Simulation Of Transient Behavior Research Articles

Improved State-Space Approach Based on Lumped Mass Matrix for Transient Analysis of Large-Scale Locally Nonlinear Structures

Due to the assumption of acceleration variation in traditional step-by-step integration methods such as Newmark, sufficiently small time steps are required to ensure numerical stability and accuracy in dynamic systems. In contrast, the state-space approach, based on piecewise interpolation of discrete load functions, does not rely on predetermined acceleration assumptions and has demonstrated high efficiency in terms of stability and accuracy. The original state-space method requires the calculation of the inverse of the structural mass in the transition matrix. However, when a lumped mass matrix is used, this computation renders the entire mass matrix singular, resulting in an invalid solution expression. To address this issue, this study proposes an improved state-space approach for the transient analysis of large-scale structural systems with local nonlinearities. In this approach, a nonlinear force corrector is introduced as an external force term applied to the linear elastic system to account for the nonlinear behavior of locally yielding components. Consequently, the original nonlinear dynamic system can be transformed into an equivalent linear elastic transient system. Furthermore, based on the lumped mass matrix, a first-order ordinary differential state-space equation for such an equivalent linear elastic transient system is derived. Simulation results from three transient system examples show that the state-space approach outperforms the Newmark method in terms of accuracy and stability for dynamic systems characterized by high frequency and low damping. The prediction results show that the state-space approach appears to be insignificantly affected by the choice of the consistent or lumped mass matrix. The numerical results show that the root-mean-square errors between the consistent and lumped matrices in the top displacement time histories of a 15-storey plane frame under various seismic intensities are all less than 1%, and in the base reaction time histories responses the discrepancies are only about 0.5%, indicating that the use of lumped mass matrices is quite reliable. When many nodes or degrees of freedom have no assigned mass, the dimensionality of the state-space equation can be significantly reduced using the lumped mass approach. Therefore, the simulation of large-scale systems can be simplified by employing the improved state-space approach with lumped mass matrices, yielding results nearly identical to those obtained using traditional methods. In conclusion, the improved state-space approach has great potential for the simulation of transient behavior in large-scale systems with local nonlinearities.

CFD based design gas rotameters: Dynamic mesh transient simulation

Rotameters are variable-area flowmeters widely used in various industries. In this paper, a comprehensive discussion has been presented on simulation of transient behavior of a rotameter using Computational Fluid Dynamics (CFD) technique. Various methods of dynamic meshing and the reasons behind choosing the diffusion-based model have been discussed. 2D and 3D simulations were compared, and in the 2D simulation, triangular and quadratic meshes were compared, and their orthogonal quality changes due to dynamic mesh methods are shown. It was illustrated that triangular mesh cells preserve their orthogonal quality far more effectively. Moreover, the CFD simulation results were validated by comparing them with experimental data obtained from a real rotameter and it was found that the 3D simulation results are more consistent with experimental data than the 2D simulation results. A difference of less than 4% has been observed between the simulation and experimental results, indicating a reasonable agreement. The CFD solver configuration and simulation steps presented in this paper can be used to optimize rotameter designs and improve their performance. Additionally, the simulation results can provide valuable insights into the behavior of the rotameter, allowing designers to identify the potential areas for improvement and optimization of the design, accordingly.

Polymer electrolyte fuel cell system level modelling and simulation of transient behavior

Abstract In this paper, a system level model for a polymer electrolyte fuel cell (PEFC) system, which is capable of characterizing transient behavior using the control volume method, is developed. Three different cases based on different mass transfer assumptions in the cathode channel are explicitly discussed, i.e., considering the presence of 1) only oxygen, 2) both oxygen and nitrogen or 3) oxygen as well as nitrogen and water vapour in the cathode control volume. The analysis illustrates that the model developed predict in adequate manner the dynamic behavior of a PEFC system. It is demonstrated that the presence of nitrogen and water vapour in the cathode volume significantly affects the PEFC’s overall performance. Additionally, the model was modified to test the PEFC system’s application for start-up, power step-up and shut-down response with the simulation results that show good agreement with experimental data.

The Improvement of Coupling Method in TINTE by Fully Implicit Scheme

TINTE is a well-established code for the pebble-bed high-temperature gas-cooled reactor (HTR), including the complicated nuclear module and thermal-hydraulic module, which has been validated by experiments and widely used in the transient behavior simulation. However, only an operator splitting scheme is employed in TINTE to couple the neutronics and thermal hydraulics, and some physical quantities are not consistent in time. As a result, the accuracy and stability are limited by the additional error term derived from the unconverged physical term. In this paper, a fully implicit coupling method was investigated in which the coupled nonlinear fields at each time step are converged using Picard iterations. A physics-based preconditioning is proposed in the work here to further improve the computational performance of the fully implicit coupling method. Seven test problems are implemented based on a practical engineering model, rather than a simple model, to evaluate the performance of the Picard method. The numerical results show that the fully implicit Picard iteration method is more accurate and more stable, which permits longer time steps and a reduction of the computational burden for solving the coupled field equations. The computational efficiency is further enhanced when the physics-based preconditioning is utilized.

Modeling of District Heating Networks for the Purpose of Operational Optimization with Thermal Energy Storage

Abstract The aim of this document is to present the topic of modeling district heating systems in order to enable optimization of their operation, with special focus on thermal energy storage in the pipelines. Two mathematical models for simulation of transient behavior of district heating networks have been described, and their results have been compared in a case study. The operational optimization in a DH system, especially if this system is supplied from a combined heat and power plant, is a difficult and complicated task. Finding a global financial optimum requires considering long periods of time and including thermal energy storage possibilities into consideration. One of the most interesting options for thermal energy storage is utilization of thermal inertia of the network itself. This approach requires no additional investment, while providing significant possibilities for heat load shifting. It is not feasible to use full topological models of the networks, comprising thousands of substations and network sections, for the purpose of operational optimization with thermal energy storage, because such models require long calculation times. In order to optimize planned thermal energy storage actions, it is necessary to model the transient behavior of the network in a very simple way - allowing for fast and reliable calculations. Two approaches to building such models have been presented. Both have been tested by comparing the results of simulation of the behavior of the same network. The characteristic features, advantages and disadvantages of both kinds of models have been identified. The results can prove useful for district heating system operators in the near future.

Development of Numerical Models for Simulations of Transient Internal States of PEFC Stack System

Overview A computational model is developed for simulations of transient behavior of PEFC stack under realistic driving conditions. This model is applied to simulations of internal states of cells stack (maximum output about 100kW) with an injector, an exhaust and a recirculation system. Transient internal states under pressure swing and purging process are calculated. The physical model is based on so-called 2D + 1D model, which takes into account overall transport phenomena in PEFC fuel cell systems, such as transport of gas species and liquid water, thermal balance, electrochemical reactions and current distributions. Injector and exhaust systems are modeled by boundary conditions. Transient internal states, including fuel flow distributions between stacked cells and cell performance change at purging process will be demonstrated. Possible applications of those simulations to designing fuel cell stack control system will be discussed. Numerical Modeling and Results Simulation is performed by our own simulation software for PEFCs, which is capable of simulating full-stack fuel cell system for fuel cell vehicles (up to 400 cells) under transient operating conditions. An important feature of this software is that macroscopic models are applied for electrochemical reactions and transport phenomena in the MEA. Heat transfer and fluid dynamics of liquid-gas two phase fluid with phase changing are also taken into account. These models are coupled with multi-dimensional heat and mass transport equations, including effective parameters for gas diffusivity and permeability in the GDL and equivalent hydraulic diameter in the flow channel. This software can also simulate cathodic degradation by carbon corrosion reactions. Computational models of cell stack (250 cm2 reaction area in each cells) are constructed. A numerical model of anode system is constructed by adding transient boundary conditions for an injector and an exhaust system with an ejector valve. An interconnect boundary condition between inflow and outflow manifolds are also introduced as a computational model of recirculation system. These transient boundary conditions represent open / close timings of injector and ejector under actual driving conditions. Distributions and consumptions of hydrogen, build-up of cross-leak nitrogen and condensate liquid water can be calculated under transient (maximum output is about 100kW) loading. Simulated results show transient behavior of the stacked cell system, for example, Build-up of cross-leak nitrogen and condensate liquid water under transient loading conditions, and their effects on the distribution of performances between stacked cells.Evolution of fuel flow distribution between stacked cells under pressure swing.Efficiency of purging process and change of performance before and after purging. Those results can be calculated for various configurations of cells and driving conditions, and relations between stacked cell behaviors and those conditions can be discussed. Those discussions can provide useful information for improving durability of stacked cells by avoiding depletion of hydrogen, and / or examinations of control schemes which can efficiently utilize hydrogen fuel.

Development of a program for transient behavior simulation of heavy-duty gas turbines

In this study, a program for analyzing the transient behaviors of heavy-duty gas turbines was developed. Focus was given to simulating a practical load-following operation. A distinct feature of our work is that all of the gas turbine components are modularized to enhance the expandability of the program. We used object-oriented programing for this purpose. Mass and energy balances and performance maps for the compressor and turbine were used for the modeling, and a multivariable numerical solving technique was used to enhance the numerical stability. The fundamental thermodynamic modeling of the program was validated by comparison of the transient response of a simulated gas turbine with that of a commercial program. PID control was adopted for simultaneous control of the rotational shaft speed and turbine exhaust temperature by the modulations of the fuel flow and the opening of the compressor inlet guide vane at the same time. The influence of the magnitude of load change and ramp rate was investigated. Stable control of the gas turbine was possible, even for a very rapid change of load.

Tetrahedral Fluid Method Applied to FDTD for Simulation of Transient Behaviors of High-Energy Collisionless Plasmas

A new method called tetrahedral fluid method is presented in this paper. It is developed based on the fluid behavior of plasmas, unlike particle-based models, such as particle in cell; this method serves the continuity of the charge regions. The modeling of a moving plasma is performed by a fluid segmentation, in which the interaction with plasma fields is treated by the finite-difference time-domain numerical technique. The development of the relation between two field and fluid lattices has been described. The important part of this relation is the determination of the common volumes in the two lattices. A comparison is made between the results obtained by this method and those using the nearest grid point method, which shows the increased accuracy and speed of the proposed technique in a sample problem.

Simulations of Transient Behavior and Parasitic Effects for 20 nm Gate Length of PD SOI nMOSFET

In this paper, we report the possible transient behavior for 20nm partially depleted silicon-on-insulator (PD SOI) n-channel MOSFET studied using Synopsys TCAD program. Tunnelling model is set at Si/SiO2 interface by varying front gate oxide. Parasitic floating body effects also have been observed in the device behavior for both linear and saturation regions, the magnitude of these effects depends on the front gate oxide thickness. For 4nm gate oxide thickness, the parasitic floating body effect is dominant at low drain voltage.

Decoherence and time reversibility: The role of randomness at interfaces

The Wigner formalism is a convenient reformulation of the Schrödinger equation that allows the simulation of transient behavior of quantum systems in the presence of general boundary conditions. Recently, a Wigner Monte Carlo technique, based on particles signs, has been generalized to two-dimensional evolution problems. In this paper, we apply this technique to study the time reversibility of the quantum evolution of a wave packet colliding with a potential wall in the presence of interface roughness, elastic, inelastic, and diffusive interactions with the environment. We show that a wall surface roughness does not necessarily involve time irreversibility. The dynamics of the packet is indeed influenced, but remains coherent, until the boundaries of the system begin to absorb information from the system. Finally, it is shown that in the presence of inelastic scattering or diffusive processes, the time-reversibility of a quantum system is destroyed, whatever the shape of the wall interface is. In particular, we show that the random nature of a process, elastic or inelastic, is responsible for the appearance of quantum decoherence.

Concept design of fusion power shutdown system for ITER

ITER is a nuclear facility. It is essential to maintain operational safety or to bring this facility to a safe state in case of accidents or incidents. During plasma operation ITER plasma will generate significant heat loads on the plasma facing components. For a few reference accidents there is the need to stop plasma reliably within a certain time. Fusion power shutdown system is the safety system to implement this termination function for ITER. It is based on the concept of massive gas injection.This paper summarizes the safety requirements, logics and the physics requirements on this system for reliable termination of ITER plasma. With regard to the quantity of gas, transient behavior simulation is shown, subsequently providing guideline for laboratory bench-testing. Conceptual engineering design of the system is given together with instrumentation and control specifications.

Oxide Film Growth Kinetics on Metals and Alloys: II. Numerical Simulation of Transient Behavior

A number of passive film growth models have been advanced in the past, each founded on different assumptions and providing different rate laws. A new, generalized growth model has been developed and the numerical simulation of the transient behavior of oxide film growth on metals and alloys is reported here. This model is based on the transport of anions and cations (in a Cr2O3 film for stainless alloy). In contrast to most models up to date, it describes the time-variant behavior of film and film/solution potential differences. Because these potential differences are expressed as exponential functions of film thickness, asymptotic behaviors can be studied and it is shown that a square root law results for very thin and very thick films. Growth rate for intermediate films and during transient periods, on the other hand, is determined by numerical integration. Parameters which were introduced by the model and whose values are unknown are conveniently regrouped into only four lumped parameters, which are easily estimated by numerical optimisation. It is shown that experimental oxide film thickness measurements on stainless alloys in high-temperature aqueous environments can be reproduced. It is further shown that the diffusional rate expressions of all the species contributing to growth are analogous, explaining why it is possible to fit growth kinetics with a single type of point defect.

Calculation of equivalent circuit constants of synchronous machines considering field transient characteristics using DC decay testing method with open and shorted field windings

AbstractIt has been reported that the calculated values of field transient behavior in a synchronous machine differ considerably from the measured values. This discrepancy is caused by the use of equivalent circuit constants in the standardized tests provided by JEC‐2130 and IEC‐60034‐4, in which the mutual leakage reactance between the damper and field windings is not accounted for. The authors have been studying a method for calculating equivalent circuit constants for the accurate simulation of transient behavior including the field winding side, by means of a standstill test with a small‐capacity DC power supply (DC decay testing method). The authors have previously presented a calculation method using operational impedances with the field windings opened, shorted, and shorted with an external resistance, obtained by the DC decay test. This paper presents a new method in which the external resistance used in our previous method is no longer needed. Instead, the field winding impedance is determined on the basis of its invariability against slip. The validity of the new method is demonstrated by comparing the calculated and measured values of the armature and field currents during a sudden three‐phase short‐circuit using 10 kVA/200 V/31.9 A/4 P/50 Hz test machines. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 178(2): 39–46, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21207

Model development and simulation of transient behavior of helium turbine system

This paper describes a performance model for the transient analysis of helium turbine system. Governing equations have been derived from integral forms of unsteady basic conservation equations. The one-dimensional model is employed for flow-paths except turbine and compressor, which are considered as zero-dimensional components and volume-less treatment is employed. Component mathematical model results in a set of ordinary differential equations and algebraic equations. The simulation code is established on MATLAB, and the ordinary differential equations are solved a variable order solver of MATLAB, ode15s. The accidents of loss of load and loss of feedwater to precooler and intercooler, the transients of recuperator and the decreasing heat transfer capacity of intermediate heat exchanger are simulated respectively. The analysis of calculated results verifies the present model. The effects of bypass valve size and thermal inertia of the recuperator wall are also studied. The simulation results show that throttle size of bypass valve has important influence on the characteristics of turbine system and should be carefully selected to satisfy the requirement of system control and safety.

Transient behavior simulation of fin-and-tube heat exchangers for the variation of the inlet temperatures of both fluids

In this article, a transient behavior simulation of fin-and-tube heat exchangers has been studied. Energy equation for fluid flow and tube wall is derived and solved numerically. The variation of the temperatures of both fluids with time and position are obtained for a step-change in the inlet temperatures of the water and air fluids. The results show that in step-change of inlet water-side temperature, outlet water-side temperature will get steady faster than air-side, while in step-change of inlet air-side temperature, outlet air-side temperature will become steady state faster than water-side. Also, the time constant (the time interval that the flow will reach steady state) of the system is not influenced by the step-change amplitude of inlet air and water temperatures. The inlet water temperature expands along the tube and after a time interval, it reaches the outlet section of the tube. But, the inlet air temperature reaches the outlet section without time delay.

A global transient, one-dimensional, two-phase model for direct methanol fuel cells (DMFCs) – Part II: Analysis of the time-dependent thermal behavior of DMFCs

Abstract A transient-thermal model based on a lumped system is newly developed and implemented in a one-dimensional (1D), two-phase rigorous direct methanol fuel cell (DMFC) model presented in Part I. In this model, the main focus lies on the investigation of the transient thermal behavior of DMFCs and its influence on methanol crossover, cell performance, and efficiency. 1D simulations are carried out and the time-dependent thermal behaviors of DMFCs are analyzed for various methanol-feed concentrations and external heat-transfer conditions. Predicting the close interactions between the evolution of the transient temperature, methanol crossover, cell voltage, and efficiency during DMFC operations, the simulations of transient behavior indicate that the insufficient cooling of DMFCs finally lead to thermal runaway, particularly under high methanol-feed concentrations. Therefore, it is concluded that an efficient cooling system is greatly needed to safeguard DMFC operations and enhance the performance of DMFCs. The present 1D DMFC model is a useful tool for attaining a better understanding of complicated physical phenomena in DMFCs, which assists in optimizing the operating conditions of such cells and material/design parameters.

界磁巻線開放時および短絡時の直流試験法による界磁過渡特性を考慮した同期機の等価回路定数算出法

It has been reported that the calculated values of field transient behavior in a synchronous machine differ considerably from the measured values. This discrepancy is caused by the use of equivalent circuit constants in standardized tests provied by JEC2130 and IEC60034-4, in which the mutual leakage reactance between the damper and field windings is not accounted for. The authors have been studying a method for calculating equivalent circuit constants for the accurate simulation of transient behavior including the field winding side, by means of a standstill test with a small-capacity DC power supply (DC decay testing method). The authors have previously presented a calculation method using operational impedances with the field windings opened, shorted, and inserted with an external resistance, obtained by the DC decay test. This paper presents a new method in which the external resistance used in our previous method is no longer needed. Instead, the field winding impedance is determined based on its invariability against slip. The validity of the new method is demonstrated by comparing the calculated and measured values of the armature and field currents during a sudden three-phase short-circuit using 10kVA-200V-31.9A-4P-50Hz test machines.

Condenser microphone as parametric electroacoustic system and its time‐domain modelling via equivalent electrical circuit in SPICE software

Condenser microphone usually works with constant polarisation voltage. Where special attentions are prescribed for spectrum of self noise there may be useful to use high frequency voltage (pump) applied to transducer instead of DC charge. When frequency of pump is orders of higher than frequency of acoustic signals, large parametric gain occurs in the transducer. Method for describing such a system via equivalent electric network with approach of electronic filter design blocks was developped and special analogies were used for convergency improvement. Method was applied to time‐domain simulation of pressure transducer with current‐mode diode discriminator in SPICE computer analysis software. Simulations of transient behavior after switching system on was demonstrated.

Development of a One-Dimensional Modular Dynamic Model for the Simulation of Surge in Compression Systems

The paper deals with the development of a nonlinear one-dimensional modular dynamic model for the simulation of transient behavior of compression systems. The model is based on balance equations of mass, momentum, and energy, which are derived through a general approach and are written by using the finite difference method. The model also takes rotating mass dynamics into account through a lumped parameter approach. Moreover, it reproduces the behavior of the system in the presence of the surge phenomenon through steady-state performance maps, which represent the compressor operation in the inverse flow region by means of a third degree polynomial curve. The model is implemented through the Matlab Simulink tool, where the system of ordinary differential equations is solved by using a fourth- and fifth-order Runge–Kutta method. A sensitivity analysis is carried out to evaluate the influence on compressor outlet pressure oscillations of the model parameters, of the supplied torque, of ambient conditions and of the shape of the compressor characteristic curves. The results show that the model proves effective in capturing the physical essence of surge phenomenon without being computationally too heavy.

Numerical Simulation of Transient Behaviors of Nematic Liquid Crystalline Flow between Parallel Plates under Magnetic Fields

Unsteady behavior of a nematic liquid crystal is simulated by using the Leslie-Ericksen theory as a constitutive equation. After a magnetic field is imposed on nematic flow between parallel plates, the director keeps a symmetric posture with respect to the channel centerline for a certain period, and afterwards, the symmetry is destroyed and the director reorients to the direction parallel to the magnetic field. When the magnetic field intensity is high, the period is short. While pressure gradient at the channel centerline shows the same behavior as that of orientation angle of the director for low magnetic field intensity, it indicates two-stepwise complicated variation before reaching the steady state for high magnetic field intensity. For liquid crystalline flow, pressure depends on the transverse position of a channel. An overshoot phenomenon of pressure near the channel wall is simulated numerically for high magnetic field intensity, which agrees with experimental results.