Sharif Digital Repository

Queuing algorithms are used in packet forwarding elements such as routers or switches and the goal is to minimize transmission delays as well as to help manage network congestions. On the other hand, over 90 to 95% of the packets in the Internet use TCP and correspond to the transfer of digital documents of one form or another (Web pages, data files, MP3 tracks etc.) ; this kind of traffic is called elastic. In this thesis, we present a new Start-time Fair Queuing (SFQ) algorithm called Weighted Start-time Fair Queuing (WSFQ) which is more efficient and achieves better fairness than SFQ in the presence of small and huge elastic traffic flows. WSFQ scheduler, like SFQ uses a start time... 

Stress tensors and their rates are fundamental quantities in modeling the behavior of materials in the field of nonlinear continuum mechanics. One of the most remarkable applications of stress rates is their presence in the rate type constitutive equations of materials.
Co-rotational rates are corresponding to the rates which are measured by an observer who is in a reference rotating coordinate system. In general, under some specific conditions for the spin of a co-rotational rate, the rate of an Eulerian tensor would be objective. In this thesis, the objective co-rotational rates of Eulerian stress tensors for isotropic elastic materials under large deformations are obtained for the... 

The DNA Molecule carries genetic information in almost all living organisms. The information coded on DNA is an instruction which organizes and conducts all living activities of the living organism. The role of DNA in biological processes is so fundamental that life, as we know, is not possible without DNA. Therefore, studying the biological properties of DNA is crucial for understanding the nature of life. In biological conditions, DNA molecule can be found in highly packed configurations. Since the DNA is a rather stiff molecule, the formation of these configurations cost a considerable amount of energy. The elasticity of DNA plays an important role here. The important question is: how... 

In this study we investigated the stress effects on the fractured block in formation and its impact on flow through the matrix. At first the effect of various stresses on reduction of sample's pore volume and thereby reduction the permeability have been studied. The equations used to describe the elasticity relations in saturated matrix block is provided by Bayot theory. In this theory, the system of related equations to stress and strain changes have been developed in matrix. We have used finite element method in COMSOL software to solve the equation system created by elasticity relations in formation. After solving the equations, volume and volumetric strain changes which are caused by the... 

In this thesis, we aim to study the elastic-plastic behavior of short fiber reinforced composites experimentally and analytically. The experimental investigation is applied to glass fiber-polypropylene composites (PP/GF). The samples are produced by injection process with maximum temperature of 190C. Fibers are randomly oriented in this procedure of fabrication. The samples are tested in tension, and the mechanical strength is measured by changes in fiber content. The Young's modulus is also investigated in these experiments.Also in this thesis, an analytical model for calculation of Young’s modulus of fiber reinforced composite materials is proposed. In this analysis a displacement field is... 

Previous studies indicate that increasing cement compressive strength can’t prevent loss of zonal isolation and casing collapse in worse case during well operations or formation loading. Since standard cements with high compressive strength can’t solve these problems, so application of elastic cements with low Young’s modulus and high Poisson’s ratio becomes important. These cements sustain generated stresses during well operations and prevent creation of point loads when formation creep occurs. For this purpose, a new additive was designed in order to improve the cement elasticity and tensile strength. It is a nanoparticle and named EX-RIPI (by Research Institute of Petroleum Industry,... 

Exoskeleton is a wearable active device used to augment human power in upper or lower extremities. The integration of robotic devices and conventional physiotherapy is becoming more and more acceptable worldwide. The main scope of this thesis is to design and develop a prototype of a light, low-cost and wearable robotic exoskeleton to rehabilitate the upper extremity in stroke patients at home. For this purpose mechanism of a wearable exoskeleton will be proposed compatible with upper extremity degrees of freedoms with minimum number of actuators which is constructed inexpensively to eliminate the demand of expensive and professional therapists. The conceptual design of such a system should... 

The concept of equivalent linearization in which the actual nonlinear system is substituted by an equivalent linear single-degree-of-freedom (SDOF) system is extended to soil-structure systems with elastic and inelastic behaviors. Equivalent linearization is one of the approximate methods widely used to estimate the response of nonlinear structures in seismic performance based design procedures as a basis of Capacity Spectrum method. Period and damping of equivalent linear model should be evaluated in a way to predict the maximum inelastic displacement demand of the structure with sufficient accuracy. In recent years, comprehensive statistical studies have been performed to improve the... 

This research deals with pricing private automobiles entering a congested area in urban CBD to reduce auto-related air pollution. There are two factors affecting the pollution level: car volumes, and the weather condition. A Markov decision process is employed to find the optimal tolls and determine the prices for different states of the system, under several weather conditions. The demand for travel is elastic. It is assumed that users change modes in the short run. In longer terms, the part of the demand with shopping, recreation, or similar flexible trip purposes change the respective demand levels according to the price level. The study attempts to estimate elastic demand functions for... 

Usually the structures are designed based on the fixed-base assumption; however this assumption is not always correct and the bed flexibility should be considered. This effect which is called soil-structure interaction, change the response of structure on both elastic and inelastic behavior. In the regulations for the design of structures on flexible base, generallysoil-structure system is replaced with an equivalent fixed base structure; in the way that the design force of soil-structure system with reasonable accuracy can be estimated by equivalent fixed-base structure. In some regulations, dynamic characteristics of equivalent structure have been determined on elastic state and the... 

In the last few decades Seismic Regulations attitude’s has been undergone significant changes from various aspects. One of these aspects is entering new topics based on the latest scientific achievements. Soil-Structure Interaction terms was from such issues which had found their ways into Seismic codes as an optional effective factor. In recent years, Based on this new attitude and Scientific achievements, Soil-Structure Interaction terms entrance into Iranian seismic code (2800 Standard), is also has been studied. Naturally, applying these criteria will impact on the design and performance of structures, because of reducing effect of soil structure interaction on decreasing base shear... 

In the framework of surface elasticity theory, the scattering of in-plane elastic waves by a multicoated nanofiber embedded in an elastic matrix is studied. Atoms near an interface experience a local environment different from it within the bulk. Going from macro to nano dimensions, surface to volume ratio increases significantly. Thus, on this scale it is important to consider the surface/interface properties as well as the bulk properties. In this dissertation, the surface/interface is modeled by a very thin layer in which the traction and displacement are discontinuous. Effects of surface/interface parameters on the dynamic stress concentration factor (DSCF) and scattering cross section... 

In recent years, Internet traffic has nearly doubled in the core network every two years, and it is expected to continue to grow in the future. With this growth, there is a need for very high capacity and economically efficient transmission systems, which optical communication is the best choice. Elastic optical network (EON) is one of the best options for achieving this goal in the future. Unlike existing networks, the transmission parameters of EON will be customizable, which will have an essential role in optimizing bandwidth assignment. The problem facing scientists today is to approach the final physical capacity of current optical fibers. The solution proposed for this challenge is... 

High-definition video streaming, multimedia file sharing, cloud computing, mobile networks and online gaming, all together, will increase the network traffic enormously every year.Long-haul optical fiber networks form the backbone of the Internet and scaling to higher throughput and lower cost per bit is essential to ensure continued growth of information technologies. For four decades, optical networks have exploited significant progress in physical technologies, including low-loss single-mode fibers (SMFs), high-performance lasers and modulators, and low-noise optical amplifiers. In parallel, communication methods have evolved to multiplex information in various physical dimensions in... 

The use of metal matrix composite materials (MMC) in aerospace structures and engine parts has become more frequent recently. These materials are composed of metal matrices reinforced by hard discontinuous ceramic fibers, usually oriented longitudinally within the matrix, and due to the stability of the mechanical properties, including the hardness and the strength at high working temperatures, have found various applications.Stress transfer from the matrix to the fiber is known as the most important mechanism governing the deformation and the fracture response of such MMCs. The stress transfer characteristic of fiber reinforced composite materials under various mechanical and thermal... 

In recent decades, the study of Nano and Micro Electromechanical Systems (NEMS and MEMS) as in micro sensors and actuators or in Atomic Force Microscopes (AFM) attracted considerable attention. Since an indispensable part of such systems is a nano or micro flexible beam, the inevitable undesirable vibrations of the flexible beam necessitate the implementation of vibration control strategies. Recently, some cutting edge experiments on statics and dynamics behavior of micro beams have been conducted that reveal some deviations from the prediction of classical theories, and hence this leads to development of strain gradient elasticity theory. In addition, the often-adopted finite dimensional... 

In the present study, we analyzed the mechanical behavior of micro rotating disk based on the most general form of strain gradient elasticity. According to the fact that micro rotating disks have an important role in MEMS, it is necessary to present an exact analysis of their mechanical behavior. It is noticed experimentally that materials behave stronger in deformation as they get smaller. The classical continuum theory is not capable to predict such behavior; therefore, it is essential to use one of the non-classical continuum theories to analyze the material in micro scales, which can capture this phenomenon. In this work, the strain gradient theory is utilized to derive the governing... 

In this thesis one of the mesh-free methods called RKPM is employed to solve the differential equations of strain-gradient elasticity. To this end the corresponding weak form is laid down. Subsequently the relevant stiffness-matrix is obtained by discretization of the weak form. To be sure about the accuracy of the relations, the problem of a plate weakened by a hole under uniform far-field tension, for which the exact solution is available in the literature, is solved. The obtained numerical result is in good agreement with the solution of Eshel and Rosenfeld. Afterwards, a plate containing a crack of finite length subjected to uniform far-field tension (mode I) is considered. This problem... 

There are experimental observations that show material response in micro-scale is dependent on some other parameters rather than Lame parameters. Strain gradient elasticity has been recently developed to take into account this characteristic of materials response. In strain gradient elasticity, characteristic length parameters enter the constitutive equations through the elastic strain energy density function. The elastic strain energy density function is assumed to be a function of the gradient of strain tensor in addition to the strain tensor. In this way, new material constant (characteristic length parameters) are introduced and entered into the constitutive equations. In recent years,... 

Hydraulic steering technology has been used in vehicles for decades. Recently, a trend has been seen towards the use of electronic steer-by-wire systems that provide greater design flexibility by customizing the connection between the steering wheel and the steering mechanism. Steer-by-Wire systems (SbW) offer the potential to enhance steering functionality by enabling features such as automatic lane keeping, park assist, variable steer ratio, and advanced vehicle dynamics control. The lack of a steering intermediate shaft significantly enhances vehicle architectural flexibility. The task of a SbW system is two-fold: turning the steered wheels by tracking the hand wheel (HW) rotation and...