Sharif Digital Repository

The computational aspects of the finite element solution procedure based on the Cosserat theory are studied for some elastic-plastic problems, in which the localization occurs. For this purpose, the equations of the Cosserat elasto-plasticity, which include effects of couple stress, micro-rotation and length scale, are presented. The Cosserat finite element formulation is derived and an algorithm for the solution procedure is proposed. For the elastic-plastic problems considered here, the mesh-independency of the Cosserat-based results is quantified and effects of the internal length and Cosserat material parameter a are investigated on the results. Also, the influence of the internal length... 

Recent investigations have revealed the significant role of reactive power in blackout events. The associated disturbances frequently emerge in the form of voltage instability and collapse. Due to the computational complexity of modeling and analysis of enormous contingencies, DC approximation of the power flow, without accounting the role of reactive power, is usually used to evaluate the contingencies and to mitigate the associated probable violations. This paper, at first, presents a linear power flow model based on an approximated version of AC power flow formulation. The proposed model is then used to develop an efficient reliability assessment approach which is capable of taking both... 

In multi-state systems (MSS) reliability problems, it is assumed that the components of each subsystem have different performance rates with certain probabilities. This leads into extensive computational efforts involved in using the commonly employed universal generation function (UGF) and the recursive algorithm to obtain reliability of systems consisting of a large number of components. This research deals with evaluating non-repairable three-state systems reliability and proposes a novel method based on a Markov process for which an appropriate state definition is provided. It is shown that solving the derived differential equations significantly reduces the computational time compared... 

This paper presents an improved firefly algorithm (FA) for fast optimization of truss structures with discrete variables. The enhanced accelerated firefly algorithm (AFA) is a simple, but very effective modification of FA. In order to investigate the performance and robustness of the proposed algorithm, some benchmark (structural optimization) problems are solved and the results are compared with FA and other algorithms. The results show that in some test cases, AFA not only finds lighter structures compared to other algorithms, but also converges faster. In the rest test cases, the optimal solutions are found with very less computational effort. The study also shows that the proposed AFA... 

In this paper, considering horizontal components of seismic excitation, an algorithm for the nonlinear multi-component analysis of building structures by the Endurance Time (ET) method is proposed, and results of the ET method for various steel moment frames with one to seven stories are verified by comparing with the results from Nonlinear Time History Analysis with use of two different sets of ground motions. In addition, several parameters such as eccentricities and irregularities in structure, rotational and lateral stiffness, effect of hysteresis type and ductility have been considered for sensitivity analysis on results, obtained by ET method. Results show that on the basis of... 

The effectiveness of the local search algorithms for shop scheduling problems is proved frequently. Local search algorithms like tabu search use neighborhood structures in order to obtain new solutions. This paper presents a new neighborhood for the job shop scheduling problem. In this neighborhood, few enhanced conditions are proposed to prevent cycle generation. These conditions allow that the neighborhood encompasses larger number of solutions without increasing the order of computational efforts  

In endurance time (ET) method structures are subjected to a set of predesigned intensifying excitations. These excitations are produced in a way that their response spectrum, while complying with a specified spectrum, intensifies with time so they can be used approximately to simulate the average effects of several ground motions scaled to different intensities. In this paper applicability of the ET method for evaluating collapse potential of buildings is investigated. A set of four steel moment frames is used for collapse assessment. The process of using ET method in collapse evaluation is explained and the results are compared with incremental dynamic analysis (IDA) results. It is shown... 

This paper presents a stochastic framework to properly involve the uncertainties associated with demand in distribution system restoration (DSR) problem. To reach this goal, these uncertainties are represented as probabilistic scenarios corresponding to different load levels. Subsequently, the associated stochastic optimization problem is formulated such that it can be readily solved using dynamic programming approach. Moreover, a clustering technique is presented that enables the dynamic programming approach to find near-optimal solutions for the stochastic load restoration problem with reasonable computational effort. The proposed framework is implemented on a real-world test system, and... 

Localization and quantification of structural damage and estimating the failure probability are key outputs in the reliability assessment of structures. In this study, an Artificial Neural Network (ANN) is used to reduce the computational effort required for reliability analysis and damage detection. Toward this end, one demonstrative structure is modeled and then several damage scenarios are defined. These scenarios are considered as training data sets for establishing an ANN model. In this regard, the relationship between structural response (input) and structural stiffness (output) is established using ANN models. The established ANN is more economical and achieves reasonable accuracy in... 

We use the dissipative particle dynamics (DPD) method to simulate the non-Newtonian electroosmotic flow (EOF) through nanochannels. Contrary to a large amount of past computational efforts dedicated to the study of EOF profile, this work pays attention to the EOF of non-Newtonian fluids, which has been rarely touched in past publications. Practically, there are many MEMS/NEMS devices, in which the EOF behaviour should be treated assuming both non-continuum and non-Newtonian conditions. Therefore, our concern in this work is to simulate the EOF through nanochannels considering both non-Newtonian fluid properties and non-continuum flow conditions. We have chosen DPD as our working tool because... 

The main objective of this work is to propose a scheme to extract intrinsic mode functions of online data with an acceptable speed as well as accuracy. For this purpose, an individual block framework method is firstly employed to extract the intrinsic mode functions. In this method, buffers are selected such that they overlap with their neighbors to prevent the end effect errors with no need for the averaging process. And in order to avoid the mode mixing problem, a bandwidth empirical mode decomposition scheme is developed to effectively improve the results. Through this scheme, an auxiliary function made of both high- and low-frequency components corresponding to noise and dominant... 

Ground reaction force (GRF) characteristics of amputee walking are important for the analysis of clinical gait data, and also to update model reference adaptive impedance (MRAI) controllers. GRF estimation is a better alternative than direct GRF measurement because of the disadvantages of load cells, such as high cost, integration difficulties due to weight and physical dimensions, the possibility of overload, and measurement noise. This paper presents four robust MRAI observer/controller combinations for GRF estimation-based control of a prosthesis and a legged robot model in the presence of parametric uncertainties and unmodeled dynamics, in which the robot model is employed to mimic... 

A new shape-based geometric method (SBGM) is proposed for generation of multi-impulse transfer trajectories between arbitrary coplanar oblique orbits via a heuristic algorithm. The key advantage of the proposed SBGM includes a significant reduction in the number of design variables for an N-impulse orbital maneuver leading to a lower computational effort and energy requirement. The SBGM generates a smooth transfer trajectory by joining a number of confocal elliptic arcs such that the intersections share common tangent directions. It is proven that the well-known classic Hohmann transfer and its bi-elliptic counterpart between circular orbits are special cases of the proposed SBGM. The... 

Depending on the reference frame, a proportional integral (PI) or proportional resonant (PR) controller is often utilized for current control in the three-phase PWM rectifiers. Due to the fact that the controller bandwidth is either smaller than the frequency of a specific order harmonic or the controller gain at that frequency is too low the harmonics cannot be effectively eliminated. Among all approaches introduced for harmonic elimination, selective harmonic compensation is believed to have a better performance. Selective harmonic compensation can successfully compensate for harmonics. It, however, forces a considerable computation burden on DSP. This paper proposes a new compensation... 

The endurance time (ET) method is a dynamic analysis in which structures are subjected to intensifying excitations, also known as ET excitation functions (ETEF). The ET method is a tool for structural response prediction. The main advantage of the ET method over conventional approaches is its much lower demand for computational efforts. The concept of acceleration spectra is used in generating existing ETEFs. It is expected that ETEF acceleration spectra increase consistently with time and remain proportional to a target spectrum. Nonlinear unconstrained optimization is commonly used to generate ETEFs. Generating new ETEFs is a complicated time-consuming mathematical problem. If the target... 

This paper deals with the identification of fractional order systems via orthogonal rational functions. These functions have widely been used in system identification of classical integer order systems. It has been shown that due to some properties such as the presence of non-exponentional aperiodic multimodes in the fractional order systems, it is much better to use fractional orthogonal rational functions in approximation of these systems. One problem which arises in this area is the estimation of fractional order of these orthogonal rational functions. In the existing methods, these parameters have been found by trial and error which requires a large amount of calculations. To reduce the... 

In this work, velocity field and temperature distribution during hot strip rolling are predicted, employing a combined upper bound and finite element analysis. At first, a velocity field is proposed utilizing the principle of volume constancy, and then the velocity field is modified by means of upper bound theorem. At the same time, a thermal-finite element analysis is utilized to determine temperature distribution within the metal as well as to calculate the flow stress of deforming material. The model is capable of considering the effects of different factors on temperature and velocity distributions such as rolling speed and interface heat transfer coefficient. In order to verify the... 

In this paper, the application of Endurance Time (ET) method to the seismic analysis of bridges is elaborated. ET method is a novel seismic analysis method based on time history analysis in which a structure is subjected to a predefined intensifying acceleration function. First, six concrete bridges were modeled in this study. Three Endurance Time Acceleration Functions (ETAFs) were applied to the models, and the average of responses was calculated. Next, the time history analysis was conducted using seven real accelerograms that are scaled using the method recommended by Federal Highway Administration (FHWA) to be compatible with the design spectrum of American Association of State Highway... 

The low-cycle fatigue performance of shear panel damper (SPD) highly depends on the geometry of its shape and the criterion considered for its design. The main contribution of the current study is to find the optimum shape of the SPD subjected to cyclic loading by considering two different objective functions. The maximum equivalent plastic strain and the ratio of energy dissipation through plastic deformation to the maximum equivalent plastic strain are selected as the first and second objective functions, respectively. Since the optimization procedure requires high computational efforts, a hybrid computational approach is used to perform two paramount phases of estimating the inelastic... 

A recent method in the seismic assessment of structures is Endurance Time Analysis (ETA). ETA is a time-history-based dynamic pushover procedure, in which structures are subjected to gradually intensifying acceleration functions called Endurance Time Acceleration Functions (ETAFs), and their performances are evaluated based on the equivalent intensity level that they can endure while satisfying required performance goals. In this paper, the accuracy of the ETA in the seismic assessment of steel moment resisting frames is compared with the Time History Analysis (THA) and Incremental Dynamic Analysis (IDA) methods. For this purpose, a set of mid-rise and high-rise frames were selected as a...