Time-varying Load Research Articles

Measuring the Systemic Risk of Clean Energy Markets Based on the Dynamic Factor Copula Model

This study is based on the stock returns of 11 subindustry markets in the international clean energy market from 2010 to 2024 and constructs a skewed t distribution dynamic factor copula model. The time-varying load factor is used to characterize the correlation between a single subindustry market and the entire system, and the joint probability of distress is calculated as a measure of the overall level of systemic risk. Two indicators, Systemic Vulnerability Degree and Systemic Importance Degree, are introduced to evaluate the vulnerability of a single subindustry market in systemic risk and its contribution to systemic risk. A conditional risk-spillover index is constructed to measure the risk-spillover level between subindustry markets. This method fully considers the individual differences and inherent correlations of the international clean energy market subsectors, as well as the fat tail and asymmetry of returns, thus capturing more information and more timely information. This study found that the correlation between subindustry markets changes over time, and during the crisis, the market correlation shows a significant upward trend. In the measurement of the overall level of systemic risk, the joint probability of distress can identify the changes in systemic risk in the international clean energy market. The systemic risk of the international clean energy market presents the characteristics of rapid and multiple outbreaks, and the joint default risk probability of the whole system can exceed 0.6. The outbreak of systemic risk is closely related to a series of major international events, showing a strong correlation. In addition, the systemic vulnerability analysis found that the biofuel market has the lowest systemic vulnerability, and the advanced materials market has the highest vulnerability. The energy efficiency market is considered to be the most important market in the system. The advanced materials market and renewable energy market play a dominant role in the risk contribution to other markets, while the geothermal market, solar market, and wind energy market are net risk overflow parties in the tail risk impact, and the developer market and fuel cell market are net risk receivers. This study provides a theoretical basis for systemic risk management and ensuring the stability of the international clean energy market.

Hall effect thruster impedance characterization in ground-based vacuum test facilities

Hall effect thrusters (HETs) are typically regarded as DC electric propulsion devices as they are operated with isolated DC power supplies. However, it is well known that the HET’s discharge current possesses oscillations of varying magnitudes and frequencies and is thus a function of time with AC characteristics. The observed oscillations are caused by plasma processes associated with ion, electron, and neutral particle dynamics that occur inside the HET’s discharge channel and in the plume as the HET electrically interacts with its local operating environment. The extent to which plasma oscillations impact HET discharge dynamics is difficult to quantify due to the complexity of analyzing AC signals, given that the HET is a nonlinear, time-variant electrical load. In this work, we overcome the challenge of nonlinearity and time-variance of HETs by conducting a small-signal impedance analysis to characterize the effective resistance and reactance of the HET discharge with a novel and versatile impedance measurement diagnostic. The impedance magnitude and phase of a 7-kW class HET were measured from 100 Hz to 300 kHz with an excitation signal of ± 2 Vpk for two discharge operating conditions on krypton: 4.5 kW, 15 A and 6 kW, 20 A. The results were used to quantify resistive, capacitive, and inducive characteristics present within the HET discharge signature. For the 4.5 kW, 15 A thruster operating condition, the breathing mode capacitance was estimated to be 12.6 µF with an inductance of 15.3 µH. Furthermore, the impedance characteristics of the breathing mode are within ± 2.4 kHz of the power spectral density plots independently generated by time-resolved oscilloscope traces indicating good agreement in the frequency domain. Thus, the impedance measurement tool is a new diagnostic for characterizing the impedance and associated AC characteristics of HETs.

Dynamic amplification analysis in acoustic wave propagation problems subject to time-variable loads

In this work, the formulation of the direct interpolation boundary element method is applied to evaluate the dynamic behavior of continuous acoustic systems subject to the excitation of time variable loads. In such a condition, the temporal variation of the load may be close to some of the system's natural frequencies, and the evaluation of dynamic amplification and damping effects is very important. Physically, the analysis undertaken in this work provides an excellent indication of the dynamic behavior of a continuous system subject to pressure waves, which can be an aqueous medium or any seismic medium. In numerical terms, the simulations carried out show the robustness of the chosen technique in solving acoustic wave propagation problems compared to other methods.

Transient lubrication of floating bush coupled with dynamics and kinematics of cam-roller in fuel supply mechanism of diesel engine

The roller-floating bush pin is a major component in the fuel supply mechanism of a diesel engine, and its tribological performance seriously affects the reliability of the equipment. In this study, a transient thermoelastic hydrodynamic lubrication analysis model of the double-layer oil film on the roller-floating bush pin is established, which is coupled with the dynamic and kinematic analysis model of the cam-roller. The transient dynamic characteristics, such as time-varying speeds and loads of the roller during the operation of the cam, were considered. Also, the elastic deformation and the thermal effect were considered in the lubrication model. The relevant experimental platform for this double-layer oil film lubrication structure was built, and the simulation model was verified through experiments. The influence of the operating conditions, including cam speed and plunger fuel pressure, on the lubrication performance of the structure is explored, and besides, the influence of the structural parameters, including length-diameter ratios and clearance ratios, is also analyzed. It shows the lubrication condition of the inner oil film is worse compared with the outer oil film. As the explored factors change, the pressure of the inner oil film is more affected than that of the outer oil film. This work could provide useful guidance in designing the roller-floating bush pin in the fuel supply mechanism.

Aerodynamic interaction mechanisms in typical wingtip-mounted tractor propeller configurations

The wingtip-mounted tractor propeller configuration has been extensively studied due to its great potential for improving wing aerodynamic performance. In the present work, the aerodynamic interaction mechanisms in typical wingtip-mounted tractor propeller configurations are analyzed in depth using the reformulated vortex particle method. The results show that the elliptical lift distribution over the entire wing is altered by the installation of the tip-mounted propeller. The lift coefficient increases with inboard-up rotation and decreases with outboard-up rotation. Three key aerodynamic interaction mechanisms, swirl recovery, slipstream distortion, and slipstream impingement, are investigated. The swirl recovery mechanism generated by the inboard-up rotating propeller positively contributes to the performance of downstream wing. For the outboard-up rotation, the propeller-induced downwash results in reduced lift and increased induced drag. The interaction between the propeller-induced spanwise velocity and the wingtip-induced crossflow results in slipstream distortion. At α = 0°, for either inboard-up or outboard-up rotation, the slipstream on the retreating side is moved toward the propeller axis and away from the propeller axis on the advancing side. With the change of the rotation direction and angle of attack, the slipstream geometry exhibits different features. The slipstream impingement induced by the propeller generates time-varying loads on the downstream wing at the blade passing frequency. The propeller-induced tip vortices start to bend close to the leading edge and their vorticity increases, and then the vortex tube begins to deform. A higher angle of attack results in larger fluctuations in the downstream wing's drag coefficient.

Research on Feedforward-Feedback Composite Anti-Disturbance Control of Electro-Hydraulic Proportional System Based on Dead Zone Compensation

Considering the complexity and difficulty of obtaining certain parameters in the electro-hydraulic proportional control system, a precise transfer function of the system was derived through parameter identification using experimental data obtained from an Amesim simulation model after establishing a basic mathematical model. This approach reduces the reliance on accurate parameters of individual components. A feedforward-feedback composite controller was designed, and its effectiveness was validated in Simulink using the system’s transfer function. Subsequently, the dead zone range of the proportional valve was determined through experiments, and a dead zone compensation strategy was designed, which reduced the time required for the proportional valve to traverse the dead zone by 89.4%. Based on the dead zone compensation, trajectory tracking experiments were conducted to validate the effectiveness of the feedforward-feedback composite controller. Under fixed disturbances, the trajectory tracking error was reduced by 53.8% compared to feedback control. Under time-varying load disturbances, the trajectory tracking error was reduced by 51.2% compared to feedback control.

A new type of time-varying terminal load energy harvester: Design, simulation, and experiments

To achieve stable power output under low-intensity, broadband, time-varying vibrations, this paper proposes a Variable Stiffness Magnetic Oscillation (VSMO) energy harvester. The VSMO energy harvester is based on an integrated concept design, where the end magnet of the basic cantilever beam is replaced by a sliding magnetic element. Utilizing the kinetic energy generated by a magnet sliding in a cavity, this VSMO device achieves broadband energy harvesting without requiring manual adjustments. The working principle of the VSMO was analyzed through dynamic equations, and experiments were conducted to verify the effects of cantilever beam thickness, length, and sliding magnet mass on the output performance. Experimental data indicate that the VSMO can achieve a maximum power output of 47.4 mW, marking a 217.4 % enhancement in power output compared to the original technology. The minimum acceleration required for operation is only 0.05g. The VSMO enables enhanced output power over a broadband frequency range of 1 Hz–30.6 Hz. In addition, the device was able to power commercial sensors and use Bluetooth for data transmission. This device holds promise for condition monitoring in vibration environments, thus facilitating distributed monitoring within the realm of the Internet of Things.

Contact fatigue life forecasting model considering micro-scale subsurface stress for aerospace spiral bevel gears

Focusing on stress distribution at subsurface layer under aerospace service condition in roughness tooth flank meshing interface, a new loaded contact fatigue life forecasting model is developed by considering micro-scale surface effect for aerospace spiral bevel gears. Firstly, tooth flank modeling considering the actual manufacturing process is used for accurate tooth flank point determination having high and uniform grid density. Then, with application geometric approximation and operation, discrete convolution and fast Fourier transformation (DC-FFT) based conjugate gradient (CG) method is applied to determine time-varying load distribution. While at normal direction of each point from the high-density tooth flank discretization after accurate interpolation is added the roughness height from the actual micro-scale geometric topography measurement, a tooth flank reconstruction is performed to determine the micro- scale geometric topography. Then, elastic half-space loaded contact model and DC-FFT method are employed to compute subsurface stress distribution for roughness tooth flank. Von Mises stress is selected as design variable and introduced into Zaretsky model to establish the contact fatigue life forecasting model. Finally, a spiral bevel gear set in aerospace industrial application is exercised to verify the impact of subsurface stress on contact fatigue life.

A MATLAB/Simulink model of a parallel hybrid PEMFC/battery powertrain for passenger cars

Abstract The transition towards sustainable transportation solutions needs the development of efficient and environmentally friendly propulsion systems. Among these, Proton Exchange Membrane Fuel Cells (PEMFCs) seem to be a promising solution for sustainable powertrain design. This study presents the development of a MATLAB/Simulink model for a parallel hybrid FC/battery system representative of a passenger car, including all the auxiliary components and sub-systems. The model incorporates electrochemical, heat transfer, and fluid dynamic processes to accurately simulate the dynamic PEMFC stack and system behaviour. By implementing user-defined initial and boundary conditions, the model offers flexibility in simulating real-world scenarios, allowing the investigation of system performance under different environmental and dynamic driving conditions, as prescribed by the latest homologation protocols. Additionally, it accounts for the membrane degradation, which is a critical aspect affecting long-term durability, performance, and efficiency. Furthermore, a Graphic User Interface (GUI) has been developed to simplify the input of the main parameters, embedding the model in a user-friendly yet comprehensive tool designed for students, researchers, and engineers to evaluate the realizability of these efficient technologies. The intrinsic adaptability to model any FC/battery power system under a generic time-varying load constitutes an additional valuable point of the presented study to enable engineering progress, advancing the energy transition via sustainable powertrain solutions.

Modeling the Dynamic Properties of Multi-Layer Glass Fabric Sandwich Panels.

Sandwich panels are key components of many lightweight structures. They are often subjected to time-varying loads, which can cause various types of vibrations that adversely affect the functionality of the structure. That is why it is of such importance to predict the dynamic properties of both the panels and the structures made of them at the design stage. This paper presents finite element modeling of the dynamic properties (i.e., natural frequencies, mode shapes, and frequency response functions) of sandwich panels made of glass fabric impregnated with phenolic resin. The model reproducing the details of the panel structure was built using two-dimensional, quadrilateral, isoparametric plane elements. Afterwards, the model was subjected to an updating procedure based on experimentally determined frequency response functions. As a result, the average relative error for natural frequencies achieved numerically was 5.0%. Finally, a cabinet model consisting of the analyzed panels was built and experimentally verified. The relative error between the numerically and experimentally obtained natural frequencies was on average 5.9%.

Fatigue Strength Analysis of Rail Fastening Baseplate

Superstructural elements of a railway track experience dynamic loads varying over time. As a consequence, this is accompanied by the initiation and accumulation of fatigue damages. Defects in intermediate rail fastening are quite difficult to identify using flaw detection methods, and some can be destructed. In particular, rail fastening baseplates often fail due to insufficient fatigue strength.Purpose: To investigate the fatigue strength of rail fastening baseplate.Methodology: Development of the mathematical model of deformation based on the finite element method (FEM). Strength analysis of the baseplate using the model of a variable-thickness slab resting on a nonlinear elastic foundation. The latter is modeled by a set of rods with nonlinear deformation and parameters determined by compression tests of the baseplate. A mathematical model developed in COSMOS software for the FE analysis of the stress-strain state of the baseplate at a time-varying load from rolling equipment and assembly forces from attaching the baseplate to the sleeper and rail.Research findings: The cyclic loading parameters are obtained at dangerous points of the baseplate and its durability is analyzed at different loads with respect to unevenness of track settlement and failure of bolt connections of rail fasteners. The influence of these factors on actual loads on the rail fastener is determined. It is found that the service life of the baseplate mainly depends on the dynamic load.

The Effectiveness of Asset Pricing Models Under Special Conditions: Evidence from the COVID-19 Pandemic, Emerging Markets, and Financial Crises

Asset pricing models are important tools in finance. However, special market conditions can make them highly ineffective. This paper evaluates the performance of classical asset pricing models (that is, CAPM and Fama-French models) under three specific scenarios: the COVID-19 pandemic, emerging markets, and financial crises. Recent research and market data are reviewed to conclude that these models have very low efficiency in times of extreme occurrences for capturing proper asset valuations. Our findings reveal that standard models lose a substantial part of their explanatory power in periods of crises while new context-specific risk factors come into existence in other exceptional conditions. This work argues for the inclusion of liquidity and tail risk considerations, especially during market stress, as we also observe a greater need for more dynamic and adaptive modeling approaches that capture time-varying risk premia and factor loadings. The findings drive home the point that standard asset pricing models cannot be applied across the board in all market conditions and stress the need for developing more flexible and situation-specific valuation tools. Therefore, this research lies in its dual contribution, both in academic terms and practical finance, by providing an understanding of the behavior of asset pricing models in extreme markets and giving indications for future model development.

Influence of Transient Static and Transient Dynamic Analysis on Deflection of Bridge Slab Using Finite Element Method in ANSYS

The deflection behavior of bridge slabs is critical for ensuring structural safety and longevity. By using the Finite Element Method (FEM) in ANSYS, engineers can simulate and analyze the effects of both transient static and transient dynamic loads on bridge slabs. This paper explores how these two types of analyses affect the deflection behavior of a bridge slab. We present the mathematical formulations, practical applications, and a case study involving a bridge slab subjected to time-varying loads. The comparative results demonstrate the significant impact that the choice of analysis method has on predicting deflection and ultimately guiding structural design decisions.

Distinguishing Time-Varying Factor Models

Time-varying factor models have been widely used to model changing relationships among economic and financial variables. The existing literature usually specifies the time-varying factor loadings as deterministic functions of time or unit root processes. This article proposes two consistent tests to distinguish between these two specifications based on a randomization approach. By setting the null hypothesis as either specification, we show that the proposed test statistics follow an asymptotic Chi-squared distribution under the respective null hypotheses and diverge to infinity in probability under the respective alternatives. Simulation studies reveal that both test statistics perform reasonably well in finite samples. We apply the proposed tests to the U.S. macroeconomic and global macroeconomic and financial datasets. The results suggest that the time-varying factor loadings as deterministic functions of time should be adopted for these two applications.

Fault diagnosis of rolling bearing under complex working conditions based on time-frequency joint feature extraction-deep learning

The same independent distribution is not obeyed for the data collected under complex working conditions such as time-varying speed or loading, and the fault characteristic information is insufficient, resulting in low accuracy by using traditional methods. To solve the above problem, a fault diagnosis method based on time-frequency joint feature extraction combined with deep learning is proposed. Firstly, the original vibration signal is processed by variational mode decomposition (VMD) to obtain several intrinsic mode functions (IMFs), then the sensitive components are selected by calculating the steepness values of each IMF. Subsequently, the characteristic features of the selected sensitive component in time-domain, frequency-domain and time-frequency domain are calculated to form the time-frequency joint feature. The sparse attention mechanism (SAM) is combined with the advantages of recurrent neural network (RNN) and convolutional neural network (CNN) to form a hybrid deep learning model (SAM-RNN-DCNN). Finally, the time-frequency joint features are combined with the hybrid model for fault diagnosis. Experimental verifications are carried out by using data sets under variable rotational speed, variable load and strong noise interference, and the analysis results show that the proposed method has high diagnostic accuracy, good diagnostic performance and robustness under complex working conditions.

Fatigue life prediction method for composite laminates based on equivalent life under time-varying loads

Fatigue life prediction method for composite laminates based on equivalent life under time-varying loads

Reliability and line loading enhancement of distribution systems using optimal integration of renewable energy and compressed air energy storages simultaneously under uncertainty

Recently, with the significant growth of load demand, further interest in renewable energy sources (RESs) incorporated into electric distribution systems (DSs) has become important. However, the output powers' stochastic fluctuations of the photovoltaic (PV) and wind turbine (WT) generation sources, as well as the load demand, result in obvious operational challenges related to the reliability and line loading of these systems. Therefore, this present study is intended to enhance the load-oriented reliability indices (LORIs), the customer-oriented reliability indices (CORIs), and the line loading index (LLI) of the DSs by assigning the optimal integration of RESs and compressed air energy storages (CAESs) through the determination of both the optimal placement and capacities of RESs and CAESs simultaneously, as well as the optimal charging and discharging powers of the CAESs and their initial state of charge based on the red-tailed hawk (RTH) optimization technique. The Monte Carlo Simulation (MCS) and the Scenario-based Reduction (SBR) uncertainty methods are implemented to address the uncertainties of RESs and loading. The influences of the linear failure rate of DS's feeders and the installation of protective devices are also demonstrated. The efficiency of the intended methodology is demonstrated on the IEEE 69-bus and IEEE 33-bus DSs associated with voltage-dependent, time-varying mixed loads. The outcomes obtained from the suggested RTH are compared to those of other reported optimization techniques to validate its effectiveness. The results reveal that the optimal integration of RESs along with CAESs in the IEEE 69-bus test DS can improve the energy not supplied (ENS), system average interruption duration index (SAIDI), system average interruption frequency index (SAIFI), average service unavailability index (ASUI), and LLI by 1.09 %, 1.86 %, 1.73 %, 1.89 %, and 64.53 %, respectively.

Observer-based decoupling control of air flow and pressure in fuel cell systems

This paper investigates the control of the air supply system for marine proton exchange membrane fuel cells. A mathematical model of the air supply system is established, and a sliding mode diagonal decoupling controller based on a state observer is proposed. The control effectiveness is validated through experiments and simulations. A decoupling control strategy for the air supply system of marine fuel cells is presented. First, the load input current of the fuel cell system is selected according to the load characteristics of the target vessel, ensuring that the load current meets the power level of the fuel cell system model. Second, the impact of the cathode pressure on the fuel cell system is analyzed. An observer-based measurement method is proposed to address the measurement issue of cathode pressure. On this basis, a decoupling control strategy combining a diagonal decoupling matrix and sliding mode variable structure control theory is proposed. Finally, the effectiveness of the decoupling controller is validated through Matlab/Simulink. The results show the proposed decoupling controller exhibits superior performance to existing controllers with environmental disturbances, continuous time-varying loads, and step loads. The maximum relative error is 2.95%, the average relative error is 0.022%, and the longest adjustment time is 0.5 s. These results indicate that the control performance of the proposed decoupling controller is superior to that of diagonal PI controllers and sliding mode controllers, thereby validating the superiority of the sliding mode decoupling controller. This article provides a useful method for the development and application of control technology for the air supply system of marine fuel cells.

Optionally Reinforced Columns Under Simulated Seismic and Time Varying Axial Loads: Advanced HYLSER-2 Testing

Steel- and composite-reinforced columns (SRC and CRC columns) provide alternative solutions for common and harsh environments. Although extensive research has been conducted on these columns, direct comparative studies of SRC and CRC columns under seismic conditions, with consistent testing and realistic load simulations, remain limited. This study examined the nonlinear seismic responses of nine ordinary steel-reinforced concrete column models constructed alternatively with normal-strength and high-strength concretes under simulated earthquakes and time-varying axial loads. A developed advanced HYLSER-2 seismic testing system was employed to conduct seismic tests. Spiral transversal reinforcement with pitches of 6.0 and 9.0 cm was used to explore the effects of concrete confinement. The HYLSER-2 seismic tests, conducted under various interactively simulated earthquake intensities and time-varying axial loads, yielded crucial experimental results. Additionally, an extensive complementary analytical study was conducted to provide comparative insights between steel-reinforced columns (SRC) and composite-reinforced columns (CRC) with novel glass fiber-reinforced (GFRP) bars. The analytical study was conducted using experimentally proven advanced nonlinear analytical micromodels. The analytical results highlight the hysteretic behavior of columns reinforced with ordinary steel and novel GFRP reinforcing bars under the simulated combined effects of reversed cyclic bending and time-varying axial loads. The findings form a critical basis for advancing seismic design strategies for SRC and CRC columns exposed to strong earthquakes and high-time variations in axial loads. Doi: 10.28991/CEJ-2024-010-10-09 Full Text: PDF

Pulsed EM Scattering by Dipoles With Time-Varying Loads

Pulsed EM Scattering by Dipoles With Time-Varying Loads