Sharif Digital Repository

Linear stability analysis is used to characterize the dynamics of a plane jet by incorporating non-modal stability analysis besides classical global temporal stability analysis. It is explained that similar shapes of different global modes are the result of non-normal characteristics of linearized Navier Stokes equations. Optimal initial disturbances and their eigenfunctions together with transient energy growth are obtained for different time horizons and Reynolds numbers of the jet in the linear unstable configuration. These structures are localized at the upstream of the jet nozzle at the boundary layer. The transient growth of the inlet perturbation in limited time bounds is found in the... 

A highly reliable tool for transient simulation is vital in the safety analysis of a nuclear reactor. Despite this fact most tools still use diffusion theory and point-kinetics that involve many approximation such as discretization in space, energy, angle and time. However, Monte Carlo method inherently overcomes these restrictions and provides an appropriate foundation to accurately calculate the parameters of a reactor. In this paper fundamental parameters like multiplication factor (K eff ) and mean generation time (t G ) are calculated using Monte Carlo method and then employed in transient analysis for computing the neutron population, proportional to K eff , during a generation time... 

In this paper, an SET and SEU tolerant latch suitable for use in embedded systems called SETUR (Single Event Transient and Upset Robust latch) is presented and evaluated. The SETUR is based on the use of a redundant feedback line and a CMOS delay element to tolerate the effect of the SETs occurring in the input line of the latch as well as SEUs occurring inside the latch. The experimental results show that the probability of an SET resulting in a soft error can be reduced up to 90% by choosing a proper delay value. The soft error rate of the SETUR due to SEUs occurring inside the latch is reduced by 95% while having lower area, power and performance overhead than the previously proposed... 

Developing an accurate network equivalent to reduce the computational time of an electromagnetic transient study has been the major task of many researches in the past two decades. However, still no accurate wide band equivalent for an asymmetrical network is addressed in the literature. This paper presents a Hybrid equivalent for the external system, assuming that it contains an asymmetrical transmission system at the terminal of this external system. The proposed method is based on the introduction of an asymmetrical transmission system in the model through application of a real constant transformation matrix in the subsystem and dealing with its asymmetrical effects in the external... 

When velocities in the piping systems change rapidly, spectacular accidents occur, due to tranient-state pressures where the elastic properties of the pipe and liquid must be considered. This hydraulic transient is commonly known as water hammer. A conventional widely-used technique for analyzing this phenomenon is the Method Of Characteristic (MOC), in which, by introducing the characteristic lines, two ordinary differential equations, in lieu of the governing partial differential equations, are produced. In the undisturbed form of the equations, the energy dissipation is evaluated by the steady or quasi-steady approximation. But, there is experimental and theoretical evidence which shows... 

This article presents a comprehensive small signal model for a virtual synchronous generator (VSG) integrated into a microgrid. In the developed state-space, active and reactive power control loops of the VSG as well as resistances, and reactances of the lines are considered. Based on this model, the robust D-stable region in terms of different values of inertia constant $(M)$, and damping coefficient $(D)$ values of the VSG is obtained. To enhance both transient performance as well as guarantee the robust D-stable operation of VSG, an optimization problem is also proposed. Moreover, a new control method is suggested to damp frequency oscillations of the microgrid. The proposed controller is... 

In this study, a nonlinear observer for high-speed estimation of the sample surface topography in a small duration of the probe transient motion utilizing a 2DOF model of TR-AFM is proposed. Since the time duration to reach the steady-state periodic motion of the oscillating probe in conventional imaging methods is relatively high, the proposed nonlinear observer in this research is able to address this limitation and estimate the surface topography throughout transient oscillation of the microcantilever. With this aim, topography estimation process utilizing a Thau observer without any linearization of the system dynamics is designed and coupled with the system dynamics to achieve sample... 

In this research, important parameters of a rack and pinion steering system in dynamic steady state and transient responses have been investigated. For this purpose, virtual model of a medium passenger car in ADAMS/Car has been used. The model has up to 121 kinematic degree of freedom and includes all components of the rack and pinion steering system. Several different experimental test results have confirmed the validity of the model. Sensitivity analysis have been done based on design of experiments (DOE) method. Two level fractional factorial designs have been selected for this purpose. Steady state cornering and step steer input are the analysis that used for this research. Understeering... 

This study introduces a new method for forecasting future states of nuclear power plants (NPPs) parameters in abnormal conditions (i.e. transients). The proposed method consists of two steps. First, the type of transients is recognized by the modular EBP based identifier. A hybrid network is then used to forecast the selected parameters of the identified transient. ARIMA model is used to estimate the linear component of the selected parameters. The neural network developed by EBP learning algorithm is then used to estimate the nonlinear component of the selected parameters. Finally, prediction of parameters is obtained by adding the estimated linear and nonlinear components. To analyze the... 

Piloted ignition of solid fuels in dump combustor geometry subject to an igniter hot jet plume is numerically investigated. The objective of this work is to gain insight into the fuel ignition and subsequent flame spreading in this turbulent flow configuration. Conjugate heat transfer between gas and solid phases is considered to study the solid fuel heating and evaporation process; Solid phase energy equation is simultaneously solved coupled with flow governing equations. Finite rate one step second order chemistry is used in simulations. The gas phase equations along with the Spalart-Allmaras turbulence model are solved with a finite volume approach in which the AUSM + scheme is used to... 

Transient liquid phase (TLP) bonding was used to join a nickel-based superalloy GTD-111 using a Ni-Si-B amorphous interlayer, MBF30. Bonding was carried out at 1100 °C with different holding time under vacuum. Joint region microstructure was studied by optical and scanning electron microscopy. Microstructural studies showed that before completion of isothermal solidification, bond region consists of four distinct zones: centerline eutectic structure due to athermal solidification, solid solution phase due to isothermal solidification, diffusion-induced boride precipitates and base metal. Complete isothermal solidification, which prevents formation of centerline eutectic constituent, occurred... 

This paper presents a behavior-based error detection technique called Control Flow Checking using Branch Trace Exceptions for PowerPC processors family (CFCBTE). This technique is based on the branch trace exception feature available in the PowerPC processors family for debugging purposes. This technique traces the target addresses of program branches at run-time and compares them with reference target addresses to detect possible violations caused by transient faults. The reference target addresses are derived by a preprocessor from the source program. To enhance the error detection coverage, three other mechanisms, i.e., Machine Check Exception, System Trap Instructions and Work Load Timer... 

Electrodeposition of the Co-Ni/Cu multilayer films was carried out in sulfate solution. Cyclic voltammetry and current transient techniques were utilized to characterize the multilayer system and to obtain the nucleation and growth mechanism. The cyclic voltammograms clearly showed that electrodeposition of cobalt-nickel alloy layer was controlled by a kinetic process, where copper ions were reduced under diffusion-controlled mechanism. In addition, the current transients revealed that nucleation mechanism was instantaneous with a typical three-dimensional growth process. The microstructure of the Co-Ni/Cu films was also changed with overpotential. In this system, the growth of multilayer... 

In the present work, the sintering behavior of high strength Al-5.6Zn-2.5Mg-1.6Cu (in wt.%) alloy compacts was investigated. Green parts made by pressing of mixed elemental powders were sintered at different temperatures between 400 and 600°C The effect of the sintering temperature on density, dimensional change and microstructure of the Al alloy was studied. Microstructural evaluation was accompanied by XRD and DSC methods in order to determine the temperature and chemical composition of the liquid phases formed during sintering. It was found that three transient liquid phases are formed at 420, 439 and 450°C Microstructural study revealed the progressive formation of sintered contacts due... 

The transient response of all optical gain clamped multi channel erbium doped fiber amplifiers (EDFA's) and optical fiber inverter is studied by a semi-classical model. Using the semi-classical theory, the electric field interacting with mater is considered classical and the medium is quantized according to the equations of motion for density matrix. The rate equations and "self consistency" equations, which yield amplitude and phase of the electric fields of oscillator and amplifying channels are derived by considering Maxwell's equations and equation of motion of density matrix. The governing equations form a system of coupled partial differential equations. The relaxation oscillation... 

By occurring failures in computer networks, routing protocols are triggered to update routing and forwarding tables. Because of invalid tables during update-time, transient loop may occur and packet-drop rate and end-to-end delay increase which means that the quality of service decreases. This paper proposes a parallel architecture for a router to recalculate and update routing table. Simulation results show that with dual-processor architecture, this update time could be up to 40% improved, depending on the network topology and the size of tables. This paper also studies the effect of this speed-up on networks' performability, i.e. the ability to deliver services at predefined level. A... 

Solid rocket performance during rapid pressure excursions differs greatly from predictions based on steady state burning rate data. Rapid pressurization following a chamber filling interval produces indicated burning rate overshoots. Transient internal ballistics of a solid propellant rocket motor during rapid pressurization part of chamber filling phase has been considered in this work. Quasi one-dimensional unsteady Euler equations with a transient propellant burning model that accounts for the effects of time rate of change of the chamber pressure on the burning rate have been used to simulate the internal ballistics of rocket motors. The compressible convective flow solver used in this... 

In this paper, the Markov models, eigenvectors and eigenvalue concepts are used to propose a methodology for analyzing the transient reliability of a system with identical components and identical repairmen. The components of the systems under consideration can have two distinct configurations, namely; they can be arranged in series or in parallel. A third case is also considered, in which the system is up (good) if k-out-of-n components are good. For all three cases, a procedure is proposed for calculating the transient probability of the system availability and the duration of the system to reach the steady state. © Sharif University of Technology, February 2007