Time-domain Code Research Articles

Coupling a time-domain code with a RANS solver for slamming analysis

ABSTRACT A time domain hydroelastic code (TD) based on strip theory is two-way coupled with a Reynolds Averaged Navier−Stokes (RANS) solver to capture the bottom slamming effects on ships. The study is conducted on an Ultra Large Container Ship (ULCS). The methodology considers the hull’s flexibility and analyses longitudinal bending in head seas. Ship motions are calculated using a body non-linear time-domain method based on strip theory. The equations of motion are solved, and time integration is performed using the fourth-order Runge–Kutta method. When a slamming event is identified, the TD code activates a RANS solver to determine the bottom slamming force. The simulations are conducted under extreme sea conditions, and the hull experiences springing and whipping responses. The numerical results of vertical bending moments are compared with experimental data. The bottom and bow flare slamming are identified from the simulations.

Improved Earthquake Source Parameters with 3D Wavespeed Models in California and Nevada

Abstract Seismic tomography harnesses earthquake data to explore the inaccessible structure of the Earth. Adjoint waveform tomography (AWT), a method of seismic tomography, updates the tomographic model by optimizing the fit between observed earthquake data and synthetic waveforms. The synthetic data are calculated by solving the wave equation through a given 3D model. An important requirement to calculating synthetics is the source information (location, centroid time, depth, and moment tensor). Errors in source information affect the quality of the synthetics produced, which in turn can limit how structure can be inferred in the AWT workflow. To test the effect of updating source information, we used MTTime (Chiang, 2020), a time-domain full-waveform moment tensor inversion code, to calculate the moment tensors and depths of 118 earthquakes that occurred in California and Nevada over a 20-yr period. We calculated 3D Green’s functions using a 3D seismic wavespeed model of California and Nevada (Doody et al., 2023b). We show that the inverted solutions provide better waveform fits than the Global Centroid Moment Tensor catalog and increase usable, well-correlated data by up to 7%. Therefore, we argue that recalculating source parameters should be considered in AWT workflows, particularly for smaller magnitude events (Mw<5.0).

Metric perturbations of Kerr spacetime in Lorenz gauge: circular equatorial orbits

We construct the metric perturbation in Lorenz gauge for a compact body on a circular equatorial orbit of a rotating black hole (Kerr) spacetime, using a newly-developed method of separation of variables. The metric perturbation is formed from a linear sum of differential operators acting on Teukolsky mode functions, and certain auxiliary scalars, which are solutions to ordinary differential equations in the frequency domain. For radiative modes, the solution is uniquely determined by the s=±2 Weyl scalars, the s = 0 trace, and s=0,1 gauge scalars whose amplitudes are determined by imposing continuity conditions on the metric perturbation at the orbital radius. The static (zero-frequency) part of the metric perturbation, which is handled separately, also includes mass and angular momentum completion pieces. The metric perturbation is validated against the independent results of a 2+1D time domain code, and we demonstrate agreement at the expected level in all components, and the absence of gauge discontinuities. In principle, the new method can be used to determine the Lorenz-gauge metric perturbation at a sufficiently high precision to enable accurate second-order self-force calculations on Kerr spacetime in future. We conclude with a discussion of extensions of the method to eccentric and non-equatorial orbits.

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.

Radar Micro-Doppler Signature Generation Based on Time-Domain Digital Coding Metasurface.

Micro-Doppler effect is a vital feature of a target that reflects its oscillatory motions apart from bulk motion and provides an important evidence for target recognition with radars. However, establishing the micro-Doppler database poses a great challenge, since plenty of experiments are required to get the micro-Doppler signatures of different targets for the purpose of analyses and interpretations with radars, which are dramatically limited by high cost and time-consuming. Aiming to overcome these limits, a low-cost and powerful simulation platform of the micro-Doppler effects is proposed based on time-domain digital coding metasurface (TDCM). Owing to the outstanding capabilities of TDCM in generating and manipulating nonlinear harmonics during wave-matter interactions, it enables to supply rich and high-precision electromagnetic signals with multiple micro-Doppler frequencies to describe the micro-motions of different objects, which are especially favored for the training of artificial intelligence algorithms in automatic target recognition and benefit a host of applications like imaging and biosensing.

Development and validation of the in-house hydrodynamics code and the DNV software for TALOS wave energy converter

The Lancaster in-house code is a time-domain analysis code, developed especially for the TALOS wave energy converter, a novel wave energy conversion technology. The TALOS wave energy converter is a point absorber-like wave energy converter, but with a unique power take-off (PTO) system. The whole PTO system is fully enclosed within the structure, hence no moving parts are exposed to the harsh marine environments.
 The PTO of the TALOS WEC consists of a mass ball within the structure, with a number of springs and dampers (for instance, direct drives or hydraulic systems) connecting between the mass ball and the structure. Such an arrangement of the PTOs would make the PTO essentially non-linear, regardless whether the actual PTO dampers are linear or nonlinear as well as the linear springs. Therefore, a time domain analysis must be established for the TALOS WEC. Towards the goal, an in-house time-domain model has been developed at Lancaster University, and now it is to be validated via the delicated numerical models built using well-established commercial software (and in the future via experimental data). Within this framework, the present paper presents an effort for validation using the DNV SESAM commercial hydrodynamic and structural analysis software. 
 To build a numerical model for TALOS WEC, the Lancaster in-house time domain code is based on the hybrid frequency-time domain approach. That is, the basic hydrodynamic parameters are analysed using the panel methods (WAMIT, HAMS, NEMOH), and then the relevant parameters are transformed for the Cummins’ time-domain equation, such as the added mass at infinite frequency, and the memory effect (the convolution terms). This work is an extension of the comparison between the in-house code and the DNV SESAM software, and the comparisons would include the relevant hydrodynamic analyses, the time-domain analyses with and without the TALOS PTO system, with the aims towards developing the validated tools of both the in-house code and the commercial software.

Nonlinear time-domain soil-structure interaction analysis of a mid-rise building benchmarked against earthquake recordings: A case study of the BRI's annex building

Performing soil-structure interaction (SSI) analysis under strong shaking scenarios using the direct method is becoming more prevalent in engineering practice. However, given the pronounced nonlinearity in soil, structure, and foundation response, the robustness of current methodologies is open to question, and benchmarking these methodologies against instrumented case histories is needed to identify the strengths and weaknesses in the approach. In this study, a three-dimensional (3D) Nonlinear (NL) Soil-Structure Interaction (SSI) analysis methodology in the time-domain code LS-DYNA is benchmarked against the actual recordings for a mid-rise building. The well-instrumented Building Research Institute (BRI) Annex Building (ANX) in Tsukuba City northeast of Tokyo, Japan, which features an adjacent free-field geotechnical downhole array, is used for this case study. A Finite Element (FE) three-dimensional 3D SSI model of the site, including the BRI ANX building, that considers both material and geometric nonlinearities is first developed. Two events are applied to the model: the 2011 Tohoku-Oki earthquake main shock (Mw9.1), referred to as the Great East Japan Earthquake (GEJE), and the 2002 Southern Ibaraki Prefecture earthquake (Mw4.9), selected from the BRI strong motion network due to the relatively wide amplitudes recorded at the site. The former event induced maximum shear strains in the soil of 1.6%, while the latter event induced maximum shear strains of 0.08%. For both horizontal and vertical components of shakings, the predicted response of the structure and adjacent free-field downhole array are compared with the corresponding recordings. The results show that the structural response is significantly affected by the accuracy of the predicted free-field ground surface response, especially at the structural fundamental period as opposed to the site period. In addition, results indicate that the fixed-base and flexible-base structural models’ responses are substantially different, mainly attributed to the SSI effects that cannot be captured in the fixed-base model. Finally, a comprehensive discussion on the minimum amount of data required to adequately capture nonlinear structural response using the proposed SSI methodology is provided in this case study.

Two-dimensional direction-of-arrival estimation based on time-domain-coding digital metasurface

Direction-of-arrival (DOA) estimation is one of the most critical technologies of radar, remote sensing, and wireless communications. The traditional DOA estimation is closely related to phased array antennas, which require complicated and expensive hardware and high-power consumption. Metasurface can manipulate electromagnetic waves without using massive transceivers, which makes it possible to realize antenna arrays in a cost-effective way. Here, we propose a strategy of DOA estimations by using a time-domain-coding digital metasurface and a single receiver. Specifically, the incident wave on the metasurface is modulated by the time-domain orthogonal codes impressed on the meta-atoms, and their amplitude and phase distributions are precisely retrieved from the signals detected by the receiver antenna. The effectiveness and accuracy of the proposed strategy are verified by experiments on a two-dimensional metasurface with individually addressable meta-atoms. The strategy features low cost and high flexibility and will facilitate various wireless applications.

Numerical modeling of T-wave generation in a horizontally inhomogeneous ocean

T-waves are generated by earthquakes and travel horizontally with the speed of sound. They are the most common earthquake sounds in the ocean and make strong contributions to the ambient sound field. However, the mechanisms of their generation remain poorly understood. Recent theoretical paper [O. A. Godin, J. Acoust. Soc. Am. 150, 3999–4017 (2021)] argues that scattering of ballistic waves from the earthquake focus by the ocean surface is a strong source of T-waves, which should be considered on par with previously identified generation mechanisms of wave diffraction on large bathymetric features and scattering by the rough seafloor. This paper compares the T-waves due to ocean surface roughness and a seamount in deep water. 3-D surface scattering and wave interaction with large-scale bathymetric features are modeled rigorously using time-domain spectral-element code SPECFEM3D, with the ocean surface being shaped by wind waves and sea swell and a large seamount located off the earthquake epicenter. These simulations are complemented by a normal-mode code to propagate the sound field from the generation region to long ranges. A mode projection technique is developed to efficiently couple SPECFEM3D to the normal mode code. The two mechanisms are found to generate T-waves of comparable amplitudes.

Numerical analysis of the limits of effective medium theory for nonlinearly propagating dispersive waves

Acoustic metameterials are an increasingly popular approach to control both linear and nonlinear sound propagation. In this approach, heterogeneous structures are engineered to behave as predetermined continuous media. A key requirement for a system to be accurately approximated as a homogeneous material for wave motion is that the smallest length scale of the waveform be much larger than any length scale associated with the microscale structure of the system. However, the fact that the characteristic length scales of a finite-amplitude waveform can change due to nonlinear effects means that care must be taken when analyzing such signals in an acoustic metamaterial. This talk presents recent work toward understanding the breakdown of effective medium theory as finite-amplitude waves propagate in a heterogeneous system and weak shocks or solitons begin to form. A finite-difference time-domain code was used to model the propagation of a pulse of sound in a one-dimensional propagation domain using both a full description of the system and an effective-medium description accounting for effective mass density, compressibility, coefficient of nonlinearity, and dispersion. The difference between the solutions of the two models provides a method by which one can estimate the error associated with using effective properties.

Joint Estimation for Time Delay and Direction of Arrival in Reconfigurable Intelligent Surface with OFDM.

Recently, the joint estimation for time delay (TD) and direction of arrival (DOA) has suffered from the high complexity of processing multi-dimensional signal models and the ineffectiveness of correlated/coherent signals. In order to improve this situation, a joint estimation method using orthogonal frequency division multiplexing (OFDM) and a uniform planar array composed of reconfigurable intelligent surface (RIS) is proposed. First, the time-domain coding function of the RIS is combined with the multi-carrier characteristic of the OFDM signal to construct the coded channel frequency response in tensor form. Then, the coded channel frequency response covariance matrix is decomposed by CANDECOMP/PARAFAC (CPD) to separate the signal subspaces of TD and DOA. Finally, we perform a one-dimensional (1D) spectral search for TD values and a two-dimensional (2D) spectral search for DOA values. Compared to previous efforts, this algorithm not only enhances the adaptability of coherent signals, but also greatly decreases the complexity. Simulation results indicate the robustness and effectiveness for the proposed algorithm in independent, coherent, and mixed multipath environments and low signal-to-noise ratio (SNR) conditions.

The influence of intra-cortical microstructure on the contrast in ultrasound images of the cortex of long bones: A 2D simulation study

Decreased thickness of the bone cortex due to bone loss in the course of ageing and osteoporosis is associated with reduced bone strength. Cortical thickness measurement from ultrasound images was recently demonstrated in young adults. This requires the identification of both the outer (periosteum) and inner (endosteum) surfaces of the bone cortex. However, with bone loss, the cortical porosity and the size of the vascular pores increase resulting in enhanced ultrasound scattering which may prevent the detection of the endosteum. The aim of this work was to study the influence of cortical bone microstructure variables, such as porosity and pore size, on the contrast of the endosteum in ultrasound images. We wanted to estimate the range of these variables for which ultrasound imaging of the endosteum is feasible. We generated synthetic data using a two-dimensional time-domain code to simulate the propagation of elastodynamic waves. A synthetic aperture imaging sequence with an array transducer operating at a center frequency of 2.5 MHz was used. The numerical simulations were conducted for 105 cortical microstructures obtained from high resolution X-ray computed tomography images of ex vivo bone samples with a porosity ranging from 2% to 24 %. Images were reconstructed using a delay-and-sum (DAS) algorithm with optimized f-number, correction of refraction at the periosteum, and sample-specific wave-speed. We observed a range variation of 18 dB of endosteum contrast in our data set depending on the bone microstructure. We found that as porosity increases, speckle intensity inside the bone cortex increases whereas the intensity of the signal from the endosteum decreases. Also, a microstructure with large pores (diameter >250 μm) was associated with poor endosteum visibility, compared with a microstructure with equal porosity but a more narrow distribution of pore sizes. These findings suggest that ultrasound imaging of the bone cortex with a probe operating at a central frequency of 2.5 MHz using refraction-corrected DAS is capable of detecting the endosteum of a cortex with moderate porosity (less than about 10%) if the largest pores remain smaller than about 200 μm.

Feasibility of a finite-difference time-domain model in large-scale acoustic simulations.

Wave-based techniques for room acoustics simulations are commonly applied to low frequency analysis and small-sized simplified environments. The constraints are generally the inherent computational cost and the challenging implementation of proper complex boundary conditions. Nevertheless, the application field of wave-based simulation methods has been extended in the latest research decades. With the aim of testing this potential, this work investigates the feasibility of a finite-difference time-domain (FDTD) code simulating large non-trivial geometries in wide frequency ranges. A representative sample of large coupled-volume opera houses allowed demonstration of the capability of the selected FDTD model to tackle such composite geometries up to 4 kHz. For such a demanding task, efficient calculation schemes and frequency-dependent boundary admittances are implemented in the simulation framework. The results of in situ acoustic measurements were used as benchmarks during the calibration process of three-dimensional virtual models. In parallel, acoustic simulations performed on the same halls through standard ray-tracing techniques enabled a systematic comparison between the two numerical approaches highlighting significant differences in terms of input data. The ability of the FDTD code to detect the typical acoustic scenarios occurring in coupled-volume halls is confirmed through multi-slope decay analysis and impulse responses' spectral content.

Light-controllable time-domain digital coding metasurfaces

Programmable metasurfaces enable real-time control of electromagnetic waves in a digital coding manner, which are suitable for implementing time-domain metasurfaces with strong harmonic manipulation capabilities. However, the time-domain metasurfaces are usually realized by adopting the wired electrical control method, which is effective and robust, but there are still some limitations. Here, we propose a light-controllable time-domain digital coding metasurface consisting of a full-polarization dynamic metasurface and a high-speed photoelectric detection circuit, from which the microwave reflection spectra are manipulated by time-varying light signals with periodic phase modulations. As demonstrated, the light-controllable time-domain digital coding metasurface is illuminated by the light signals with two designed time-coding sequences. The measured results show that the metasurface can well generate symmetrical harmonics and white-noise-like spectra, respectively, under such cases in the reflected wave. The proposed light-controllable time-varying metasurface offers a planar interface to tailor and link microwaves with lights in the time domain, which could promote the development of photoelectric hybrid metasurfaces and related multiphysics applications.

Data-based energy coverage measurements to discover the potentials of regional energy storage

Abstract The fluctuations of the renewable energies request storage systems as a significant part of the future energy system. Here we introduce a measurement approach regarding the regional coverage of the power demand and the influence of a storage system on it. The approach is based on measured power data with given uncertainties. The resolution is 15 minutes in the time domain and 2-digit ZIP code regions in the spatial domain. An idealized storage model is introduced, and one storage is implemented for each region. As a result, the regional coverage is measured for different storage parameters and the respective measurement uncertainty is assessed to be below the set goal of 2 %. Furthermore, the inverse problem, i. e. the storage dimensioning, is investigated by estimating the necessary storage parameters to achieve a desired regional coverage. Findings: electrical self-sufficiency can be reached in 23 out of 95 regions, in contrast in 56 regions the regional coverage is increased insignificantly regardless of the parameters of the storage system. This illustrates the necessity of power transport to enable the increase of regional coverage. Concluding, the proposed measurement approach enables the estimation of storage parameters and can be used for storage dimensioning.

Attenuated orthotropic time-domain half-space BEM for SH-wave scattering problems

SUMMARY In this paper, the complete formulation of the attenuated orthotropic time-domain half-space boundary element method was proposed to analyse the transient SH-wave scattering problems. By changing the spatial variable, the time-domain half-space Green's functions were obtained in an isotropic-like analytical process by solving the singular form of the scalar wave equation. The stress-free boundary condition of the ground surface was satisfied using the wave source image theory. To include the materials damping, the Barkan approach was used in the formulation to attenuate the half-space by applying a constant logarithmic reduction into the modified boundary integral equation. The closed-form attenuated orthotropic half-space scalar kernels were obtained in the time-domain for displacement/traction fields by analytical integration of Green's functions. The method was easily implemented in a time-domain computer code to analyse the seismic homogenous orthotropic medium. To elaborate the model of the heterogeneous problems, a substructuring approach was presented to satisfy the continuity conditions at the interface depending on the position of the node and normal. Then, several seismic problems including a surface canyon, an underground cavity, a subsurface inclusion, and an alluvial valley subjected to SH wave was solved and compared with the literature to validate the surface response in the case of isotropic convergence. To evaluate the accuracy and the time of analysis for the proposed method versus the full-space boundary element models, a comparative test was carried out for two examples. A favourite agreement was testified between the responses to verify the proposed method's capability for simple modelling of the orthotropic topographic features. Finally, the ability of the method to analyse the problems with high degrees of freedom was investigated by solving a half-space model including 25 subsurface orthotropic circular inclusions.

Acoustic barcode based on the acoustic scattering characteristics of underwater targets

Underwater target acoustic scattering research is an important branch in the field of underwater acoustic engineering. The application of acoustic scattering characteristics to acoustic coding has great prospects for underwater target identification, navigation and positioning. Based on the echo sound path difference, discontinuous ribbed structures are designed for polydirectional time-domain coding. For the frequency-domain case, geometric Bragg scattering characteristics are transformed into vertical graphical representation through azimuthal coordinate transformation to enable coding. Then, the periodically ribbed structure and the spiral groove structure are designed for frequency-domain coding in the middle- and high-frequency ranges, respectively. Low-frequency coding is completed by exploiting the subsonic Rayleigh wave resonance characteristics of polymer sphere combinations from frequency and azimuthal dimensions. In addition, the use of acoustic scattering characteristics for acoustic coding can be further developed to specifically design coding methods for different structural forms and expand the application scope of underwater acoustics.

Modal pencil method for the radiation of guided wave fields in composite plates of finite size

Quantitative prediction of guided wave fields (GW) radiated by ultrasonic sources in parts is a generic problem whose solutions are important in many applications of quantitative non-destructive evaluation (NDE) and structural health monitoring (SHM). NDE and SHM methods relying on GW are very challenging to optimize due to the complexity of GW propagation. This is all the more difficult since composite materials are concerned and since the finite size of the piece cannot be ignored. A simulation tool for choosing the most appropriate transducer in a given application, for optimally positioning transducers by considering the actual testing configuration etc., would be most valuable to practitioners. In order to develop such a tool, a semi-analytical model is derived to predict the modal radiation by an arbitrary source of surface stress of guided waves in a finite anisotropic plate. Considering arbitrary source of surface stress allows us to cover most ultrasonic sources. Considering composite plates of finite size allows us to deal with configurations of industrial interest involving complex wavepaths from the source to field points. Considering modal solutions allows one to more easily interpret complex waveforms. This model combines and generalizes an approximate model predicting the modal Green's function for GW in an infinite anisotropic plate and an approximate model predicting the modal Green's function for GW in a finite isotropic plate. It is validated by comparing its predictions to those computed by a time-domain finite element code for elastodynamics.

Numerical simulation of a vessel’s manoeuvring performance in regular waves

ABSTRACT This paper deals with the numerical simulation of a ship’s manoeuvring performance in regular waves. This is made possible by employing the time-domain code ELIGMOS, which adopts a hybrid formulation to couple seakeeping and manoeuvring contributions. The first and steady second-order wave-induced forces are incorporated, implementing a multidimensional interpolation scheme to account for their dependency on the instantaneous heading and forward speed values. Two methods have been adopted for the calculation of the added resistance regarding the size of the wavelength (short or long wave seas), namely a far and a near-field one, using the hydrodynamic software NEWDRIFT+ and NEWDRIFT v.7. Low-frequency hydrodynamic manoeuvring forces are incorporated by adopting the expressions suggested by the Japanese MMG (Manoeuvring Modelling Group) method. Validation of the suggested methodology is attempted through the comparison of numerical simulations of turning circle tests in calm water and in regular waves with available experimental evidence for the S-175 container ship.

Range Dividing MIMO Waveform for Improving Tracking Performance.

A multiple-input multiple-output (MIMO) method that shares the same frequency band can efficiently increase radar performance. An essential element of a MIMO radar is the orthogonality of the waveform. Typically, orthogonality is obtained by spreading different signals into divided domains such as in time-domain multiplexing, frequency-domain multiplexing, and code domain multiplexing. This paper proposes a method of spreading the interference signals outside the range bins of interest for pulse doppler radars. This is achieved by changing the pulse repetition frequency under certain constraints, and an additional gain can be obtained by doppler processing. This method is very effective for improving the angular accuracy of the MIMO radar for a small number of air targets, although it may have limitations in use for many targets or in high clutter environments.