two polarized waves: transverse electric and transverse magnetic waves. Due to the nonlinear effect and the temperature and magnetic flux density tunable properties of indium antimonide, the light intensity, temperature, and magnetic flux density can be used to adjust the reflection phase difference and polarization form. Discussing the influences of these three physical quantities on the phase difference, the incident linear polarization wave and reflected circular polarization wave conversion, the circular polarization conversion of two different rotations and the circular polarization separation of a certain bandwidth can be simultaneously realized with the specific modification of the light intensity or temperature. Moreover, four disparate input logic levels composed of temperature and magnetic flux density achieve specific encoding outputs for all polarization forms. This signifies that polarization selection through control of the logic codes can be easily attained. The consequences show that this form of polarization conversion and encoding is tailored and can be accurately manipulated. Hence, this provides a novel foundation for tunable and diverse polarization splitters and selectors.

A Cross-Scanning Crack Damage Quantitative Monitoring and Imaging Method

Quantitative crack monitoring and detection is one of the focuses of attention to structural health monitoring due to its concealment and harmfulness. However, in the existing guided wave- and PZT-sensor-array-based structural health monitoring approaches, the crack orientation information is often ignored, which is a key issue in crack monitoring, making it hard to effectively evaluate the crack damage. In this study, the circular piezoelectric array and active Lamb wave RAPID imaging technology are introduced and improved to study the quantitative monitoring of crack damage. A cross scanning method is proposed to determine the crack orientation based on response signal variations of different degrees on monitoring paths with different angles. Accordingly, the signal difference coefficient (SDC) value of the detected monitoring paths of parallel or approximately parallel crack damage are corrected to strengthen the reconstruction information of crack orientation. After that, image reconstruction and quantitative evaluation of crack damage can be realized based on the enhanced SDC information. Several experiments are carried out on the aluminum plate to validate the proposed method. The cracks at different positions and orientations are monitored and evaluated through the proposed method. The experimental results show that crack orientation can be well identified through the proposed cross scanning crack monitoring and imaging method.

Performance Assessment of a Combined Circular Aquaculture Cage Floater and Point Absorber Wave Energy Converters

Performance Assessment of a Combined Circular Aquaculture Cage Floater and Point Absorber Wave Energy Converters

The Normal Velocity of the Population Front in the “Predator–Prey” Model

The propagation of one and two-dimensional waves of populations are numerically investigated in the framework of the “predator-prey” model with the Arditi - Ginzburg trophic function. The propagation of prey and predator population waves and the propagation of co-existing populations’ waves are considered. The simulations demonstrate that even in the case of an unstable quasi-equilibrium state of the system, which is established behind the front of a traveling wave, the propagation velocity of the joint population wave is a well-defined function. The calculated average propagation velocity of a cellular non-stationary wave front is determined uniquely for a given set of problem parameters. The estimations of the wave propagation velocity are obtained for both the case of a plane and cellular wave fronts of populations. The structure and velocity of outward propagating circular cellular wave are investigated to clarify the local curvature and scaling effects on the wave dynamics.

새로운 6-단자 급전 회로망을 이용한 이중 직교 원형 편파를 갖는 소형 고이득 GPS 안테나

In this study, the design, fabrication, and performance of a small high-gain global positioning system (GPS) antenna that generates dual orthogonal circular polarization using a novel six-terminal feeding network is presented. The proposed six-terminal feeding network has two input quadrature terminals and four output terminals having the same amplitude and 0° or 180° phase. Therefore, it is suitable for feeding the four radiating elements with linear polarization characteristics. To realize a dual orthogonal circular polarization antenna with miniaturization and high gain characteristics, the six-terminal power supply network is positioned in the center of the antenna, and the four radiating elements are arranged on the outside. The radiating element selected after considering the limited space is a three-dimensional printed dipole element with linear polarization characteristics which is rotated by 90°. The antenna size designed optimally in the GPS L1 band (1,575.42±12 MHz) is less than 130 mm (0.68 λ0) in diameter (D) and 40 mm (0.21 λ0) in height. The antenna’s input return loss and port isolation characteristics measured with a vector network analyzer are excellent within the band at 17.1 dB and 18.0 dB or more, respectively. In addition, the antenna gain, and efficiency measured in the anechoic chamber are 8.69 dBi (@bore-sight) or more and 83 % or more, respectively. Furthermore, it can be seen that the axial ratio characteristics of the dual orthogonal circular polarization wave are very good, with less than 0.2 dB (@bore-sight) and less than 1.8 dB (@ 3 dB beam width).

Ultra Low Frequency Wave Produced by Underwater Oscillating Sphere and Its Measurement

When an underwater target moves in viscous fluid, it may cause the periodic movement of the surrounding fluid and generate ultra-low-frequency (ULF) gravity waves. The initial domain of the gravitational surface wave propagating above the moving target is named circular wave. This article studies the ULF circular wave generated by underwater oscillating sphere, which will provide basis for underwater long-range target detection. Firstly, the circular wave caused by the sphere oscillation in a finite deep fluid is studied based on the theory of linear potential flow. Meanwhile, the multipole expansion theory is established to solve the circular wave field. Secondly, the interface wave generated by the target oscillation in a two-layer fluid are numerically analyzed by comparison with the free surface fluctuation of a single-layer fluid. The results show that the amplitude of the internal interface displacement (AIID) is smaller than that of the free surface (AFSD). When the sphere is in the lower layer, the layering effect of the fluid has significant influences on the AFSD. Finally, the results of the pool experiment verified that the wave generated by the oscillating sphere is the surface gravity wave. Furthermore, the change trend of the test result is consistent with the simulation result.

Analysis of new wave-curved tensegrity dome

The cable dome is one of the most widely used structures in cable-strut tensile structures. A new topology of the tensegrity system is firstly proposed, and combined with the circular wave surface, a new wave-curved tensegrity dome is then obtained, which has less members than traditional cable domes. Concurrently, based on a combination of Levenberg-Marquardt and quasi-Newton (LM-QN) methods, a form-finding algorithm for cable-strut tensile structures is proposed to solve nonlinear equilibrium equations by addressing the least-squares problem. Subsequently, the configuration of the wave-curved tensegrity dome is analyzed using the proposed form-finding method, which demonstrates that an integral feasible prestress exists in the new structural form. Finally, the static and dynamic performance of the new structure are analyzed, and its possible defects and weaknesses are subsequently examined based on the analysis results. The numerical results show that the proposed wave-curved tensegrity dome satisfies the necessary requirements and has high research relevance as a type of light roof. Moreover, the LM-QN method can overcome the singular problem of the tangent stiffness matrix and be used as one of the form-finding methods for the verification of new structures.

Interior Elastic Scattering by a Non-Penetrable Partially Coated Obstacle and Its Shape Recovering

In this paper, the interior elastic direct and inverse scattering problem of time-harmonic waves for a non-penetrable partially coated obstacle placed in a homogeneous and isotropic medium is studied. The scattering problem is formulated via the Navier equation, considering incident circular waves due to point-source fields, where the corresponding scattered data are measured on a closed curve inside the obstacle. Our model, from the mathematical point of view, is described by a mixed boundary value problem in which the scattered field satisfies mixed Dirichlet-Robin boundary conditions on the Lipschitz boundary of the obstacle. Using a variational equation method in an appropriate Sobolev space setting, uniqueness and existence results as well as stability ones are established. The corresponding inverse problem is also studied, and using some specific auxiliary integral operators an appropriate modified factorisation method is given. In addition, an inversion algorithm for shape recovering of the partially coated obstacle is presented and proved. Last but not least, useful remarks and conclusions concerning the direct scattering problem and its linchpin with the corresponding inverse one are given.

Analysis of angular dependent spatial frequency response of Digital Coplanar Translational Laminography

Digital Coplanar Translational Laminography (CTL) is a radiographic imaging technique that permits the visualization of structural details and discontinuities in the object of interest after 3D reconstruction from multiple projections. This technique becomes quite competitive regarding to computed tomography in situations of arrangements in which no complete rotation of the X-ray source - detector system around the object of interest is accessible or in cases where the part to be inspected has a high geometric asymmetry. The literature does not show many studies about image quality for different laminographic reconstructions and scan geometries. Different methodologies for measuring the Modulation Transfer Function (MTF) as a key image quality parameter for the aforementioned technique are applied. The MTF was measured using different approaches, pre-sampled MTF or sampled MTF, both based on the Fourier Transform-Magnitude Spectrum or the square wave MTF using line pattern gauges. A sample known as Siemens Star with a converging line pattern was used to measure the circular square wave MTF. This circular MTF provides information on the direction dependent contrast transfer related to the measurement conditions, the reconstruction algorithms and the translational scan direction. For this purpose, several MTF measurements were performed in multiple angular directions. Beyond it, reconstructions were performed using different algorithms (Filtered Shifted Average) and an iterative one (MART – Multiplicative Algebraic Reconstruction Technique). Image quality analysis were done for both reconstruction methods. MART showed an image quality improvement for challenging conditions for laminographic arrangements, such as for a low number of projections (down to 25 for this study, considered as limited view, limited angle reconstruction). Additionally, a Gadolinium gauge with parallel line patterns within a range from 10 μm to 1000 μm was used to compare the square wave MTF with the sine MTFs, measured by the different techniques mentioned above. The results show that the MTF techniques presented are equivalent in this study. The results obtained for this Gadolinium plate showed a good agreement between MTF10% and SRbimage measurements. The methodology presented here is used for the characterization of the frequency dependent spatial resolution of measurements obtained from different laminographic reconstruction techniques and scan geometries. Finally, from the acquisitions and reconstructions obtained with the Siemens Star, cross laminographic images were calculated in order to improve the direction dependent sensitivity. The results showed a better detectability for cross laminographic images in comparison to unidirectional coplanar translational laminographic images covering dead zones.

Broadband Circularly Polarized Microstrip Array Antenna With Curved-Truncation and Circle-Slotted Parasitic

This article presents a novel broadband circularly polarized microstrip array antenna for synthetic aperture radar system onboard Cessna 172 Aircraft. The antenna employs an 8×8 matrix formation with uniform element separation of 0.5λ and single-proximity-coupled strip-line feeding. The circular polarization wave generation uses a curved-truncation patch and a circle-slotted parasitic patch above it to realize the broadband circularly polarized antenna. The broadening of the axial ratio bandwidth (ARBW) applies a fully serial sequential rotation on the array feeding network. Meanwhile, the gain is improved using a circle-slotted parasitic patch with a floating cover on the top layer. The prototype antenna has a physical aperture of 270 mm × 300 mm with 1.1-kg weight. The prototype produces an impedance bandwidth of 1.5 GHz (29.3%), ARBW of 1.62 GHz (30.6%), a gain of 18.17 dBic, and aperture efficiency of 26%. The azimuth and elevation beamwidth achieved 12.7° and 13.0° at the center frequency, respectively.

Quad‐Band Transmissive Metasurface: Quad‐Band Transmissive Metasurface with Linear to Dual‐Circular Polarization Conversion Simultaneously (Adv. Theory Simul. 8/2021)

In article number 2100117, Si Jia Li and co-workers propose a quad-band dual-circular transmissive metasurface to manipulate polarization of transmissive wave. The metasurface manipulates linear incident polarization waves into left-hand and right-hand circular polarization waves in different frequency bands simultaneously. The proposed transmissive metasurface can be utilized in multiband wireless communications and multifunctional dual-circularly polarized antenna systems.