Field Inside Research Articles

From coupled plane waves to the coupled-mode theory of guided-mode resonant gratings

We establish the connection between two theoretical approaches widely used for describing resonant optical properties of guided-mode resonant gratings (GMRGs): the coupled plane waves model and the coupled-mode theory (CMT). In the coupled plane waves model (also known as the multiple interference model), the field inside and outside the structure is represented as a small number of propagating plane waves; these waves are coupled at one or several interfaces, which leads to a system of linear equations. In the CMT, the optical properties are described by several coupled differential equations, each corresponding to one of the modes supported by the structure. The approach proposed in the present work provides explicit expressions for the coupling coefficients of the CMT through “local” reflection, transmission, and diffraction coefficients of the coupled plane waves model, which is important for the design and analysis of optical resonators, filters, and sensors based on GMRGs.

Imaging phonon eigenstates and elucidating the energy storage characteristics of a honeycomb-lattice phononic crystal cavity

We extend gigahertz time-domain imaging to a wideband investigation of the eigenstates of a phononic crystal cavity. Using omnidirectionally excited phonon wave vectors, we implement an ultrafast technique to experimentally probe the two-dimensional acoustic field inside and outside a hexagonal cavity in a honeycomb-lattice phononic crystal formed in a microscopic crystalline silicon slab, thereby revealing the confinement and mode volumes of phonon eigenstates—some of which are clearly hexapole in character—lying both inside and outside the phononic-crystal band gap. This allows us to obtain a quantitative measure of the spatial acoustic energy storage characteristics of a phononic crystal cavity. We also introduce a numerical approach involving toneburst excitation and the monitoring of the acoustic energy decay together with the integral of the Poynting vector to calculate the Q factor of the principal in-gap eigenmode, showing it to be limited by ultrasonic attenuation rather than by phonon leakage to the surrounding region.

Polarization and built-in electric field improve the photocatalytic overall water splitting efficiency of C2N/ZnSe heterostructures

The type-II van der Waals heterostructure photocatalytic system is an ideal band alignment structure for photocatalysis, which the high separation efficiency of photo-induced carriers. By using density functional theory, we predicted two van der Waals (vdW) heterostructures, namely, C2N/ZnSe-ML (monolayer) and C2N/ZnSe-BL (bilayer), respectively. For the C2N/ZnSe-BL heterostructure, it has an intrinsic type-II band alignment, in which the built-in electric field formed at the interface accelerates the separation of photogenerated electrons-holes in heterostructure. The photogenerated electrons-holes have been accumulated near the conduction band minimum of C2N and the valence band maximum of ZnSe-BL, respectively, which the C2N/ZnSe-BL heterostructure is a type-II photocatalytic heterostructure. However, the C2N/ZnSe-ML heterostructure has a strong built-in electric field from C2N to ZnSe-ML because the ZnSe-ML is a polarized material with a polarized electric field inside, which form a type-Ⅱ heterostructure and realizes the effective separation of photogenerated electron-holes under the action of built-in electric field. The C2N/ZnSe-X (ML, BL) heterostructures are both direct band-gap semiconductors and they can harvest sunlight efficiently almost in the range of the solar spectrum. These distinguishing features confirm that the C2N/ZnSe-X (ML, BL) heterostructures have promising prospects in the sphere of photocatalysis for the separation of photogenerated electron-holes.

Time-domain simulation of acoustic wave scattering and internal propagation from a gas bubble of various shapes.

Acoustic scattering and resonances of gas bubbles are computed using a time-domain simulation based on numerical solutions of the conservation laws. The time histories of scattered pressure and fluid velocity, outside and inside the bubble, are obtained simultaneously from an immersed-boundary method allowing for the investigation of exterior and interior fields for non-spherical geometries. The acoustic resonances of the bubble are investigated for various bubble sizes, shapes, and inner gas parameters and compared in limiting cases to the partial wave scattering solutions for spherical bubbles. The dynamics of the gas motion and its associated contribution to resonance response has received little attention in previous analytical and numerical formulations. In this study, the acoustic propagation and motion inside the interior gas is investigated with respect to the monopole resonance with the combined time-domain simulation and immersed-boundary method. For the non-spherical prolate and oblate shapes, the scattering and resonance behaviors are compared with the approximate analytical results based on the shape factor method. The simulation method can be extended to less-understood shapes relevant to underwater and physical acoustics, such as "pancake-shaped" or "cigar-shaped" bubbles, as well as to spatial and time-dependent forcing.

Experimental Analysis of the Influence of Gear Design and Catch Weight on the Fluid–Structure Interaction of a Flexible Codend Structure Used in Trawl Fisheries

This study evaluated the behavior of different codend designs to provide the basic information that is relevant for improving the gear selectivity, energy efficiency, to better understand the fish behavior inside the codend, and prevent the probability of the fish escaping. Three different codends were designed from the standard codend commonly used in the Antarctic krill fisheries based on modified Tauti’s law and evaluated. The first and the third codends were designed with four-panel and two-panel nettings, respectively, both made of diamond meshes. While, the second one was a four-panel diamond mesh design with cutting ratio 4:1(N [NBNBN]16). We measured the drag force, codend shape, fluttering codend motions, and the flow field inside and behind the different codends composed of different catch weights under various flow velocities in flume tank. The power spectra density was undertaken to analyze the time evolution of measured parameters. The results showed that the drag force and the codend motion increased and decreased, respectively, with the number of net panels and the cutting ratio. Due to the catch weight and flow velocity, which caused significant codend motions and deformation, a complex interaction occurred between the fluid and the structure, and there was a strong correlation between the codend drag, the codend motions, and the turbulent flow inside and behind the codend. The study showed that the use of the four-panel codend with cutting ratio and the two-panel codend resulted in drag reductions of 6.07% and 6.41%, respectively, compared to the standard codend. The velocity reduction and turbulent kinetic energy were lower inside and behind the four-panel codend than inside and behind the two-panel codend, indicating that turbulent flow through the two-panel codend is more important than through the four-panel codend. The results of the power spectral density analysis showed that the drag and codend motions were mainly low frequency in all codends, with another component related to turbulent flow street. In addition, the two-panel codend showed more unstable behavior with more pendulum motion compared to the four-panel codends, resulting in a smaller mesh size in this codend that could affect swimming energy and thus influence fish escape, making it the least selective codend. The results of this study provide fundamental insights useful for understanding and improving the hydrodynamic performance and selectivity of trawls in the Antarctic krill fishery, especially to reveal the masking effects of the number of net panels on codend design.

Basic investigation of sound field inside and outside ear canal under ultrasound irradiation

In recent years, high-intensity airborne ultrasonic waves applied technology has been developed actively. Accordingly, there are concerns about the effects of sound wave exposure by high-intensity airborne ultrasonic waves. The sound wave intensity is different near the entrance of the ear canal and near the eardrum, because the wavelength is short in the ultrasonic region. Therefore, it is necessary to know accurately the sound pressure at the eardrum position in considering the effects of sound waves exposure by ultrasonic waves. However, it is difficult to measure sound pressure near the eardrum with a probe microphone because it may damage the ear canal and eardrum. Against this background, we have studied the sound pressure characteristics inside and outside the ear canal when airborne ultrasonic waves are exposed to the mannequin head (KEMAR) with a pseudo-ear canal created to imitate the dimensions of the human ear canal. And, we have attempted to estimate the sound pressure value near the eardrum using the sound pressure in the ear canal. In this report, we verified the sound field inside and outside the ear canal and the sound pressure near the eardrum, when ultrasonic waves (frequency:20 kHz) were irradiated to the mannequin head.

Visualizing screening in noble-metal clusters: static vs. dynamic.

The localized surface-plasmon resonance of metal nanoparticles and clusters corresponds to a collective charge oscillation of the quasi-free metal electrons. The polarization of the more localized d electrons opposes the overall polarization of the electron cloud and thus screens the surface plasmon. By contrast, a static electric external field is well screened, as even very small noble-metal clusters are highly metallic: the field inside is practically zero except for the effect of the Friedel-oscillation-like modulations which lead to small values of the polarization of the d electrons. In the present article, we present and compare representations of the induced densities (i) connected to the surface-plasmon resonance and (ii) resulting from an external static electric field. The two cases allow for an intuitive understanding of the differences between the dynamic and the static screening.

Mechanism of avalanche charge domain transport for nonlinear mode of GaAs photoconductive semiconductor switches

Photoconductive semiconductor switch is of significance in the fields of ultafast electronics, high-repetition rate and high-power pulse power system, and THz radiation. The mechanism of the nonlinear mode of the switch is an important area of study. In this work, stable nonlinear wave forms are obtained by a semi-insulating GaAs photoconductive semiconductor switch triggered by a 5-ns laser pulse with pulsed energy of 1 mJ at a wavelength of 1064 nm under a bias of 2750 V. Based on two-photon absorption model, the photogenerated carrier concentration is calculated. The theory analysis and calculation result show that the photogenerated carrier can compensate for the lack of intrinsic carrier, and lead to the nucleation of photo-activated charge domain. According to transferred-electron effect principium, the electric field inside and outside the domain are calculated, indicating that the electric field within the domain can reach the electric field which is much larger than intrinsic breakdown electric field of GaAs material, and results in strong impact avalanche ionization in the bulk of the GaAs switch. According to the avalanche space charge domain, the typical experimental phenomena of nonlinear mode for GaAs switch are analyzed and calculated, the analysis and calculations are in excellent agreement with the experimental results. Based on drift-diffusion model and negative differential conductivity effect, the transient electric field in the bulk of the switch is simulated numerically under the optical triggering condition. The simulation results show that there are moving multiple charge domains with a peak electric filed as high as the intrinsic breakdown electric field of GaAs within the switch. This work provides the experimental evidence and theoretical support for studying the generation mechanism of the nonlinear photoconductive semiconductor switch and the improvement of the photo-activated charge domain theory.

Direct connection between Navier and spherical harmonic kernels in elasticity

<abstract> <p>Linear isotropic elasticity is an interesting branch of continuum mechanics, described by the fundamental laws of Hooke and Newton, which are combined in order to construct the governing generalized Navier equation of the displacement within any material. Implying time-independence and in the absence of external body forces, the latter is reduced to the corresponding form of a homogeneous second-order partial differential equation, whose solution is given via the Papkovich differential representation, which expresses the displacement field in terms of harmonic functions. On the other hand, spherical geometry provides the most widely used framework in real-life applications, concerning interior and exterior problems in elasticity. The present work aims to provide a little progress, by producing ready-to-use basic functions for linear isotropic elasticity in spherical coordinates. Hence, we calculate the Papkovich eigensolutions, generated by the spherical harmonic eigenfunctions, obtaining connections between Navier and spherical harmonic kernels. A set of useful results are provided at the end of the paper in the form of examples, regarding the evaluation of displacement field inside and outside a sphere.</p> </abstract>

The analysis of oil circuit cooling of a screw compressor

The paper analyses the fluid flow and heat transfer in the heat exchanger connected to the SMARTRONIC screw compressor housing manufactured by ATMOS Chrást s.r.o. using numerical simulations. The work aims to describe the flow in all parts of the exchanger and to determine the distribution of the temperature field inside.

Магнитное поле внутри и вне тороидального плазменного шнура с током, удерживаемого в равновесии внешним аксиальным полем

We provide a description of magnetic field inside and outside of a circular plasma ring carrying an electric current. The equilibrium is maintained by an external axial magnetic field. The provided solution is close to that by Titov and Demoilin (1999), but our solution is more general and accurate

Overlooked species diversity in the hyper‐arid Sahara Desert unveiled by dryland‐adapted lizards

AbstractAimHyper‐arid sandy and rocky fields rank among the least biologically diverse habitats of the desert biome, yet knowledge of local biodiversity patterns is also extremely poor. In the Sahara Desert, palaeoclimate oscillations affected the extent of hyper‐arid habitats, but it is unclear how these dynamics determined the evolution and distribution of local specialists. Herein, we assessed cryptic diversity, diversification patterns and spatial connectivity within a Sahara‐widespread group of dryland‐adapted lizards.LocationSahara‐Sahel ecoregions.TaxonAcanthodactylus scutellatus species group.MethodsInter‐ and intraspecific phylogenetic structure, divergence times, spatial genetic patterns and cryptic diversity were assessed using nuclear and mitochondrial loci. The effects of topography and land cover on phylogeographic structure and diversity were tested with generalized linear models. Interspecific hybridization was evaluated using 11 microsatellites across the group's major sympatry zone, predicted based on ecological niche models.ResultsSpecies of Acanthodactylus scutellatus group exhibit Late Miocene origins, followed by extensive intraspecific divergence throughout the Pliocene. The northern Sahara worked as a major diversification hotspot, harbouring a patchwork of small‐ranged, divergent lineages. These lineages are parapatric or sympatric and present concordant nuclear and mitochondrial differentiation, suggesting species status. Genetic connectivity increases in southern latitudes, with wide‐ranging lineages spanning from the Red Sea to the Atlantic coast. Within these potential corridors, mountain outskirts and sand fields in the Sahara interior seemingly acted as origins for recent population expansions. Genetic diversity and connectivity are favoured by terrain roughness and soft‐sand cover respectively. Three species inhabit the Atlantic Sahara sympatry zone without evidence of gene flow.Main conclusionsOverlooked species‐level diversity within a major specialist group of Sahara drylands exposes the recurrent knowledge shortfalls present in hyper‐arid desert environments. Humidity and sandy habitat shifts triggered potential successions of population isolation and re‐connectivity, which favoured cladogenesis in northern desert regions and population expansions across southern east–west corridors.

A non-intrusive model order reduction boundary element method for frequency sweeps of 2D acoustics

The boundary element method (BEM) is one of the most commonly used prediction technique to deal with acoustic problems governed by the Helmholtz equation. Its use, however, suffers from the disadvantage that the system of equations have to be reformulated for each frequency of interest due to the frequency-dependent property. This undermines the inherent efficiency of frequency sweep analyses of BE models. Additionally, the resulting coefficient matrices are fully-populated, non-symmetric and complex-valued. In order to enable the traditional BEM to be applied for large-scale acoustic problems, a non-intrusive model order reduction boundary element method under the offline–online framework is proposed. In the offline phase, a proper reduced-order basis (ROB) which spans a projection subspace is constructed via the Arnoldi method. A newly developed cutoff technique is incorporated to reduce the involved computational complexity. Once a low-rank ROB is determined, the full-order frequency-decoupled system matrices based on the series expansions are projected onto the spanned subspace column-by-column to favor the memory requirement. In the online phase, fast frequency sweep analyses with a direct solver can be achieved because only the compact reduced-order model needs to be confronted. The high performance of the proposed solution approach in the context of single-frequency and multi-frequency analyses of two-dimensional (2D) BE models with the number of degrees of freedom ranging from a few hundred up to one hundred thousand is demonstrated by various problem settings, including both interior and exterior acoustic fields with different boundary conditions.

Avalanche photodiodes with multiple multiplication layers for coherent detection

We demonstrate a novel avalanche photodiode (APD) design which fundamentally relaxes the trade-off between responsivity and saturation-current performance at receiver end in coherent system. Our triple In0.52Al0.48As based multiplication (M-) layers with a stepped electric (E-) field inside has more pronounced avalanche process with significantly less effective critical-field than the dual M-layer. Reduced E-field in active M-layers ensures stronger E-field allocation to the thick absorption-layer with a smaller breakdown voltage (Vbr) resulting in less serious space-charge screening effect, less device heating at high output photocurrent. Compared to the dual M-layer reference sample, the demonstrated APD exhibits lower punch-through (− 9 vs. − 24 V)/breakdown voltages (− 43 vs. − 51 V), higher responsivity (19.6 vs. 13.5 A/W), higher maximum gain (230 vs. 130), and higher 1-dB saturation-current (> 5.6 vs. 2.5 mA) under 0.95 Vbr operation. Extremely high saturation-current (> 14.6 mA), high responsivity (7.3 A/W), and decent O-E bandwidth (1.4 GHz) can be simultaneously achieved using the demonstrated APD with a 200 µm active window diameter. In coherent FMCW LiDAR test bed, this novel APD exhibits a larger signal-to-noise ratio and high-quality 3-D images than the reference dual M-layer and high-performance commercial p-i-n PD modules, while requiring significantly less optical local-oscillator (LO) power (0.5 vs 4 mW).

A new planar microwave sensor for fat-measuring of meat based on SRR and periodic EBG structures

In this paper, we present a novel microwave sensor for sensing the fat of the meat with high sensitivity. We developed the sensor based on a bent transmission line (TL) and connected it to a split ring resonator (SRR). The gaps in the SRR make hot spots. The material under test (MUT) is placed on the hot spot to increase sensitivity. The sensor is surrounded by electromagnetic bandgap (EBG) elements in the final structure to enhance the Q-factor and save the electric field inside of the sensor. The full-wave simulation resonated with this sensor at triple frequencies of 3.4, 4.08, and 4.55 GHz. Additionally, the proposed EBG unit cell covers 2.4–5 GHz. So, it can make an electromagnetic shield for all resonances, and the final sensor shows dual-band characteristics at 3.48 and 4.67 GHz. The sensor is used to recognize the fat ratio in various meat samples. After Full-wave analysis, the equivalent circuit is extracted and modeled with the aid of an advanced design system (ADS). Next, the relation between the frequency shift of S 21 nulls and fat percentages is calculated based on a proposed closed-form formula. In the end, comparing full-wave analysis, experimental results, and the equivalent circuit model shows perfect matching. • Introducing the first planar sensor for detecting the percentage value of the fat in themeat. • Unlike bulky waveguide sensors the proposed sensor is compact but shows a high Q-factor. • The compact planar sensor is based on a transmission line (TL) and split ring resonator (SRR). • The frequency shifts are used for recognizing the material under test and experimental results confirmed simulation. • The circuit model is determined for confirming simulation and experimental and also the interpolated formula is investigated for calculating permittivity as a closed form formula.

3D stochastic interferometer detects picometer deformations and minute dielectric fluctuations of its optical volume

AbstractSpeckle metrology harnesses the interferometric properties of disordered light to achieve remarkable sensitivities. Often relying on time-domain analysis, it is rate-limited by the acquisition of speckle images. In the present work instead, we use a frequency-domain approach which spans 8 to 10 frequency decades up to 100 MHz, and reveals minute changes of speckle decorrelation spectra. We built a 3D stochastic interferometer using a centimeter-sized quartz-powder cavity with arbitrary shape and high Lambertian reflectivity. Filled with a coherent monochromatic photon gas, it creates statistically isotropic and homogeneous 3D interference patterns, whose variations arise from cavity deformations or fluctuations of the dielectric tensor field inside. Speckle decorrelation depends neither on where the perturbation sits nor on where it is measured. With an average 62 m photon transit path and a finesse of 10500, cavity deformations are detected with a power noise floor of 4 × 10−3 pm2, i.e., 2.7 pm at 1 kHz. We also demonstrate a 100-fold sensitivity gain compared to conventional light scattering techniques when probing thermal motions of single and multiply scattering colloids.

Noise and Critical Sound Levels During Non-Invasive Ventilation of a Preterm Infant in the Incubator.

Preterm birth and the subsequent necessary treatment in neonatal intensive care units (NICU) subjects the preterm infant to non-physiological noise exposure with potentially adverse consequences for short- and long-term development. Adjusters to improve the acoustic environment for the preterm infant need to be defined. Sound pressure level measurements during routine procedures in a NICU were performed by ¼" microphones placed inside and outside the incubator. The microphones need to be suitably positioned to measure sound pressure levels that are representative for the sound field inside and outside the incubator. The sound pressure level spectra generated by respiratory support and corresponding monitor alarms were compared. Inside the incubator, higher sound level pressures (in dBA) were generated primarily by the use of the system components of the incubator itself than outside, whereas when the incubator was closed, it had an insulating effect on sounds generated in the NICU. Non-invasive ventilation resulted in an increase in sound pressure levels from 50 to 60 dBA in the neonate's environment, with sound pressure levels increasing particularly in the frequency range above 1 kHz. Preterm infants are exposed to high sound levels, especially in the non-physiological high-frequency range, particularly during non-invasive ventilation. The continuous sound exposure could be further reduced to some extent by an optimized design of the incubator.

The effect of crosswind and installation of wind-break deflector on the performance of natural draft dry cooling tower (NDDCT)

Natural draft dry cooling tower (NDDCT) is one of the most important components of combined power plant cycles in arid regions. Studies illustrate that being subjected to the wind blowing has negative effects on NDDCT's thermal performance. These adverse effects mostly inflict on the outlet port of the tower and the installation platform of the radiators at the bottom of the tower. In this study, the influence of installing wind-break deflectors at the outlet port of the tower and addressing the negative impacts of the wind is investigated. Considering the buoyancy effect for a 3-D incompressible flow, the flow field inside and outside the tower as well as the heat transfer from the radiators is modeled through simultaneously solving the Navier-Stokes and energy equations via computational fluid dynamics method (CFD). The performance of NDDCT is compared with and without the presence of deflectors where considered variables are height of the deflector and wind velocity. The improvement in the rate of the heat released to the atmosphere from the radiators will be estimated. Generally, in the presence of deflector walls the thermal performance of the tower enhances. With the increase in the height of deflectors, the thermal performance will further improve. For instance, the enhancement in the tower's performance with a 12-m deflector at wind velocities of 5 and 20 m/s is 57 and 44 percent, respectively.

Electromagnetic Fields around Black Holes in Einstein Æther Gravity

Axial symmetry and stationary properties of spacetime allow to find exact analytical solutions of differential equations describing fields and particles in a gravitational background. The present work is mainly devoted to derivation of exact solutions of Maxwell’s equations for magnetic fields generated by current loops around static black holes (BHs) in Einstein-aether gravity based on the spacetime symmetries in both regions: (i) interior and (ii) exterior to the current loop for a proper observer. The spacetime symmetries are applied in separating variables to solve the second order ordinary differential equation for vector potential of electromagnetic field and the equations of motion of test particles around the aether BH. We also study effects of the aether field on innermost stable circular orbits (ISCOs) of the test particles assuming the current loop position is placed there. It is obtained that the ISCO radius, as well as dipole magnetic moment of the current loop decrease with the increase of the aether parameter c14. Moreover, the performed analysis indicates that the aether field causes a decrease in the magnetic field inside and outside the current loop due to the change of its position.

Characteristics of the flow field inside and around a square fish cage considering the circular swimming pattern of a farmed fish school: Laboratory experiments and field observations

The management of marine aquaculture requires an accurate assessment of environmental impacts to achieve an environmentally sustainable system. To study the influence of farmed fish schools on the flow field around a fish cage, a pseudo-fish-school structure model was developed to simulate the circular swimming patterns of farmed fish in laboratory experiments. The drag force acting on the fish cage and current velocities inside and around a modeled square fish cage were investigated. A field survey was also conducted. As a result, the non-uniformity and complexity of the flow field of a stocked cage were observed in laboratory experiments. Circular swimming patterns were found to have obvious influences on the flow field distribution. The drag force of the square fish cage decreased by 11.8% because of the presence of the fish-school structure. Based on the field measurements, the averaged velocity inside and downstream of the full-scale fish cage was reduced by 46.2% and 57.8%, respectively. An acceleration was also observed in the current velocity below the fish cage. The vertical mixing of seawater may be because of the water blockage resulting from the cage and the circular swimming pattern of farmed fish schools. The findings presented in this study can be used as benchmarks for numerical models in assessing the potential environmental impact of fish farm sites.