Sharif Digital Repository

Railway managers have a strong economic incentive to minimize track maintenance costs, while maintaining safety standards and providing adequate service levels to train operators. The objective of this study is to apply a procedure for making optimal maintenance decisions in Iranian Railways. This study consists of two parts. First, a cumulative damage model, based on a Markov process, is applied to model the deterioration of the track. For this reason, tracks are categorized into six classes, so that those tracks with similar traffic loads and geographical location are collected into one class. The track survey data from 215 blocks (4,228 km) of the ten divisions of the Iranian Railway... 

The dynamics of a two dimensional plane jet injected at the base of a step, parallel to the wall, in backward facing step flow geometry is numerically studied. The objective of this work is to gain insight into the dynamics of the igniter flow field in solid fuel ramjet motors. Solid fuel ramjets operate by ingestion of air and subsequent combustion with a solid fuel grain such as polyethylene. The system of governing equations is solved with a finite volume approach using a structured grid in which the AUSM+ scheme is used to calculate the convective fluxes. The Spalart and Allmaras turbulence model is used in these simulations. Experimental data have been used to validate the flow solver... 

A variable air volume (VAV) air handling system can significantly reduce fan power under partial load conditions. The focus of this study is developing a modeling program to predict the potential energy saving by the secondary HVAC system strategies. Comprehensive steady-state models are established to describe the energy performance of different control strategies for secondary HVAC systems. With building loads, minimum fresh air flow rate, pressure losses, supply air temperature control, dampers control and coil characteristic as inputs, the objectives were to develop a simulation model to evaluate performance of an air handling system. The potential energy savings of constant air volume... 

Massively multi-user virtual environments (MMVE) incorporate computer graphics, sound and haptics to simulate the experience of real-time interaction among multiple users in a shared three-dimensional virtual world. Such applications therefore deal with the distribution of updates among their users to provide them with a common sense of time and place while interacting in the virtual environment. As MMVEs grow larger in terms of size and numbers of users, they tend to rely on P2P architecture as a scalable way of exchanging the huge bulk of updates among their users without the need for the exponential increase in their central servers. While much work has been done on P2P architectures for... 

Resistance spot welding was used to join austenitic stainless steel sheets. Mechanical properties of the spot welds were evaluated using tensile shear test. Mechanical behaviour was described by peak load, failure energy and failure mode. The relationship between weld fusion zone attributes and failure behaviour was studied. Generally, it was observed that increasing fusion zone size is accompanied by an increase in load carrying capacity and energy absorption capability. However, when expulsion occurs, despite almost constant weld fusion zone size, energy absorption capability reduces significantly due to increase in electrode indentation depth. Considering the failure location and failure... 

In this paper we investigate the effects of superconducting shielded core reactor (SSCR) on the operation of induction generators. Large number of induction generators used in wind power plants generally increase the short circuit current level of the system. The reason is that they provide parallel reduced-impedance paths for the fault currents. Taking advantage of SSCR in wind power plants allows for using these generating units in large numbers with no concerns about the system fault current increments. The study is performed on a test system using the conventional model of an induction generator and a new effective model for SSCR based on its shielding behavior. Mathematical equations... 

The main concern of steganography (image hiding) methods is to embed a secret image into a host image in such a way that it causes minimum distortion to the host; to make it possible to extract a version of secret image from the host in such a way that the extracted version of secret image be as similar as possible to its original version (this should be possible even after usual attacks on the host image), and to provide ways of embedding secret images with larger size into a given host image. In this paper we propose a method that covers all above mentioned concerns by suggesting the idea of finding from an image data base, the most suitable host for a given secret image. In our method,... 

In this paper, we propose a new approach in computing interference distribution and outage probability for uplink CDMA cellular networks. In this approach, we model time-varying interference as a discrete-time Markov chain with flexible approximation and find its steady state distribution. Our approach is based on tracing the time variation of the effective factors in interference such as spatial locations, activity status, and shadowing status for the connected users. To this end, we approximate shadowing with several discrete levels by Lloyd algorithm. Moreover, we incorporate correlation properties of the shadowing status in our model. Then, we map the effective factors onto the... 

Power system equipments are normally designed for a specific system and load situation. This situation could change significantly as a result of the system expansion or aging. Among the system equipments, circuit breakers are more sensitive to the system expansion and they must be able to isolate the faulty components if a fault occurs. As the system load increases in the future, the power network must be expanded. In this situation, either the switching equipments must be replaced in order to be able to respond to the new fault current or the fault current must be limited to the interrupting capability of the existing circuit breakers. Application of Fault current Limiters (FCL) serves as a... 

The spatial sparsity of targets in the radar scene is widely used in multiple-input multiple-output (MIMO) radar signal processing, either to improve the detection/estimation performance of the radar or to reduce the cost of the conventional MIMO radars (e.g., by reducing the number of antennas). While sparse target estimation is the main challenge in such an approach, here, we address the design of a compressive-sensing-based MIMO radar, which facilitates such estimations. In particular, we propose an efficient solution for the problem of joint power allocation and antenna placement based on minimizing the number of transmit antennas while constraining the coherence of the sensing matrix.... 

In recent years, declining fossil fuel reserves and increasing environmental concerns led to higher utilization of renewable energy source (RES). One of the RES is Solar energy which is abundantly found in different areas of the globe, particularly in Iran. The aim of this research is to select a suitable site for constructing a solar power plant to generate electricity-hydrogen in southern Iran, Kerman province. For this purpose, a new hybrid Multi criteria decision making method is used. The Stepwise Weight Assessment Ratio Analysis (SWARA)method is used to weigh the criteria and the Measurement of alternatives and ranking according to Compromise solution (MARCOS)method is used to rank... 

Radon is released from soil and building materials and can accumulate in residential buildings. Breathing radon and radon progeny for extended periods hazardous to health and can lead to lung cancer. Indoor air conditions and ventilation systems strongly influence indoor radon concentrations. This paper focuses on effects of air change rate, indoor temperature and relative humidity on indoor radon concentrations in a one family detached house in Stockholm, Sweden.In this study a heat recovery ventilation system unit was used to control the ventilation rate and a continuous radon monitor (CRM) was used to measure radon levels. FLUENT, a computational fluid dynamics (CFD) software package was... 

Rare earth elements (REEs) are becoming significant due to their huge applications in many industries, large-scale mining and refining activities. Increasing usage of such metals pose negative environmental impacts. In this research ICP-MS has been used to analyze soil samples collected from former ex-mining areas in the depths of 0–20 cm, 21–40 cm, and 41–60 cm of residential, mining, natural, and industrial areas of Perak. Principal component analysis (PCA) revealed that soil samples taken from different mining, industrial, residential, and natural areas are separated into four clusters. It was observed that REEs were abundant in most of the samples from mining areas. Concentration of the... 

The paper presents the experimental and analytical model analysis of a steel-girder arch bridge. The field test is carried out by ambient vibration testing under traffic excitations. Both the peak picking method in the frequency domain and the stochastic subspace identification method in the time domain are used for the output-only model identification. A good agreement in identified frequencies has been found between the two methods. It is further demonstrated that the stochastic subspace method provides better mode shapes. The three-dimensional finite element models are constructed and an analytical model analysis is then performed to generate natural frequencies and mode shapes in the... 

Reynolds-Averaged Navier-Stokes (RANS) approach with the k-ε closure model is employed for the first time to simulate direct Solid-Phase Micro-Extraction (SPME) computationally. Simulations are performed by using COMSOL Multiphysics in order to examine methods to decrease the extraction time. Experiments are also conducted to support data obtained from the numerical framework. Di-n-Butyl Phthalate (DNBP) and etched steel wire are chosen as the analyte and the adsorbent, respectively. Stirring rate, fiber's location, stirrer magnet's size, and the method of sample rotation are examined to decrease the extraction time. In addition, the effects of adding a baffle to the vial and implementing a...