Sharif Digital Repository

Earthquakes are of self-organized critical phenomena, the phenomena that exhibit scaling behavior without tuning external parameters. A number of models have been proposed to describe such features of earthquakes, the most known one is the Burridge-Knopoff model. The Burridge-Knopof model is a spring-block system where the blocks slip on a plate which has friction and are attached through some other springs to a moving plate. Because of the moving plate the blocks become under increasing tension but do not move until the tension overcomes the static friction. As a result, the blocks will move and then stop repeatedly. These movements are avalanche-like and their intensity obeys a power-law... 

Oil pollutants are among the important sources of pollution in countries with oil and gas industries such as Iran. Soil pollution by these pollutants is one of the environmental issues and hazards. All kinds of organic and inorganic pollutants in the soil may penetrate the underground water or enter the atmosphere in the form of gas, which respectively enters the food chain through absorption by plants or enter the body of living organisms through breathing. They include humans, which are a threat to the health and life of living beings. For example, many of these pollutions cause diseases such as cancer, suppression of the body's immune system, or even obesity; therefore, it is necessary to... 

The design, optimization, and construction of systems that are capable of swimming in the low Reynolds conditions are urgently needed in the light of the recent human, medical, industrial and scientific needs. Therefore, it is necessary to inspire the movement of microorganisms to design and construct artificial micro swimmer as well as to achieve the best geometry for swimming. In order to achieve the optimum microstructure geometry, it is necessary to know the behavior and hydrodynamic interactions of the micro swimmer with the fluid and its outer channel. In this study, we numerically simulate the movement of a microorganism with a spiral tail in a circular channel using Computational... 

With the growing use of computer systems, especially in form of web applications, software development and customer requirements has become more complex. Modern computer systems are supposed to achieve non-functional requirements in addition to functional requirements. Non-functional requirements are also called software quality attributes. One of the most important quality attributes for today's software systems is performance. Today’s softwares due to the growing number of their users, need more efficiency than ever to properly meet the performance request.Storage components and databases are the most important factors in causing performance bottlenecks in web applications. Most of... 

Experiments conducted in recent two decades indicated critical behavior in neural activity at different scales. Theoretically occurrences of these critical and power-law behavior can significantly facilitate brain activities correspondent to computation and memory tasks, but attaining the critical point essentially demands externally fine-tuning which has not been established yet. This fine-tuning often lies with placing system at transition point. Recent studies of group showed that a transition from synchronous to asynchronous phase could be achievable by a change in external parameters. At the very transition point, neuronal avalanches statistically demonstrate a power-law behavior which... 

A complte set of characteristic parameters of the sandpile models is still unknown. We have studied the existence of ”weak chaos” critical exponent in different sandpile models and we have shown that it is a characteristic exponent of deterministic models. We have shown that BTW and Zhang models do not belong to the same universality class (contrary to Zhang’s previous conjecture and contrary to Ben-Hur & Biham’s results.) Also we have shown that directed models, specificly Ramaswamy-Dhar’s directed model form a different universality class. ”Weak chaos” exponent in also studied in massive models and we have shown that by increase of dissipation, the exponent decreases rapidly to an... 

We study the Abelian Sandpile Model and its relation with surface growth. ese two models are related through their field theories and equations of motion. It has been shown that the different features of different sandpile models can be expressed in terms of the noise term in the surface growth equation. A mapping between the simplest sandpile model, the BTW model, and a surface growth has already been introduced. is surface growth has not been studied in details so far. In this thesis we study different features of this surface growth corresponding to the BTW model, continuous sandpile model and also massive abelian sandpile model. We also consider different boundary conditions  

The four-page article by Bak, Tang and Wiesenfeld in 1987 was a beginning to a new wave of physicists’ efforts to explain and describe the concept of complexity; a not-so-well-defined concept that resists against the reductionist tools and methods of physics. The Self-organized Criticality theory presented in that article via a simple model, known as sandpile model, was first of all an effort to explain the numerous occurrence of power law distribution in nature. SOC was introduced to tell us why so many natural phenomena like Earthquakes, landslides, forest fires, extinction and other seemingly non-related catastrophic events, more or less obey the scale-less power law distribution; A... 

In this research, we want to address the question of universality classes in BTW and Manna sandpile models. So far, number of works has been devoted to this issue but the the answer remained unsolved. We will try another approach to study this question by perturbing the original models. To this end, we introduce three models that have evolution rules between BTW model and Manna model. By simulating this models, we observe that in the presence of perturbation, the probability dis- tribution has two regimes of behaviour which are separated by a new characteristic scale. The regime of small avalanches is described by the exponent of BTW model and the regime of large avalanches by the exponent... 

The transport behavior of water molecules inside a model carbon nanotube is investigated by using nonequilibrium molecular dynamics simulations. The shearing stress between the nanotube wall and the water molecules is identified as a key factor in determining the nanofluidic properties. Due to the effect of nanoscale confinement, the effective shearing stress is not only size sensitive but also strongly dependent on the fluid flow rate. Consequently, the nominal viscosity of the confined water decreases rapidly as the tube radius is reduced or when a faster flow rate is maintained. The effect of ion concentration and temperature rise on fluid flow and shearing stress is also investigated....