فهرست مطالب

Scientia Iranica
Volume:15 Issue: 8, 2008

  • تاریخ انتشار: 1387/11/11
  • تعداد عناوین: 16
|
  • Sensitivity Based Health Monitoring of Structures with Static Response
    S. Chakraborti Page 1
    A sensitivity based parameter identification method is presented to detect the damage of existing structures, using applied sets of static forces at one subset of degrees of freedom and measured displacements at a subset of degrees of freedom that may overlap completely, partially, or not at all. The algorithm follows an output error approach, which minimizes the deviation between a measured and a theoretical displacement, in lieu of the commonly used force error function. An iterative scheme is developed utilizing a first order Taylor series expansion to linearize the associated non linear problem. The algorithm automatically adjusts the structural element stiffness parameters, in order to improve the comparison between a measured and a theoretical response in an optimal way. The measured input required in the present study is artificially generated. The effect that a noisy displacement measurement has on an identification procedure is also studied. A procedure is also identified, in order to select the limited number of DOF required to perform successful parameter identification, reducing the impact of measurement errors on the identified parameters. The algorithm is elucidated by a numerical example on frame structures.
  • Strong Ground Motion Effects on Seismic Response Reduction by TLCDs
    S.M. Zahrai Page 2
    Nowadays, various types of passive control systems are being used as an effective solution to reduce the seismic responses of structures. One type of these systems, the Tuned Liquid Column Damper (TLCD), suppresses the input seismic energy by a combined action, including the movement of liquid mass in the container, a restoring force on the liquid, due to gravity loads, and the damping, due to liquid movement through the orifices. In this paper, the possible effects of seismic excitation characteristics, such as frequency content and soil condition, on the seismic performance of TLCDs, are investigated, using nonlinear time-history analyses. For this purpose, a ten-story building was modeled as an elastic MDOF structure and used for numerical analyses. For the time-history analyses, among the past strong ground motion records of Iran, 16 records with different characteristics were selected. The results of this study show that these characteristics play a substantial role in the performance of TLCDs and they should be, accordingly, considered in the designing of TLCDs. In some cases, TLCD is able to reduce structural displacement up to 50\%, while, in most cases, the effectiveness of TLCD in reducing structural acceleration is not significant. However, it should be mentioned that, in real applications, de-tuning may occur, due to the inelastic behavior of structures, which can reduce effectiveness. This study also shows that the displacement reduction capacity of TLCDs is highly dependent on excitation characteristics, while the acceleration reduction capacity is not that sensitive.
  • A Method for Calculation of Face Gradients in Two-Dimensional, Cell Centred, Finite Volume Formulation for Stress Analysis in Solid Problems
    N. Fallah Page 3
    In this paper, a procedure is proposed for the evaluation of displacement gradients in a two-dimensional, cell centred, finite volume formulation for stress analysis in linear elastic solid problems. Temporary elements with isoparametric formulations are used for calculation of the gradients at the cell boundaries. In this way, stress continuity across the common face of the two adjacent cells will be guaranteed. The formulation is verified by three test cases, in which the proposed formulation shows good predictions.
  • Performance Evaluation of Masonry Buildings Using a Probabilistic Approach
    A. Bakhshi Page 4
    In many countries, there are traditional houses made of stone, adobe and brick, which perform relatively weakly in earthquakes. To mitigate casualties in future earthquakes, it is necessary to evaluate the current status of these buildings and propose effective methods to retrofit them. One method of evaluating the performance of buildings in earthquakes is through the use of fragility curves. These diagrams show the probability of exceeding a specific state of damage versus seismic intensity parameters, such as PGA, MMI, Ia, CAV etc. Fragility curves will provide an important basis for analytical methods, based on probabilistic approaches. Much work has been done on bridges, concrete and steel structures, while limited studies have been conducted on masonry buildings. Considering the prevalence of these buildings and the high seismic activity in some regions where they are built, fragility curves for different types of masonry buildings are developed in this research. The results obtained show the probable damage to those types of masonry building chosen in this research for different earthquake intensities and restates that, by providing horizontal and vertical ties and retrofitting these buildings, by the methods proposed in some seismic code provisions (such as FEMA 356 and 357), their behavior in earthquakes are apparently effected and structural damage reduced. Furthermore, results indicate that soil type does not significantly influence the seismic behavior of masonry buildings.
  • Use of Artificial Neural Networks in Predicting Highway Runoff Constituent Event Mean Concentration
    M. Kayhanian Page 5
    In this paper, the large amount of highway runoff characterization data that were collected in California, during a 3-year monitoring season (2000-2003), were assessed in order to develop an Artificial Neural Network (ANN) model for predicting the Event Mean Concentration (EMC) of the constituent. The initial data analysis performed by a Multiple Linear Regression (MLR) model revealed that the Total Event Rainfall (TER), the Cumulative Seasonal Rainfall (CSR), the Antecedent Dry Period (ADP), the contributing Drainage Area (DA) and the Annual Average Daily Traffic (AADT) were among the variables having a significant impact on the highway runoff constituent EMC. These parameters were used as the basis for developing an Artificial Neural Network (ANN)model. The ANN model was also used to evaluate the impact of various site and storm event variables on highway runoff constituents'' EMCs. The ANN model has proven to be superior to the previously developed MLR model, with an improved R^2 for most constituents. Through the ANN model, one was able to see some non linear effects of multi variables on pollutant concentration that, otherwise, would not have been possible with a typical MLR model. For example, the results showed that copper EMC is more sensitive at higher Annual Average Daily Traffic (AADT), with respect to ADP, compared with lower range AADT.
  • Free Water Surface Oscillations in a Closed Rectangular Basin with Internal Barriers
    A.R. Kabiri Samani Page 6
    Thee enclosed basin has certain natural frequencies of seiche, depending on the geometry of the water boundaries and the bathymetry of water depths. Therefore, the variation in the water surface at a point becomes irregular, as caused by the combination of several natural frequencies, which may be considered as the superposition of sinusoidal frequency components of different amplitude. This paper is mainly concerned with the motion of an incompressible irrotational fluid in a closed rectangular basin with internal impervious barriers. An analytical solution is presented for predicting the characteristic of generated waves in these types of basin. The equations of free water surface oscillations and its boundary conditions are reduced to a system of linear equations, which is solved by applying the small amplitude water wave theory. The flow potential, wave amplitude, flow patterns and the natural period of waves generated in the basin with impervious internal barriers are found, based on the basin geometry. It is shown that the natural period of the basin is strongly dependent on the location of the barriers and the size of the barrier opening.
  • Numerical Simulation of Free-Surface Waves and Wave Induced Separation
    B. Firoozabadi Page 7
    The present study is concerned with the numerical simulation of free-surface waves and wave induced separation in the presence of an intrusion. The results of several simulations are reported. The first study was performed for a NACA0024 surface piercing hydrofoil over a range of several Froude numbers (0.19, 0.37, 0.55), along with wave breaking at Fr = 1.0 The NACA0024 foil was of particular interest, as it almost has no separation at large depths; thus the effect of the free-surface wave and the wave induced separation could be studied. Free- surface waves and wave induced separation results were evaluated and compared with both the available experimental data and the previous numerical results. The wave breaking flow was also successfully simulated and results were presented. The second series of simulations were carried out for a circular cylinder in order to investigate the shape effects on the wave-induced separation. Results suggest that, at high Froude numbers, the free-surface waves are a function of Froude and not the shape of a body. Flow features with regard to separation, free-surface elevations and drag coefficients were also studied.
  • Investigation of the Effect of Pad Geometry on Flat and Rounded Fretting Fatigue
    G.H. Farrahi Page 8
    In this paper, the effect of geometrical parameters on the fretting fatigue of a half-plane in contact with a flat and rounded pad is studied. This is accomplished by calculating and comparing the stress states and stress intensity factors of fretting cracks for a number of pad geometries. The pad geometry is represented by the radius of its rounded corners and the width of its central flat part. The distribution of dislocation method is employed to calculate the stress intensity factors of fretting induced cracks of different lengths for different values of geometrical parameters. The results of this study can be insightful for improving the geometrical design of an aero-engine compressor disk, as similar contact and damage are prone to occur in its dovetail region.
  • Comprehensive Simulation of Surface Texture for an End-Milling Process
    B.M. Imani Page 9
    The analysis and simulation of the manufacturing process require extensive and complicated computations. Nowadays, computer resources and computational algorithms have reached the stage where they can model and simulate the problem efficiently. One of the important processes in manufacturing is machining. In this research, the end-milling process, which is one of the most complex and widespread processes in machining, is chosen. The most important parameters in end-milling are surface roughness and surface location errors. Comprehensive simulation software is developed to model the end-milling process, in order to anticipate the finishing parameters, such as surface roughness and errors. The proposed algorithm takes into account cutting conditions, such as feed, doc, woc and tool run out etc. In addition, the dynamic simulation module of the software can accurately model the flexible end-mill tool, the milling cutting forces and regeneration of the waviness effects, in order to construct a realistic surface texture model. The software can accurately determine the most commonly used index of surface roughness parameters, such as Ra, P.T.V. and surface errors.
  • A Meshless Boundary Element Method Formulation for Transient Heat Conduction Problems with Heat Sources
    M.R. Hematiyan Page 10
    In the boundary element formulation of heat conduction, the heat source effect imposes an additional domain integral term on the system of integral equations. With this term, an important advantage of the boundary element method as a boundary-only formulation will be lost. This paper presents an accurate method for the evaluation of heat source domain integrals, with no need of domain discretization. Transformation of the domain integral into the corresponding boundary integral is carried out using Green''s theorem. Both time dependent and time-independent fundamental solutions are considered. The methodology can be implemented in general and for similar situations. Numerical examples will be presented to demonstrate the accuracy and efficiency of the presented method.
  • Optical and Structural Properties of Copper Doped CdS Thin Films Prepared by Pulsed Laser Deposition
    S.M. Mahdavi Page 11
    This paper presents the study of the structural and optical properties of copper doped cadmium sulfide thin films prepared by pulsed Nd:YAG laser. The copper concentration in the targets was varied from 0.05% to 5% in weight and they were heated at a temperature of 500 C in air for one hour. It was observed that annealing un-doped and doped thin films at temperatures above 160 C leads to an abrupt increase in the optical transmission, changing from dark brownish to transparent yellowish. For annealed samples at 300 C, increasing Cu concentration from 0.0% to 2%, leads to an increase in the optical transmission up to 80%, at a wavelength of about 700 nm. XRD patterns showed the hexagonal phase of cadmium sulfide and a number of CdO peaks relating to the partial oxidation of cadmium. The XPS spectra of the annealed samples indicate an accumulation of Cu at the surface. Therefore, Cu impurity can enhance CdS oxidation and the size of the crystallites. The highest value of the photo-current for the samples is related to the lowest Cu concentration, i.e: 0.05%.
  • Unaxisymmetric Heat Transfer in the Axisymmetric Stagnation-Point Flow of a Viscous Fluid on a Cylinder with Simultaneous Axial and Rotational Movement Along with Transpiration
    A.B. Rahimi Page 12
    The unaxisymmetric heat transfer of an unsteady viscous flow, in the vicinity of an axisymmetric stagnation-point of an infinite circular cylinder, with simultaneous axial and rotational movement, along with transpiration, U_o, is investigated, when the angular velocity, axial velocity and wall temperature or wall heat flux vary arbitrarily with time. The impinging free stream is steady and with a strain rate of \overline{k}. An exact solution of the Navier Stokes equations and energy equation is derived in this problem. A reduction of these equations is obtained by the use of appropriate transformations for the most general case, when the transpiration rate is also time-dependent. However, results are presented only for uniform values of this quantity. The general self-similar solution of unsteady unaxisymmetric heat transfer is obtained, in which unaxisymmetry is due to the sinusoidal variation of the temperature, with respect to the surface position of the rotating cylinder, and unsteadiness is because of the sinusoidal variation of the temperature of each point of the cylinder surface, with respect to time and, also the rotation of the cylinder. All the solutions are presented for Reynolds numbers, Re=\overline{k}a^2/2\nu, ranging from 0.1 to 10.0 for different values of Prandtl number and selected values of the dimensionless transpiration rate, S=U_0/\overline{k}a, where a is the cylinder radius and \nu is the kinematic viscosity of the fluid. The local coefficient of heat transfer (Nusselt number) is found to be independent of time and place, though the cylinder wall temperature or wall heat flux are both functions of time and place.
  • Fuzzy Hierarchical Queueing Models for the Location Set Covering Problem in Congested Systems
    H. Mahlooji Page 13
    In hierarchical service networks, facilities at different levels provide different types of service. For example, in health care systems, general centers provide low-level services, such as primary health care, while specialized hospitals provide high-level services. Because of the demand congestion at service networks, the location of servers and their allocation of demand nodes can have a strong impact on the length of the queue at each server, as well as on the response time to service calls. This study attempts to develop hierarchical location-allocation models for congested systems by employing a queueing theory in a fuzzy framework. The parameters of each model are approximately evaluated and stated as fuzzy-numbers. The coverage of demand nodes is also considered in an approximate manner and is stated by the degree of membership. Using queueing theory and fuzzy conditions, both referral and nested hierarchical models are developed for the Location Set Covering Problem (LSCP). To demonstrate the performance of the proposed models, a numerical example is solved in order to compare the results obtained from the existing probabilistic models and the new fuzzy models developed in this paper.
  • A Methodology for Optimizing Statistical Multi-Response Problems Using Fuzzy Goal Programming
    M. Amiri Page 14
    This paper presents a method for optimizing statistical multi response problems. The method is based on fuzzy goal programming and it enjoys a strong mathematical foundation. In this method, the decision maker''s comments are considered objectively. The LINGO programming environment is used to test the developed method. The method performance is evaluated by comparing the results with those of other existing methods
  • Numerical Simulation of Buoyancy Affected Turbulent Air Flow in a Room
    A. Nouri, Borujerdi Page 15
    In this paper, a three-dimensional steady state incompressible turbulent air flow is considered in a large single room. The buoyancy affected turbulent air flow is numerically simulated by solving governing equations. The turbulence modeling includes both k-\varepsilon and zero-equation models and their results are compared to the experimental data. The paper reviews several aspects, such as displacement of radiator system performance, temperature and flow field distribution and comfort conditions. The results show that the best temperature distribution and comfort condition are obtained when the radiator is installed under the window and its height is equal to or greater than that of the window.
  • Quality Function Deployment, Value Engineering and Target Costing, an Integrated Framework in Design Cost Management: A Mathematical Programming Approach
    F. Jariri Page 16
    hIn this paper, the need to incorporate three famous design cost management methods, called: Quality Function Deployment (QFD), Value Engineering (VE) and Target Costing (TC) into a single model has been addressed. Each method performs very well in cost management procedures as design activities. These methods have been incorporated into a mathematical programming model, in order to achieve the maximum benefit of each method. The model, essentially, optimizes customer satisfaction subject to target cost. The tool is a mixed integer zero-one nonlinear programming. The unified model has been proposed to prevent a non-optimal solution when methods interact with each other. The practitioner should be confident that the quality solution would be achieved in contrast to when the methods are applied sequentially. A simple automobile design example was formulated and solved to show the performance of the model.