Time-domain Model Research Articles

Implications of spring-like air compressibility effects in floating coaxial-duct OWCs: Experimental and numerical investigation

The paper analyses the spring-like air compressibility effect in coaxial-duct oscillating-water-column (CD-OWC) wave energy converters (WECs). This is accomplished through a novel non-linear time-domain model for OWC WECs implemented in the object-oriented language Modelica. Good agreement was observed between numerical and physical model testing results. The air chamber volume significantly affected the spring-like air compressibility effect and the converter performance. Another significant factor is the damping level of the power take-off system. Both fixed and floating CD-OWC versions were numerically investigated, focusing on the compressibility effect. Differences were found mainly due to the additional degrees of freedom in floating configurations. In general, numerical and experimental results showed that air compressibility might positively or negatively affect device power performance in regular waves, depending on whether the frequency is within or out of an interval defined by critical frequencies. Some particular points were observed and categorised as Equicompressum Nullum and Equicompressum critical points, representing different characteristics. Knowledge of these critical values might be important in OWC control. Finally, some applications of the CD-OWC concept are discussed, including considerations on the power output level provided by biradial and Wells air turbines.

Insight into the Molecular Mechanism for Enhanced Longevity of Supramolecular Vesicular Photocatalysts.

Supramolecular self-assembly is a promising strategy for stabilizing the photo-sensitive components in photocatalysis. However, the underlying correlation between the enhanced photostability and supramolecular structure at the molecular level has not yet been fully understood. Herein, we develop a biomimetic vesicular membrane-based polyporphyrin photocatalyst exhibiting excellent photocatalytic stability with at least activity time of 240 h in hydrogen generation. Time-domain ab Initio modelling together with transient absorption spectroscopy, visual frontier orbitals and Gibbs free energy calculation disclose that the ordered aggregation of porphyrin units in the vesicle membrane facilitates "hot" electron relaxation, a longer photo-generated carrier lifetime and the rapid dissipation of photo-generated charges, which contribute to the longevity. This work deepens the molecular-level understanding on photostability and photocatalytic mechanism of supramolecular photocatalysts.

A Time-Domain Analysis Model for Modular-Multilevel-Based Grid-Forming Converters in Isolated Power Supply

The modular multilevel based grid-forming converter (MMGC) is considered to be a viable solution in isolated island power supply. However, the existing analysis model cannot meet the requirement of its performance evaluation and circuit parameters design. This paper proposes a high-accurate time-domain steady-state analysis model for MMGC, where the coupled relationships among the load, the MMGC, and the dc-side system can be fully reflected; the amplitudes and phase angles of electrical quantities, including their harmonic components, can be directly calculated. Then, the influence of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> filters on submodule capacitor are studied. The study found that the submodule capacitance is highly correlated to the filter capacitance and almost uncorrelated to the filter inductance. It is also found that the filter capacitance selection should consider the load characteristic; a larger filter capacitance is required in the inductive load supply scenario. Finally, the studies are verified by both simulation and experiment.

A Minimum ZVS Current Control Strategy of Semi Bridgeless Active Rectifier for Wide Operation Range Based on LCC-S Compensated WPT System

To meet the requirement of the battery charging characteristics and electric vehicle interoperability, wide power range and wide voltage range are important for the wireless electric vehicle (EV) charging system. Hence, ensuring zero voltage switching (ZVS) and high efficiency over the wide operating range is a hot issue but not well resolved. To address the current state of research, an efficiency-oriented control strategy is proposed for LCC-S compensated Wireless Power Transfer (WPT) system to achieve minimum ZVS current over a wide voltage and power regulation range. Based on the accurate harmonic-considered time-domain model, a universal design method for system parameters under the worst ZVS current conditions within wide operation range is proposed. Then, the universal explicit expression at the unit voltage gain point is given. Moreover, based on the simplified fitting method of piecewise bi-quadratic interpolation (PBQI), minimum ZVS current and minimum non-active current can be ensured over the full output voltage and output power range. A 3.3 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> W prototype has been built to verify that the converter can realize ZVS within the whole voltage range, the measured peak efficiency is about 95.2%.

Deep Independent Recurrent Neural Network Technique for Modeling Transient Behavior of Nonlinear Circuits

This paper introduces a novel macromodeling method based on a recurrent neural network called deep independently recurrent neural network (DIRNN). The proposed method applies to time-domain modeling of nonlinear circuits and components, resulting in better training. It overcomes the vanishing and exploding gradient problems encountered with conventional RNNs. In conventional RNNs, all neurons in each layer are involved in recurrent connections that cause unnecessary connections, increasing the model’s complexity over time and making it hard to train for long-time sequences. To solve this problem, the proposed DIRNN’s neurons are independent of each other in recurrent connections, because each neuron only receives connections from its own previous hidden state. The validity of the proposed method is verified by modeling two nonlinear circuit examples, namely, a multi-stage driver terminated by a multi-line interconnect, and an on-chip voltage generator.

An improved pedestrian dead reckoning algorithm based on smartphone built-in MEMS sensors

With the advantages of low cost, small size and lightweight, the smartphone has played a more and more important role in pedestrian indoor positioning. In particular, the application of Pedestrian Dead Reckoning (PDR) in the smartphone built-in MEMS sensors makes the application of smartphone more extensive. However, the zero-bias instability of the smartphone built-in MEMS sensors leads to the rapid accumulation of pedestrian trajectory calculation error. To solve this problem, we used the bias drift model and Kalman filter (KF) to denoise the original MEMS data. The dual-feature step detection model of peak domain and time domain was established to provide accurate step information for step length estimation and heading correction. Based on the Weinberg model, the three-steps constraint step length estimation (TCSLE) model was proposed to estimate step length accurately. Then, based on the improved heuristic drift elimination (iHDE), the adaptive drift elimination (ADE) model was proposed to identify different walking states. The correction models under different walking states were established to correct the heading angle accurately. Finally, the pedestrian trajectory was reconstructed using accurate step length and heading information. To verify the performance of the PDR algorithm based on the above model, three experimenters with different heights and genders were recruited, and three mobile phones with different sensor performance were selected. The experimenters moved smoothly and steadily with hand-held mobile phone, and 18 sets of experiments were carried out along two paths. The experiment results shown that the step length deviation was less than 1.4871 %, the horizontal positioning error was less than 1.6070 m, and the relative positioning error was less than 1.1816 %D. The proposed PDR algorithm has strong adaptability and robustness, and meets the needs of pedestrian indoor positioning.

DIGITAL DOUBLES IN MANAGEMENT

The work considers some digital models of real devices that have interface means by which the simulated object interacts with the external environment. Such models are called «digital twins». Two types of models used in creation of digital twins are considered. These are time series models and domain models. Domain models use the laws of a given domain. They use mathematical, logical and probabilistic properties inherent in a particular domain. Time series models are universal for various domains and do not depend on the physical nature of variables of a specific series. Such models can be used to predict the behavior of a simulated object. As examples, a delay prediction model in a video traffic control system and a model for recharging batteries of wireless sensor networks are considered.

Global maximum flow based time-domain simulation method for evaluating power system resilience

With climate change, extreme weather is occurring frequently, which can cause serious damage to energy supply systems. It is of great significance to study the performance of power transmission system under extreme weather for prevention and reinforcement before and after disasters. This paper proposes a time-domain simulation model for quantifying power system resilience under extreme typhoon scenario. Firstly, the power system resilience assessment method based on global maximum flow (GMF) is presented, which is applied to map the overall system operation performance via creating a network indicator while considering component fragility, system topology, and power flow. A specific resilience expression (RES) is proposed to describe percentage improvement of each component in the resilience metrics by considering the overall system performance. Secondly, power system vulnerability model and time-domain simulation methodology are established. The proposed method is demonstrated by four case studies using the IEEE-30 node test system under specific typhoon scenarios. From the GMF based RES index, the component repair strategy is formulated accordingly. The proposed restoration strategy, compared with the traditional expert strategy can effectively measure the resilience contribution of each component to the whole power system, and minimizes the power system performance losses due to extreme typhoon. The proposed approach quantitatively evaluates real-time system resilience to extreme typhoons, and it can also identify the system critical areas and assess the benefits of different resilience enhancement strategies. The resilience assessment framework can also be applied to other extreme scenarios and contribute to planning and construction of strong smart grid in the future.

Discrete time domain modeling and design of current mode controlled flyback LED driver

In this paper, modeling for a current mode controlled (CMC) flyback LED driver with a stabilizing ramp is performed in a step-by-step procedure. Discrete time state equations for the system are derived and linearized with respect to a steady-state operating point. At this operating point, the switching control law, the condition that determines the duty ratio, is also linearized. In the next step, a closed-loop system model is derived by combining the two models of the flyback driver and the switching control law. The root locus analysis in the z-plane is used to investigate the characteristics of the combined linearized system and obtain design guidelines for feedback loops. The feasibility of the proposed design is confirmed through the experimental results for the CMC flyback LED driver.

Laser–tissue interaction simulation considering skin-specific data to predict photothermal damage lesions during laser irradiation

Abstract This study aimed to develop a simulation model that accounts for skin-specific properties in order to predict photothermal damage during skin laser treatment. To construct a computational model, surface geometry information was obtained from an optical coherence tomography image, and the absorption coefficient of the skin was determined through spectrophotometry. The distribution of the internal light dose inside the skin medium was calculated using the light propagation model based on the Monte Carlo method. The photothermal response due to the absorption of laser light was modeled by a finite difference time domain model to solve the bio-heat transfer equation. The predicted depth and area of the damaged lesions from the simulation model were compared to those measured in ex vivo porcine skin. The present simulation model gave acceptable predictions with differences of approximately ∼10% in both depth and area.

A Coupled Hydrodynamic–Structural Model for Flexible Interconnected Multiple Floating Bodies

Evaluating the structural safety and seakeeping performance of very large floating structures (VLFS) using the rigid module flexible connector (RMFC) method remains challenging due to the complexity of the coupled hydrodynamic–structural responses in this system. In this study, a coupled hydrodynamic–structural frequency–time domain model is developed based on the RMFC method employing the planar Euler–Bernoulli beam elements to investigate the dynamic responses of multi-module floating systems. To reveal the dynamic characteristics of the systems, the coupled hydrodynamic–structural responses are investigated using a frequency–time-domain numerical model with viscous correction, in which the mass and stiffness attributes of connectors are incorporated into the system. Given the effects of hydrodynamic interaction, consideration is given to the case of three modular boxes connected by flexible beams aligned in series in shallow water to validate the present model. Higher efficiency and accuracy can be found in the system using viscous correction in potential flow theory and introducing state–space model to replace the convolution terms in the Cummins equation for the time domain. Moreover, this model can be extended to a considerable number of floating modules, which provides possibilities to analyze N-module floating systems.

Time-domain simulation of sum-frequency hydrodynamic performance of a TLP-type floating wind turbine in multidirectional seas

Previous studies (Ren et al., 2022) by the authors have confirmed that the second-order diffracted wave loads are very sensitive to the wave directional spreading of the so-called storm-swell mixed seas where the storm-driven local seas combined with long-period swells propagating from different angles. In this paper, particular attention is paid to estimate the sum-frequency hydrodynamic forces and motion responses of a TLP-type floating wind turbine (FWT) in such complex sea states. A time-domain potential flow model is developed for the second-order interaction between the multidirectional irregular waves and moored structures. The boundary value problems for the scattered potential are solved by a higher-order boundary element method (HOBEM). The fourth-order Adams–Bashforth–Moulton method is used to update the wave surface and the Newmark method combined with the Newton–Raphson iteration scheme is adopted to solve the body motion equations.For validation, a freely floating hemisphere in unidirectional bichromatic waves is investigated and good agreement is found by comparing with Kim and Yue's frequency-domain results. Numerical tests are conducted on the TLP-type FWT with different combinations of incident angles and spreading factors of the storm-swell mixed seas. The effects of the wave headings and directional spreading on the sum-frequency hydrodynamic response are discussed. It is noted that when the swells approach the storm seas from large relative angles, the sum-frequency vertical response of the FWT can be up to seven times greater than that in the collinear mixed seas. In addition, the present model can extend the rotor-floater-tether coupled dynamic simulator of FWT to more realistic seas such as storm-swell mixed seas.

Tunable resonance Raman scattering of quantum dots in a nonlinear excitation regime.

Controllable tuning of electron-phonon coupling strength and excited state dynamics is important for the understanding of resonance Raman scattering in low-dimensional semiconductors. Here, we report a significant and reversible field-induced modulation in absolute resonance Raman intensity of quantum dots using ionic liquid gating. Meanwhile, a potential-dependent nonlinear relationship is present between Raman intensity and excitation power density. By exploring the parameter space within a time domain model, we find that the Raman intensity variation is mainly determined by the homogeneous linewidth. We further propose that the Fermi level positions and exciton species play key roles in the excited state decay rates.

Global Sensitivity Analysis for Impedance Spectrum Identification of Lithium-Ion Batteries Using Time-Domain Response

Accurate modeling is of great importance for describing battery dynamics under different working conditions. To achieve high accuracy and obtain insights into battery operations, it is critical to bridge the time-domain equivalent circuit model and frequency-domain electrochemical impedance spectroscopy. However, the practical battery running data are recorded at a very low sampling frequency, it is thus challenging to identify the frequency-domain impedance spectrum using time-domain response data. To solve this problem, this article proposes a systematic battery time-domain modeling approach considering impedance spectrum identification. First, a probabilistic approach for global sensitivity analysis is proposed to evaluate the impact of different parameters on model performance. The parameter sensitivity of both integer-order and fractional-order models are evaluated using the Sobol indices obtained by Monte Carlo simulations. Then, a Bayesian optimization algorithm is proposed to identify model parameters to reduce the evaluation times of the battery model. Finally, the experimental data conducted on LiFePO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{4}$</tex-math></inline-formula> cells are employed to verify the proposed method. The results indicate that the total Sobol indices show a miscellaneous sensitivity of parameters on impedance spectra, which consolidates the understanding of impedance characteristics, and the higher order integer-order models with well-initialized low sensitive parameters are more suitable for predicting frequency-domain characteristics.

Time domain response of a piezoelectric layer

A number of one-dimensional models have been developed to inform the design of piezoelectric transducers. The majority of these models are in the frequency domain. In this paper, we develop a one-dimensional time-domain model for the mechanical response of a piezoelectric layer. Secondary effects, resulting from feedback between the acoustic and electric variables, are included in the model. Our approach utilizes Green's function for the Helmholtz equation with radiation boundary conditions and the methods of complex analysis. The model predictions are validated by comparison with a finite-difference time-domain numerical simulation of the governing acoustic equations in and outside the layer. This time-domain model enables efficient calculation of the secondary piezoelectric action effects and provides the mechanical response to an arbitrary electrical source.

Mutual Information for Electromagnetic Information Theory Based on Random Fields

Traditional channel capacity based on the discrete spatial dimensions mismatches the continuous electromagnetic fields. For the wireless communication system in a limited region, the spatial discretization may results in information loss because the continuous field can not be perfectly recovered from the sampling points. Therefore, electromagnetic information theory based on spatially continuous electromagnetic fields becomes necessary to reveal the fundamental theoretical capacity bound of communication systems. In this paper, we propose analyzing schemes for the performance limit between continuous transceivers. Specifically, we model the communication process between two continuous regions by random fields. Then, for the white noise model, we use Mercer expansion to derive the mutual information between the source and the destination. For the close-form expression, an analytic method is introduced based on autocorrelation functions with rational spectrum. Moreover, the Fredholm determinant is used for the general autocorrelation functions to provide the numerical calculation scheme. Further works extend the white noise model to colored noise and discuss the mutual information under it. Finally, we build an ideal model with infinite-length source and destination which shows a strong correpsondence with the time-domain model in classical information theory. The mutual information and the capacity are derived through the spatial spectral density.

A Variable Switching Frequency Space Vector Pulsewidth Modulation Technique Using Virtual Flux Ripple

The variable switching frequency space vector pulse width modulation technique based on current ripple prediction (CRP-VSFSVPWM) can effectively suppress electromagnetic noise, but its effectiveness depends on the accuracy of load parameters. In this paper, a virtual flux ripple prediction model is developed. Similar to the current ripple prediction model, it is also a time-domain ripple prediction model, but it does not require load parameters and has low complexity. Furthermore, a variable switching frequency space vector modulation strategy based on three-phase virtual flux ripple is designed in this paper. The proposed method is fully validated by comparison with the fixed switching frequency space vector modulation and CRP-VSFSVPWM on a three-phase inverter. Experimental results confirm that the proposed technique can significantly suppress electromagnetic noise while maintaining current harmonic performance, and its performance is not affected by load parameters.

Nonlinear dynamics of pure intrinsic thermo-acoustic modes

In this paper the nonlinear dynamics of the pure Intrinsic Thermo-Acoustic (ITA) modes are experimentally investigated for burner stabilized premixed flames by decoupling them from the acoustic modes. Thereto, a setup is used with up- and downstream mufflers that provide close to anechoic boundary conditions. This enables the investigation of the self-excited oscillations of the flame, without the influences of acoustic reflections. An experimental bifurcation analysis is conducted to systematically investigate the different asymptotic oscillatory states and their transitions. By increasing the upstream velocity, the flame looses stability through a supercritical Hopf bifurcation and reveals limit cycles with a single dominant frequency. For increasing upstream velocity, a low frequent mode appears, resulting in quasi-periodic oscillations. This regime is characterized by a beating phenomenon with simultaneous amplitude and frequency modulations. When the upstream velocity is even further increased, a period doubling bifurcation is observed. To our best knowledge, the experimental investigation of the nonlinear dynamical behavior of pure ITA modes and, in particular, the low frequency oscillation, has not been reported before, and it provides valuable insights towards developing accurate time-domain models of the nonlinear acoustic behavior of premixed flames. Finally, a phenomenological model of two coupled oscillators is introduced that is able to qualitatively reproduce all observed nonlinear oscillations of the flames.

Halevi's extension of the Euler-Drude model for plasmonic systems

The nonlocal response of plasmonic materials and nanostructures is often described within a hydrodynamic approach, which is based on the Euler-Drude equation. In this paper, we reconsider this approach within an extension proposed by Halevi [Phys. Rev. B 51, 7497 (1995)]. After discussing the impact of this extended model on the propagation of longitudinal volume modes, we reevaluate within this framework the Mie scattering coefficients for a cylinder and the corresponding plasmon-polariton resonances. Our analysis reveals a nonlocal, collisional, and size-dependent damping term, which influences the resonances in the extinction spectrum. A transfer of the Halevi model into the time domain allows to identify a contribution to the current, which shares similarities with Cattaneo-kind diffusive-wavelike dynamics. After a comparison to other approaches commonly used in the literature, we implement the Halevi model into the discontinuous-Galerkin time-domain finite-element Maxwell solver and identify an oscillatory contribution to the current. Such an implementation of the Halevi model in time domain is of particular importance for applications in nanoplasmonics where nanogap structures and other nanoscale features have to be modeled efficiently and accurately.

Feasibility of integrating a very large floating structure with multiple wave energy converters combining oscillating water columns and oscillating flaps

The installation of wave energy converters (WECs) along the facing-wave side of in-development or pre-existing offshore structures provides an effective cost-sharing solution. In this study, a time-domain numerical model based on the modal expansion theory and the nonlinear potential flow theory, is applied to optimize the size and layout of an in-line array of oscillating water column (OWCs) and oscillating flaps (OFs) deployed along the facing-wave side of a very large floating structure (VLFS). The front/back-wall end of each OWC chamber is hinged by an OF device, and thus the number of OWCs is identical with that of OFs. Comparison with a typical OWC-VLFS integration shows that the addition of OFs can significantly reduce the vertical hydroelastic response of the VLFS. Additionally, wave energy extraction from the design of OFs hinged at the back-wall end of OWCs is enhanced due to more wave energy gathered in the chamber. Under the premise of the same space, the array layout demonstrates both higher conervsion efficiency and hydroelastic reduction. The energy conversion is generally enhanced by increasing the gap, the OWC width, the OF height. The maximum efficiency increases firstly with wave height for weak wave nonlinearity and then decreases for strong wave nonliearity.