Time-domain Formulation Research Articles

Signal modulation waveform recognition method based on STF-Net

Signal modulation waveform recognition is one of the key technologies in the field of spatial spectrum cognition and an important means to realize the monitoring and control of spectrum resources of low-orbit satellites. In view of the problems of large number of parameters and high computational complexity in the current modulation waveform recognition methods based on deep learning, a lightweight signal modulation waveform recognition method based on space-time fusion network (STF-Net) is proposed. The signal is preprocessed into dual-channel data in the form of time domain and frequency domain, and the signal spatial features are extracted and feature redundancy is reduced by convolutional neural network (CNN). Then, the long short-term memory network (LSTM) is used to extract the timing information and output the recognition result. The experimental results show that when the signal-to-noise ratio is greater than 0 dB, the average recognition accuracy of the modulation waveform of the proposed method reaches 91.79%; compared with the same method, the number of parameters of the proposed method is reduced 96%and the efficiency is increased by 2.7 times.

Explainable AI-driven machine learning for heart disease detection using ECG signal

The majority of countries globally find that cardiovascular diseases have the greatest death rate. Early detection of heart disease is necessary to prevent deaths from it. Big data for medical diagnostics has made it easier to construct sophisticated machine learning (ML) and deep learning (DL)-based models for the early automatic identification of heart disease. Using three common ECG datasets, i.e., the Physionet Challenge 2016, the PASCAL Challenge Competition, and the MIT-BIH - random forest (RF) and extreme gradient boosting (XGBoost), two tree explainer classifiers are proposed for the detection of heart disease in this study. During pre-processing variety of techniques like bandpass filtering, denoising, butterworth filtering and zero-phase filtering of the signal are carried out. After the signals are restored to the time domain form using IDWT, DWT and EWT are utilized to extract features. SHapley Additive exPlanations (SHAP) is used for the feature importance which identifies important features that have more impact on the prediction output of the models. It is demonstrated from the simulation results that EWT with XGB performs well in the three datasets considered. RF explainer algorithm with EWT feature performs best and results in an AUC of 95.36 % for the PASCAL Challenge Competition dataset. However, with AUC values of 97.44 % and 98.25 % for the Physionet Challenge 2016 and MIT-BIH datasets, respectively, the XGB explainer algorithm with EWT features performs better than (DWT+XGB), (DWT+RF), and (EWT+RF). When all three datasets are compared to the current models, overall the suggested model performs better and has a higher AUC.

A new frequency domain method to solve the potential flow problem

The potential flow theory is a reliable tool widely-used in ocean engineering to solve the fluid hydrodynamic problems such as the wave hydrodynamic problems. The Rankine source and its image about the seabed is often chosen as the Green's function to solve the potential flow problems for its simple form, easy singular-integral treatment and few boundary conditions in the boundary integral equations. However, traditionally, the boundary element method based on it is only executed in the time domain by time stepping technique which requires lots of computation due to the time domain form of the damping zone's boundary condition. To apply this Green's function in frequency domain, this paper derived a parameterized frequency-domain formula for the boundary condition of the damping zone, and successfully applied the Rankine source and its image about the seabed in frequency domain. This paper's work possesses a great potential to promote the efficiency in solving the potential flow problems.

Self-stabilizing economic nonlinear model predictive control applied to modular systems

Recent advances have been made in self-stabilizing Economic Nonlinear Model Predictive Control (eNMPC) formulation without pre-calculated setpoints, which leverages norm-based steady-state optimality conditions to enhance system robustness. To enable practical implementation, a generalized time-domain formulation is proposed, accommodating the discrete-time nature of control instrumentation and the continuous-time nature of first-principles models. A case study involving a modular membrane reactor illustrates the applicability of self-stabilizing eNMPC in real-world industrial scenarios.

Information Design and Sharing in Supply Chains

We study the interplay between inventory replenishment policies and information sharing in the context of a two-tier supply chain with a single supplier and a single retailer serving an independent and identically distributed Gaussian market demand. We investigate how the retailer’s inventory policy impacts the supply chain’s cumulative expected long-term average inventory costs [Formula: see text] in two extreme information-sharing cases: (a) full information sharing and (b) no information sharing. To find the retailer’s inventory policy that minimizes [Formula: see text], we formulate an infinite-dimensional optimization problem whose decision variables are the MA([Formula: see text]) coefficients that characterize a stationary ordering policy. Under full information sharing, the optimization problem admits a simple solution and the optimal policy is given by an MA(1) process. On the other hand, to solve the optimization problem under no information sharing, we reformulate the optimization from its time domain formulation to an equivalent z-transform formulation in which the decision variables correspond to elements of the Hardy space H2. This alternative representation allows us to use a number of results from H2 theory to compute the optimal value of [Formula: see text] and characterize a sequence of ϵ-optimal inventory policies under some mild technical conditions. By comparing the optimal solution under full information sharing and no information sharing, we derive a number of important practical takeaways. For instance, we show that there is value in information sharing if and only if the retailer’s optimal policy under full information sharing is not invertible with respect to the sequence of demand shocks. Furthermore, we derive a fundamental mathematical identity that reveals the value of information sharing by exploiting the canonical Smirnov–Beurling inner–outer factorization of the retailer’s orders when viewed as an element of H2. We also show that the value of information sharing can grow unboundedly when the cumulative supply chain costs are dominated by the supplier’s inventory costs. Funding: R. Caldentey acknowledges the University of Chicago Booth School of Business for financial support. Supplemental Material: The online appendix is available at https://doi.org/10.1287/moor.2023.008 .

On-the-Fly Unsteady Adjoint Aerodynamic and Aeroacoustic Optimization Method

An on-the-fly unsteady adjoint-based aerodynamic and aeroacoustic optimization methodology is presented, aiming to achieve practical engineering applications to explore high-efficiency and low-noise design for aerodynamic shapes. Firstly, a novel on-the-fly hybrid CFD-CAA approach is developed with a close integration of unsteady Reynolds-averaged Navier–Stokes equations and a fully viscous time-domain FW-H formulation. Subsequently, an adjoint-based sensitivity analysis method is proposed for unsteady aerodynamic and aeroacoustic problems with either stationary or moving boundaries, wherein a unified architecture for discrete-adjoint sensitivity analysis of both aerodynamics and aeroacoustics is achieved by integrating the on-the-fly hybrid CFD-CAA approach. The on-the-fly approach facilitates direct evaluation of partial derivatives required for solving adjoint equations, eliminating the need for explicitly preprocessing flow and adjoint variables at all time levels in a standalone adjoint CAA solver and consequently substantially reducing memory consumption. The proposed optimization methodology is implemented within an open-source suite SU2. Results show that the proposed on-the-fly adjoint methodology is capable of achieving highly accurate sensitivity derivatives while significantly reducing memory requirements by an order of magnitude, and further demonstrations of single-objective and coupled aerodynamic and aeroacoustic optimizations highlight the potential of the proposed method in exploring high-efficiency and low-noise design for aerodynamic shapes.

Numerical prediction of mean flow and acoustic field of a supersonic impinging jet

The three-dimensional turbulent mean flow and acoustic field of a supersonic jet impinging on a solid plate is studied computationally using the general purpose CFD code Ansys Fluent. A pressure-based coupled solver formulation with the second order weighted central-upwind spatial discretization is applied to compute transient solutions. Cold and hot jet thermal conditions are considered. Mean flow characteristics are investigated by a steady-state modeling approach. Acoustic radiation of impingement tones is simulated using a transient time-domain formulation. The effects of turbulence in steady-state are modeled by the SST k-ω turbulence model. The Wall-Modeled Large-Eddy Simulation (WMLES) model is applied to compute transient solutions. The near-wall mesh on the impingement plate is fine enough to resolve the viscosity-affected near-wall region all the way to the laminar sublayer. Nozzle-to-plate distance is parameterized in the model for automatic re-generation of the mesh and results. Steady-state predictions of hover lift loss and mean jet velocity distributions are compared with experimental data, and favorable agreement is reported. The transient solution reproduces the mechanism of impingement tone generation by the interaction of large scale vortical structures with the impingement plate. The acoustic near-field is directly resolved by Computational Aeroacoustics (CAA) to accurately propagate impingement tone waves to near-field microphone locations. Calculated impingement tone frequencies and sound pressure levels agree with experimental values.

A Method to Assess Granger Causality, Isolation and Autonomy in the Time and Frequency Domains: Theory and Application to Cerebrovascular Variability.

Concepts of Granger causality (GC) and Granger autonomy (GA) are central to assess the dynamics of coupled physiologic processes. While causality measures have been already proposed and applied in time and frequency domains, measures quantifying self-dependencies are still limited to the time-domain formulation and lack of clear spectral representation. We embed into the linear parametric framework for computing GC from a driver X to a target process Y a measure of Granger Isolation (GI) quantifying the part of the dynamics of Y not originating from X, and a new spectral measure of GA assessing frequency-specific patterns of self-dependencies in Y. The measures are illustrated in theoretical simulations and applied to time series of arterial pressure and cerebral blood flow obtained in syncope subjects and healthy controls. Simulations show that GI is complementary to GC but not trivially related to it, while GA reflects the regularity of the internal dynamics of the target process. In the application to cerebrovascular interactions, spectral GA quantified the physiological response to postural stress of slow cerebral blood flow oscillations, while spectral GC and GI detected an altered response to orthostasis in syncope subjects, likely related to impaired cerebral autoregulation. The new spectral measures of GI and GA are useful complements to GC for the analysis of interacting oscillatory processes, and detect pathophysiological responses to postural stress which cannot be traced in the time domain. The thorough assessment of causality, isolation and autonomy opens new perspectives for the analysis of coupled processes in both physiological and clinical investigations.

Modeling and experiments on the vibro-acoustic analysis of ring stiffened cylindrical shells with internal bulkheads: A comparative study

The vibro-acoustic response of ring stiffened cylindrical shells with internal bulkheads under forced excitation is presented. The numerical analysis model is established using the Jacobi Ritz-Boundary element method. The first-order shear deformation theory, multi-segment technique and artificial spring technology are applied to establish the theoretical model, and the Jacobi orthogonal polynomials are introduced to represent the displacement functions. The Newmark-β integration method is used to obtain the vibration response of the structure, and the time-domain Kirchhoff boundary integral formulation is applied to describe the exterior acoustic field. The vibration and sound radiation of the ring stiffened cylindrical shell under impact and narrow band random loading were measured. The comparative study reveals that the results obtained from the proposed method agree well with the experimental results. The stiffeners and bulkheads have a significant effect on the vibration characteristics of the structure. Additionally, some physical insights into the resonant peaks and sound pressure directivity of the cylindrical shells are provided.

Pencirian Isyarat Data Muzik yang Terhasil dari Alat Muzik Tradisional Menggunakan Ciri Fraktal

Musical instruments are usually distinguished by their sound produced through human perception which may lead to misinterpretation due to auditory perception bias and other disturbances. Therefore, recognition using music signals is carried out to help characterize signals from different musical instruments. Fractal analysis is a mathematical tool used to study complex and irregular patterns in various systems. In this study, fractal analysis was used to study and analyze musical notes signal data from different instruments. The fractal analysis method used is the box counting method. The traditional musical instruments involved in this study are the seruling, cak lempong, kompang, and gambus. Matlab software was used to analyze the musical data signal. First, the fractal dimension of the music data signal in the form of time domain is calculated. Then, the mean and standard deviation of the fractal dimension values were determined to recognize different musical instruments. Additionally, different image resolutions and box sizes are also used to calculate the fractal dimension of musical instrument from the time domain data. The error of best fit line, E will also be calculated to ensure the reliability of box counting dimensions using leastsquares regression method. The results show that the value of the fractal dimension for all 18 data is between 1.2636 to 1.7543. In terms of musical instrument recognition, the recognition of seruling and kompang using mean fractal dimension is successful. However, this method is less suitable for identification for cak lempong and gambus due to the high standard deviation. By using different image resolutions and box sizes, the accuracy of fractal dimension values will be affected. Increasing these parameters increases the accuracy of the results. The results of the study show that an image with a resolution of 1024 x 1024 pixels and a scaling factor of 9 is suitable to analyze the fractal characteristics of musical instrument data signals by using the box counting method.

Explicit Definitions for the Electromagnetic Energies in Electromagnetic Radiation and Mutual Coupling

It is still difficult to accurately evaluate the reactive electromagnetic energy and the radiative electromagnetic energy of a radiator, because there are no explicit expressions for them. This paper proposes to borrow the energy concept in the charged particle theory and separate the total electromagnetic energy of a radiator into three parts: a Coulomb–velocity energy, a radiative energy and a macroscopic Schott energy. Consequently, the Poynting vector is considered to include a real radiative power flow by the radiative energy and a pseudo power flow caused by the fluctuation of the reactive energy. The energies involved in the electromagnetic mutual coupling are separated in a similar way. All energies are defined with explicit expressions in which the vector potential plays an important role. The time domain formulation and the frequency domain formulation of the theory are consistent with each other. The theory is verified with the Hertzian dipole. Numerical examples demonstrate that this theory may provide proper interpretations for electromagnetic radiation and mutual coupling problems.

Time-domain multiple traces boundary integral formulation for acoustic wave scattering in 2D

We present a novel computational scheme to solve acoustic wave transmission problems over two-dimensional composite scatterers, i.e. penetrable obstacles possessing junctions or triple points. The continuous problem is cast as a local multiple traces time-domain boundary integral formulation. For discretization in time and space, we resort to convolution quadrature schemes coupled to a non-conforming spatial spectral discretization based on second kind Chebyshev polynomials displaying fast convergence. Computational experiments confirm convergence of multistep and multistage convolution quadrature for a variety of complex domains.

Maximizing the surge amplitude of a floater through an adaptable mooring tightening technique

Abstract
 A technique to optimize the response of Wave Energy Converters (WECs) by maximizing the amplitude of motion along the surge direction is presented. This is achieved by utilizing an adaptable mooring tightening technique for a floater moored with tension legs (tendons). To gain a deeper understanding of the effect of sea states and mooring cable lengths on the surge response of the system, a series of numerical simulations were conducted for various wave conditions while varying the length of the mooring cables. WEC-Sim [1] was used to solve the multi-body dynamics of a rectangular cuboid floater by solving the equations of motion using a time-domain formulation. The dynamics of the mooring cables were simulated using the MoorDyn model with a lumped-mass formulation [2]. To validate the accuracy of the numerical methods, a series of experimental tests were performed in a small-scale wave tank. It was observed that tightening the mooring cables by decreasing their length amplifies the surge motion of the floater while the mooring forces in the heave direction rise due to the increased tension in the cables. Stretching the cables further was found to (i) decrease the surge amplitude and (ii) drastically increase the mooring forces, threatening the integrity of the cables. Therefore, there is an optimum value of the length of the cables that maximizes the surge amplitude of the floater while ensuring that the cables will not break. The impact of other mooring cable parameters such as diameter and material properties were also evaluated. More specifically, increasing the stiffness by increasing the diameter or the tensile modulus of elasticity was found to reduce the floater’s surge amplitude. For the geometry used in this study, the optimum length, diameter and properties of the cables are provided for several sea states. The current results lay the foundations for the design of new types of WECs that harness the surge motion of a floater rather than the heave which is the most common approach for floating WECs.
 References
 [1]: Kelley Ruehl, David Ogden, Yi-Hsiang Yu, Adam Keester, Nathan Tom, Dominic Forbush, Jorge Leon, Jeff Grasberger, and Salman Husain. (2022, September), WEC-Sim (Version v5.0.1), DOI: 10.5281/zenodo.7121186.
 [2]: Hall, M., & Goupee, A. (2015). Validation of a lumped-mass mooring line model with DeepCwind semisubmersible model test data. Ocean Engineering, 104, 590–603. https://doi.org/10.1016/j.oceaneng.2015.05.035

Structural pounding of three adjacent multi degree of freedom system under subsurface blast and seismic action by considering and ignoring the effect of soil structure interaction

A lack of spacing between the adjacent structures is mostly observed in metropolitan cities due to more land cost, which has led to more serious problem called as structural pounding in many recent earthquakes. Large impact forces and short acceleration pulses are mostly noticed during the structural pounding, and which is hazardous to structural health of the buildings involved in this phenomena. In the present paper three adjacent multi-degree-of-freedom linear elastic systems subjected to seismic and subsurface blast actions are analyzed by considering and ignoring the effect of soil structure interactions. Three dimensional buildings are analyzed and designed for deciding the column and beam sizes in SAP 2000 NL software considering all possible load combinations of dead load, live load and earthquake load as per IS 1893:2016 Part I. The subsurface indirect method is used for studying the effect of soil structure interaction. The SSI spring stiffness’s and damping constants are calculated as per FEMA 356. These buildings are converted into the lumped mass model as per the principles of dynamics of structures. The entire analysis of three adjacent lumped mass models with insufficient separation is carried out in time domain form using MATLAB programming. The outputs are measured in the forms of top storey displacement time history plots, top floor level pounding force time history plots and base shear force time history plots. From the outputs, it is noticed that the SSI amplifies the displacement response of building than that of fixed base system. SSI also overrates the pounding response of structure. Blast force response is much smoothens than seismic response of structure.

Time-Domain Meshless Method Simulation of Metasurfaces With Generalized Sheet Transition Conditions

This paper presents a new time-domain formulation of a meshless method based on the generalized sheet transition conditions (GSTCs). This formulation is applicable for modeling a kind of dispersive metasurface and a time-varying metasurface. This is the first time a meshless method is developed for analysis of the metasurfaces using GSTCs. The significance of time-domain meshless methods originates from their capabilities for multi-scale and conformal modeling in addition to analysis of problems in a broad frequency-range. Here, for simplicity, the meshless formulation is presented for analysis of 1-D problems consisting of monoisotropic metasurfaces. The extension of this method to 2-D and 3-D problems or bianisotropic metasurfaces is straightforward. The scheme is verified by numerical examples.

Equilibrium Violation From the Complex Stiffness

Abstract Evidence from cyclic tests on metals, elastomers, and sandy soils reveals that damping forces are nearly rate-independent and structural (hysteretic or rate-independent) damping was widely adopted since the 1940s. While there is no time-domain constitutive equation for a linear spring connected in parallel with a rate-independent dashpot, the dynamic stiffness (transfer function) of this mechanical network can be constructed in the frequency domain; and it was known since the early 1960s that this mechanical network exhibits a non-causal response. In view of its simplicity in association with the wide practical need to model rate-independent dissipation, this mechanical network was also implemented in time-domain formulations with the label complex stiffness where the force output, P(t) is related in the time domain to the displacement input, u(t), with P(t) = k(1 + iη)u(t). This paper shows that the complex stiffness, as expressed in the time domain by various scholars, is a fundamentally flawed construct since in addition to causality it violates equilibrium.

Time-domain viscoelastic analysis of laminated composite plates by using a unified formulation

This paper presents the time-domain viscoelastic formulation based on the Carrera unified formulation (CUF) to obtain the response of composite structures submitted to bending loads. The governing equations are obtained through a modified form of the principle of virtual displacement (PVD) for a viscoelastic analysis. The problem is solved using the Laplace transform instead of direct integration to reduce the computational cost. The results demonstrate the strong capability of the time-domain viscoelastic formulation. Overall, the proposed methodology allows the direct implementation of new optimized high order deformation theories including the thickness stretching effect that may be implemented in futures works.

A Numerical Scheme for Fast and High Accuracy Simulation of Motion-Induced Eddy Current Testing Signals

ABSTRACT In this paper, an efficient numerical scheme and corresponding fast solver are proposed and implemented for rapid and high-accuracy numerical calculation of the motion-induced eddy current testing (MIECT) forward problems. First, inspired by the time-domain numerical formulation of the MIECT problem, a DFT-based numerical scheme in the frequency-domain was proposed through DFT of the transient excitation magnetic field signals at conductor surface caused by the moving permanent magnet of the MIECT probe. Second, a fast-forward simulator was implemented by further improving the existing Ar code for single-frequency ECT problem and adopting databases approach using the unflawed field information solved and stored a priori. As the key of the fast simulation of the MIECT signal perturbed by defects, a scheme to calculate the pickup signal perturbations was proposed and validated based on the reciprocity theorem. Finally, the validity of the numerical schemes and corresponding fast-forward solver was verified through comparing simulation results with experimental signal. Compared with the full analysis domain methods, the fast simulator can save computational burden up to 1000 times on the premise of not affecting the numerical precision, which enables its being adopted in the inverse analysis of MIECT signals for defect sizing.

A new time-domain boundary element formulation for generalized models of viscoelasticity

The contribution is concerned with the novel algorithmic formulation for generalized models of viscoelasticity under quasi-static conditions within the framework of the boundary element method (BEM). The proposed update algorithm is constructed for a generic rheological model of linear viscoelasticity that can either be straightforwardly simplified to recover the basic Kelvin and Maxwell models or readily furthered towards the generalized models of viscoelasticity through the serial or parallel extensions. In contrast to the scarce existing rate formulations of inelasticity developed within BEM, the put forward non-iterative formulation exploits the hybrid semi-implicit update of strain-like kinematic history variables. The challenge arising from the indispensable domain integrals is overcome through the mesh-free Cartesian transformation method (CTM), complemented by the radial point integration (RPIM) technique. The excellent performance of the proposed approach is demonstrated in comparison with the corresponding analytical and finite element results for boundary-value problems with uniform and non-uniform strain fields under different representative modes and types of loading.

Development and design of a deep tank damper with submerged suspended plate

An appendage, in the form of a submerged suspended plate (SSP) in a liquid-containing deep tank, is proposed for converting functional liquid storage tanks into passive tank dampers for structural vibration mitigation. The oscillating frequency of the SSP is designed to be tuned to the structural frequency so that the tank damper is independent of detuning effects that could be caused by liquid level fluctuations. Further, the impulsive, rather than the sloshing, liquid mass is utilized here. This alleviates the disadvantage of low energy dissipation associated with the relatively smaller proportion of sloshing liquid mass of a deep tank. The theoretical model and working mechanism of the deep tank damper with submerged suspended plate (DTD-SSP) are developed and the expression for the tuning frequency derived. A time-domain formulation for the structure-DTD-SSP system is presented and the design and effectiveness of the DTD-SSP for an example structure under base excitation examined. Further, the performance sensitivity to tuning ratio and comparison with the traditional tuned mass damper (TMD) are presented. Results indicate that with the proposed design, liquid storage tanks installed for other functional requirements can be converted into DTD-SSPs that can achieve significant response reductions comparable with that of TMDs.