Sharif Digital Repository

In the 21st century, the world population is growing at a spiraling pace. Much of this growth is occurring in developing countries, where access to food, sanitary water, education, and health is severely limited. The ever-increasing urbanization highlighted the need for sustainable development based on human-environment interaction. The excessive and unplanned expansion of cities has resulted in numerous environmental predicaments due to clearly emphasized economic issues at the expense of social and environmental concerns. The most significant features of megacities are indicated in this paper and the environmental, social, physical, and economic criteria are addressed in order to attain... 

We consider the nonlinear Euler differential equation t2 x″ + g (x) = 0. Here g (x) satisfies x g (x) > 0 for x ≠ 0, but is not assumed to be sublinear or superlinear. We present implicit necessary and sufficient condition for all nontrivial solutions of this system to be oscillatory or nonoscillatory. Also we prove that solutions of this system are all oscillatory or all nonoscillatory and cannot be both. We derive explicit conditions and improve the results presented in the previous literature. We extend our results to the extended equation t2 x″ + a (t) g (x) = 0. © 2006 Elsevier Inc. All rights reserved  

In this work we study the problem of whether all trajectories of the system ẋ=y-F(x) and ẏ=-g(x) cross the vertical isocline, which is very important for the existence of periodic solutions and oscillation theory. Sufficient conditions are given for all trajectories to cross the vertical isocline. © 2005 Elsevier Ltd. All rights reserved  

This paper deals with the question of nonexistence of limit cycles in two famous classes of predator-prey systems. We present some sufficient conditions for the nonexistence of limit cycles in these systems. Our results extend and improve the results presented by Moghadas. © 2005 Elsevier Inc. All rights reserved  

In the present work, the feasibility of friction stir welding (FSW) of poly(methyl methacrylate) sheets was studied experimentally and theoretically by employing thermomechanical simulations. The effect of processing parameters including tool plunge depth, tilt angle, tool rotational speed (w), and transverse velocity (v) was investigated to determine suitable conditions to attain sound and defect-free joints. It is shown that a low tool plunge depth of 0.2 mm and a tilt angle of 2° provide suitable material flow to gain sound joints. By controlling the heat input into stir zone by increasing the tool rotational speed and decreasing linear velocity, the formation of defects can be minimised.... 

Process optimization to prepare T-joints of poly methyl methacrylate (PMMA) and AA5754 alloy by friction stir welding (FSW) was performed. Effects of processing parameters including tool rotational velocity, traverse speed, tilt angle and plunge depth (TP) were studied. The material flow and formation of interaction layer upon FSW were evaluated and the tensile strength and hardness of the joints were examined. It is shown that by controlling the heat input into stir zone through increasing the rotational velocity or decreasing travelling speed, less defects are formed while the thickness of the interaction layer increases. The optimum condition is attained at 1600 rpm/30 mm min−1 with a... 

We present a new semi-solid state procedure for efficient joining of dissimilar materials. The process called fed friction stir processing (FFSP) and works based on in-situ injection of colloidal nanoparticles in the welding line during processing. To present the efficiency of the process, friction stir welding of AA6062 aluminum alloy and poly(methyl methacrylate) (PMMA) through injection of alumina nanoparticles is presented. Microstructural features and mechanical characteristics of the weldments are elaborated. It is shown that in-situ feeding of the alumina nanoparticles during FFSP changes the thermo-mechanical regimes of the bonding zone and decreases the thickness of interaction... 

Effects of frustum, cubic and triangle tool pin profiles on the friction stir welding (FSW) of poly(methyl methacrylate) (PMMA) were studied by experimental analysis and thermomechanical simulations. It is shown that employing pins with a low contact surface area form macrocracks and voids along the joint line. Edged-type pins locally stir the polymer and cause tunnel defects which are formed at the root of the joint. In contrast, pins with relatively smooth and large surface area would yield a defect-free weldment if an appropriate processing condition is employed. Mechanical examinations in longitudinal (LS) and transverse (TS) directions indicate that the highest tensile strength (LS = 59... 

In this study, alumina-reinforced poly(methyl methacrylate) nanocomposites (PMMA/Al2O3) containing up to 20 vol% nanoparticles with an average diameter of 50 nm were prepared by friction stir processing. The effects of nanoparticle volume fraction on the microstructural features and mechanical properties of PMMA were studied. It is shown that by using a frustum pin tool and employing an appropriate processing condition, i.e. a rotational speed of 1600 rpm/min and transverse velocity of 120 mm/min, defect free nanocomposites at microscale with fine distribution of the nanoparticles can successfully been prepared. Mechanical evaluations including tensile, flexural, hardness and impact tests... 

The relation between process parameters and properties of polycarbonate (PC) sheet after friction stir welding (FSW) was investigated. Effects of FSW tool rotation speed (ω), travelling speed (V), tilt angle (α) and plunge depth on material flow, joint tensile strength, flexural strength, impact energy and hardness were investigated. It is shown that at low and high levels of heat input sound joints cannot be attained. At low heat inputs, the local temperature is not high enough to decrease the viscosity of the polymer to fill the interface, and thus voids and large planar cracks are formed in the joint root and stir zone. At high heat inputs, expelling of materials due to high peak... 

In this research, the feasibility of friction-stir welding (FSW) for dissimilar lap-joining of an aluminium-magnesium alloy (AA5058) and poly-methyl-methacrylate sheets to attain sound and defect-free joints was examined. The inter-mixing flow patterns between the metal and polymer counterparts during FSW were predicted by employing three-dimensional finite element models. It is shown that the bonding mechanism between the dissimilar materials is mechanical interlocking at the interface which controls the joint strength depending on the processing parameters. The most suitable dissimilar lap-joining regarding microstructural soundness is attained at w= 1600 rev min−1 and v = 25 mm min−1.... 

Considering the wide spread locations of wind farms in Iran, it is important to develop a suitable decision support system (DSS) to fulfill proper management of wind farms. Extensive literature survey indicates that there are no integrated forms of DSS to manage a set of wind farms. The existing wind farms are performing independently, and there is no practical method for exchanging the online data. DSS can contribute to optimal operation of wind farms, operation and maintenance scheduling, pricing policy, etc. In this study, a geographic information system and RETSCREEN software were linked to the designed DSS to achieve a more suitable result. Also, a huge number of data are constantly... 

Considering the wide spread locations of wind farms in Iran, it is important to develop a suitable decision support system (DSS) to fulfill proper management of wind farms. Extensive literature survey indicates that there are no integrated forms of DSS to manage a set of wind farms. The existing wind farms are performing independently, and there is no practical method for exchanging the online data. DSS can contribute to optimal operation of wind farms, operation and maintenance scheduling, pricing policy, etc. In this study, a geographic information system and RETSCREEN software were linked to the designed DSS to achieve a more suitable result. Also, a huge number of data are constantly... 

A typical scientist has no responsibility other than to explain how a natural event occurred. However, when a philosopher asks about the conditions under which a scientist’s explanation is true, he is, in fact, raising an ultimate question, the concept which Karl Popper used for the first time. Answering this question requires that no elements are neglected in the explanation, and no significant factors in the explanation are overlooked. In other words, in explaining a phenomenon, at any level of its explanation, there should be no remainder. These requirements can be achieved through the full explanation. In the present article, by drawing on concepts such as theory-ladenness of... 

To differentiate between the laws of nature and accidental generalizations, we must adopt a view of necessity that is capable of being realized in relationships existing among natural objects. In neo-Sadraian Islamic philosophy, metaphysical necessity is accepted as part of the cause-effect relationship. This paper compares the neo-Sadraian interpretation of necessity and necessitarian theories about the laws of nature, particularly essentialism and universal theory. By resorting to specific forms al-shûrat al-naw’iyyah, the origin of the essential properties of natural objects, I argue that neo-Sadraians have proposed a perspective on metaphysical necessity that is epistemologically a... 

A typical scientist has no responsibility other than to explain how a natural event occurred. However, when a philosopher asks about the conditions under which a scientist’s explanation is true, he is, in fact, raising an ultimate question, the concept which Karl Popper used for the first time. Answering this question requires that no elements are neglected in the explanation, and no significant factors in the explanation are overlooked. In other words, in explaining a phenomenon, at any level of its explanation, there should be no remainder. These requirements can be achieved through the full explanation. In the present article, by drawing on concepts such as theory-ladenness of... 

To differentiate between the laws of nature and accidental generalizations, we must adopt a view of necessity that is capable of being realized in relationships existing among natural objects. In neo-Sadraian Islamic philosophy, metaphysical necessity is accepted as part of the cause-effect relationship. This paper compares the neo-Sadraian interpretation of necessity and necessitarian theories about the laws of nature, particularly essentialism and universal theory. By resorting to specific forms al-shûrat al-naw’iyyah, the origin of the essential properties of natural objects, I argue that neo-Sadraians have proposed a perspective on metaphysical necessity that is epistemologically a... 

Reformed epistemology, based on the reliability of the functions of cognitive faculties, holds that one can believe in God directly without argument or evidence. Alvin Plantinga, one of the proponents of this view, replaces justification with warrant in the classical definition of knowledge and considers true belief, which has a warrant as knowledge. On this basis, he considers it reasonable to believe in the existence of God because it has a warrant and arises from our healthy cognitive faculties without the need for an independent argument. Some defenders of the cognitive science of religion also seek to provide evidence from cognitive science to explain religious beliefs in such a way as... 

Some proponents of new atheism, including Dawkins, claim that religion, especially Islam, is the cause of the war because it encourages divisiveness and labeling. In this article, in the first step, I will show that Islam is not the cause of war, but rather it is a misconception of Islam that is the cause of divisiveness and eventually war. Then, I will demonstrate that the COVID-19 pandemic and its impact on societies and religious behaviors somehow could, and still can, modify this misconception and blur divisions. This effect occurs in two ways: firstly, a pandemic creates common human problems for all people worldwide, making people more aware of their common points that Islam refers to... 

The need for supporting the ever-growing number of multi-computers in the contemporary Internet has persuaded researches worldwide to search for suitable underlying network topologies with desirable properties. Even in the presence of speedy optical technologies, the arrangement of nodes in a network can be highly influential. Among all the topologies investigated so far, k -ary n-cubes have been reported to be widely adopted in the literature. But in hybrid networks, where the network is composed of a mixture of different types of topologies, the type and size of k-ary n-cubes can greatly affect the performance factor of the network. In this paper, we study and compare the performance...