Sharif Digital Repository

In this research, we consider the problem of finding longest paths in some special classes of graphs. This problem is NP-hard in general case. It has been proven that there is no constant factor polynomial time approximation algorithm for this problem, unless P = NP. Therefore, the problem is usually solved for special classes of graphs.In this research, we focus on finding longest paths in solid grids, and propose a factor 3/4 approximation algorithm. Our algorithm improves the best current result which has an approximation factor of 2/ 3 . As a side result, we also propose a linear time algorithm for finding Hamiltonian cycle in a subset of Hamiltonian solid grids which improves the... 

The Darcy’ law is valid for low velocity or low Reynolds number where merely viscous force is responsible for pressure loss in the porous media. At higher velocity, because of inertial forces and turbulency, the pressure loss would be higher than Darcy’ law estimates; therefore at these flow regimes the Darcy’ law is no longer valid. Hence, consideration of Non-Darcy flow effect is necessary to sufficiently model high gas velocity effects. The velocity is high in gas reservoirs especially those ones that are considered for underground gas storage. In this study, the effects of Non-Darcy flow on several parameters such as; estimation of differential pressure, flowing bottom hole pressure,... 

Transport of drilling cutting to the surface is the most important function of drilling fluid. Insufficient hole cleaning leads to accumulation of cuttings in the annulus especially of a Horizontal wellbore. This may cause severe drilling problems including reduction in rate of penetration, stuck pipe, premature bit wear, increase the mud lost, high drag and torque. In this research work a mathematical model is developed to simulate cutting transport process in the horizontal drilling wells. A three-layer model was used to simulate cutting transport process in horizontal wellbores: a stationary at the bottom of hole, a moving bed and a suspension layer on top of these layers. In this model,... 

Quantum cryptography can provide complete security. Using the quantum mechanics postulates, we can achieve a provable security which is not possible in classical cryptography. When the first completely secure protocol for distributing key was introduced, it turned a great deal of attention toward quantum cryptography and researcher focused on finding quantum mechanical counterpart of other classical cryptographic protocols. Among these protocols, quantum secret sharing is one of the most useful and important ones. In this thesis, first we introduce the basic postulates of quantum mechanics. Then, we introduce some known quantum secret sharing protocols and discuss their properties and... 

Complex systems are composed of a large number of subsystems behaving in a collective manner. In such systems, which are usually far from equilibrium, collective behavior arises due to self-organization and results in the formation of temporal, spatial, spatio-temporal and functional structures. The dynamics of order parameters in complex systems are generally non-stationary and can interact with each other in nonlinear manner. As a result, the analysis of the behavior of complex systems must be based on the assessment of the nonlinear interactions, as well as the determination of the characteristics and the strength of the fluctuating forces. This leads to the problem of retrieving a... 

Wellbore storage effect is one of the biggest problems of petroleum engineers in analyzing the reservoir properties. In this phenomenon, at the beginning time of production from a reservoir, the fluid is initially produced from the well and not from the reservoir. Due to this effect, the pressures are distorted and could not be used in reservoir analysis. In this thesis the wellbore storage effect would be removed in naturally fractured reservoirs using deconvolution method. Using this method the range of data used could be increased. In this method special techniques are used in order to determine reservoir behaviour.
Using the method presented in this thesis, petroleum engineers could... 

In this study, Wave Endurance Time (WET) is introduced as an applicable method for assessment of offshore structures under extreme waves. In this method, artificial records called Wave Functions (WFs) are used instead of usual extreme wave records. WFs are designed so that their excitations gradually increase with time. Therefore, one WF can be replaced by a large number of sea surface records with different excitation levels. The main advantage of proposed method is that it assesses the structural performance under various extreme waves with low computational cost especially in comparison with three-hour simulasions. Moreover, reliability of structures can be evaluated based on the time... 

Steam-Assisted Gravity Drainage (SAGD) is one of the best and the most effective methods of enhanced oil recovery (EOR) which is used in the production from Heavy Oil Reservoirs which constitute large amount of oil throughout the globe. In a typical process of SAGD, steam is injected into a horizontal well and the oil is produced several meters deeper in the ground. In order to increase the Recovery of this process, it is better to review on the Pre-heating methods around the injecting and producing wells. Pre-heating is important when the viscosity of the oil is large enough to prevent oil from any motion at the beginning of the SADG operation, and makes it necessary to to use a Pre-heating... 

Due to unusual response of structures of buildings with irregularity in their architecture to dynamic excitations, to have a comprehensive knowledge of their behavior during earthquakes is of utmost importance. Building irregularities are in general divided into two major categories, namely Irregularity in Plane and Irregularity in Elevation, and one of the forms of the latter is where there is a split in the levels of the building. In this work, through a series of case studies, the effects of P- phenomenon on the behavior of steel split-level building structures were studied. 5, 10 and 15 story buildings with 5 and 6 bays, and level splitting of 30, 60, 90, 120 and 150 cm were studied. All... 

Mesoscopic hydrodynamic equations are solved employing a VOF based method to investigate the equilibrium shape of nanodroplets positioned over various topographic geometries of the supporting substrate for three-dimensional systems. By taking into account liquid-liquid and liquid-solid interactions a complex distribution for inter-molecular forces over the substrates (the disjoining pressure) is observed. In this research we show that motion of nanodroplets not only caused by contact angle difference in drplets two sides, but also depend on disjoining pressure parameters.Geometries with increasing complexities, from wedges to three dimensional edges and wedges, were explored with the main... 

Observations made after previous earthquakes have shown the crucial role of irregularities in initiating damage in buildings with irregularity, either in Plan or in Elevation. Since in existing Building Codes, there is no provision for the distribution of the Equivalent Static Loads for such buildings, designers have no option but to resort to Dynamic Analysis. In this work, the seismic behaviour of a group of two-dimensional steel building structures with irregularity in elevation, namely Split-Level buildings, was studied. The studied structures consist of a series of 5- and 15-storey frames having two parts with splitting distance of 150 cm. Both, moment-resisting and braced frames were... 

With the road transportation vehicles being widely used, concerns have been increased regarding their unwanted effects. Air pollution, climate change, and oil supply security are among these effects. Many alternative fuels and propulsion systems have been proposed to be used instead of conventional ICE vehicles in recent decades. This project intends to conduct a Techno-Economic comparison between different vehicle powertrains so as to select the suitable choice to substitute conventional ICE vehicles. The vehicle options are ICE vehicles using both gasoline and CNG as fuels, Hybrid Electric Vehicles (HEVs), Plug-in Hybrid Electric Vehicles (PHEVs), and Electric Vehicles (EVs). The amount of... 

Base plates as the connecting components of steel structures to their foundations, have a crucial role in the behavior of these structures. In order to limit the thickness of base plates hence their cost within reasonable values, various types of attachments (stiffeners) are normally added to them. In this project 6 different configurations, Models, for the base plate and its attachments were considered and studied through finite element analysis. Each Model was studied under the combination of biaxial moments and a range of compressive loads including 0, 0.05 P/Py, 0.10 P/Py and 0.15 P/Py, where Py is the plastic load of the column. The moment-rotation curves of all models under various... 

Reducing air pollutant emissions from vehicles is a difficult and time consuming procedure which needs utilizing of modern emission control equipment and setting strict regulations for car factories and drivers as well. How efficient these policies are and proposing operative solutions for air quality improvement depend on developing a sustainable model which is used in vehicle emission predictions and their share in total emissions. Emission factors are not constant values as they can be affected by various operational and ambient parameters. Development of Emission inventories calls for a huge number of measurements using different vehicles in diverse conditions. Thus, recognition of... 

The city of Tehran is dealing with air pollution problems for years. Providing a solution for this problem requires information about different pollutants emission sources. In this subjects, emission factors and emission models are essential tools for decision makers. Among different mobile emission sources in Tehran, motorcycles are of high importance. In this project, for the first time in Iran, exhaust gas pollutants emission of 66 motorcycles with different ages and engine displacement volumes were measured according to Euro-3 standard certification procedure. Measurement results showed that none of the tested motorcycles passed the Euro-3 certification limit on CO emission. Calculations... 

Homogeneous Charge Compression Ignition (HCCI) engines are the new generation of internal combustion engines which have diesel-like thermal efficiency, while they have low nitrogen oxide and soot emission. One of the major problems of HCCI engines is combustion timing control due to its dependence on reactivity of the charge and chemical kinetics. Moreover, limiting operation range between knock and misfiring is the other drawback of using these kind of engines. In the present work, partially homogeneous HCCI combustion in a single cylinder diesel engine is modeled using Computational Fluid Dynamics (CFD) coupled with chemical kinetics. Simulation includes the engine cycle from exhaust valve... 

Today air pollution has become one of the major urban management concerns. Air pollution could have dangerous effects on human health in short time or long time in many situations. Numerical modelling is one of the most applicable among different ways for predicting pollutant concentration like field measurement, physical model in wind tunnel, and simulation. Modelling of pollutant dispersion is a way for estimating concentration and aggregation of the pollutant near the surface and with different distances from pollution source. In different dispersion models, one of simplest is the Gaussian model. In this approach it is supposed that emission plume obeys from Gaussian distribution and is... 

Due to the important role of base plates, both in transferring forces from the structure to the foundation, and also in transferring vibrations from soil to the structure, it is necessary to have sufficient knowledge on their behaviour and performance under monotonic and cyclic loading regimes. In fact, the behaviour of the supports of any structure cannot be identified without identifying the behaviour of its base plates. Numerous configurations with/without various types of attachments have been proposed and used for base plates. In this work, altogether six commonly-used types of base plates were studied and their behaviour under monotonic and cyclic loadings was obtained using commercial... 

Past earthquakes show that irregular structures have been more vulnerable than their regular counterparts. One of the common types of irregular buildings in urban areas is split-level ones. On the other hand, beam-to-column connections in steel structures are mostly of "semi-rigid" type. In this work, effects of different levels of rigidity of connections, 100%, 75%, 50% and 25%, on the behavior of a number of two-dimensional split-level steel structures, with 1.5 m difference in levels of floors, 2, 3, 5 and 6 bays, and 5, 10 and 15 stories, were studied. All frames were analyzed “with” and “without” considering the P-∆ phenomenon, with two methods, ‘equivalent static load,’ and ‘modal...