Time-domain Numerical Simulations Research Articles

Simulation and reconstruction for 3D elastic wave using multi-GPU and CUDA-aware MPI

3D finite-difference time-domain numerical simulation and reconstruction based on the domain decomposition technique are essential parts of high-performance computation for reverse-time migration and full-waveform inversion. However, the low GPU utilization in computing for small-sized models and the tremendous memory consumption for large-sized models may result in low computational efficiency and high memory costs. This paper proposes a contiguous memory management (CMM) method and a variable-order wavefield reconstruction (VWR) method. The CMM allocates the memory of many small-sized arrays used for MPI communications on a larger-sized contiguous memory block, which aims to reduce the number of MPI communications between subdomains and improve the communication bandwidth, thus reducing the MPI time overhead and improving the GPU utilization. Meanwhile, the VWR can flexibly set the number of layers of boundary wavefield used for source wavefield reconstruction according to the host memory capacity and accuracy requirements. Since one layer of boundary wavefield could be stored using the VWR, the memory consumption of host memory can be significantly alleviated. Numerical experiments show that GPU utilization in computing for the model with a size of 1213 can be improved from 25% to 90% using the CMM method, and the VWR method can reduce memory consumption by about 86% while maintaining good accuracy in wavefield reconstruction. In addition, the issue of how to obtain a domain decomposition scheme with optimal performance is discussed in this paper.

The time-domain Cartesian multipole expansion of electromagnetic fields

Time-domain solutions of Maxwell’s equations in homogeneous and isotropic media are paramount to studying transient or broadband phenomena. However, analytical solutions are generally unavailable for practical applications, while numerical solutions are computationally intensive and require significant memory. Semi-analytical solutions (e.g., series expansion), such as those provided by the current theoretical framework of the multipole expansion, can be discouraging for practical case studies. This paper shows how sophisticated mathematical tools standard in modern physics can be leveraged to find semi-analytical solutions for arbitrary localized time-varying current distributions thanks to the novel time-domain Cartesian multipole expansion. We present the theory, apply it to a concrete application involving the imaging of an intricate current distribution, verify our results with an existing analytical approach, and compare the proposed method to a finite-difference time-domain numerical simulation. Thanks to the concept of current “pixels” introduced in this paper, we derive time-domain semi-analytical solutions of Maxwell’s equations for arbitrary planar geometries.

CFD investigation on wind power harvesting from small-scale wind turbines for powering residential buildings in New Zealand

ABSTRACT As an alternative solution, wind power could be harnessed by designing and implementing small-scale wind turbines for distributed power generation. In this work, 3-dimensional time-domain numerical simulations, based on the Reynolds-Average Navier-Strokes (RANS) model, are conducted for two categories of small-scale wind turbines, i.e. drag force- (Savonius) and lift force-driven (Darrieus) ones, while placing them at a flat surface and a step height, respectively. The present results confirmed that placing a small-scale wind turbine at a step height can improve the maximum power output, respectively, approximately 219.2% for the Savonius wind turbine and 121.0% for the Darrieus wind turbine operating at their optimum stage; moreover, the corresponding peak power spectral density of the turbine torque is improved by 173.9% and 83.6%, respectively. In addition, the power generated from the wind flow could reduce the CO2 emission. It has been observed that the Savonius turbine could reduce the annual CO2 emission by 3,738.9–19,857.8 kg and that of Darrieus turbines by 4,919.6–26,128.8 kg. This study demonstrated the benefits of adopting forward facing steps to improve wind turbine performance in a typical urban environment.

Time-domain numerical simulation for multi-ships moving in waves with forward speed

With the development of the shipping industry, the impact of hydrodynamic interaction between two or more ships on navigational safety has received widespread attention. However, limited research has been conducted on multi-ships moving in waves with forward speed due to the increased complexity of hydrodynamic interactions caused by speed. Therefore, this paper establishes a numerical model of multi-ship hydrodynamics under the wave impact based on the time-domain Rankine source panel method. The hydrodynamic performance and motion characteristics of ships advancing in waves are investigated. An artificial damping layer is applied to free surface boundary conditions to absorb reflected waves. The fourth-order Runge-Kutta method is used for time integration to solve the motion of the ship. The hydrodynamic and motion response of the classical Wigley III ship model are calculated at each wavelength, and the simulation results are compared with other calculations to verify the reasonable reliability of the numerical method employed in this study. On this basis, sensitivity analysis of different parameters is conducted to reveal the influence of various parameters such as speed, wave direction, transverse distance, and water depth on the motion response of the Wigley III ship model in waves.

Design Optimization of a Mooring System for an Offshore Aquaculture Platform

In the present paper, the mooring system of an offshore aquaculture platform is designed and optimized, applying a shallow water mooring system design methodology which combines the Non-dominated Sorting Genetic Algorithm-II (NSGA-II) and the cable–platform coupled model. One mooring design is first given using the proposed methodology. Two reference mooring systems are modified based on an NSGA-II design, and AIP criteria and expertise. The hydrodynamic performance of the offshore aquaculture platform with these three mooring systems is compared via use of a potential flow time–domain numerical simulation. A physical model-scale experiment is carried out to validate the numerical coupled model. Both numerical and experimental results confirm the effectiveness of present model. The comparative analysis shows that the mooring system designed using NSGA-II can provide a relative radical solution compared to the other two mooring designs when considering a limited budget.

Adiabatic sound transport in acoustic waveguides with time-varying Helmholtz resonators

We investigate the dynamics of acoustic waveguides with time-varying Helmholtz resonators and the ensuing wave propagation features. We focus on the numerical modeling of such a system with emphasis on the time-varying dispersion properties and emerging wave phenomena due to slow time modulation. We show that a propagating wave packet experiences a transformation that preserves the wavenumber content, resulting in frequency conversion that follows the time evolution of the dispersion bands. The conditions for such a transformation to be “adiabatic” are derived analytically, which allows the identification of the limiting modulation speed required to avoid undesired reflections or mode conversions. The predictions from the dispersion investigations are confirmed by time-domain numerical simulations, which illustrate the possibilities for frequency conversion and temporal signal compression or decompression of impinging signals. The framework presented herein may open new avenues in the context of time-varying phonic waveguides, with possible applications in communication, sound isolation, and frequency conversion.

Programmable hardening and softening cubic inductive shunts for piezoelectric structures: Harmonic balance analysis and experiments

It has been well explored in the literature that intentionally introduced (designed) nonlinearities offer significant advantages in vibration absorption and isolation problems. In the context of using piezoelectric shunt damping for vibration attenuation in flexible structures, synthetic impedance circuits with digital control enable precise tuning capabilities and eliminate the need for bulky analog circuit components when targeting low frequencies, and they have been studied mostly for linear circuits. In this work, we explore nonlinear synthetic impedance-based Duffing shunts with hardening and softening nonlinearities, connected to a cantilever under base excitation in its geometrically and materially linear regime. We demonstrate a shunt of nonlinear impedance equivalent to a positive or negative cubic inductor connected in parallel to a linear inductor and a linear resistor. This shunt, along with the inherent piezoelectric capacitance, provides digital analog of a hardening- or softening-type nonlinear stiffness, linear stiffness, linear damping, and mass, as a synthetic Duffing circuit. Electromechanical modeling of a piezoelectric cantilevered beam shunted to the Duffing oscillator with hardening or softening inductance is performed. Numerical simulations for single-term and multi-term harmonic balance solutions are performed for periodic response. These approximate analytical predictions are compared against time-domain numerical simulations and are experimentally validated for various mechanical excitation amplitudes and nonlinear coefficients. Overall, a very good agreement is obtained between the model simulations and experimental results, in addition to demonstrating the unique digital programming capabilities of this nonlinear shunt.

A novel concept of floating absorber for motion mitigation in floating offshore wind turbines

This paper proposes the innovative concept of floating absorber for motion mitigation in floating wind turbines on spar supports. The floating absorber consists in a spar floater coupled with the floating wind turbine by an elastic spring and a viscous damper in parallel. It is conceived to act as a resonant mass tuned to the pitch frequency of the floating wind turbine and, for this purpose, its parameters are calibrated based on the well-established fixed point tuning procedure typically applied for tuned mass dampers. For validation, extensive time-domain numerical simulations are performed on a multi-degree-of-freedom computational model of the coupled system, considering thirty-six different configurations of the floating absorber and fifteen different combinations of wind-wave conditions. The results demonstrate a considerable effectiveness of the floating absorber in reducing the pitch motion of the floating wind turbine. A main advantage over conventional tuned mass devices is that the absorber is not housed in the nacelle of the wind turbine but floats close to the floating wind turbine. This not only ensures reduced costs of installation and maintenance of the floating absorber but allows, also, to increase its mass without causing extra weight on the floating wind turbine, exploiting hydrodynamic added mass as well, with consequent benefits in terms of motion mitigation in the floating wind turbine. Another relevant result is that, for a given mass of the floating absorber and a given attachment point of the parallel spring-dashpot, the performances of the floating absorber do not vary significantly with its geometry. This is important to facilitate design and manufacturing of the floating absorber, confirming the robustness of the proposed concept and its great potential for motion mitigation in floating wind turbines.

Topology optimization on the attenuation degree of evanescent waves in periodic pile barriers

Periodic pile barriers can be used for ambient vibration mitigation owing to their bandgaps. There are no propagating modes in the bandgaps; only evanescent modes exist. The commonly used real band structure cannot be used to identify the evanescent waves. Therefore, the complex band structure becomes significant and can be used to analyze the evanescent modes in the bandgap. Based on the genetic algorithm, both single- and multi-objective topology optimizations were performed to maximize the minimum imaginary part of the wavenumbers at a specified frequency. The results showed that a bandgap can be opened when the minimum imaginary part of the wavenumbers is maximized. The opened bandgap width and filling fraction can affect the degree of wave attenuation in the periodic pile barriers. Both frequency- and time-domain numerical simulations were performed, which validated that the attenuation capability of the periodic pile barriers is related to the minimum imaginary part of the evanescent wavenumbers. We also found that all the optimized pile designs could be classified into three forms, and the mechanisms of their bandgaps were analyzed in detail. The optimized designs exhibited higher levels of wave attenuation than conventional circular or square piles.

Electromagnetically Induced Transparency Analog of Asymmetric Perovskite Metamaterial in the THz Spectral Region

The analogy of electromagnetically induced transparency (EIT) in perovskite metamaterials is characterized by the numerical simulations in finite-difference time-domain (FDTD). The perovskite metamaterials consist of two cut wire resonators (CWRs) and a disk resonator (DR) on a polyimide substrate. The analysis revealed the characteristic dynamics of the electromagnetic field, the near-field couplings of CWRs and DR, and the EIT-like spectral features of perovskite metamaterials as functions of the asymmetry parameter and polarization direction. The strong coupling and destructive interference of bright and bright–dark transitions in perovskite metamaterials displayed EIT-like transparency at 653.5 GHz with a high Q-factor of approximately 1470, a sensitivity of 531 GHz/RIU and a figure of merit of around 780. In addition, perovskite metamaterials exhibited slow light with a group delay of about 106 ps and a group index of approximately 3100. These results may provide an important perspective for understanding the coupling mechanism and applications of perovskite materials in slow-light devices, THz sensors, and tunable switching in THz spectral region.

Impact of film thickness in laser-induced periodic structures on amorphous Si films

We report self-organized periodic nanostructures on amorphous silicon thin films by femtosecond laser-induced oxidation. The dependence of structural periodicity on the thickness of silicon films and the substrate materials is investigated. The results reveal that when silicon film is 200 nm, the period of self-organized nanostructures is close to the laser wavelength and is insensitive to the substrates. In contrast, when the silicon film is 50 nm, the period of nanostructures is much shorter than the laser wavelength, and is dependent on the substrates. Furthermore, we demonstrate that, for the thick silicon films, quasi-cylindrical waves dominate the formation of periodic nanostructures, while for the thin silicon films, the formation originates from slab waveguide modes. Finite-difference time-domain method-based numerical simulations support the experimental discoveries.Graphical abstract

GENERAL SOLUTION OF THE EXCITATION PROBLEM OF A SYMMETRIC FLAT DIELECTRIC STRUCTURE BY PLANE ELECTROMAGNETIC WAVES

General solution of the excitation problem of a symmetric flat dielectric structure by two laser pulses is obtained. The symmetrical geometry consists of two dielectric prisms separated by a vacuum channel for electron acceleration (so called “sandwich”). Each prism can be illuminated with a separate laser pulse; electric filed amplitudes of pulses can differ. The general solution consist from a symmetric distribution and asymmetric one of a longitudinal electric field across the vacuum channel. In the case of a general solution, the effect of the asymmetric part on the total amplitude of the accelerating and defocusing fields is also analyzed. We determined also conditions when a symmetric or asymmetric solution only is realized. For these cases, the obtained analytical solutions are compared with results of full time-domain numerical simulations of bilateral excitation of dielectric prisms with laser pulses.

Photonic-crystal-based high-performance ring resonator using a topological interface state: design and analysis.

We propose a photonic crystal ring resonator for the enhancement of quality factor that supports two-dimensionally bounded topological edge states. Crystal parameters are obtained through finite-difference time-domain numerical simulation to get the enhanced quality factor using the topological properties of the photonic crystal. Topological edge states are created when two regions with dissimilar band topologies come together at an interface and are contained within a slab of dielectric material. These edge states can move along sharp edges without backscattering. The transmission dropout issue arises whenever the quality factor is enhanced in a conventional photonic system and is eliminated remarkably by employing the present approach. Such nanoscale photonic crystal structures promote robust interactions between quantum emitters and photonic edge states.

Efficient terahertz optical filtering with large-area all-metal and polymer-metal woven wire meshes.

Many components for terahertz (THz) optical filtering are mechanically fragile and are hard to produce with large aperture, making them unsuitable for applications where larger THz beam diameter is required. In this work, the THz optical properties of industrial-grade, readily available and inexpensive woven wire meshes are studied using THz time-domain spectroscopy and numerical simulations. These meshes are meter-sized, free-standing sheet materials that are principally attractive for the use as robust, large-area THz components. Our results show that such meshes can act as efficient, tunable THz bandpass filters due to sharp plasmonic resonance supported by the interwoven metallic wires. Further, the meshes that combine metallic and polymer wires act as efficient THz linear polarizers with a polarization extinction ratio (field) above 60:1 for frequencies below 3 THz.

A homogenized model accounting for dispersion, interfaces and source points for transient waves in 1D periodic media

A homogenized model is proposed for linear waves in 1D microstructured media. It combines second-order asymptotic homogenization (to account for dispersion) and interface correctors (for transmission from or towards homogeneous media). A new bound on a second-order effective coefficient is proven, ensuring well-posedness of the homogenized model whatever the microstructure. Based on an analogy with existing enriched continua, the evolution equations are reformulated as a dispersive hyperbolic system. The efficiency of the model is illustrated via time-domain numerical simulations. An extension to Dirac source terms is also proposed.

Synthesis Methods and Optical Sensing Applications of Plasmonic Metal Nanoparticles Made from Rhodium, Platinum, Gold, or Silver.

The purpose of this paper is to provide an in-depth review of plasmonic metal nanoparticles made from rhodium, platinum, gold, or silver. We describe fundamental concepts, synthesis methods, and optical sensing applications of these nanoparticles. Plasmonic metal nanoparticles have received a lot of interest due to various applications, such as optical sensors, single-molecule detection, single-cell detection, pathogen detection, environmental contaminant monitoring, cancer diagnostics, biomedicine, and food and health safety monitoring. They provide a promising platform for highly sensitive detection of various analytes. Due to strongly localized optical fields in the hot-spot region near metal nanoparticles, they have the potential for plasmon-enhanced optical sensing applications, including metal-enhanced fluorescence (MEF), surface-enhanced Raman scattering (SERS), and biomedical imaging. We explain the plasmonic enhancement through electromagnetic theory and confirm it with finite-difference time-domain numerical simulations. Moreover, we examine how the localized surface plasmon resonance effects of gold and silver nanoparticles have been utilized for the detection and biosensing of various analytes. Specifically, we discuss the syntheses and applications of rhodium and platinum nanoparticles for the UV plasmonics such as UV-MEF and UV-SERS. Finally, we provide an overview of chemical, physical, and green methods for synthesizing these nanoparticles. We hope that this paper will promote further interest in the optical sensing applications of plasmonic metal nanoparticles in the UV and visible ranges.

Olver plasmon: an accelerating surface wave with various orders.

In this Letter, we introduce a new, to the best of our knowledge, class of accelerating surface plasmonic wave: the Olver plasmon. Our research reveals that such a surface wave propagates along self-bending trajectories at the silver-air interface with various orders, among which Airy plasmon is regarded as the zeroth-order one. We demonstrate a plasmonic autofocusing hot-spot by the interference of Olver plasmons and the focusing properties can be controlled. Also, a scheme for the generation of this new surface plasmon is proposed with the verification of finite difference time-domain numerical simulations.

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.

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 the 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.

Modeling sea state induced atmospheric sound transmission loss

This work presents a numerical study on atmospheric sound propagation over rough rigid surfaces. The intent is to simulate acoustic propagation over water. Methods to estimate sea state induced atmospheric sound transmission loss and relative uncertainties are evaluated. In previous studies, a flat surface with an equivalent impedance was used to account for the effect of surface roughness on sound transmission loss. Equivalent impedances were estimated based on time-domain numerical simulations of atmospheric sound propagation above pseudorandom sea surfaces coherent with a Pierson-Moskowitz spectra. Estimation of equivalent impedances using time- and frequency- domain approaches are compared for cases up to sea state 4. Acoustic excess attenuation due to propagation over a rough surface was predicted by the authors, in a previous work, by correcting the excess attenuation of a propagation over a flat, perfectly reflecting surface. The correction factor was frequency and sea state dependent and included an additional term to account for the uncertainties characteristic of different sea states. Excess attenuation predictions estimated using the equivalent impedance approach are compared with those obtained from the authors’ previous study. Implications of the use of these methods for source detectability determinations are discussed.