Near-field Area Research Articles

Application of Radar Image Fusion Method to Near-Field Sea Ice Warning for Autonomous Ships in the Polar Region

Mastering the real-time dynamics of near-field sea ice is the primary condition to guaranteeing the navigation safety of autonomous ships in the polar region. In this study, a radar image fusion process combining marine radar and ice radar is proposed, which can effectively solve the problems of redundant information and spatial registration during image fusion. Then, using the fused radar images, this study proposes a set of near-field sea ice risk assessment and warning processes applicable to both low- and high-sea-ice-concentration situations. The sea ice risk indexes in these two situations are constructed by using four variables: sea ice area, sea ice grayscale, distance between sea ice and the own-ship, and relative bearing of sea ice and the own-ship. Finally, visualization processing is carried out according to the size of the risk index values of each piece of sea ice to achieve a better near-field sea ice risk assessment and warning effect. According to the example demonstration results, through the radar image fusion process and the set of near-field sea ice risk assessment and warning processes proposed in this study, the sea ice risk distribution in the near-field area of the ship can be well obtained, which provides effective support for the assisted decision-making of autonomous navigation in the polar region.

All‐metallic near‐field convergent lens design using cross‐ Jerusalem ‐slot elements

The near-field convergent lens has aroused wide attention in the field of microwave passive imaging, medicine, and beam shaping in recent years as it can converge electromagnetic energy with high efficiency. The main objective of the paper is to exhibit the design process of a multiple-conductor-layers convergent lens using all-metallic elements. A quad-layer metal cross-Jerusalem-slot structure is proposed to obtain compact unit cells without dielectric substrate, and then an all-metallic near-field convergent lens based on the cells is constructed. To validate the proposed method, an L-band lens sample with 20 × 20 cells is designed and numerically simulated. Results show that the waves radiated from a horn antenna to the lens are focused in the near-field area, 500 mm away from the lens, with the full width at half maximum of the focal spot of less than 200 mm. In addition, it is observed that the overall size of the lens has a significant effect on the near-field convergent performance. Therefore, when the array is small, how to adjust the type of array elements to achieve a better convergence effect is analyzed in detail.

Textile-integrated metamaterials for near-field multibody area networks

Textile-integrated metamaterials for near-field multibody area networks

Analysis on space transmission model of the Microwave Wireless Power Transfer system

Abstract This paper systematically analyzes the main link that imposes restrictions on the overall efficiency of the Microwave Wireless Power Transfer (MWPT) system–space transmission. Three typical transmission models of aperture transmission in the near-field area, radiation-focused transmission in the middle-field area, and Friis transmission in the far-field area of radiation were analyzed and calculated. Then, the applicable conditions and efficiency of three-models were compared. And the influence of antenna electrical parameters on the efficiency of the MWPT system was discussed. In the end, three typical space transmission models of aerial vehicles, satellites, and islands were studied. The influence of the medium on the transmission efficiency was emphatically analyzed. This paper aims to summarize the general laws of space transmission of microwave energy. Provide a theoretical reference for related scientific research and engineering practice.

Alteration in Structure of Underexpanded Freejet Through Gas-Dynamic Perspective

Studying the underexpanded jet has been essential for many fields of research because it might occur intentionally or spontaneously wherever a high-speed gas flow is possible. Therefore, an in-house compressible flow solver is employed to simulate an underexpanded sonic freejet. The behavior of the jet is examined in the near-field area, immediately downstream of the nozzle exit plane, to determine the parameters influencing the characteristics of the jet. The Mach reflection phenomenon is thoroughly studied, and a criterion is defined for the transformation from regular reflection to Mach reflection. Furthermore, the location and height of the Mach disk are found to be governed by the exit pressure ratio, the stagnation temperature, and the type of gas used. Although the temperature does not have a significant impact, the pressure ratio and specific heat ratio are found to affect the jet structure. The results obtained from the solver are compared with existing experimental results, highlighting the utility of the solver in probing the compressible flowfields.

Numerical simulation of seismic wave triggered by low-frequency sound source with 3D staggered-grid difference method in shallow water

Ship seismic wave is often used for targeting ship location. This elastic wave that traveled in the seabed layer is triggered by low-frequency ship noise. At present, most of the theoretical studies of ship seismic wave were conducted in the frequency domain and the wave components such as P-wave, S-wave, and normal mode wave cannot be distinguished. In this study, the propagation characteristics were studied by numerical simulation in the time domain based on 3D staggered-grid finite difference method whose availability was firstly verified through a theoretical solution. The numerical simulation results show that seismic waves in the seabed layer were dominated by transmitted S-wave and interface wave in the near-field area, and transmitted P-wave is not obvious here, while seismic waves were dominated by normal mode and interface wave in the far-field area, and when the sound source is moved away from the seafloor, the amplitude of normal mode wave becomes larger, but the amplitude of interface wave becomes smaller.

Investigation of Earthquake Significant Duration on the Seismic Performance of Adjacent Steel Structures in Near-Source

According to the recent Earthquake records, the earthquake duration is longer in some areas, resulting into more structural damage. One of the important factors in reducing earthquake damages is the separation gap between two adjacent structures. This case study investigated the effect of significant duration of the earthquake on two adjacent steel moment-resisting structures with different heights and near to active fault. The pounding between pairs of three 3, 6 and 9-story steel moment frames was evaluated using a nonlinear time history analysis method considering the reduced stiffness and strength. The results showed that for the intended type 3-soil, the risk of pounding and collapse amplification among the 3- and 6-story buildings are higher than others. This is due to the necessity of the Iranian standard 2800 to calculate the separation gap by the nonlinear methods for the buildings with height more than 8 stories. Also, the analysis of the significant duration of the applied earthquakes demonstrated that this parameter is a determining and effective factor in the pounding of structures, especially the adjacent buildings with defferent heights. It is noteworthy all of the analysis was done by 9 earthquake records. This study recommended using the nonlinear method to calculate separation gap while designing two adjacent steel moment-resisting structures with different heights in the near-field area and on the soft soil.

Evaluation of Compact Boundary of Outage Probability in HetNet

In this paper, we investigate the outage probability of per‐tier in heterogeneous networks (HetNet) in the presence of aggregated interference from coexisting ad hoc networks. In ad hoc networks, a receiving node suffers the cross‐tier interference signal from cellular cell and aggregated interference signal from other transmitting nodes. In order to evaluate the effect of cross‐tier interference on the outage probability of ad hoc networks, we analyze the restricted region in cellular cell that the received signal interference noise ratio (SINR) of a receiving node does not exceed the SINR threshold considering the path loss model. Further, to evaluate the effect of aggregated interference of ad hoc networks, we divide the plane area into near‐field area and far‐field area according to whether the signal of a transmitting node could cause an outage independently or not. Based on this division, we derive the compact lower and upper boundaries of outage probability of per‐tier. The computer simulations validate the results of theoretical analysis and show the effect of different factors, such as transmit power, intensity of ad hoc nodes, on the outage probability.

Near-Field to Far-Field RCS Prediction on Arbitrary Scanning Surfaces Based on Spherical Wave Expansion.

Near-field to far-field transformation (NFFFT) is a frequently-used method in antenna and radar cross section (RCS) measurements for various applications. For weapon systems, most measurements are captured in the near-field area in an anechoic chamber, considering the security requirements for the design process and high spatial costs of far-field measurements. As the theoretical RCS value is the power ratio of the scattered wave to the incident wave in the far-field region, a scattered wave measured in the near-field region needs to be converted into field values in the far-field region. Therefore, this paper proposes a near-field to far-field transformation algorithm based on spherical wave expansion for application in near-field RCS measurement systems. If the distance and angular coordinates of each measurement point are known, the spherical wave functions in an orthogonal relationship can be calculated. If each weight is assumed to be unknown, a system of linear equations as numerous as the number of samples measured in the near electric field can be generated. In this system of linear equations, each weight value can be calculated using the iterative least squares QR-factorization method. Based on this theory, the validity of the proposed NFFFT is verified for several scatterer types, frequencies and measurement distances.

Analytic spherical harmonic gradients for real-time rendering with many polygonal area lights

Recent work has developed analytic formulae for spherical harmonic (SH) coefficients from uniform polygonal lights, enabling near-field area lights to be included in Precomputed Radiance Transfer (PRT) systems, and in offline rendering. However, the method is inefficient since coefficients need to be recomputed at each vertex or shading point, for each light, even though the SH coefficients vary smoothly in space. The complexity scales linearly with the number of lights, making many-light rendering difficult. In this paper, we develop a novel analytic formula for the spatial gradients of the spherical harmonic coefficients for uniform polygonal area lights. The result is a significant generalization, involving the Reynolds transport theorem to reduce the problem to a boundary integral for which we derive a new analytic formula, showing how to reduce a key term to an earlier recurrence for SH coefficients. The implementation requires only minor additions to existing code for SH coefficients. The results also hold implications for recent efforts on differentiable rendering. We show that SH gradients enable very sparse spatial sampling, followed by accurate Hermite interpolation. This enables scaling PRT to hundreds of area lights with minimal overhead and real-time frame rates. Moreover, the SH gradient formula is a new mathematical result that potentially enables many other graphics applications.

Dual‐steel frame consisting of moment‐resisting frame and shape memory alloy braces subjected to near‐field earthquakes

SummaryIn this paper, concentric braced frames are combined with moment‐resisting frame (MRF) as a dual system subjected to near‐field (NF) pulse‐like and far‐field ground motions. The braced frame in this system configuration consists of steel buckling‐restrained braces (BRB model), braces with shape memory alloy (SMA model), or combination of BRB and SMA braces (COMBINED model). Some prototype structures of the proposed systems are designed according to the code recommendations. Then, the nonlinear models of the considered structures are developed in SeismoStruct software, and nonlinear time history analysis (NLTHA) is implemented. NLTHA is performed subjected to earthquake record sets at maximum considered earthquake (MCE) and design base earthquake (DBE) levels, and responses of the systems are investigated and compared with each other. Among the examined models, the SMA and COMBINED models exceed the CP level subjected to NF‐MCE record set. Therefore, more investigation is needed for using short‐segment SMA braces in the dual‐steel frames in NF area.

Fragility Analysis of Gantry Crane Subjected to Near-Field Ground Motions

A gantry crane located in a near-field earthquake-prone area is selected in this paper as an example, and the nonlinear finite element (FE) model is used considering the material nonlinearity including plastic hinges and the second order (P − Δ ) effect with a comprehensive consideration of the components including sill beams, support beams, legs, and trolley girders. The local displacement ratio (LDR) and deflection ratio (DR) are proposed as demand measures (DMs) of the gantry crane, which are utilized to construct a probabilistic seismic demand model (PSDM). Then, the capacity limit states for the gantry crane are defined in this study by performing pushover analysis (POA), known as serviceability, damage control, and collapse prevention, respectively. Moreover, the operating capacity of the crane during an earthquake is further investigated and quantified by operating seismic peak ground acceleration, which is defined as the maximum acceleration when the failure probability is 50%. Finally, the fragility curves and the failure probability of the gantry crane are derived by the above definitions, all of which are pioneering in the seismic design of gantry cranes subjected to near-field ground motions. Some major conclusions are drawn that the horizontal component of an earthquake has a more notable effect on the structural damage of the gantry crane compared to the vertical component, and incremental dynamic analysis can take seismic uncertainty into account and quantify the deformation of gantry crane in more detail.

Ultrawideband Circularly Polarized Antenna for Near-Field SAR Imaging Applications

In this article, an ultrawide band circularly polarized crossed dipole antenna is proposed for near-field synthetic aperture radar imaging. The presented antenna comprises two orthogonal dipoles with elliptical arms vacated by rotated elliptical slot. The slots and a modified feeding configuration enhance both impedance and axial ratio bandwidth. The antenna has 4.75:1 impedance bandwidth (1.6–7.6 GHz) and 4.1:1 axial ratio bandwidth (1.8–7.1 GHz). To reduce the coupling in quasi-monostatic imaging scenarios, a planar circularly symmetric structure is used that decreases coupling about 10 dB. Due to importance of antenna time-domain behavior in imaging systems, transient response is evaluated in near-fields and also far-fields, which shows a good fidelity factor. The electric field distribution in a region of near-field area is almost uniform in different frequencies that improve imaging. The far-field features like radiation pattern, gain, and axial ratio are also satisfactory. Finally, an oil well monitoring system using the proposed antenna is simulated and based on the results, existed perforations in the well wall are detected. Compared to previous antennas, this antenna has several advantages such as higher impedance and axial ratio bandwidth, planar structure, low front to side ratio which mitigates mutual coupling, and satisfactory transient response.

Effects of gap thickness and emitter location on the photoluminescence enhancement of monolayer MoS2 in a plasmonic nanoparticle-film coupled system

Abstract Plasmonic nanocavities comprised of metal film-coupled nanoparticles have emerged as a versatile nanophotonic platform benefiting from their ultrasmall mode volume and large Purcell factors. In the weak-coupling regime, the particle-film gap thickness affects the photoluminescence (PL) of quantum emitters sandwiched therein. Here, we investigated the Purcell effect-enhanced PL of monolayer MoS2 inserted in the gap of a gold nanoparticle (AuNP)–alumina (Al2O3)–gold film (Au Film) structure. Under confocal illumination by a 532 nm CW laser, we observed a 7-fold PL peak intensity enhancement for the cavity-sandwiched MoS2 at an optimal Al2O3 thickness of 5 nm, corresponding to a local PL enhancement of ∼350 by normalizing the actual illumination area to the cavity’s effective near-field enhancement area. Full-wave simulations reveal a counterintuitive fact that radiation enhancement comes from the non-central area of the cavity rather than the cavity center. By scanning an electric dipole across the nanocavity, we obtained an average radiation enhancement factor of about 65 for an Al2O3 spacer thickness of 4 nm, agreeing well with the experimental thickness and indicating further PL enhancement optimization. Our results indicate the importance of configuration optimization, emitter location and excitation condition when using such plasmonic nanocavities to modulate the radiation properties of quantum emitters.

Approach to Precisely Model an Underwater Electric Dipole Source with TSS-FDTD

AbstractDue to the fields in the near-field area of an electric dipole source which is sharply and rapidly varying with distance, a novel approach to introduce an underwater low-frequency excited source is proposed in this paper. The approach is built up to combine the theoretical solution for an electric dipole in a conductive medium with the total-field scattered-field boundary to reduce the error in modeling the source. The spatial distributions of the magnetic field amplitude at a distance of 1 km above the sea surface are discussed. The numerical results verified the accuracy and validity of the approach by comparing with the results of the Sommerfeld numerical integration method.

Fluvial and oceanographic influences on suspended sediment dispersal in the Magdalena River Estuary

The Magdalena River is the main contributor of fresh water and sediment to the Caribbean Sea. Hydrological inputs from the Magdalena River are 10,287 and 4068 m3 s−1 at high and low flows, respectively. Sediment delivery from the Magdalena River is among the largest in the world, with 142 × 106 t yr−1 of transported suspended matter. We used MODIS images to develop a regional model and assess the spatial and temporal variability of suspended sediment concentration at the mouth of the Magdalena River, map the extent of the turbidity plume using its morphometric properties, and identify the main fluvial and oceanographic factors associated with plume variability. We analyzed monthly average suspended sediment concentration data for the period 2003–2017. MODIS images were calibrated with in situ measurements of suspended sediment concentrations in surface waters of the study area. A finite mixing model was used to identify turbidity thresholds and define areas within the plume. Calculated suspended sediment concentrations indicate high turbidity in the Magdalena River mouth (178.6 ± 78.7 mg L−1), values that are of the same order of magnitude as those measured in the Yangtze, Huang He and Amazon River mouths. Calculated plume areas (diffuse, solid and mixing) and their shapes suggest that the Magdalena River plume has a limited area, related to conditions such as wind speed and direction, streamflow, and sediment transport. Calculated areas and shapes demonstrate the ability to generate a near-field area, which indicates the presence of a clear convergence front. The data also emphasize the importance of sediment transport from the Magdalena River to the Caribbean Sea. Such fluvial transport is important in the biogeochemistry of estuarine systems, especially in turbid estuaries, where there is marked variability in the distribution of sediment in surface waters.

Theoretical analysis and experimental verification of wideband multiple‐input multiple‐output imaging radar

Multiple-input multiple-output (MIMO) radar is a novel radar system which can achieve more individual observation echoes than the number of the actual antennas by transmitting orthogonal waveforms. At present, pattern theory is usually utilised to evaluate the azimuth imaging indicator of the MIMO imaging radar system. Although it cannot describe the two-dimensional performance, it is quite effective to analyse the azimuth resolution of far-field targets. However, the pattern based on the far-field condition becomes invalid for targets in the near-field area, thus it cannot explain the appearance of near-field azimuth grating lobes. In this article, spatial spectrums and generalised ambiguity functions (GAFs) of near-field targets are calculated. By describing the near-field GAF (NGAF) approximately, the relationship between the distortions of NGAF and the spatial spectrum is studied qualitatively, which can explain the appearance of azimuth near-field grating lobes. In addition, the calibration method of amplitude and phase errors by an isolated corner reflector is presented in this article. Finally, a simulation and an experiment are carried out to verify the conclusions.

Fusion model of inductive thermography and ultrasound for nondestructive testing

Aluminum alloy is widely used in industry. Generally, the ultrasound testing as one of the non-destructive testing technologies has been applied to the metal sheet inspection for guarantying the quality of the production. However there exist challenges if only single ultrasound testing is applied. The near-field area of ultrasonic method remains limitation for detection. This paper investigates a robust data fusion model for comprehensive defects detection by hybriding inductive thermography and ultrasound testing in dynamic scanning. The proposed data fusion architecture is based on feature layer construction and it is able to cover the entire inspection from the surface, near surface and inside of the specimen. The experiment result demonstrates that the measurement of defects position and contour is accurate and reliable. It is easier to achieve automatic inspection through visualization and feature extraction. This method is effective and reliable for comprehensive detect and measurement metal plate.

Design, Fabrication, and Evaluation of Multifocal Point Transducer for High-Frequency Ultrasound Applications.

The present study illustrates the design, fabrication, and evaluation of a novel multifocal point (MFP) transducer based on polyvinylidene fluoride (PVDF) film for high-frequency ultrasound application. The fabricated MFP surface was press-focused using a computer numerical control (CNC) machining tool-customized multi-spherical pattern object. The multi-spherical pattern has five spherical surfaces with equal area and connected continuously to have the same energy level at focal points. Center points of these spheres are distributed in a linear pattern with 1 mm distance between each two points. The radius of these spheres increases steadily from 10 mm to 13.86 mm. The designed MFP transducer had a center frequency of 50 MHz and a −6 dB bandwidth of 68%. The wire phantom test was conducted to study and demonstrate the advantages of this novel design. The obtained results for MFP transducer revealed a significant increase (4.3 mm) of total focal zone in the near-field and far-field area compared with 0.48 mm obtained using the conventional single focal point transducer. Hence, the proposed method is promising to fabricate MFP transducers for deeper imaging depth applications.

Computational and experimental modelling study of the unsteady airflow over the aircraft carrier HMS Queen Elizabeth

This paper describes a comprehensive experimental and computational modelling study of the aerodynamic environment around the UK's new Queen Elizabeth Class (QEC) aircraft carriers. The study has been performed to support the integration of the F-35B Lightning II multi-role fighter with the UK Royal Navy's flagship, HMS Queen Elizabeth. Unsteady airwakes have been generated using Computational Fluid Dynamics (CFD) and have been incorporated into the F-35/QEC Integration Flight Simulator at BAE Systems Warton and into the HELIFLIGHT-R research simulator at the University of Liverpool.A small-scale experiment has also been conducted in which a 1.4 m long (1:200) scale model of the QEC was submerged in a water channel and Acoustic Doppler Velocimetry was used to measure the unsteady flow around the ship. Delayed Detached Eddy Simulation CFD was used to model the flow in the water channel and the computed unsteady flow field has been compared with the experimental measurements. The results show generally excellent agreement between the model-scale experiment and CFD. Building on this, full-scale 30-s CFD airwakes have been generated for the nearfield area surrounding the QEC, and for about 400 m astern of the ship to capture the disturbed air flow along the fixed-wing approach glideslope.