Time-domain Simulation Code Research Articles

Fatigue assessment of a new concept for a floating barge with pendulum for offshore wind turbines in MOST

Among floating foundations for offshore wind turbines, the barge is considered one of the most cost-effective platforms, owing to its ease of manufacturing, transport, and installation. However, the barge platform is susceptible to wave-induced stress due to its extensive waterplane area, potentially leading to fatigue in critical structural components such as the platform, tower, nacelle components and moorings. This paper delves into the prospect of mitigating fatigue damage in the barge concept by introducing a pendulum ballast, which can be installed at the offshore site using polyester ropes. The primary focus of this research is the assessment of the ‘barge-pendulum’ concept, aimed at reducing load cycles that could lead to fatigue failure, with a specific emphasis on the tower base as a representative indicator of device damage. The analysis is conducted using MOST, a time-domain simulation code developed in Matlab-Simscape environment. The findings show that the barge-pendulum concept has a poorer performance in terms of fatigue life, due to the increased sensitivity to wave forces.

Simulation and experimental results of plasma opening switch operation with inductive and electron beam diode loads in different plasma regimes

This paper reports the experimental results of a plasma opening switch (POS) that operates at hydrogen and carbon plasma regimes confirmed based on the conduction bipolar limit of the plasma species and the availability of plasma species during the conduction based on the density profiles. The generator is the compact capacitor bank developed using the synchronization of four field distortion spark gap switches. The plasma source is coaxial cable plasma guns characterized using the Faraday cup plasma diagnostic. The experimental results are compared with the time domain simulation code in which the POS is represented as the dynamic resistance governed by the physical equations of the bipolar model. The experiments and simulations are compared for inductive load and dynamic load of accelerating gap of 5 mm between the anode and cathode at the load end. From the results, POS can be operated based on the availability of plasma species with proper synchronized triggering between the plasma gun firing and generator firing that can operate at low and high values of currents.

Development of a Time-Domain Simulation Code for Cross-Flow Vortex-Induced Vibrations of a Slender Structure with Current Using the Synchronization Model

In this paper, the numerical analysis method for the cross-flow vortex-induced vibration (CF VIV) analysis based on the proposed procedure for CF VIV analysis of slender structures is developed. In order to consider the changes in the incoming flow according to the static configuration of the slender structures due to the current, the proposed procedure has three stages. A slender structure is modeled as the lumped-mass line, and the dynamic relaxation method known as the numerical technique for a slender structure with large geometric nonlinearity is applied in the static analysis. The comparison studies with a commercial program are carried out to validate the developed code. The vortex-induced force on slender structures is considered with the synchronization model. To verify the developed CF VIV analysis procedure and numerical method for a slender structure, VIV analysis of the tensioned flexible risers under a uniform and shear current is performed. The simulated results of CF RMS displacement show good agreement with the results of the model test It is found that a tensioned riser vibrates with one dominant frequency in resonance with the nth mode, even though multi-frequencies components of the vortex shedding along the riser due to the shear current occurs.

Fuel-Optimal Thrust-Allocation Algorithm Using Penalty Optimization Programing for Dynamic-Positioning-Controlled Offshore Platforms

This research, a new thrust-allocation algorithm based on penalty programming is developed to minimize the fuel consumption of offshore vessels/platforms with dynamic positioning system. The role of thrust allocation is to produce thruster commands satisfying required forces and moments for position-keeping, while fulfilling mechanical constraints of the control system. The developed thrust-allocation algorithm is mathematically formulated as an optimization problem for the given objects and constraints of a dynamic positioning system. Penalty programming can solve the optimization problems that have nonlinear object functions and constraints. The developed penalty-programming thrust-allocation method is implemented in the fully-coupled vessel–riser–mooring time-domain simulation code with dynamic positioning control. Its position-keeping and fuel-saving performance is evaluated by comparing with other conventional methods, such as pseudo-inverse, quadratic-programming, and genetic-algorithm methods. In this regard, the fully-coupled time-domain simulation method is applied to a turret-moored dynamic positioning assisted FPSO (floating production storage offloading). The optimal performance of the penalty programming in minimizing fuel consumption in both 100-year and 1-year storm conditions is demonstrated compared to pseudo-inverse and quadratic-programming methods.

Application of the Memristor in Reconfigurable Electromagnetic Devices

The recently popularized memristor, short for memory resistor, is investigated in this paper for its potential as a new and attractive method for enabling reconfigurable radio-frequency (RF) devices. The charge- or flux-controlled resistance of this “fourth circuit element” allows for easy reconfigurability and maintains its configured state in the absence of controlling signals. A specialized finite-difference time-domain simulation code is developed and employed to design devices with embedded memristors. The time-domain code allows observation of the nonlinear memristor switching characteristics and real-time functionality of the reconfigurable device. Several different reconfigurable RF devices are designed here to demonstrate the versatility of the memristor and determine the behavior of systems which utilize them.

Prediction of hydrodynamic performance of an FLNG system in side-by-side offloading operation

Floating liquefied natural gas (FLNG) is a type of liquefied natural gas (LNG) production system that shows prospects in exploitation of stranded offshore gas fields. The dynamic performance of an FLNG system in side-by-side configuration with a LNG carrier under the combined actions of wave, current and wind can be quite complex. This paper presents a comprehensive study on the hydrodynamics of an FLNG system with a focus on the nonlinear coupling effects of vessels and connection systems based on the concept FLNG prototype recently designed for South China Sea. In this study, the hydrodynamic characteristics of the two floating vessels connected through hawsers and fenders are investigated using a state-of-the-art time-domain simulation code SIMO, considering their mechanical and hydrodynamic coupling effects. The simulation model consisting of FLNG and LNG carrier is developed and calibrated by a series of model tests including a tuned damping and viscous levels. The hydrodynamic performances of the two floating vessels under an extreme sea state during side-by-side offloading operation are obtained, and their relative motions and the force responses of the connection hawsers and fenders are analyzed. Sensitivity studies are conducted to clarify contributions from the pretension and the stiffness of the connection hawsers. The effects on the hydrodynamic performance of the vessels and on the loads of the connection system are also investigated.

Power system equilibrium tracing and bifurcation detection based on the continuation of the recursive projection method

This study presents the continuation scheme of the recursive projection method (RPM) for power system equilibrium tracing and bifurcation detection using time-domain simulation code (TDSC). For a given operational condition, a basic RPM algorithm has been demonstrated to be very effective to extract power system steady-state and small-signal stability information from TDSC. It is now combined with arc-length continuation technique to mitigate singularity problems when the operational parameter variations are considered. Along the equilibrium tracing path, saddle node bifurcations (SNBs) and Hopf bifurcations (HBs) can be detected conveniently from byproducts of the RPM procedure. The proposed method takes advantage of available resources and avoids full state-space linearisation and large-scale eigenspectrum computation. The significance of the proposed approach is validated by a test example on New England 39-bus system.

Application of FDTD to HRRP Generation of a Cavity Model for NCTI Purposes

The application of a time-domain electromagnetic simulation code to a realistic and challenging problem like the high-resolution range profile (HRRP) generation of an inlet cavity model is presented in this paper. HRRPs can be used to accomplish noncooperative target identification (NCTI) of aircrafts by means of radar, and the database needed in this technique tends to be populated with predicted data obtained with software tools. Most codes employed with these purposes are frequency domain methods, which need multiple simulations at different adequately chosen frequencies, as well as some postprocessing, to get a radar signature. Instead, this paper focuses on finite difference time domain (FDTD) analysis of the response of a cavity, because of the relevance of these parts of an aircraft in the overall signature, and numerical results are compared with measurements performed by the authors in an anechoic chamber. This work also shows the advantages of using an electromagnetic code based on time domain for HRRP generation.

RPM-Based Approach to Extract Power System Steady State and Small Signal Stability Information From the Time-Domain Simulation

Recursive projection method (RPM) is investigated to extract power system steady state and small signal stability information from time-domain simulation code (TDSC). RPM takes outputs from TDSC to identify an unstable/slow invariant subspace of the full state space. On this subspace, a Newton type of method is applied to enhance convergence for unstable/slowly-converging modes. While on the complement, fixed point iteration of TDSC is kept to evolve stable/fast-decaying modes. Dominant eigenspectrum of the integration scheme is also obtained as byproduct, from which system small signal stability features can be reconstructed. Thus, RPM can enhance convergence of fixed point iteration and enable TDSC to perform small signal stability analysis. At the same time, modeling and numerical advantages of original TDSC are preserved and extra programming costs are saved. The approach is validated by examples on WECC nine-bus, New England 39-bus, and 230-bus systems.

Time-Domain Simulation of Inductive Output Tubes

In this paper, we discuss the development of a 3-D time-domain formulation and simulation code for modeling inductive output tubes (IOTs). This formulation relies on the integration of equivalent circuit equations in time coupled with the Lorentz force equations for particle trajectories. In the case of IOTs and klystrons, the equivalent circuit is a simple inductance-resistance-capacitance model. The same formulation using the equivalent circuit equations for Curnow cavities has been used to simulate coupled-cavity traveling-wave tubes. The connection between the equivalent circuit equations and the forces on the electrons used in the Lorentz force equations is through a scaling of a radio-frequency (RF) field model in which the amplitude is proportional to the cavity voltage. The RF field model can be obtained analytically [as derived in two dimensions by Kosmahl and Branch] or by means of a field map generated by electromagnetic structure simulators. The electron trajectories are integrated in these RF fields as well as using magnetostatic focusing fields and the space-charge fields. The space-charge fields are obtained by mapping the charge to a grid and then solving Poisson's equation. The new code is called NEMESIS, and we discuss the presently implemented IOT model and future development plans. Data on an IOT under development at Communications and Power Industries (CPI; K5H90W-2) have been provided for benchmarking the simulation. Comparison of NEMESIS with both anticipated performance predictions developed using scaling laws developed in-house at CPI and with actual tube performance has been good, and these comparisons are discussed in detail.

Near field distribution in two dimensionally arrayed gold nanoparticles on platinum substrate

Theoretical and experimental results for near field properties in the vicinity of two dimensionally aligned gold nanoparticles are presented. The numerical analysis is based on finite difference time domain simulation code. The simulated system consists of gold particles with a radius of 100nm, deposited on platinum substrate. The near field distribution on the substrate surface and its magnitude are found to depend on the interparticle distance. The experimental results obtained confirm the theoretical findings and demonstrate that the produced near field can result in a permanent substrate surface nanomodification and selective nanoparticle removal.

Enhanced near field mediated nanohole fabrication on silicon substrate by femtosecond laser pulse

Investigation of the process of nanohole formation on silicon surface mediated with near electromagnetic field enhancement in vicinity of gold particles is described. Gold nanospheres with diameters of 40, 80 and 200 nm are used. Irradiation of the samples with laser pulse at fluences below the ablation threshold for native Si surface, results in a nanosized surface modification. The nanostructure formation is investigated for the fundamental ( λ = 800 nm, 100 fs) and the second harmonic ( λ = 400 nm, 250 fs) of the laser radiation generated by ultrashort Ti:sapphire laser system. The near electric field distribution is analyzed by an Finite Difference Time Domain (FDTD) simulation code. The properties of the produced morphological changes on the Si surface are found to depend strongly on the polarization and the wavelength of the laser irradiation. When the laser pulse is linearly polarized the produced nanohole shape is elongated in the E-direction of the polarization. The shape of the hole becomes symmetrical when the laser radiation is circularly polarized. The size of the ablated holes depends on the size of the gold particles, as the smallest holes are produced with the smallest particles. The variation of the laser fluence and the particle size gives possibility of fabricating structures with lateral dimensions ranging from 200 nm to below 40 nm. Explanation of the obtained results is given on the basis simulations of the near field properties using FDTD model and Mie's theory.

Prediction of Relative Motion and Probability of Contact Between FPSO and Shuttle Tanker in Tandem Offloading Operation

Excessive relative motions between Floating Production Storage Offloading Unit (FPSO), and tanker, which are termed as excessive surging and yawing events, are identified as the “failure prone situation” in tandem offloading. These events have contributed to the initiation of tanker drive-off in most collision incidents that happened in the North Sea in recent years. To estimate and reduce the probability of excessive surging and yawing events in tandem offloading, a simulation-based approach, which is based on a state-of-the-art time-domain simulation code SIMO, is presented in this paper. A typical North Sea FPSO and a DP shuttle tanker simulation models are setup in SIMO and calibrated by full-scale measurements. The simulated relative distance and relative heading between FPSO and tanker are analyzed by fitting their extreme values into statistical models which give out probabilities of excessive surging and yawing events. Sensitivity studies are performed to pinpoint contributions from various technical and operational factors. Measures to minimize the occurrence of excessive surging and yawing events are identified in design and operational perspectives.

Saw-tooth instability in storage rings: simulations and dynamical model

The saw-tooth instability in storage rings is studied by means of a time-domain simulation code which takes into account the self-induced wake fields. The results are compared with those from a dynamical heuristic model exploiting two coupled non-linear differential equations , accounting for the time behavior of the instability growth rate and for the anomalous growth of the energy spread. This model is shown to reproduce the characteristic features of the instability in a fairly satisfactory way.

Multimedia modules for electromagnetics education

Multimedia technology provides a valuable resource to enhance the teaching and learning experience. The ability to combine practical applications, visualization of complex mathematical and abstract subjects, virtual labs, and guided use of simulation software is among the advantages of technology-based education. In this article we describe the main features of several multimedia tutorials for electromagnetics education developed by the Computer Applications in Engineering Education Center at the University of Utah. The electromagnetics CD-ROM includes lessons on vectors and coordinate systems, electrostatics, Maxwell's equations, electromagnetic waves, dielectric materials, and transmission lines. Several virtual labs were developed and software packages on electrostatic, transient and steady-state analysis of transmission lines, and two-dimensional finite-difference time-domain (FDTD) simulation code are included. Tutorials can be used both as standalone tutors and as demonstrations to complement classroom lectures. © 1997 John Wiley & Sons, Inc. Comput Appl Eng Educ 5: 257–267, 1997