Time-domain Simulation Method Research Articles

Drifting of a dead ship in wind

ABSTRACT The drift characteristics in wind of a dead ship that cannot sail on its own are investigated experimentally and theoretically. First, a free-drift test of a bulk carrier model in wind is conducted. Next, a time-domain simulation method is developed for predicting the drift motion of the dead ship in wind. Tank tests and motion simulations show that the dead ship drifts with a large hull drift angle over in wind, and the converged drift angle is independent of wind speed and initial ship heading angle. To explain why the dead ship drifts with a large drift angle in wind, a theoretical formula is derived for investigating the motion stability of the dead ship in wind. It is theoretically confirmed that the ship drifts stably with a large drift angle over in wind, whereas drift angles of and result in unstable motion. Thus the reason for the drift behaviour of the dead ship is clarified.

Efficiency of room acoustic simulations with time-domain FEM including frequency-dependent absorbing boundary conditions: Comparison with frequency-domain FEM

Recent wave-based room acoustic simulations in the time domain can incorporate frequency-dependent absorbing boundary conditions (BCs), by which time responses including a broad frequency component are calculable with a single computational run. However, their performance over the frequency-domain method remains poorly understood. This paper presents a discussion of the capabilities of a recently developed implicit time-domain finite element method (TD-FEM) for room acoustics simulation by comparison with a fourth-order accurate frequency-domain FEM (FD-FEM). First, the implicit TD-FEM accuracy is examined via an impedance tube problem, having frequency-dependent absorbing BCs at the tube end, where a specific acoustic admittance ratio of rigidly backed porous sound absorbers is imposed. Results indicate that the implicit TD-FEM has the same solution convergence rate as that of FD-FEM. The solution converges to the FD-FEM solution when using a sufficiently small time interval. A performance comparison of both methods is then made using two real-scale 2D room acoustic problems in an office room and a complexly shaped concert hall. Numerical results show that the implicit TD-FEM can be useful for acoustic simulation in practical-sized rooms at broad frequency ranges with markedly smaller computational costs than those of FD-FEM while maintaining similar accuracy.

Polydopamine-assisted in situ growth of three-dimensional ZnO/Ag nanocomposites on PET films for SERS and catalytic properties

Surface-enhanced Raman scattering (SERS) spectroscopy as an effective method in sensitive detection applications has attracted more and more attentions recently. In this work, polydopamine was taken to achieve the modification of PET film surface for in-situ growth of three-dimensional (3D) ZnO/Ag nanocomposites. Ag nanoparticles on cluster-like ZnO nanorods surface were applied as Raman hot spots for target molecules detection. SEM, XPS, and XRD were used to characterize the microstructures and morphologies of 3D SERS substrates. pATP molecules were selected to measure the sensitivity, uniformity, and durability, the surface-enhanced enhancement mechanism of Raman scattering was analyzed by finite difference time domain (FDDT) simulation method. Malachite green (MG) was further used as the demonstrative pesticide residue, and the catalytic activity was studied by the deoxidization of 4-nitrophenol (4-NP). This work has indicated that SERS substrates exhibited the advantages of facile fabrication process, low cost, high sensitivity, good stability and excellent uniformity, which can be used for the rapid Raman enhanced spectroscopy analysis of environmental pollutants.

A time-domain simulation method to predict insertion loss of a dissipative muffler with exhaust flow

Insertion loss (IL) is a more useful acoustic index than noise reduction and transmission loss for muffler design, but it is harder to predict because the prediction of IL requires knowing the characteristics of the muffler itself and the acoustic source. A muffler with porous material and complex meanflow makes the prediction even more challenging. Although calculation of noise reduction and transmission loss with the transient computational fluid dynamics (CFD) can naturally solve the flow–acoustic interaction problem, the published CFD methods have not been adopted to evaluate the IL of the dissipative muffler with exhaust flow. In this paper, a time-domain simulation method is developed to predict the IL of a complex engine muffler with porous material. The noise reduction and load impedance of the muffler were first calculated using a three-dimensional CFD model, in which the porous material was simulated by the governing equations modified with the material porosity and flow resistivity. The IL was then predicted from the noise reduction, the load impedance, and the engine source extracted from the measured exhaust noise. This developed method explores the application of the three-dimensional CFD method in calculating noise reduction, load impedance, and insertion loss of complex dissipative mufflers with exhaust flow.

Effect of HfO2 Passivation Layer on Light Extraction Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes

Diverse applications of ultraviolet (UV) light in the field of purification, sterilization, detection, surface disinfection, sensing, and many others have been studied mostly on AlGaN light-emitting-diodes (LEDs). However, the external quantum efficiency (EQE) of AlGaN deep UV LEDs is limited due to strong induced polarization fields, high dislocation density, severe electron leakage, difficulty in p-type doping, low internal quantum efficiency (IQE), IQE droop, and low light extraction efficiency (LEE). Alternatively, AlInN semiconductor has been recently gained tremendous attention for high power UV LEDs [1-3]. Our group has successfully demonstrated the first AlInN core-shell nanowire UV LEDs with a relatively high IQE of ~52%[1]. However, the light extraction study of such nanowire LEDs is unexplored. In this context, we have studied the LEE of a single AlInN nanowire UV LED at 220 nm - 400 nm wavelength regime. Moreover, we report that the LEE of a single AlInN nanowire UV LED could be increased by ~60% using HfO2 passivation compared to conventional nanowire LED at 282 nm.The schematic structure of AlInN nanowire UV LED is shown in Figure 1(a). The nanowire is composed of 200 nm n-GaN, 100 nm n-AlxIn1-xN quantum barrier (QB)/40 nm i-AlyIn1-yN quantum well (QW)/100 nm p-AlxIn1-xN QB, and 20 nm p-GaN layer. We have estimated the LEE of nanowire LEDs using the finite difference time domain (FDTD) simulation method. Silicon (Si) is used as a substrate, and the n-GaN layer is tapered with a diameter from 70 nm (bottom) to 100 nm (top). The diameter is set to be 100 nm for the rest of the regions. We have placed a single TM-polarized dipole source at the QW center to calculate LEE for a nanowire LED in the simulations. The estimated LEE of 282 nm wavelength single nanowire AlInN UV LED with various HfO2 passivation layers thickness is provided in Figure 1(b). The LEE of such LED without any passivation is ~25.2%, whereas the maximum recorded LEE is ~40.6% with the utilization of 35nm thickness HfO2 passivation layer. This study provides a promising approach for enhancing the light extraction efficiency of nanowire UV LEDs. Reference R. T. Velpula, B. Jain, M. R. Philip, et al., Sci. Rep., 10(1) 2547-2556 (2020).R. T. Velpula, B. Jain, H.Q.T. Bui, et al., Opt. Mater. Express, 10(2), 472-483 (2020).B. Jain, R. T. Velpula, M. Tumuna, et al., Opt. Express, 28(15), 22908-22918 (2020). Figure 1

Electrically Tunable Terahertz Focusing Modulator Enabled by Liquid Crystal Integrated Dielectric Metasurface

Active lenses with focal tunable properties are highly desired in the modern imaging systems from the visible to the microwaves. In this paper, we demonstrate a terahertz (THz) lens with electrically switchable focal length. It is composed of a large-birefringence liquid crystal (LC) layer infiltrating a dielectric metasurface. When the birefringence of LC is tuned with an external bias, the phase shift of a single meta-unit will change. With parameter sweep using the finite-different time-domain (FDTD) simulation method, meta-units with varying geometries are optimized to achieve a focal length switchable metalens. The numerical results show that the focal length can be switched between 8.3 mm and 10.5 mm at bias OFF and ON states, respectively, which is consistent with the design. A feasible fabrication procedure of the lens is further discussed. Such a device can be designed beyond the THz band to the visible or the microwaves, and may be widely applied in integrated imaging systems.

Nonpolarizing narrow band metamaterial transmission filter based on electromagnetically induced transparency at visible wavelengths

A nonpolarizing narrow band metamaterial transmission filter based on EIT operating at visible wavelengths is demonstrated theoretically and experimentally. The two separated resonance components that are able to generate EIT effect are connected using a SiO2 film layer and the hardness of the metamaterial filter is further improved by adding a glass substrate. The theoretical results are calculated by using the finite-difference time-domain simulation method and a metamaterial filter sample is fabricated and measured to verify the simulation spectrum. The full width at half-maximums of simulation and measured spectra are 0.74 nm and 1.61 nm respectively. Electric field distributions are also calculated to illustrate the physical mechanism. The novel method might be helpful to improve the processability of the narrow band metamaterial transmission filter, and also to further promote the applications of this kind of metamaterial filter.

Numerical Analysis of Hydrodynamic Loads on Passing and Moored Ships in Shallow Water

In this study, hydrodynamic interactions between passing and moored ships were studied by applying a time-domain numerical simulation method. The boundary value problem for a fluid domain was formulated based on a potential flow theory. A numerical method was developed based on a finite element method with an efficient re-mesh algorithm. Regarding the free-surface boundary conditions, both double-body and free-surface models were considered for examining the free-surface effect on the hydrodynamic forces due to the passing ship. First, numerical results were validated by comparison with the model test results of Kriebel et al. (2005), where generic Series 60 hulls were considered as the target model for the passing and moored ships. In addition, hydrodynamic pressure fields and force time-series were investigated to understand the passing ship problem. Second, a series of numerical simulations were performed to study the effects of the passing ship speed, separation distance, water depth, and relative vessel size, which were used to compare the peak values of hydrodynamic forces. The applicability and limitations of the double-body and free-surface models are discussed for predicting passing ship loads.

Accurate time-domain simulation of power electronic circuits✰

Modern power electronic circuits contain numerous switches driven by high frequency controllers and can cause discontinuities in time-domain simulation methods. This paper presents numerical problems resulting from discontinuities when simulating power electronic circuits. Limitations in existing methods are analyzed. Three new methods are proposed to improve accuracy and computational performance.

OPF Calculation with Small-Signal Trajectory Constraints

In order to avoid the complexity of obtaining the sensitivity in the usual optimal flow with small signal stability, which is easily result from the mere consideration of active power output optimization or the change of element model, a new model is proposed to improve the small-signal stability of the system by constraining the perturbation trajectory curve. Based on the time-domain simulation method, a power flow optimization model with small-signal trajectory constraint is established by extracting the damping ratio of the perturbation trajectory and forming constraints. The optimization calculation results of Western Systems Coordinating Council 3-generator 9-node system and China Electric Power Research Institute 6-generator 22-node system prove that the corresponding minimum damping ratio of the system increases with the increase of the constraint value of approximate damping ratio, which demonstrates the effectiveness of the model.

Time domain simulations of piston-like resonant flow in the gap of an oscillating twin-hull using inviscid and viscous flow solvers

A numerical study of a semi-circular twin-hull section under heave oscillation is presented. Two different time domain simulation methods were used: Firstly, a boundary element method based on potential theory which incorporates fully nonlinear free surface boundary conditions by using a mixed Eulerian Lagrangian scheme. Secondly, a finite volume method in combination with a volume of fluid scheme for capturing the free surface. In order to evaluate the effects of viscosity, simulations with the finite volume method were carried out using inviscid as well as viscous flow assumptions. Hydrodynamic mass and damping coefficients were derived from first order Fourier coefficients and validated against results from linear theory and experiments. A detailed comparison of nonlinear results from both simulation methods was carried out in vicinity of the piston-mode resonance frequency and a very good agreement was found. The phase angles corresponding to the Fourier coefficients varied strongly with frequency, which went along with a phase shift of the fluid motion in the gap. Influence of viscosity on the flow was found to be present but had relatively low impact on the hydrodynamic forces.

Investigating cross-flow vortex-induced vibration of top tension risers with different aspect ratios

The vortex-induced vibration (VIV) of marine risers with large aspect ratio (length-to-diameter ratio) is a very complicated fluid-structure interaction phenomenon, which is the main cause of fatigue failure of marine risers. In order for the safe and stable operation of the riser systems, it is crucial to investigate the effect of the aspect ratio on the VIV response of the risers. In this study, the cross-flow VIV characteristics of a top tension riser with different aspect ratios are analyzed using a time domain simulation method. First of all, a riser model considering the fluid-structure interaction was established to calculate the natural vibration characteristics and compare with the theoretical result. Then, the cross-flow VIV response of the riser model in uniform and shear flows were respectively calculated and compared with the results of an experimental model. The numerical results were in good agreement with the experimental results. Subsequently, the developed riser model can be used to predict the cross-flow VIV response with different aspect ratios. Finally, the effects of the model scale and aspect ratio on VIV response were analyzed. The analysis results may provide a basis for selecting appropriate model scale and aspect ratio in establishing the riser simulation model.

Parameter effects on high-speed UAV ground directional stability using bifurcation analysis

A loss of ground directional stability can trigger a high-speed Unmanned Aerial Vehicle (UAV) to veer off the runway. In order to investigate the combined effects of the key structural and operational parameters on the UAV ground directional stability from a global perspective, a fully parameterized mathematical high-speed UAV ground nonlinear dynamic model is developed considering several nonlinear factors. The bifurcation analysis procedure of a UAV ground steering system is introduced, following which the simulation efficiency is greatly improved comparing with the time-domain simulation method. Then the numerical continuation method is employed to investigate the influence of the nose wheel steering angle and the global stability region is obtained. The bifurcation parameter plane is divided into several parts with different stability properties by the saddle nodes and the Hopf bifurcation points. We find that the UAV motion states will never cross the bifurcation curve in the nonlinear system. Also, the dual-parameter bifurcation analyses are presented to give a complete description of the possible steering performance. It is also found that BT bifurcation appears when the UAV initial rectilinear velocity and the tire frictional coefficient vary. In addition, results indicate that the influence of tire frictional coefficient has an opposite trend to the influence of initial rectilinear velocity. Overall, using bifurcation analysis method to identify the parameter regions of a UAV nonlinear ground dynamic system helps to improve the development efficiency and quality during UAV designing phase.

Reinforcement Learning Algorithm and FDTD-Based Simulation Applied to Schroeder Diffuser Design Optimization

This paper aims to propose a novel approach to the algorithmic design of Schroeder acoustic diffusers by employing a deep learning optimization algorithm and a fitness function, which are based on a computer simulation of the propagation of acoustic waves. The deep learning method employed for the research consists of a deep policy gradient algorithm. It is used as a tool for carrying out a sequential optimization process, which seeks to maximize the fitness function based on parameters characterizing the autocorrelation diffusion coefficient of the designed acoustic diffuser. As the autocorrelation acoustic diffusion coefficients are calculated based on the polar response of a diffuser, the finite-difference time-domain (FDTD) simulation method is used to obtain a set of impulse responses, which are necessary to calculate the polar responses of the optimized Schroeder diffusers. The results obtained from the optimization derived from the deep learning algorithm were compared with the outcomes of a similar algorithm by employing a genetic algorithm and based on random selection of the Schroeder diffuser well-depth pattern. We found that the best result was achieved by the deep policy gradient, as it produced outcomes that, in terms of the provided autocorrelation diffusion coefficient, were statistically better than the properties of the designs supplied by two other baseline approaches.

A Direct Bounded Control Method for Transient Stability Assessment

Abstract The paper proposes a direct method for transient stability assessment, which is more efficient than traditional time-domain simulation methods. To achieve this objective, a new bounded control law for the model of a power system is designed. This yields superior closed-loop transient performances when compared to those achievable with traditional automatic voltage regulators and power system stabilizers. The designed control law allows defining an energy-based Lyapunov function which is instrumental in assessing transient stability properties of the post-fault system. A case study on a single machine infinite bus power system model is presented to illustrate the merits of the proposed method.

Analysis on subsynchronous oscillation stability for large scale offshore wind integration

As the offshore wind installations are moving further from shore, the transmission distance also increases. Synthesize the cost of construction and maintenance, as well as the technical maturity, grid-connection via AC network will still be a preferred power delivery method for offshore wind power projects in the near future. However, according to the “8•9” blackout in the UK, there is sub-synchronous oscillation (SSO) risk in the AC network connected offshore wind projects, which was attributed to the mismatch between the power electronic source and the power grid. In this article, both eigenvalue analysis and time-domain simulation method are used to evaluate the operating SSO stability of grid integration of large scale offshore wind power. It suggests that transmission system via long distance cables naturally forms a weak AC network, and the SSO characteristics of AC grid connected offshore wind farm is closely related to the wind turbine parameters, offshore distance and the active power output level. At last, the principle of parameter configuration in offshore wind turbines is proposed based on different offshore distances.

Perbaikan CCT Pada Multi Machine Infinite Bus Dengan Supercapacitor Energy Storage Menggunakan Critical Trajectory

Transient stability is an important aspect in maintaining the continuity and reliability of the electrical power system when a sudden large disturbance occurs. However, there is a time operation's limitation of the protection system to eliminate disturbance before the system becomes unstable and loses its synchronization. Critical Clearing Time (CCT) is the toleration time to eliminate the fault to keep the system stable. To improve system stability, installing Supercapacitor Energy Storage (SCES) can be one of the methods for extending the CCT values. SCES absorb large amounts of electricity simultaneously when a fault occurs. This paper proposed a determination of SCES' controller effect in extending the CCT. SCES becomes a dummy load and extends the CCT value depending on the controller's ability to respond to a fault. Controller modeling is applied to SCES, which is installed at the bus generator with determined capacity. The modified Fouad and Anderson 9-bus 3-machine system with a single machine to an infinite bus is used to validate the proposed method. Moreover, the critical trajectory method, which is known as a faster calculation and better accuracy than the time domain simulation method, is used to obtain the CCT value. The result shows that the faster controllers work against fault, the higher the CCT value improvement of system CCT. The highest improvement occurs when the controller works at 0.001s.

Efek Penambahan SCES Pada Sistem Multimesin dengan Damping dan Kontroler Berdasarkan Metode Critical Trajectory

The system can maintain its synchronization under a transient condition. The transient stability system can be evaluated using the critical trajectory method by calculating the Critical Clearing Time (CCT). The advantage of using a critical trajectory method is that the CCT can be found directly and more accurate than the time domain simulation method. This paper proposed the addition of Supercapacitor Energy Storage (SCES) and damping to enhance the transient stability on multi machine system. SCES is an electrical energy storage device that quickly stores and supplies large amounts of electricity, while damper winding damp the oscillations during unstable steady state conditions. Within the addition of SCES and damping in the system, the stability will last longer than before. The stability system is seen by the extended CCT. Furthermore, multi-machine to infinite bus used to validate the proposed method. The test system also includes the Automatic Voltage Regulator (AVR) and the governor

Polarimetric Phased Array Weather Radar Data Quality Evaluation Through Combined Analysis, Simulation, and Measurements

This letter combines a time-domain modeling and simulation method for evaluating the impacts of system modules on polarimetric data quality of phased array weather radars with both theoretical analysis and actual measurements. In the presented phased array radar system simulator (PASIM), the distributed weather returns are modeled by randomly distributed scatterers, and Next-Generation Radar (NEXRAD) Level-II data or user-defined weather scenarios are utilized as weather truth fields. Based on a specially designed patch element, a dual-polarization phased array mobile demonstration system (Ten Panel Demonstrator or TPD) is simulated. In addition, the biases of differential reflectivity, copolar correlation coefficient, and differential phase along beam direction away from the broadside in principal plane and nonprincipal plane are used as data quality metrics. Moreover, theoretical analysis, system simulation, and actual TPD proof of concept measurements in a stratiform precipitation and a convective precipitation are presented, respectively, then similarities and discrepancies between simulations and measurements are compared and explained.

Feasibility study on new energy multi-feed direct current grid connection

To adapt to the characteristics of new energy long-distance transmission, this paper proposes a scheme of multi-fed voltage source converter grid connection and line-commutated converter transmission for a regional large energy base. First, the feasibility of this multi-infeed direct current (DC) transmission scheme is discussed. Then, the performance characteristics of the multi-infeed DC and other traditional transmission technologies are compared and analysed. In addition, the economic benefits of the multi-infeed DC project are evaluated. To study the characteristics of new energy multi-infeed DC power transmission mode, a long-distance energy transmission system based on multi-infeed DC power transmission mode is built using DIgSilent software. On the basis of the continuous power flow method, the static characteristics of the system are studied. Based on the time-domain simulation method, the transient characteristics of the system are studied. The results show that multi-infeed DC has great advantages in large-scale regional new energy long-distance transmission.