Sharif Digital Repository

IEEE802.16 protocol is being developed and 802.16 modems will be used in every house to connect to the Internet through base-stations in both urban and rural areas. Since user bandwidth for connection to the Internet is limited, a large fraction of each modem’s bandwidth is not used. In this thesis, we propose an architecture to make a PPMN (Public Peer-to-peer Mesh Network) of all users in a city. Users can be wired or wireless under any authority. They have to forward intra-city traffic themselves. As a result, it is not required to transfer intra-city traffic through BSs except for critical information. The architecture is composed of a routing algorithm, a forwarding incentive... 

The necessity of electrical energy, high cost of big power plants and high voltage transmission lines and also progressing of restructuring science has lead to the increasing of tendency to use the distributed generation in electrical industry. Among of different types of distributed generations, fuel cell power plants because of many advantages such as high efficiency, low emission, modular structure and ect., have a high potential to be used in power systems. The fuel cells produce electrical energy by chemical reactions. Due to the necessity of having the knowledge related to the chemical reaction which are effective on fuel cell, to study the fuel cell performance, this type of... 

Gene expression using microarrays technology has special importance in biology studies especially in cancer researches. Using this technology we can assess expression level of several thousand genes simultaneously. One of the major applications of this technology is diseases’ diagnosis which cancers are the most important types of them. The problem in classifying microarray data is its high dimension; although number of samples hardly exceeds a hundred. This severe asymmetry and high dimensionality, results in reduction of accuracy, speed, stability and generalization of classification methods. Thus, one of the most important issues in cancer diagnosis is selection of related and useful... 

“Sustainable development” is a frequently used phrase, but the idiom has been experienced little since it was coined by the Brundtland, due to its intrinsic complications. The issue is going to compromise two key components; economy and ecology. One of the solutions based on concept of sustainable development that has been proposed for maintaining water quality at acceptable levels is to cap pollutant emissions at a sustainable level, then establish an economic market in which the right to discharge pollutants is traded according to market supply and demand mechanisms. Water quality trading or pollution trading is one such kind of market-based program that has also been described using names... 

This dissertation accounts hydrodynamic coefficients for the hull form of Dolphine Craft using an Experimental-Numerical method. First, a short preface is mentionded about Dolphine craft and its charcterstics. Then, different methods for determining the hydrodynamic coeeficient are reviewed and the method used in this dissertation is introduced. After that, the selected methods will numerically be modeled. For accurate modeling, virtual labarotory is organizing for performing these tests During modeling, six degree freedom equation including the kinematic and Dynamics of body element are extractred and consequently, they are expressed for the rigid body and control volume of fluid. The... 

Transparency is a measure of performance in haptic devices. In order to improve transparency and reduce the difference between the impedance transmitted to the user and the target impedance it is necessary to compensate for the dynamics of the haptic device. Due to stability reasons improvement of transparency is limited. Passivity as a stability criterion has been used widely in design and analysis of haptic devices. Since passivity is a conservative criterion, it acts as an obstacle in improving transparency of the haptic interfaces. In this project instead of passivity, robust stability of the interaction is studied in the presence of parametric uncertainties due to variations in user... 

Stochastic Partial Differential Equations have many applications in other area of science. In this thesis we investigate two pproaches in SPDE.The first approach is semigroup and the second is variational method.Our main purpose is stability of these equations  

Cracks significantly change the vibration response of structures. Cracks which exist in structures as a defect in the material or created by repeated loads, reduce the natural frequencies and change the vibration mode shapes. Therefore with measuring the vibration response of structures, the existence and position of cracks are detected. This research analytically investigates the free vibration and elastic buckling of a beam made of functionally graded materials (FGM) with open edge cracks using Reddy's beam theory and the rotational spring model. It is assumed that the material properties vary along the beam thickness according to a power law distribution in terms of volume fraction of... 

In this thesis, implementing parametric resonance for the purpose of improvement in sensitivity of MEMS gyroscopes is studied. Based on a parametric study on effect of each factor on the sensor’s performance, the desired values for each parameter is determined. Stability of periodic orbits is studied using Floquet Theory. In addition, three expositions are defined and proved in order to make Floquet Theory applicable for stability analysis of origin. Based on this analysis, the relation between stabilities in the system and occurrence of parametric resonance is illustrated. Due to the complexity of dynamics of a parametrically excited MEMS gyroscope, bifurcations are observed in performance... 

In the present study, a new version of haptic devices is presented in which user’s hand is in contact with the device just while collision with virtual surface. In this system, the position of user’s finger can be traced by using a camera and the haptic device follows user’s finger at the distance of about two centimeters. While the haptic device approaches to the virtual surface, gradually reduces its speed and stops about two centimeters distant of virtual plane. Then user’s hand collides with the haptic device which is static. Greater fidelity, transparency, more stability and also no need to large actuators are of the advantages of this system. In this project, first we present a new... 

In this thesis we consider the dynamics of a general predator-prey model which generalizes several known predator-prey, assuming that the intrinsic growth rate of the pray, the predation rate, and the removal functions are given in an unspecified form. Using the Lyapunov method, we derive sufficient conditions for the stability of the equilibria. also, we consider a delayed periodic predator- prey model and sufficient conditions are derived for the existence, uniqueness and global asymptotic stability of positive periodic solutions of the model.

 

The application of Stochastic Differential Equations in branches like nonlinear control,robatic systems, financial mathematics and etc. has grown impressively nowadays. In this thesis, we are going to introduce this equations and study their stability. First, we will present methods to check the stability of nonlinear systems dx(t) = f(x(t); t)dt using Lyapunov’s theorem.Then we will study stability of autonomous systems using it’s results. In the case of instability, we will study the possibility of stabilizing the stochastic equations dx(t) = Ax(t)dt + Σn i=1 Bix(t)dWi(t) by using Brownian motions and these methods. In the end, we will study the stabilizablity conditions of instable... 

Neural field models with delays define a useful framework for modeling macroscopic parts of the cortex involving several populations of neurons. Nonlinear delayed integrodi fferential equations describe the spatio-temporal behavior of these fields. We also present a study of the numerical computation of these solutions in a special case. Another original contribution of ours is the definition of a Lyapunov functional and the result of stability it implies. We illustrate our work on a variety of examples that are relevant to modeling in neuroscience.
 

In this thesis, an extended nonlinear model of peripheral milling process (2D) is presented. For the first time, cutting forces are described through a complete third order polynomial function of the axial depth of cut. Also, cubic nonlinearity is considered for the structural stiffness as a function of tool tip displacement. To complete the model, process damping and tool wear effects are also considered. A set of experiments including modal tests to determine natural frequencies and structural damping ratios; and also cutting force measurements at different feed rates to determine nonlinear cutting force coefficients, are carried out. Also, measurement of critical axial depth of cut at... 

Vibration is one of the most important factors in design, operation and lifetime of drill strings. There for, investigation of vibrations in drill string, like every other rotary machine, is very important. The main goal of this research is developing a nonlinear model for a drill string system dynamics in an inclined well. Effects of drilling mud flow rate, weight on bit, angular velocity along with viscous damping, on the stability and vibration of a drill string are studied. Dynamic equation of the model is developed considering axial displacement and lateral bending geometric nonlinear coupling. The effect of drilling mud flow is modeled using Paidoussis formulations. Equation of motion... 

Power Amplifiers are key components in new modern communication systems such as space and airborne applications, point to point networks and phase array modules. There are miscellaneous challenges in designing power amplifiers based on application and type of the system; high power, gain, bandwidth, linearity, efficiency, reliability, cost and size. Compromising among existence trade-offs is the state of the art for designing PAs. Meanwhile, technology improvements pay the way toward developing appropriate process in the realm of cost, cut-off frequency, voltage breakdown, thermal and reliance features. Throughout this thesis, challenges in designing high power amplifiers in the pHEMT GaAs... 

One of most important type of dams are concrete gravity dams, This structures maintain their stability against the load from their weight. The study of stability of dams is very important issue because Iran is placed on a seismic zone and instability of constructed dams will cause many problems for us. There are two method in study of concrete gravity dams: in first method which is named rigid approach the body of dam consider as a rigid body and by equilibrium equations and traditional strength of material concepts the safety factors of stability and stresses are determined. In second method that is named flexible approach, we should prepare a finite element method from the dam, reservoir... 

The increased need for experimental verification of the seismic performance of conventional and novel structural systems has resulted in highly sophisticated dynamic test procedures. Hybrid simulation is one of the most efficient experimental methods for assessment of dynamic and rate-dependent behavior of large-scale structural systems under earthquake excitation. Compared to earthquake simulations using shake tables, hybrid simulation may have significant advantages in terms of cost, scale, geometry, and required physical mass of structures. However, recent hybrid simulations have been limited to simplified structural models with only a few degrees of freedom. Currently, the major... 

80% of people in all over the world, experience Low Back Pain (LBP) once in their lives. LBP leads to dysfunction of spine. About 25% of LBP relates to the intervertebral disks which in the critical cases, a Posterior Lumbar Surgery (PLS) on the one or more lumbar disks should be done. Due to the some procedure such as retracting, cutting or denervation of muscles, PLS can hurt trunk muscles and spine. Thus, investigation of LBP and postoperative complications of PLS can help us in recognition of causes of LBP and PLS complications and modification of PLS approaches. The objective of this research was investigation of effects of LBP and PLS on the biomechanical function of spine. Pattern of... 

Using biomechanical models is one of the most effective ways to analyze and assessment of spinal loads in occupational and daily activities. In spite of many advances in the spine modeling, personal factors especially body weight are ignored in most studies. Epidemiological studies show that high body weight is a very strong risk factor of low back pain (LBP) and disc degeneration. There are only few studies that evaluate the relationship between spinal loads and body weight. The goal of this study is analyzing the effect of body weight on spinal load. A detailed musculoskeletal model of lumbar spine with 18 degree of freedom and 76 muscles was developed. This model consists of two parts...