Hierarchical higher-order dissipative methods for transient analysis

This article develops an isogeometric independent coefficients (IGA-IC) reduced order method for transient nonlinear heat conduction analysis. Herein, we first exactly represent the geometric model via isogeometric analysis (IGA), and therein provide an accurate solution for the semi-discretized equations. Next, our proposed GSSSS-1 time-stepping framework is employed to solve the transient nonlinear temperature in space and time domains. We advance our independent coefficients (IC) reduced order method to efficiently solve IGA-based transient nonlinear heat conduction problems. We extend the IC method to significantly reduce the original full IGA-discretized formulations and calculate the reduced equilibrium formulations in each Newton–Raphson iteration. Thereby, hugely improving the efficiency and guaranteeing the accuracy simultaneously. Illustrative numerical examples validate this proposed IGA-IC method is reliable, accurate, and efficient; especially, the larger the scale of the problem, the more advantages the proposed IGA-IC will inherit.

Thermal analysis is required in the design of the structure of atmospheric reentry vehicles, which are subjected to severe aerodynamic heating. However, prediction of aerodynamic heating is difficult owing to highly complex physical phenomena. Inverse heat conduction analysis, the methodology to estimate heat flux on boundaries and entire temperature distribution from limited number of temperature measurement, is expected to solve this problem and to contribute to improving structural integrity. Present study develops computational method of transient inverse heat conduction analysis using finite element method and pseudo-inverse matrix. The developed inverse analysis code is applied to a reentry vehicle in order to examine the present method and to discuss the computational stability and regularization methods. Sequential function specification (SFS) method and rank reduction are employed to improve the accuracy and stability of the inverse analysis. The results of the numerical simulation reveal that the present method works effectively in solving inverse problem stably by using the combination of SFS method and rank reduction. Especially, the rank reduction is quite efficient at regularization. SFS method requires proper number of future time steps.

In this paper, a new explicit time integration method for transient heat conduction analysis is proposed based on the uniform trigonometric B-spline function. The algorithmic calculation procedure for solving the heat conduction problem is given. The influence of algorithm parameters on accuracy and stability is determined by theoretical analysis and numerical experiments. With adjustable parameters, the proposed method can obtain high-order calculation accuracy and a large stability region. The local truncation error analysis of transient heat conduction shows that the proposed method has at least second-order accuracy. The numerical examples verify the effectiveness of the proposed method for linear and nonlinear heat conduction problems.

We are investigating countermeasure techniques against electric noise on telecommunication and related equipment. There are many types of electric noise, and we focus on the characterization of transient noise such as the switching noise of switching power supplies. We are developing a method of transient signal analysis based on linear predictive coding (LPC) for the purpose. Unfortunately, as standard LPC contains Yule-Walker (YW) approximation, which replaces a given time series with a periodic time series for simplification, it is not suitable for transient signal analysis. Therefore, our method of transient signal analysis does not contain YW approximation. Instead, we use the local linearization technique, which was developed in the field of quantum mechanics. Our method makes it possible to obtain instantaneous frequencies and instantaneous decay rates simultaneously with high precision from a small number of samples. We discuss our LPC method avoid using YW approximation and using the local linearization technique and its application to repetitive ringing signals, which mimics the switching noise of a switching power supply, to show the efficiency of our method.

PurposeThis paper aims to present an adaptive approach of the generalized finite element method (GFEM) for transient dynamic analysis of bars and trusses. The adaptive GFEM, previously proposed for free vibration analysis, is used with the modal superposition method to obtain precise time-history responses.Design/methodology/approachThe adaptive GFEM is applied to the transient analysis of bars and trusses. To increase the precision of the results and computational efficiency, the modal matrix is responsible for the decoupling of the dynamic equilibrium equations in the modal superposition method, which is used with only the presence of the problem’s most preponderant modes of vibration. These modes of vibration are identified by a proposed coefficient capable of indicating the influence of each mode on the transient response.FindingsThe proposed approach leads to more accurate results of displacement, velocity and acceleration when compared to the traditional finite element method.Originality/valueIn this paper, the application of the adaptive GFEM to the transient analysis of bars and trusses is presented for the first time. A methodology of identification of the preponderant modes to be retained in the modal matrix is proposed to improve the quality of the solution. The examples showed that the method has a strong potential to solve dynamic analysis problems, as the approach had already proved to be efficient in the modal analysis of different framed structures. A simple way to performh-refinement of truss elements to obtain reference solutions for dynamic problems is also proposed.

Large-scale AC-DC hybrid power systems have become a basic pattern for power transmission in China. Currently, time-domain simulation is widely used for transient stability analysis, which has the disadvantages of high computational burdens and lack of system stability margin information. An alternative method for transient stability analysis is the direct method, which assesses transient stability from the energy point of view. Direct method requires much less computation efforts. In addition, the stability margin of the system can be obtained. Due to the challenges in system modelling, energy function construction, and determination of critical energy value, it is challenging to apply direct method for transient stability analysis of AC-DC hybrid power systems. This paper is a step in exploring the application of direct method in AC-DC hybrid power systems.

This paper presents a new efficient method for transient state analysis in magnetodynamic systems of linear eddy current problems. This method employs the Fourier transform and the high-order frequency sensitivity of the harmonic finite element method. By taking into account the time-constant of the magnetodynamic system, the Fourier integral of continuous frequency is converted into the Fourier series of discrete frequency. With the results of Fourier series expansion of converted input waveform, the responses of each sinusoid are superposed to give the total response of the magnetodynamic system. But, if the frequency band of input waveform is broad, it takes a long computational time since all responses for each sinusoid must be calculated. Therefore, the high-order frequency sensitivity method is employed to estimate the response variation to frequency. The proposed algorithm is applied to an induction heating system to validate its numerical efficiency.

In this thesis a new method for the analysis of power transients in a nuclear reactor is developed, which is more accurate than the present state-of-the-art methods. Transient analysis is important tool when designing nuclear reactors, since they predict the behaviour of a reactor during changing conditions, such as a control-rod movement, induced by an operator, or an accident scenario. The current methods for transient analysis apply deterministic solvers to calculate the neutronic response, but not only do these deterministic solvers always need to discretise the problem, they also often apply more fundamental approximations such as homogenization or diffusion. Therefore a stochastic method is needed, which only has a statistical uncertainty. A challenge for applying the Monte Carlo method on transient analysis, is the different types of particles which must be simulated in a single simulation. It is common for a neutronics calculation to simulate the prompt neutrons, however the simulation of precursors is new. The main challenge when simulating precursors lies in their long lifetime, which is seconds, whereas the prompt neutrons have a lifetime of microseconds. This has been solved by dividing the transient problem into time intervals and forcing the precursors to produce a delayed neutron is all time intervals. This will ensure prompt-neutron chains in all intervals. The prompt neutrons will form prompt-neutron chains, which poses another challenge for the Monte Carlo method. The chain length of these prompt neutrons varies a lot and the can also split into many branches, which averages out in a real power reactor, but is difficult to simulate on a computer. To improve the chain-length statistics, a new variance reduction technique is developed, which dictates that a single neutron will emerge from each interaction. This emerging neutron can be the result of a scattering event or a fission event. Finally, a dynamic simulation scheme, which can run in parallel, has been devised, simulating all time intervals consecutively. Also, a method for sampling the initial conditions is created and this scheme is implemented in a purpose-built Monte Carlo code and in a general-purpose Monte Carlo code. The two codes have been tested in several cases and they behave as expected, agreeing with deterministic method where expected, but deviating when the deterministic methods are no longer valid. In order to simulate realistic transients in a power reactor, feedback has to be taken into account. To achieve this, the dynamic Monte Carlo method has been coupled to a thermal-hydraulics code, using an explicit scheme. The results of this coupled simulation are compared to a state-of-the-art coupled diffusion calculation in a NURISP-benchmark calculation and the results agree well, except for a small deviation towards the end. The exact source of this deviation should be further investigated, but the difference is small, when realising that there is already a deviation in the steady-state results. The results demonstrate the feasibility of performing a fully dynamic transient analysis, using only Monte Carlo for the neutronics part of the calculation. Transients can now be calculated, with detailed modelling of any complex geometry and with continuous energy, which is especially useful for newly developed reactor types and one-of-a-kind research reactors.

A transient TRansport Analysis method for Modulation (t-TRAM) has been developed. This method consists of two methods. One is an instantaneous modulation analysis method using the Hilbert transform (HT) with the analytical mode decomposition (AMD). In the modulation experiments, plasma transport can be changed periodically by modulated external input. Profiles of the modulation amplitude and phase change in time due to the change in the plasma transport. The HT with AMD method can evaluate an instantaneous analytic signal in the presence of changes in the modulation amplitude and phase in time. The other is a transport analysis method to evaluate the diffusion coefficient and the convection velocity from the analytic signal. The t-TRAM has enabled us to evaluate a momentum diffusion coefficient and momentum convection velocity in the presence of a momentum source. The t-TRAM is applied to momentum modulation experiments in JT-60U. The evaluation reveals that a normalized momentum flux has a linear dependence on a normalized momentum gradient during a half cycle of the modulation.

Stability analysis programs are a primary tool used by power system planning and operating engineers to predict the response of the system to various disturbances.Important conclusions and decisions are made based on the results of stability studies.The conventional method of analyzing stability is to calculate the transient behaviour of generators due to a given disturbance.Direct methods of stability analysis identify whether or not the system will remain stable once the disturbance is removed by comparing it with a calculated threshold value.Direct methods not only avoid the time consuming solutions required in the conventional method, but also provide a quantitative measure of the degree of system stability.This additional information makes direct methods very attractive when the relative stability of different plans must be compared or when stability limits must be calculated quickly.Direct methods of transient stability analysis of a multi machine power system, using a function describing the system's transient energy, are discussed.By examining the trajectory of the disturbed system, the following fundamental questions are dealt with: the concept of a controlling unstable equilibrium point (U.E.P), the manner in which some generators tend to lose synchronism, and identifying the energy directly responsible for system separation.Resolving this issue will substantially improve transient stability analysis by direct method.

The Cornering feedback from the vehicle is important information to control the path and the yaw stability, especially in limit handling situation. To improve this cornering feedback, in-depth analysis of vehicle cornering motion should be advanced. But, due to the transient characteristics of vehicle corner-ing motion, effective analysis of cornering feedback has remained as a very challenging task. In this article, 1) important properties of transient cornering motion have been studied and 2) yaw de-composition based new transient motion analysis method is proposed. This new analysis method enables separate analysis of vehicle yaw motions by its sources, from path following and from yaw spinning, which is the key element in objectification of cornering feedback. Additionally, an in-depth analysis of yaw center migration and its effect on driver feeling has been investigated. Based on these analysis methods, subjective vs. objective correlation study has been conducted and major cornering feedbacks includes carving feel have been objectively analyzed and quantified. In this article, side slip motion compliance & resistance in path following also has been investigated to analyze vehicle turn-in response. Additionally, based on this newly proposed transient motion analysis, its application idea includes active control of vehicle motion in various situations is also proposed.KeywordsTransient MotionYaw Rate DecompositionCornering Feedback

Summary Non‐stable populations exhibit short‐term transient dynamics: size, growth and structure that are unlike predicted long‐term asymptotic stable, stationary or equilibrium dynamics. Understanding transient dynamics of non‐stable populations is important for designing effective population management strategies, predicting the responses of populations to environmental change or disturbance, and understanding population processes and life‐history evolution in variable environments. Transient perturbation analyses are vital tools for achieving these aims. They assess how transient dynamics are affected by changes to vital rates, population structure, or underlying variables that affect these. These changes could be imposed deliberately by population managers, or driven by environmental variables. Methodological approaches to transient perturbation analysis are diverse, and different methods are suited to different applications: choosing a method to use may be challenging. Here, I review existing methods for prospective transient perturbation analysis, and identify a number of key considerations for ecologists when choosing a method. These include the approach taken in calculating the perturbation, the type of model being analysed, the perturbation structure, the population response of interest, nonlinear response to perturbation, standardization for asymptotic dynamics, the initial population structure, and the time frame of interest. I discuss these with reference to the application of transient perturbation analyses in both population management and comparative analysis. The diversity of transient perturbation analyses available means that existing approaches are applicable to a wide range of population management and comparative analysis scenarios. It is important, however, for ecologists using these methods to know exactly what is being measured. Despite a wealth of existing methods, I identify some areas that would benefit from further development.

This paper describes a reduced finite element formulation for mildly non-linear transient heat transfer analysis problems based on the Lanczos algorithm. In the proposed reduced formulation, all material non-linearities of irradiation boundary elements are included using the pseudo-force method. Numerical time integration of the reduced formulation is conducted by the Galerkin method. The results of numerical examples demonstrate the applicability and the accuracy of the proposed method for transient heat transfer analysis when restricted to mildly non-linear situations. In particular, the example of a bridge under environmental thermal condition shows that the proposed method is about 20 times faster, without loss of solution stability and accuracy, than a direct integration method for the unreduced formulation. Copyright © 1999 John Wiley & Sons, Ltd.

Recent trends in the scaling down of the conventional transistor size and three-dimensional stacked integrated circuit have increased the importance of transient thermal-awareness in IC floor planning. To this end, this paper will compare the performance of two transient thermal analysis simulation methods: the latency insertion method (LIM) and the Gauss-Seidel (GS) method. By simulating an example thermal problem, we can compare and evaluate the performance of these two analysis methods. The simulated results demonstrate a great superiority in analyzing the speed of LIM from the GS method.

Stiffness-tolerant methods for transient analysis of stiff Markov chains