Frequency-domain Simulations Research Articles

Convolution-based time-domain simulation for fluidelastic instability in tube arrays

A convolution-based numerical algorithm is presented for the time-domain analysis of fluidelastic instability in tube arrays, emphasizing in detail some key numerical issues involved in the time-domain simulation. The unit-step and unit-impulse response functions, as two elementary building blocks for the time-domain analysis, are interpreted systematically. An amplitude-dependent unit-step or unit-impulse response function is introduced to capture the main features of the nonlinear fluidelastic (FE) forces. Connections of these elementary functions with conventional frequency-domain unsteady FE force coefficients are discussed to facilitate the identification of model parameters. Due to the lack of a reliable method to directly identify the unit-step or unit-impulse response function, the response function is indirectly identified based on the unsteady FE force coefficients. However, the transient feature captured by the indirectly identified response function may not be consistent with the physical fluid-memory effects. A recursive function is derived for FE force simulation to reduce the computational cost of the convolution operation. Numerical examples of two tube arrays, containing both a single flexible tube and multiple flexible tubes, are provided to validate the fidelity of the time-domain simulation. It is proven that the present time-domain simulation can achieve the same level of accuracy as the frequency-domain simulation based on the unsteady FE force coefficients. The convolution-based time-domain simulation can be used to more accurately evaluate the integrity of tube arrays by considering various nonlinear effects and non-uniform flow conditions. However, the indirectly identified unit-step or unit-impulse response function may fail to capture the underlying discontinuity in the stability curve due to the prespecified expression for fluid-memory effects.

Iterative methodology for response based analysis of an FPSO mooring system

Response based analysis (RBA) has been developed for prediction of extreme N-year return period responses and design metocean conditions of offshore structures. For applying the RBA, the behaviour of the offshore system subjected to a long history of metocean conditions needs to be predicted, and then, the probabilistic analysis is applied to estimate its long-term responses. Due to the large number of analysis cases required, the structural simulation is usually performed either by simplifying the structural model or by using computationally efficient tools, such as frequency-domain (FD) analysis. These approaches usually decrease the accuracy of predictions mainly when they are utilized for nonlinear systems. On the other hand, employing time-domain (TD) simulations leads to more accurate results but it is computationally expensive. Application of RBA for a weathervaning FPSO, which is the subject of the present study, makes TD analysis an essential requirement because of a highly nonlinear behaviour of the system. In the present study, an efficient methodology is proposed that aims at reducing the computational efforts of RBA by joint application of TD and FD simulations in combining the structural and statistical analyses through a single process, such that the number of time-consuming TD simulations is minimized. After initial screening using the results from FD simulations, the methodology identifies the response events (storms) that contribute the most to the N-year response and sets out an iterative process in which only those events that are most important are analysed by fully-coupled TD simulations. Within such events, a similar approach is also applied to intervals (sea states) where only the most contributing intervals are analysed in TD, and the remaining intervals are left for a less accurate FD analysis without sacrificing the overall accuracy. The proposed methodology provides a robust framework for distinguishing between “mild” and “severe” response events, without specifying any predefined limits for the metocean parameters or making a subjective judgement. Although it is developed for the mooring system of a weathervaning FPSO, it should also be applicable to any type of offshore structure and any structural response. This paper is the first part of the study and concentrates on the development of the efficient methodology to optimize the application of RBA to FPSO mooring systems, whilst its detailed application is subject of the second part of the study.

An optimal frequency-domain finite-difference operator with a flexible stencil and its application in discontinuous-grid modeling

We have developed an optimal method to determine expansion parameters for flexible stencils in 2D scalar-wave finite-difference frequency-domain (FDFD) simulation. Our stencil only requires the involved grid points to be paired and rotationally symmetric around the central point. We apply this method to the transition zone in discontinuous-grid modeling, in which the key issue is designing particular FDFD stencils to correctly propagate the wavefield passing through the discontinuous interface. Our method can work in an FDFD discontinuous grid with arbitrary integer coarse- to fine-grid spacing ratios. Numerical examples are developed to determine how to apply this optimal method to discontinuous-grid FDFD schemes with spacing ratios of 3 and 5. The synthetic wavefields are highly consistent to those calculated using the conventional dense uniform grid, and the memory requirement and computational costs are greatly reduced. For velocity models with large contrasts, our discontinuous-grid FDFD method can significantly improve the computational efficiency in forward modeling, imaging, and full-waveform inversion.

Novel methodology for direct speed control of a permanent magnet synchronous motor with sensorless operation

A new formulation of the direct speed control (DSC) method for permanent magnet synchronous motor (PMSM) is proposed. The proposed DSC method employs a direct voltage control to synthesise the d and q axes stator voltage. In this method, no q-axis current loop regulation is required. Thus, a new concept of the dynamic limiter (DL) is proposed to limit the undesirable current rises. The mathematical analyses, frequency and time domain simulations, and experimental test results highlight the simplicity and effectiveness of the proposed DSC method. Furthermore, sensorless operation of the method can make it a reliable alternative for low-cost drive of PMSM.

RF wave propagation simulation for ICRF antenna in EAST

ABSTRACT The propagation performance of radiofrequency (RF) waves excited by ICRF antenna in plasma has a great influence on the coupling effect. In this paper, the propagation characteristics of fast wave (FW) and slow wave (SW) under tokamak experimental condition are calculated, and the influence of angular frequency w 0, antenna power spectrum k// and tokamak total magnetic field B 0 is illustrated based on the typical operating parameters in EAST. By applying perfect matched layer (PML) boundary processing method to simulate single-pass absorption and using finite element method (FEM) to solve Maxwell equations in frequency domain, the propagation and coupling simulation of RF waves in cold plasma for EAST ICRF antenna in B-window were carried out to obtain the polarization characteristics of fields and the relationship between the coupling impedance and the gap length from antenna to the FW cut-off layer. The analysis results in this paper provide useful reference and guidance for the study of propagation coupling of ICRF waves in EAST and other magnetic confined fusion devices.

Adaptive Extended Kalman Filter Based Dynamic Equivalent Method of PMSG Wind Farm Cluster

In order to reduce the dimension of wind farm model and improve the simulation efficiency under the premise of ensuring the accuracy, this article proposes a dynamic equivalent method for permanent magnet synchronous generator (PMSG) based wind farm cluster utilizing the adaptive extended Kalman filters. Combining the dynamic model of PMSG, the equivalent wind turbine parameters that need to be identified are incorporated into the extended state variables, and an online parameters identification method based on extended Kalman filter (EKF) theory is put forward. To eliminate the influence of random noise on the accuracy of identification, the adaptive technology is introduced into EKF to estimate the process and measurement noise covariance in real time through the Sage–Husa estimator. Thus, a wind farm cluster equivalent model is obtained. Finally, the effectiveness and robustness of the proposed method are verified by time-domain simulation and frequency-domain simulation in IEEE 39-bus system.

A phase-locked loop using ESO-based loop filter for grid-connected converter: performance analysis

An extended state observer (ESO)-based loop filter is designed for the phase-locked loop (PLL) involved in a disturbed grid-connected converter (GcC). This ESO-based design enhances the performances and robustness of the PLL, and, therefore, improves control performances of the disturbed GcCs. Besides, the ESO-based LF can be applied to PLLs with extra filters for abnormal grid conditions. The unbalanced grid is particularly taken into account for the performance analysis. A tuning approach based on the well-designed PI controller is discussed, which results in a fair comparison with conventional PI-type PLLs. The frequency domain properties are quantitatively analysed with respect to the control stability and the noises rejection. The frequency domain analysis and simulation results suggest that the performances of the generated ESO-based controllers are comparable to those of the PI control at low frequency, while have better ability to attenuate high-frequency measurement noises. The phase margin decreases slightly, but remains acceptable. Finally, experimental tests are conducted with a hybrid power hardware-in-the-loop benchmark, in which balanced/unbalanced cases are both explored. The obtained results prove the effectiveness of ESO-based PLLs when applied to the disturbed GcC.

Optimal FOPID Speed Control of DC Motor via Opposition-Based Hybrid Manta Ray Foraging Optimization and Simulated Annealing Algorithm

In this study, a fractional-order proportional–integral–derivative (FOPID) controller was used for controlling the speed of direct current (DC) motor. The parameters of the controller have optimally been adjusted using a new meta-heuristic algorithm, namely the opposition-based (OBL) hybrid manta ray foraging optimization (MRFO) with simulated annealing (SA) algorithm (OBL-MRFO-SA). The proposed novel OBL-MRFO-SA algorithm aims to improve the original MRFO algorithm in two ways. Firstly, it provides MRFO a better exploration capability with the aid of opposition-based learning. In this way, it can avoid local minimum stagnation. Secondly, it enables MRFO to have a better exploitation capability with the aid of hybridization using simulated annealing algorithm. The hybridization helps accelerating the convergence rate of MRFO. A time domain objective function which takes the performance criteria (maximum overshoot, steady-state error, rise time and settling time) into account has been used to design the FOPID based speed control system for DC motor with OBL-MRFO-SA algorithm. The performance of the proposed novel algorithm has been assessed through various analyses such as time and frequency domain simulations, robustness and load disturbance rejection. Compared to other state-of-the-art optimization algorithms, OBL-MRFO-SA has shown superior exploration and exploitation capabilities. The performance of the developed algorithm has also been demonstrated to be better by using a physical setup.

Определение LC-параметров корпусов ИМС с помощью электромагнитного моделирования

At the IC design phase it is required to consider all possible factors to ensure the produced device’s compliance with specified requirements. For instance, it is necessary to select an optimum package and to lower its impact on IC’s final characteristics. Electromagnetic simula-tion is widely used to calculate electrical parameters of the electric motors, printed circuit boards, and microcircuits. In this work, full-wave three-dimensional finite element method simulation in frequency domain was used to characterize LGA package. S-parameters matrix obtained by modeling was converted to an equivalent RLGC transmission line model to determine the electrical parameters of the package – self-capacitance and inductance of the leads, mutual capa-citance of the adjacent leads, – on which the package bandwidth and crosstalk depend. Correct-ness of the obtained results was confirmed by comparison with the measurements results. It has been found that discrepancy between the results of modeled and measured capacitance was less than 5 %. The inductance modeling results turned out to be less accurate, which, however, is not significant for low and medium frequency applications.

Multi-Frequency Sonoreactor Characterisation in the Frequency Domain Using a Semi-Empirical Bubbly Liquid Model

Recently, multi-frequency systems were reported to improve performance in power ultrasound applications. In line with this, digital prototyping of multi-frequency sonoreactors also started gaining interest. However, conventional time-domain simulations used to consider the multiple frequencies led to many challenges and limitations. In this study, a multi-frequency sonoreactor was characterised using frequency domain simulations in 2-D. Using Parseval’s theorem, the root-mean-squared acoustic pressure was compared against experimental validation results using sonochemiluminescence (SCL). A semi-empirical, modified Commander and Prosperetti model was used to describe the bubbly-liquid effects in the sonoreactor. The studied system consists of a hexagonal 16.5 L sonoreactor capable of operating at 28, 40 and 70 kHz. Four frequency combinations were studied: 28-40, 28-70, 40-70 and 28-40-70 kHz. The root-mean-squared acoustic pressure fields showed good qualitative agreement with SCL results in terms of antinode predictions. The empirical phase speed calculated from SCL measurements was found to be useful in terms of circumventing uncertainties in bubble parameter specifications. Case study results have also shown that the root-mean-square acoustic pressure is a promising tool to characterise the cavitation potential in the multi-frequency sonoreactor.

Parasitic High Q-Factor Open-Box Modes With 3-D Printed Dielectric-Filled Metal Waveguides

High Q-factor open-box mode resonances have been found in the microwave measurements of several 3-D printed dielectric-filled metal-pipe rectangular waveguides (MPRWGs). These parasitic Fabry-Perot eigenmodes are confined by the conductive walls in the transverse plane of the MPRWG and partially confined by the air-dielectric and dielectric-air boundaries in the longitudinal direction. The excitation of open-box modes was previously speculated to be due to the inhomogeneous and/or anisotropic nature of the 3-D printed dielectric-fillers. This has now been confirmed, by representing the inhomogeneous and anisotropic nature of the woodpile-like dielectric structure (physical realm), with an anisotropic dielectric constant tensor (simulation realm). Analytical and numerical eigenmode solvers, previously used by the authors with MPRWGs, are applied here to parallel-plate waveguides (PPWGs) and circular waveguides (CWGs); identifying all the individual parasitic open-box modes. With the former, its TM11 mode exhibits an ultra-high Q-factor of approximately 2,300 at X-band, which is considerably higher than those found with other modes and in other waveguide structures. Finally, a numerical full-wave frequency-domain simulator that employs the dielectric constant tensor is introduced in this paper. This new modeling technique independently confirms that open-box modes are excited in 3-D printed dielectric-filled MPRWG, PPWG and CWG structures. This paper provides the foundations for accurately modeling parasitic resonances associated with inhomogeneities and anisotropy in 3-D printed microwave components; not just the metal-walled waveguide structures considered here, but the methodology could also be extended to generic 3-D printed dielectric waveguides and substrate-based transmission lines.

Passivity-Based Dual-Loop Vector Voltage and Current Control for Grid-Forming VSCs

This letter proposes a passivity-based dual-loop vector voltage and current control method for grid-forming voltage-source converters (GFM-VSCs). A passive output impedance of GFM-VSC is guaranteed in both the voltage control mode and the current-limiting mode with a wide range of time delay. The frequency-domain analysis, simulation and experimental tests validate the effectiveness of the approach.

Simulation of Drive-Induced Oscillations in Helix Traveling-Wave Tubes

Drive-induced oscillations (DIOs) in helix traveling-wave tubes (TWTs) may occur during large-signal amplifier operation. As the kinetic beam energy is reduced, the beam-wave synchronism possibly shifts into the π-point, thus enabling simultaneous interaction with the forward and the backward circuit mode. There, asymmetries in the beam-wave interaction region introduce an additional coupling mechanism and lower the oscillation threshold compared with backward-wave oscillations (BWOs). Consequently, DIOs are likely to arise. In addition even without asymmetries, drive-induced BWOs are observed. A hybrid timeand frequency-domain simulation approach is presented and applied to analyze this phenomenon in a helix TWT. In addition, an effective suppression technique is proposed.

Frequency-domain calculation of Smith-Purcell radiation for metallic and dielectric gratings.

The intensity of Smith-Purcell radiation from metallic and dielectric gratings (silicon, silica) is compared in a frequency-domain simulation. The numerical model is discussed and verified with the Frank-Tamm formula for Cherenkov radiation. For 30 keV electrons, rectangular dielectric gratings are less efficient than their metallic counterparts, by an order of magnitude for silicon, and two orders of magnitude for silica. For all gratings studied, radiation intensity oscillates with grating tooth height due to electromagnetic resonances in the grating. 3D and 2D numerical models are compared.

Optical microprism cavities based on dislocation-free GaN

Three-dimensional growth of nanostructures can be used to reduce the threading dislocation density that degrades III-nitride laser performance. Here, nanowire-based hexagonal GaN microprisms with flat top and bottom c-facets are embedded between two dielectric distributed Bragg reflectors to create dislocation-free vertical optical cavities. The cavities are electron beam pumped, and the quality (Q) factor is deduced from the cavity-filtered yellow luminescence. The Q factor is ∼500 for a 1000 nm wide prism cavity and only ∼60 for a 600 nm wide cavity, showing the strong decrease in Q factor when diffraction losses become dominant. Measured Q factors are in good agreement with those obtained from quasi-3D finite element frequency-domain method and 3D beam propagation method simulations. Simulations further predict that a prism cavity with a 1000 nm width will have a Q factor of around 2000 in the blue spectral regime, which would be the target regime for real devices. These results demonstrate the potential of GaN prisms as a scalable platform for realizing small footprint lasers with low threshold currents.

Sub‐synchronous oscillations in wind farms – an overview study of mechanisms and damping methods

Sub-synchronous oscillation (SSO) causes significant damage and performance degradation in wind farms (WFs). The root causes and mitigation methods of SSO have been identified as a result of many studies. An overview that analysed and summarised those findings in different perspectives helps optimise the existing solutions and to find alternative approaches to mitigate SSO. Therefore, a comprehensive overview of the SSO analysing techniques, mechanisms, and mitigation strategies are presented in this study. This overview is focused on the WFs based on type 3 and 4 wind turbine generators and the WFs connected to high-voltage DC transmission systems. The dominant SSO modes in each application have been presented along with the identified root cause, influencing factors, and the mitigation methods. Besides that, a comprehensive comparison between the existing mitigation strategies is presented to identify the alternative approaches and improvements. The identified improved methods are presented along with the time and frequency domain simulation results to validate their applicability. Finally, an insight into the future direction of this research is presented along with the conclusions.

Ultrawideband Frequency-Selective Absorber Designed With an Adjustable and Highly Selective Notch

In this article, the working mechanism of a wideband absorber designed with an adjustable and highly selective notch band is studied, in which the narrow notch band is independently controlled by the lower lossless layer of the absorber, whereas the upper lossy layer loaded with lumped resistors realizes absorption. We present two instances with geometrically controlled and electrically controlled notch bands, respectively. Without decreasing absorption performance, the notch position can be flexibly adjusted throughout the entire frequency band by simply modifying the dimension of the lossless frequency-selective surface (FSS) or changing the capacitance of the varactor, i.e., using geometric control or electrical control. The narrow notch band allows two wide absorption bands to be retained on both sides; therefore, good stealth performance is still guaranteed. Equivalent circuit models (ECMs) are proposed to further explain the principle. The frequency-domain simulation, ECM, time-domain simulation, and experimental results are in good agreement and validate the adjustability and high selectivity of the notched absorbers. At the end of this article, an FSA-backed monopole antenna is simulated and measured, which clearly illustrates that these FSAs can serve as the ground plane for antennas and realize out-of-band RCS reduction.

Rapid Simulation of Photonic Integrated Circuits Using Verilog-A Compact Models

Silicon-based electronic-photonic integration offers several opportunities for integrated circuits (IC) designers to innovate systems architectures that leverage the advantages of optical-domain signal processing and low-loss transmission in optical fibers. However, photonic integrated circuit (PIC) design tools have evolved from the numerical Maxwell field solvers and are completely disjointed from the electronic circuit simulation tools such as SPICE and Cadence Spectre. Thus there is a gap that needs to be filled by the IC community where they can instantiate photonic building blocks to form PICs and perform co-simulation with interfacing (Bi)CMOS electronic circuits. In recent work, Verilog-A compact models have been developed for photonic device building blocks that enable transient simulations of hybrid electronic-photonic components such as lasers, modulators and detectors. However, frequency sweeps of radio-frequency (RF) photonic filters remain unwieldy due to the long simulation times of stepped frequency transient simulations. In this work, we present complex frequency chirp based methods for rapid frequency-domain simulation of PICs. The trade-offs involved in selecting the simulation parameters for a given frequency response accuracy and simulation time are studied along with the impact of windowing and frequency chirp profile. The presented method can result in over $1000\times $ improvement in simulation time of frequency sweeps of higher-order RF photonic filter topologies.

Noise-Source Model for Frequency-Domain EMI Simulation of a Single-Phased Power Circuit

In recent years, power electronic converters have been downsized significantly by high-density packaging techniques. As a result, parasitic couplings between power electronic components have become larger. For the prediction of electromagnetic interference (EMI) noise considering such couplings, a 3-D electromagnetic simulation is more suitable than a circuit simulation using a huge number of parasitic models. The most-practical electromagnetic simulation method is the finite element method, which is based on a frequency domain. The switching noise must be modeled in the frequency domain for an EMI noise simulation. In this article, we propose a new noise-source model for a frequency-domain EMI noise simulation. We modeled a transistor and a freewheeling diode by voltage and current sources based on three model topologies: transistor-OFF-diode-ON topology (OFF-topology); transistor-ON-diode-OFF topology (ON-topology); and transistor-on-diode-ON topology (recovery topology). We also used OFF-topology to simulate the turn-OFF EMI noise as caused by the turning-OFF of a transistor. Meanwhile, we used a combination of ON-topology and recovery topology to simulate the turn-ON EMI noise including recovery noise as caused by the turning-ON of a transistor. After introducing the details of the model, we compared the measured and simulated results so as to show the validity of the model.

Frequency-domain elastic-wave modeling for polygonal topography using rotated average-derivative difference operators

Modeling of seismic wave propagation in areas with irregular topography is an important topic in the field of seismic exploration. As a popular numerical method for seismic modeling, the finite difference method is nontrivial to consider the irregular free surface. There have been extensive studies on the time-domain finite difference simulations with irregular topography; however, the frequency-domain finite difference simulation considering irregular topography is relatively less studied. The average-derivative approach is an optimal numerical simulation scheme in the frequency domain, which can produce accurate modeling results at a relatively low computational cost. Nevertheless, this approach can only deal with the modeling problems with a flat free surface. To address this issue, we design a new frequency-domain finite difference scheme by introducing the polygonal representation of topography into the average-derivative method. The irregular topography is represented by line segments with various slopes. An extension of the conventional average-derivative difference operator in the local rotated coordinate system is used for formulating the spatial derivatives aligned with the topographic line segments. As a result, new average-derivative difference schemes are obtained for irregular topography. In this way, the average-derivative optimal method is generalized to the model with irregular topography. Numerical examples show the effectiveness of the presented method.