Sharif Digital Repository

Variable speed wind turbines (VSWTs) provide the capability of wide speed operation and independent control of active and reactive power. In contrast to the fixed speed WTs, they have higher efficiency, power quality and controllability. VSWTs are mainly divided in two categories: WTs based on doubly fed induction generator (DFIG) and WTs based on full converter with permanent magnet synchronous generator.
At now, among the different alternatives to obtain VSWTs, DFIGs are the most commonly used. This is because the voltage source converter (VSC) in DFIG has to handle a fraction of the total power under steady state conditions, and thus the size and cost of the converter is reduced.... 

Throughout history, engineers have always been searching for ways to increase the strength-to-weight ratio in the design of engineering structures. Using lattice structures has been a method implemented in different applications to achieve this goal. Herein, two novel hybrid lattice structures are designed, and their compressive quasi-static mechanical behavior is evaluated experimentally and numerically. In one of the designed structures, using dual-phase strengthening, FCC unit cells are incorporated as a reinforcement phase into a BCC matrix. In the multifunctional lattice structure, FCC and BCC unit cells are combined with their central nodes connected. All of the designed structures are... 

Recently, the severe plastic deformation (SPD) is known as one of the most used methods in decreasing the grain size and increasing the strength of metals. In this study, aluminum 1100was subjected toconstrained groove pressing (CGP) at room temperature. The CGPed samples in nanoscale structure, with different CGPed passes were direct rolled and cross-rolled in different rolling strains. Finally, all of the samples were annealed at temperatures of 100, 200 and 300 ° C for 30 min. It was observed that with increasing number of CGP passes and rolling strain the structure became more laminate and finer. With increasing rolling strain, the cross-rolling had greater effect on the microstructure... 

Tipping points occur in diverse systems in various disciplines such as ecology, climate science, economy, sociology, and engineering. Critical thresholds in system parameters or state variables at which a tiny perturbation can lead to a qualitative change in the system exist in many subsystems in complex systems. These thresholds are called tipping points, and these subsystems are called tipping elements. Additionally, many systems with tipping points can be modeled as networks of coupled multistable subsystems. Domino-like tippings are called tipping cascades. Considering that these tipping cascades are primarily unprecedented, it is essential to study the dynamics and control of these... 

Recent studies in various fields such as the environment, climate, economics, and neuroscience have shown that many systems exhibit dynamic responses based on catastrophic shifts—sudden and unexpected changes in the system's state and behavior in response to environmental changes. One of the simplest cases of these shifts occurs when the dynamical bifurcation diagram of a system has a tipping point, which serves as a critical threshold for system parameters, allowing small disturbances to lead to significant changes in the system's state. Research on many natural systems has revealed that tipping elements as the subsystems of a complex system are not independent, and the tipping of one...