Combined Time Domain Research Articles

Effective light scattering and charge separation in nanodiamond@g-C3N4 for enhanced visible-light hydrogen evolution

Development of heterostructured photocatalysts that promotes charge separation of semiconductor materials is crucial towards efficient photocatalytic hydrogen evolution. Here nanodiamond (ND)@graphitic carbon nitride (g-C3N4) nanosheet heterostructures were synthesized via a facile one-step pyrolysis approach. The ND@g-C3N4 heterostructures exhibit enhanced photocatalytic activity for hydrogen evolution from water splitting as compared with pristine g-C3N4. The optimal heterostructures with 10 wt% ND loading content show the best performance in photocatalytic H2 evolution under visible light irradiation, which is more than 5.6 times higher than that of the pristine g-C3N4. The increased light-trapping induced by the scattering of the NDs promotes more localized charge carriers generation, which is clarified by the combined experimental results and finite difference time domain simulations. Meanwhile, the effective charge separation at the ND@g-C3N4 interface due to the energy level matching can synergistically induce the enhanced photocatalytic activity of g-C3N4.

The Role of an Intense Front on the Connectivity of the Western Mediterranean Sea: The Cartagena‐Tenes Front

AbstractA combined space and time domain Self‐Organizing Maps analysis of Altimetry‐derived finite‐size Lyapunov exponents is used to extract characteristic spatial patterns of mesoscale features and to classify the western Mediterranean in regions of different variability of mixing activity. In particular, a recurrent and intense front is unveiled between Cartagena, Spain, and Tenes, Algeria, in the eastern boundary of the Alboran Sea, the here called Cartagena‐Tenes front (CTF). The temporal evolution and the frequency of occurrence of the spatial patterns and the regional variability of the mixing structures together with additional complementary Lagrangian diagnosis have been used to investigate the effect of the CFT on the regional flow connectivity. The seasonal behavior in the flux of particles through the domain associated with the degree of activation of the CTF shows that surface connectivity between the Alboran Sea and the western Mediterranean is stronger during winter and spring when the CTF is not active. Furthermore, the analysis shows that CTF becomes stronger than the Almeria‐Oran front over particular time periods, controlling the transport processes in this area. The impact of these fronts on transported substances has been analyzed based on Chlorophyll‐a satellite distribution, showing how the Lagrangian Coherent Structures, separate regions of different connectivity properties, affecting on the dynamical and biological behavior.

Study of the ultrasonic propagation law in the gas–liquid two-phase flow of deepwater riser through numerical simulation

In the drilling of deepwater wells, early detection of gas kicks is a crucial and difficult well-control task. A common and advantageous method is the underwater ultrasonic monitoring of the gas–liquid two-phase flow of deepwater risers. This study focuses on the problems of the early-stage ultrasonic monitoring of gas kicks in deepwater risers, and combines theoretical analysis and experimental verification to establish a method for finite element numerical simulation of ultrasonic wave propagation in the gas–liquid two-phase flow of a deepwater riser annulus. The acoustic-solid coupling module of COMSOL Multiphysics software was adopted. Through pattern analysis of ultrasonic wave propagation under different conditions, the sensitivity analysis was carried out by combining time domain and frequency domain analysis. Through analysis, two characteristic parameters of ultrasonic signals sensitive to the gas–liquid ratio, namely, the amplitude standard deviation (S) and frequency domain mean (μ) were selected. Their degrees of sensitivity to gas bubble size and density were analyzed to determine the optimal installation location of an ultrasonic receiver probe. And the optimal installation locations were located at 66°–88° and −66°−−88° of the arc area. Finally, a quantitative relationship between the ultrasonic signal characteristic parameters (S and μ) and the void fractions of risers (β) was established by using two-factor regression analysis. It was proved that its relative error was 4.5%, which was less than the acceptable error threshold of 5%, thus meeting the engineering requirements.

Annealing Induced Morphology of Silver Nanoparticles on Pyramidal Silicon Surface and Their Application to Surface-Enhanced Raman Scattering.

This paper reports on a simple and cost-effective process of developing a stable surface-enhanced Raman scattering (SERS) substrate based on silver (Ag) nanoparticles deposited on silicon (Si) surface. Durability is an important issue for preparing SERS active substrate as silver nanostructures are prone to rapid surface oxidation when exposed to ambient conditions, which may result in the loss of the enhancement capabilities in a short period of time. Here, we employ the galvanic displacement method to produce Ag nanoparticles on Si(100) substrate prepatterned with arrays of micropyramids by chemical etching, and subsequently, separate pieces of such substrates were annealed in oxygen and nitrogen environments at 550 °C. Interestingly, while nitrogen-annealed Si substrates were featured by spherical-shaped Ag particles, the oxygen annealed Si substrates were dominated by the formation of triangular shape particles attached with the spherical one. Remarkably, the oxygen-annealed substrate thus produced shows very high SERS enhancement compared to the either unannealed or nitrogen annealed substrate. The hitherto unobserved coexistence of triangular morphology with the spherical one and the gap between the two (source of efficient hot-spots) are the origin of enhanced SERS activity for the oxygen-annealed Ag particle-covered Si substrate as probed by the combined finite-difference time domain (FDTD) simulation and cathodoluminesensce (CL) experiment. As the substrate has already been annealed in an oxygen environment, further probability of oxidation is reduced in the present synthesis protocol that paves the way for making a novel long-lived thermally stable SERS substrate.

A Modified Gain Model and the Corresponding Design Method for an LLC Resonant Converter

In this paper, a modified gain model and the corresponding design method for an LLC resonant converter are proposed. To derive the gain model, two important factors-resonant factor and load factor are considered and discussed in detail by combining time domain and frequency domain. Compared with fundamental harmonic approximation (FHA) method, the accuracy of the gain model is highly improved. Referring to the modified gain model, the corresponding design method is also obtained that can inherit conciseness and practicability of the FHA method. Thus, the design method can be used to replace the FHA method in practical design procedures. Finally, the simulation results are given and a prototype of the LLC resonant converter used for power electronic traction transformer is built to verify the accuracy and validity of the proposed gain model and the corresponding design method.

Horn effect prediction based on the time domain boundary element method

A time domain boundary element method (TBEM) is applied to predict the horn effect. As the time response calculated by the time domain boundary integral equation contains the resonance components, when transformed to the frequency domain, the result will corrupt at the characteristic frequencies. To overcome this problem, a Burton–Miller-type combined time domain integral equation in half-space is applied. The resonance components are excluded in the time domain calculation, thus the corruptions are avoided in the frequency domain. As a result, the horn effect can be predicted very well at all frequencies. Compared to the frequency domain boundary element method for predicting the horn effect, the TBEM is more efficient due to the lower cost of forming coefficient matrices and solving equations. A numerical simulation is carried out to demonstrate the efficiency of the TBEM, and two experiments are conducted to validate the proposed method in predicting the horn effect. Both numerical and experimental results indicate that the proposed method is reliable and efficient in predicting the horn effect.

Multichip LED Modules With V-Groove Surfaces for Light Extraction Efficiency Enhancements Considering Roughness Scattering

Although multichip light-emitting diode (LED) modules are becoming popular in high-power lighting applications, better light extraction efficiency is always desirable in the industry. In this paper, V-grooves fabricated by the low-cost dicing process have been utilized on the top face of the multichip LED module to enhance the output power. An analytical model including the consideration of surface has been established by using a fractal theory. The combined finite difference time domain and ray tracing method has been used to simulate the light extraction enhancement considering the surface roughness scattering effects. The results show 12% enhancements in the experiments when the measured surface roughness of the V-groove has RMS roughness of $0.25~\mu \text{m}$ , which is in good agreement with the simulation result of 13.7%. The multichip LED models have been applied in indoor downlight systems and it is found that the luminous efficiency has been enhanced by 9.6% under a current of 350 mA (5.8 W) when compared with multichip LED modules without V-grooves.

Synchrophasors data analytics framework for power grid control and dynamic stability monitoring

This article develops a framework for phasor analytics necessary to enable advanced wide-area control and monitoring applications. The key component in this building is the smart phasor measurement unit for control (PMU/C) which from the substation can feed accurate fundamental and low order harmonics phasors to the phasor data concentrator at a higher than the standard rate of one point per cycle. The second piece of the building is the dynamic generator state estimation, which is mandatory to enhance the observability of dynamic phenomena and thus, improves control performance and protection dependability. Following dynamic state estimation, a combined time and frequency domain processing of voltage, angle and frequency measurements based on S-transform is proposed as a good mean for extracting critical features which enable crisper information that are more easily interpretable than the raw phasor time-series. Gluing all the proposed pieces at both substation and supervisory levels, it is possible to build a smart wide-area situational awareness system, able to close the loop through educated and well informed operators for handling geomagnetic disturbance (GMD) impacts on the grid and/or through fast control and automation devices for dealing with stability issues.

A combined rheology and time domain NMR approach for determining water distributions in protein blends

We present a combined time domain NMR and rheology approach to quantify the water distribution in a phase separated protein blend. The approach forms the basis for a new tool to assess the microstructural properties of phase separated biopolymer blends, making it highly relevant for many food and non-food related applications. First, we determine the relaxation rate of absorbed water, and the viscoelastic properties of the separated phases as function of the water content. Next, the same properties are measured for the protein blends. Finally, predictions for water distribution obtained from rheological experiments are made via the polymer blending law, and compared to a more direct assessment of the water distribution with time-domain NMR relaxometry (TD-NMR). In this study, the protein blend consists of soy protein isolate (SPI) and vital wheat gluten (WG). We demonstrate that predictions for water distribution are similar for both TD-NMR and rheological measurements. It turns out that water does not distribute homogenously over the phases. Independent of the SPI and WG ratio, more water is absorbed by the SPI phase relative to the WG phase, which largely determines the resulting rheological properties of the blends.

Classification of Brainstem Auditory Evoked Potentials Using Artificial Neural Network Based on Time and Frequency Domain Features

This article presents a novel technique of automated classification of brainstem auditory evoked potentials (BAEPs). Brainstem auditory evoked potentials are the evoked potentials used to measure the brain activity that occur in response to the auditory stimulus to the ear and to record the response by placing electrodes on the surface of the scalp. Brainstem auditory evoked potentials reflect neuronal activity in the auditory nerve pathway from the cochlea to the auditory cortex and are used in the diagnosis of neurological diseases leading to hearing loss. In the proposed automated system, the features of BAEPs are extracted in time domain based on latency, and in frequency domain, using fast Fourier transform and discrete wavelet transform. The features extracted from BAEPs are used as variables to build a feed-forward multilayer perceptron, an artificial neural network model for the classification of BAEPs. The stratified 10-fold cross-validation technique is used in training and testing the model to estimate the performance of the system. The combined time and frequency domain features along with physiological parameters as input to artificial neural network resulted in a very good average accuracy of 90.74% in classifying the normal and abnormal BAEP signals.

A measure of monopole inertia in the quantum spin ice Yb2Ti2O7

An important and continuing theme of modern solid state physics is the realization of exotic excitations in materials (e.g. quasiparticles) that have no analogy (or have not yet been observed) in the actual physical vacuum of free space. Although they are not fundamental particles, such quasiparticles do constitute the most basic description of the excited states of the "vacuum" in which they reside. In this regard the magnetic textures of the excited states of spin ices, magnetic pyrochlore oxides with dominant Ising interactions, are proposed to be modeled as effective magnetic charge monopoles. Recent inelastic neutron scattering experiments have established the pyrochlore material Yb$_2$Ti$_2$O$_7$ (YbTO) as a quantum spin ice, where in addition to the Ising interactions there are substantial transverse terms that may induce quantum dynamics and - in principle - coherent monopole motion. Here we report a combined time domain terahertz spectroscopy (TDTS) and microwave cavity study of YbTO to probe its complex dynamic magnetic susceptibility. We find that the form of the susceptibility is consistent with monopole motion and a magnetic monopole conductivity can be defined and measured. Using the unique phase sensitive capabilities of these techniques, we observe a sign change in the reactive part of the magnetic response. In generic models of monopole motion this is only possible through introducing inertial effects, e.g. a mass dependent term, to the equations of motion. Analogous to conventional electric charge systems, measurement of the conductivity's spectral weight allows us to derive a value for the magnetic monopole mass, which we find to be approximately 1800 electron masses. Our results establish the magnetic monopoles of quantum spin ice as true coherently propagating quasiparticles of this system.

Evaluation of TDR for Quantifying Heat-Pulse-Method-Induced Ice Melting in Frozen Soils

The heat pulse (HP) method is the standard for measuring thermal properties of unfrozen soils, but its application in partially frozen soils is confounded by melting and refreezing of ice that changes the properties being measured, and the most widely used data analysis models do not account for these latent heat fluxes. This study combined time domain reflectometry (TDR) and HP methods to quantify HP-induced ice melting through: (i) TDR-estimated liquid water content (θl) before and after the HP; and (ii) heat capacity (Cv) calculated from TDR-estimated θl and ice content (θi) and HP-estimated apparent heat capacity (Ca). Different heat pulse durations (8–60 s) and strengths (280–2100 J m−1) during soil thawing and freezing (−35 to 25 to −35°C) were investigated on loamy sand soil samples with a water content of 0.25 m3 m−3. The results showed that the first approach was restricted by the TDR measurement uncertainty and the difficulty in synchronizing TDR measurements with the peak ice melting. Comparison of Cv and Ca showed: (i) ice melting is limited below −5°C and the HP-estimated Ca may approach the real values; (ii) application of the HP method between −5 and 0°C is largely affected by ice melting, the amount of melting ice is largest between −1.5 and −0.5°C during soil thawing but small during freezing; (iii) the maximum estimated ice melting was 0.005 to 0.02 m3 m−3 but caused significant problems in estimating heat capacity; (iv) Cv and Ca are hysteretic because of different θi at the same temperature during freezing and thawing; and (v) a good relationship between Cv and Ca and energy input was found, which may guide the optimized heat application for frozen soil and extend the possibilities for estimating frozen soil heat capacity.

Erratum: Light-matter interaction and lasing in semiconductor nanowires: A combined finite-difference time-domain and semiconductor Bloch equation approach [Phys. Rev. B91, 045203 (2015)

Erratum: Light-matter interaction and lasing in semiconductor nanowires: A combined finite-difference time-domain and semiconductor Bloch equation approach [Phys. Rev. B<b>91</b>, 045203 (2015)

Hybridization of time synchronous averaging, singular value decomposition, and adaptive neuro fuzzy inference system for multi-fault bearing diagnosis

Bearing diagnosis has attracted considerable research interest; thus, researchers have developed several signal processing techniques using vibration analysis to monitor the rotating machinery’s conditions. In practical engineering, features extraction with most relevant information from experimental vibration signals under variable operation conditions is still regarded as the most critical concern. Therefore, actual works focus on combining Time Domain Features (TDFs) with decomposition techniques to obtain accurate results for defect detection, identification, and classification. In this paper, a new hybrid method is proposed, which is based on Time Synchronous Averaging (TSA), TDFs, and Singular Value Decomposition (SVD) for the feature extraction, then the Adaptive Neuro-Fuzzy Inference System (ANFIS) which gathers the advantages of both neural networks and fuzzy logic is applied for the classification process. First, TSA is used to reduce noises in the vibration signal by extracting the periodic waveforms from the disturbed data; thereafter, TDFs are applied on each synchronous signal to construct a feature matrix; afterwards, SVD is performed on the obtained matrices to remove the instability of statistical values and select the most stable vectors. Finally, ANFIS is implemented to provide a powerful automatic tool for features classification.

Accurate Numerical Analysis of Nonlinearities Caused by Multipactor in Microwave Devices

Due to our conformal hybrid method combining Time Domain Finite Integration Theorem with Particle-in-Cell (Conformal TDFIT-PIC), the improvement of precision in multipactor analysis is first validated in this letter. Based on such a highly accurate method, two important nonlinear phenomena caused by multipactor are discussed in time domain and frequency domain respectively. The major novelty of our work is that multipactor is found as an important source of passive intermodulation (PIM) through self-developed conformal TDFIT-PIC code. Moreover, this work is valuable to research the source and the reduction approach of PIM. Numerical examples are given to verify the effectiveness and accuracy of our analysis.

Multiple interfacing between classical ray-tracing and wave-optical simulation approaches: a study on applicability and accuracy.

In this study the applicability of an interface procedure for combined ray-tracing and finite difference time domain (FDTD) simulations of optical systems which contain two diffractive gratings is discussed. The simulation of suchlike systems requires multiple FDTD↔RT steps. In order to minimize the error due to the loss of the phase information in an FDTD→RT step, we derive an equation for a maximal coherence correlation function (MCCF) which describes the maximum degree of impact of phase effects between these two different diffraction gratings and which depends on: the spatial distance between the gratings, the degree of spatial coherence of the light source and the diffraction angle of the first grating for the wavelength of light used. This MCCF builds an envelope of the oscillations caused by the distance dependent coupling effects between the two diffractive optical elements. Furthermore, by comparing the far field projections of pure FDTD simulations with the results of an RT→FDTD→RT→FDTD→RT interface procedure simulation we show that this function strongly correlates with the error caused by the interface procedure.

Analysing the intermittent flapping flight of a Manx Shearwater, Puffinus puffinus, and its sporadic use of a wave-meandering wing-sailing flight strategy

Flights of Manx Shearwaters over the Irish Sea were investigated using GPS (n=6) and simultaneous high sample rate triaxial accelerometry (n=1). This pelagic species executes flight through intermittent bursts of flapping flight interspersed with gliding phases while meandering low over the waves. To facilitate the analysis and interpretation of body-mounted accelerometry in these challenging circumstances we introduce a combined time and frequency domain technique allowing accurate separation of flapping from gliding, measurement of wing-beat frequency and determination of flapping duty cycle. Considerable fluctuations in cycle period and time-averaged flapping duty cycle were found. Our approach offered high temporal precision, which was crucial as half the flapping bursts were briefer than 0.8s and half the cycle times shorter than 2.55s. Flapping duty cycles exceeding 38% were likeliest for short range flights and ascending flights. At higher duty cycles, cycle time decreased and wing-beat frequency and amplitude was only moderately elevated. Near-continuous flapping was only observed during steep ascents and strong headwinds. During a long-range foraging flight with good GPS coverage duty cycles between 7% and 63% were observed. We posit that flapping was modulated in order to maintain a steady airspeed in somewhat variable wind and wave conditions as part of a complex wave-meandering wing-sailing flight strategy that was often effective in reducing locomotion costs. Periods of very low duty cycle flight appear to have benefited from instantaneous crosswinds exceeding 10ms−1 with an estimated three-fold reduction in biomechanical power. Accelerometry offers a very practical tool for studying flight performance and the methods herein described can be readily adapted to other species that intermittently beat their wings.

High-resolution long-reach distributed Brillouin sensing based on combined time-domain and correlation-domain analysis

A new scheme for distributed Brillouin sensing of strain and temperature in optical fibers is proposed, analyzed and demonstrated experimentally. The technique combines between time-domain and correlation-domain analysis. Both Brillouin pump and signal waves are repeatedly co-modulated by a relatively short, high-rate phase sequence, which introduces Brillouin interactions in a large number of discrete correlation peaks. In addition, the pump wave is also modulated by a single amplitude pulse, which leads to a temporal separation between the generation of different peaks. The Brillouin amplification of the signal wave at individual peak locations is resolved in the time domain. The technique provides the high spatial resolution and long range of unambiguous measurement offered by correlation-domain Brillouin analysis, together with reduced acquisition time through the simultaneous interrogation of a large number of resolution points. In addition, perfect Golomb codes are used in the phase modulation of the two waves instead of random sequences, in order to reduce noise due to residual, off-peak Brillouin interactions. The principle of the method is supported by extensive numerical simulations. Using the proposed scheme, the Brillouin gain spectrum is mapped experimentally along a 400 m-long fiber under test with a spatial resolution of 2 cm, or 20,000 resolution points, with only 127 scans per choice of frequency offset between pump and signal. Compared with corresponding phase-coded, Brillouin correlation domain analysis schemes with equal range and resolution, the acquisition time is reduced by a factor of over 150. A 5 cm-long hot spot, located towards the output end of the pump wave, is properly identified in the measurements. The method represents a significant advance towards practical high-resolution and long range Brillouin sensing systems.

An Observer-Based Adaptive Iterative Learning Control Using Filtered-FNN Design for Robotic Systems

An observer-based adaptive iterative learning control using a filtered fuzzy neural network is proposed for repetitive tracking control of robotic systems. A state tracking error observer is introduced to design the iterative learning controller using only the measurement of joint position. We first derive an observation error model based on the state tracking error observer. Then, by introducing some auxiliary signals, the iterative learning controller is proposed based on the use of an averaging filter. The main control force consists of a filtered fuzzy neural network used to approximate for unknown system nonlinearity, a robust learning term used to compensate for uncertainty, and a stabilization term used to guarantee the boundedness of internal signals. The adaptive laws combining time domain and iteration domain adaptation are presented to ensure the convergence of learning error. We show that all the adjustable parameters as well as internal signals remain bounded for all iterations. The norm of output tracking error will asymptotically converge to a tunable residual set as iteration goes to infinity.

Crustal and upper mantle S-wave velocity structures across the Taiwan Strait from ambient seismic noise and teleseismic Rayleigh wave analyses

Although orogeny tapers off in western Taiwan large and small earthquakes do occur in the Taiwan Strait, a region largely untouched in previous studies owing mostly to logistical reasons. But the overall crustal structure of this region is of particular interest as it may provide a hint of the proto-Taiwan before the orogeny.By combining time domain empirical Green’s function (TDEGF) from ambient seismic noise using station-pairs and traditional surface wave two-station method (TS) we are able to construct Rayleigh wave phase velocity dispersion curves between 5 and 120s. Using Broadband Array in Taiwan for Seismology (BATS) stations in Taiwan and in and across the Strait we are able to derive average 1-D Vs structures in different parts of this region. The results show significant shear velocity differences in the upper 15km crust as expected. In general, the highest Vs in the upper crust observed in the coastal area of Mainland China and the lowest Vs appears along the southwest offshore of the Taiwan Island; they differ by about 0.6–1.1km/s. For different parts of the Strait, the upper crust Vs structures are lower in the middle by about 0.1–0.2km/s relative to those in the northern and southern parts. The upper mantle Vs structure (Moho – 150km) beneath the Taiwan Strait is about 0.1–0.3km/s lower than the AK135 model. The overall crustal thickness is approximately 30km, much thinner and less variable than under the Taiwan Island. The inversion of seismic velocity structures using shorter period band dispersion data in the sea areas with water depth deeper than 1000m should take water layer into consideration except for the continental shelves.