Sharif Digital Repository

The propagation of a Gaussian beam in underdense plasma with upward increasing density ramp is analyzed. In this work are shown that the spot size oscillations of laser beam increases and its amplitude shrinks with proper plasma density ramp. This causes the laser beam to become more focused and penetrations deep into the plasma by reduction of diffraction effect. The related focusing parameters are optimized to get the best possible focusing at the relativistic threshold intensity of Nd-glass laser and the effect of the laser intensity in the self-focusing parameters is also investigated. The analytical calculations are presented and showed more reliable results in comparison to the... 

Introduction: One of the main objectives of reservoir engineering studies is to increase the production of hydrocarbon reservoirs with an optimal method. One of the artificial lift methods in wells is the gas lift. This system increases the oil production flow rate by reducing the pressure at the bottom of the well and increasing the pressure at the wellhead. In this method, by injection of high-pressure gas into the well's column, the average density of the well fluid is reduced, and through this, the well is reactivated. Method: In the current study, the simulation of a gas lift system in the horizontal and inclined wells was investigated. The pressure changes at the end of a simulated... 

Propagation of Gaussian X-ray laser beam is presented in collisional quantum plasma and the beam width oscillation is studied along the propagation direction. It is noticed that due to energy absorption in collisional plasma, the laser energy drops to an amount less than the critical value of the self-focusing effect and consequently, the laser beam defocuses. It is found that the oscillation amplitude of the laser spot size enhances while passing through collisional plasma. For the greater values of collision frequency, the beam width oscillates with higher amplitude and defocuses in a shallower plasma depth. Also, it is realized that in a dense plasma environment, the laser self-focusing... 

Nanostructured materials have attracted much attention in recent decades. Nowadays, there are numerous nanomaterials with several applications. The ultrasonic spray pyrolysis method is a cost-effective and adaptable technique based on an aerosol process for synthesizing nanoparticles and depositing thin films. The technique is capable of synthesizing metal, oxide, and composite nanomaterials with precisely controllable morphologies and chemical compositions using metal salts in aqueous solvents. More importantly, it is popular, as evident from the growing number of studies being conducted on the technique. Here, we review studies conducted on basic principles and applications of the... 

Here we report on the production of highly directed ion blocks by plasma interaction of ultraviolet wavelength light produced from a KrF laser. This may support the requirement to produce a fast ignition deuterium-tritium fusion at densities not much higher than the solid state by a single shot petawatt-picoseconds ultraviolet laser pulse. Using double Rayleigh initial density profiles, we are studying numerically how the nonlinear force necessary to accelerate plasma blocks may reach the highest possible thickness. Propagation of plasma blocks and the volumetric hot electrons can be shown in detail. Results of computations for wavelengths of two lasers are compared, which show that the... 

In this work we evaluate the interaction of high intense laser beam with a steepened density profile. During laser interaction with underdense plasma by freely expanding plasma regime, modification of density profile is possible. In this paper we have investigated the ultra short laser pulse interaction with nonisothermal and collisionless plasma. We consider self-focusing as an effective nonlinear phenomenon that tends to increase when the laser power is more than critical rate. By leading the expanded plasma to a preferred location near to critical density, laser reflection is obtained, so the density profile will be locally steepened. The electromagnetic fields are evaluated in this new... 

Within elasticity theory, the reduced form of a displacement field is obtained for general cross-ply composite laminates subjected to a bending moment. The firstorder shear deformation theory of plates and Reddy's layerwise theory are then utilized to determine the global deformation parameters and the local deformation parameters appearing in the displacement fields, respectively. For a special set of boundary conditions an elasticity solution is developed to verify the validity and accuracy of the layerwise theory. Finally, various numerical results are presented within the layerwise theory for edge-effect problems of several cross-ply laminates under the bending moment. The results... 

The interaction of laser pulses of picosecond duration and terawatt to petawatt power accelerated for the very fast undistorted plasma blocks for deuterium DD or deuterium tritium fast ignition is investigated. Based on the direct and instant conversion of laser energy into mechanical motion by nonlinear (ponderomotive) forces, any thermal pressure generation is delayed by the collision process. Following the studying of the classical collision frequency, it is found that the quantum modified collision at higher energies results in a correction by about 15% reduction of the delay  

In this paper, we derive a theoretical model for the higher-order four-wave mixing (FWM) power in wavelength division multiplexing networks with non-zero dispersion shifted fibers for the first time. We have investigated the higher-order FWM power theoretically and by numerical simulations. Dividing the fiber into a finite number of elements and applying the boundary conditions allow us to derive an expression for the second-order power penalty. At the end of each element, we derive the first-order FWM power for all wavelengths and use these values to calculate the second-order FWM power in the next element. Consequently, for each channel we can compute the total second-order FWM power... 

Periodic solutions and their existence are one of the most important subjects in dynamical systems. Fractional order systems like integer ones are no exception to this rule. Tavazoei and Haeri (2009) have shown that a time-invariant fractional order system does not have any periodic solution. In this article, this claim has been investigated and it is shown that although in any finite interval of time the solutions do not show any periodic behavior, when the steady state responses of fractional order systems are considered, periodic orbits can be detected  

In most existing works on the flocking problem, results were obtained for the case where a double integrator dynamic is used to describe motion of agents and virtual leader. These results, however, may not be generalised to agents with general linear dynamic. In this study, it is shown that in the flocking of agents with a linear dynamic, the velocity convergence of agents does not generally lead to the position convergence of informed agents and the position convergence is possible only for a specified group of linear dynamics. In this study, a control protocol is proposed to flock agents with specified linear dynamics and guarantee the velocity convergence of all agents as well as average... 

In this study, flocking of multi-agent systems with virtual leader and linear dynamics is investigated. It is assumed that the dynamics of agents in each dimension is represented by a general second-order linear model. The proposed control protocol is composed of two terms, one for feed backing each agent's states and one for cooperative control. At first, the flocking problem of multi-agent systems with identical agents is discussed and then the proposed protocol is generalised to flock heterogeneous multi-agent systems. Under the proposed algorithm, the velocity convergence of whole group to a velocity of virtual leader is guaranteed while the connectivity of network is preserved and... 

In this work, we study the flocking problem of multi-agent systems with uncertain dynamics subject to actuator failure and external disturbances. By considering some standard assumptions, we propose a robust adaptive fault tolerant protocol for compensating of the actuator bias fault, the partial loss of actuator effectiveness fault, the model uncertainties, and external disturbances. Under the designed protocol, velocity convergence of agents to that of virtual leader is guaranteed while the connectivity preservation of network and collision avoidance among agents are ensured as well. © 2017 ISA  

This paper investigates a new algorithm for flocking of multi-Agent systems with virtual leader. In this algorithm, unlike the existing algorithms, receiving of information by agents are not continuous and it is assumed that each agents receives information of neighbors and virtual leader as sampled data in fixed time intervals. A sufficient condition on sampling period is derived to guarantee the velocity convergence of agents to that of virtual leader, preserve network connectivity, and ensure collision avoidance. Finally, a numerical simulation is provided to illustrate the theoretical results. © 2017 IEEE  

Dual active bridge isolated bidirectional DC converter (DAB-IBDC) has a poor efficiency at light loads and non-unity voltage conversion ratio. In this paper the current stress of the converter is analyzed and the effect of maximum phase shift ratio and leakage inductance on the current stress of the converter is investigated. In order to increase the efficiency of the converter and reduce the current stress, the current stress optimization has been done and the design procedure is presented. Simulation results are used to validate the results of the theoretical analysis. © 2017 IEEE  

This paper studies the leader-follower flocking of multi-agent systems for the linear second-order dynamics, subject to the external disturbance problem. It is assumed that the dynamic of the leader is Lipschitz-type. Also, the velocity is the output of the system, and full-state information is not available for feedback. A distributed full-order observer is employed to estimate every agent's states and external disturbance. A control protocol for each agent is designed based on the measurement of its output/velocity and relative velocity of its neighbors. Under the proposed protocol, the velocity convergence of whole agents to the velocity of the virtual leader is guaranteed as well as the... 

In this paper we will present some approaches on Reynolds stress production by vortex transport phenomena and nonlinear voracity generation in momentum equation. First of all we represent a history of recent works to describe how fluid particle motions can be associated with Reynolds stress through either displacement or acceleration terms. In the next section we will describe how vortex stretching causes the Reynolds stress production and what is the dominant effect near and far from the boundary where viscous effects have to be considered. On the other hand, some vortex considered methodologies such as those synthesize boundary layer, as a collection of vortical objects seem to be... 

In this paper we will present some approaches on Reynolds stress production by vortex transport phenomena and nonlinear vorticity generation in momentum equation. First of all we represent a history of recent works to describe how fluid particle motions can be associated with Reynolds stress through either displacement or acceleration terms. In the next section we will describe how vortex stretching causes the Reynolds stress production and what is the dominant effect near and far from the boundary where viscous effects have to be considered. On the other hand, some vortex considered methodologies such as those synthesize boundary layer, as a collection of vortical objects seem to be... 

This paper provides an approach based on the genetic algorithm for the dimensional synthesis of a six-bar mechanism for a shaper machine. The main purpose of the optimization algorithm is to maintain the velocity of the mechanism’s slider constant within a specified range of the rotational motion of the input link. Therefore, first, an objective function is defined for the slider. Then, the velocity function of the slider is calculated using a set of mathematical relationships and the mechanism’s kinematic constraints. In order for this function to reach the objective function, a cost function is defined. This cost function is minimized, and the output approaches the objective function by...