Global Time Domain Research Articles

Receding horizon control for persistent monitoring tasks with monitoring count requirements

AbstractThis article presents a reachability‐based receding horizon control (RHC) method for addressing persistent monitoring problems with count requirements. An agent is assigned to monitor multiple targets in a given environment to minimize the average uncertainty metric of all targets, while ensuring the monitoring count requirements of specific targets within predetermined time windows. To account for the spatial and temporal constraints in the monitoring requirements, a persistence predicate within the signal temporal logic (STL) specifications is introduced, which incorporates cumulative target state signals to effectively describe the monitoring count constraints. Considering the complexities arising from global time domain information requirements in STL constraints validation, an STL formula segmentation method based on completion progress is proposed. Subsequently, a reachability‐based controller for the agent is developed by solving a short‐term RHC problem while ensuring the satisfaction of the STL formulae. Simulation results are provided to illustrate the performance of proposed method.

Numerical Investigation into the Dynamic Responses of Floating Photovoltaic Platform and Mooring Line Structures under Freak Waves

Floating photovoltaics (PVs) are progressively constructed in the ocean sea; therefore, the effect that freak waves have on their structural design needs to be considered. This paper developed a dedicated numerical model coupling the floating PV platform and mooring line structures to investigate their dynamic responses under freak waves. A feasible superposition approach is presented to generate freak wave sequences via the combination of transient waves and random waves. A large floating PV platform moored by twenty lines for a water depth of 45 m was designed in detail according to the actually measured ocean environmental and geological conditions. The global time domain analyses of the floating PV mooring structures were implemented to obtain dynamic responses, including PV platform motions and the mooring line configuration and tension under freak waves. A comparison of the response results with those caused by random waves was conducted to illustrate the intuitive evidence of the freak wave effects, which offer a significant reference for the preliminary design of the floating PV platform and mooring line structures.

Dynamic Response Analysis of High-Speed Maglev-Guideway System

PurposeMaglev train travel is an efficient, modern and unconventional mode of transportation of passengers that has many advantages over the conventional railway transportation. In practice, maglev trains are primarily used for city transportation and connection with the airports. They often travel on elevated guideway bridges. This paper is concerned with developing a computationally efficient and accurate numerical method for the dynamic response of a maglev train traversing an “infinitely” long multi-span guideway bridge.MethodsThis study is based on numerical analysis in the time domain. Each guideway span is modelled as a simply supported beam with rotational springs connecting to the adjacent spans. The maglev vehicle is modelled by employing the multi-body system. The vehicle and the guideway are coupled via the electromagnetic force. In the numerical analysis, a computational scheme in conjunction with the MEM is proposed for the global time-domain simulations.ResultsThe accuracy of the proposed computational model is validated by comparison with available data for a maglev test line in the literature. Thereafter, parametric studies are conducted to examine the effects of train speed, stiffness of the suspension system and the coupling connection between adjacent guideway beams, and guideway irregularity on the dynamic response of the train. Results show that a stiffer guideway coupling connection helps to reduce the vertical acceleration of the car body and the vertical displacement of the guideway for the parameters considered in the study.ConclusionsThe computational model presented in this study in conjunction with the moving element method has the advantage of computationally efficient analysis and accurate prediction of the dynamic responses of a maglev train traversing an “infinitely” long multi-span guideway bridge over the conventional finite-element method, especially when the train is travelling at high speeds. The method can be extended to further consider more complex and practical cases.

(Retracted) Cross-spectral human behavior recognition based on deep convolutional networks for global temporal representation

The purpose of behavior recognition is to recognize the actions of the human body in action. It plays a great role in surveillance, video recommendation, and human-computer interaction with video. With the rise of neural networks, behavior recognition has also continued to develop and progress and has reached a relatively advanced level. However, behavior recognition is still insufficient in recognizing complex human movements and recognizing videos in different bands. To solve this problem, this paper establishes a convolutional neural network (CNN) cross-spectral human behavior recognition algorithm based on global time domain representation. It adopts the method of time-domain feature extraction, construction of optimized convolutional neural network layers, and global time-domain cross-spectrum construction. It also uses videos from the unified compliance framework (UCF)-sports and UCF-11 datasets for experiments. Experiments show that the algorithm achieves an average accuracy of 90% in the behavior recognition of UCF-sports. It still maintains an average accuracy rate of >90 % in the more complex behavior recognition of UCF-11, and the highest accuracy rate is 93%.

Windowed space–time least-squares Petrov–Galerkin model order reduction for nonlinear dynamical systems

Projection-based reduced-order models (PROMs) restrict the full-order model (FOM) to a low-dimensional subspace. Space–time PROMs in particular restrict the FOM to a temporally space–time trial subspace, and compute approximate solutions through a residual orthogonalization or minimization process. One such technique of interest is the space–time least-squares Petrov–Galerkin method (ST-LSPG), which reduces both the spatial and temporal dimensions. However, ST-LSPG is computationally expensive, because it requires solving a dense space–time system with a space–time basis that is calculated over the entire global time domain, which can be unfeasible for large-scale applications. To address these challenges, this paper presents the windowed space–time least-squares Petrov–Galerkin method (WST-LSPG) for model reduction of nonlinear parameterized dynamical systems. The proposed WST-LSPG approach addresses the aforementioned challenges by (1) dividing the time simulation into windows, (2) devising a unique low-dimensional high-fidelity space–time trial subspace for each window, and (3) minimizing the discrete-in-time space–time residual of the dynamical system over each window. In this formulation, the problem confines coupling within each window, but solves space–time residual minimization problems sequentially across the windows. WST-LSPG is equipped with hyper-reduction techniques to further reduce the computational cost. Numerical experiments for the one-dimensional Burgers’ equation and the two-dimensional compressible Navier–Stokes equations for flow over a NACA 0012 airfoil demonstrate that WST-LSPG is superior to ST-LSPG in terms of accuracy and computational gain by as much as 99%.

Orthogonal Basis Functions in Discrete-Time Electromagnetics and Their Implementation to Compute the Matrix-Type UTD Transition Function for PEC Wedge Diffractions

A set of orthogonal global time-domain (TD) basis functions of numerical robust is presented and examined for the applications of discrete time electromagnetics (DT-EM). The matrix equations or integrals arising from the expansion of TD field signals by this basis set can be effectively computed since they can be diagonalized by eigenvalue and vector decomposition method. This characteristic is particularly useful in the computation of TD solutions transformed from the corresponding frequency domain formulations such as in the computation of transition functions for the implementation of DT uniform geometrical theory of diffraction (DT-UTD). It may not only avoid the TD field signal divergence along propagation, but also simplify the computational complexity by avoiding the numerical integration of matrix integral. In this paper, the characteristics are examined via the implementation of plane wave propagation and DT-UTD for the diffraction from curved wedges. Numerical examples are presented to demonstrate their validity in DT-EM applications.

Predicting Short Term Extreme Response of Spar Offshore Floating Wind Turbine

In this paper, the short term extreme response of spar offshore 5MW NREL benchmark wind turbine is predicted. The spar is installed in a water depth of 320 m. The coupled wind and wave analysis is performed by coupling aerodynamic software FAST (Jonkman and Bull Jr,2005) and hydrodynamic software ANSYS-AQWA(2010). The global time domain responses of the spar type OWT is calculated for 600 s. The wave spectrum has significant wave height 6m and peak spectral period 10s and follows Pierson- Moskowitz spectrum. For safe operation, the structures should survive against different environmental conditions. The OWT can fail either in the operational regime or in the harsh environmental conditions. Therefore the dynamic simulation is carried out for two wind speeds, i.e., one in operational (hub height wind speed as 11.5 m/s) regime and another in idling condition (30 m/s) using the above wave parameters. Monte Carlo method is used to simulate the OWT responses in irregular wave loading condition. After obtaining the time-domain nonlinear responses, the 3-hr short term extreme responses are obtained which are useful for design of OWT. The 3-hr extreme response is obtained using Global Maxima Method (GMM). In Global Maxima Method, one maximum is taken from each time series and the maxima are fitted to two Generalized Extreme Value distributions, viz., Gumbel and Weibull distribution. The results show that Weibull fit is on a conservative side than Gumbel fit. Since the calculation of extremes is dependent on time domain simulations, a comparative study is also done for 100 and 20 samples so as to understand the sensitivity of extreme values due to lower sample size.

Multiaxial fatigue analysis of the stress joint for a deepwater steel catenary riser

The object of this paper is to present a more accurate and useful method for fatigue analysis and life prediction for a stress joint of a deepwater steel catenary riser. The stress joint was used to connect the riser and platform, and it was subjected to ocean environmental loadings and erosion; thus, the fatigue problem is critical. First, global time domain dynamic analysis of the steel catenary riser was performed, and then local analysis was performed for the stress joint. Finally, the fatigue life of the stress joint was evaluated according to the multiaxial fatigue theory, and a preliminary parametric study was also carried out. Results show that taking stress value of the structure as the only criterion is not enough, and fatigue analysis is necessary when the structure is subjected to an alternating load. Compared with uniaxial analysis, multiaxial fatigue analysis can reflect the damage of the structure more realistically. Especially for deepwater steel catenary risers, multiaxial fatigue problems should be given close attention.

A global time domain circuit simulation of a microwave rectenna

AbstractThe paper presents a global time domain simulation of a microwave rectenna studied for wireless energy transfer. The novelty of the work is to take into account both distributed electromagnetic parts of the antenna and the rectifier circuit including lumped elements. From a 3D finite element time domain electromagnetic modelling of the structure an equivalent circuit of the antenna is deduced: the input impedance is obtained as a function of frequency over a broad band. Then a rational approximation gives a corresponding PSPICE representation. The electromotive force induced between the ports of the antenna during the microwave illumination is directly computed from the 3D transient scattering problem. The resulting equivalent circuit of the antenna is finally incorporated into the electronic simulator PSPICE, together with the lumped components of the rectenna (ideal diodes and load). Thus a global non‐linear time domain analysis of the whole structure becomes available. The results obtained with the methods presented in the paper are compared with those resulting from other techniques. The approach developed in the work could efficiently improve the design stage of rectennas devoted to microwave power transfer. Copyright © 2006 John Wiley & Sons, Ltd.

Long-term seafloor geomagnetic station in the northwest Pacific: A possible candidate for a seafloor geomagnetic observatory

Abstract For two years, geomagnetic variations have been measured at the seafloor in the northwest Pacific. The seafloor data consist of the geomagnetic vector field measured by a three-component fluxgate magnetometer and the absolute scalar total force measured by an Overhauser (1953) magnetometer with attitude measurements for both orientation and tilt. Using the attitude data, the geomagnetic data at a site in the northwest Pacific (41o06′08″N, 159°57′47″E, -5580 m), hereafter referred to as NWP, were converted into the same reference frame as land and satellite measurements. Short-period variations of the converted vector data were examined by Hamano’s (2002) global time domain analysis method, which showed compatibility of the seafloor geomagnetic observatory data with the existing land observatory network. The smooth and gradual change of the Earth’s main field (i.e., the geomagnetic secular variation) was also found consistent with those predicted by the latest International Geomagnetic Reference Field (IGRF-10; IAGA, 2005) and by Ørsted Satellite (Olsen, 2002) for not only the scalar field but also the vector field. This means that observation of the geomagnetic vector secular variation is now feasible on the seafloor.

The Haar wavelets operational matrix of integration

The Haar wavelets operational matrix of integration P is derived, which is similar to those previously derived for other types of orthogonal functions such as Walsh, block-pulse, Laguerre, Legendre and Chebyshev. A general procedure of forming this matrix P is summarized. This matrix P can be used to solve problems such as identification, analysis and optimal control, like that of the other orthogonal functions. However, in the process of solving practical problems, the different resolution bases can be selected on the local time domain but on the global time domain, in order to understand more clearly system behaviour on the local time domain. This advantage results from the property of Haar wavelet localization, and the other orthogonal functions do not have this property. A numerical example will be used to demonstrate this point.

Comparison of modal parameter estimation techniques on aircraft structural data

Five global time domain parameter estimation methods, least squares, double least squares, total least squares, correlation fit and Smith least squares were compared using the same set of impulse responses taken from the ground vibration test of an aircraft structure. The behaviour of the modal parameter estimates was investigated for changes in the solution order and number of data points analysed. The techniques produced comparable results for the dominant modes, although there were some differences in the estimates of the other modes. The correlation fit and Smith least squares approaches found equivalent estimates to the other methods while using much smaller computational model orders.