Sharif Digital Repository

In this paper, the problem of designing optimal conflict-free maneuvers for multiple aircraft encounters is studied. The proposed maneuvers are based on changes of heading, speed and maneuvering time. The optimality of maneuvers among the conflict-free constraint is based on minimization of a certain cost function based on kinetic energy of either aircraft involved. Some suitable priority weight factors are incorporated into the cost function so that optimal resolution maneuvers are such that aircraft with lower priorities assume more responsibility in resolving the conflicts. As for considering aircraft turning dynamics, the circular 3-arced path with constant speed are proposed for each... 

This paper deals with numerical modeling of water flow which is generated by the break of a dam. The problem is solved by applying a new Incompressible Smoothed Particle Hydrodynamics (ISPH) algorithm based on projection method. The proposed ISPH model has two steps. In the first step, the incompressibility of fluid is maintained regarding to the changes of intermediate and initial particles densities at the first half-time step (stability step). In the second step, by computing the divergence of the intermediate secondary velocity at the second half-time step (accuracy step), the incompressibility is satisfied completely. In fact, by using this method both stability and accuracy are... 

Toxic gases resulted from fire events in subways stations are more dangerous than high temperature radiation from it. So, a well designed smoke exhaust system must be installed in subway station to control the smoke's propagation and discharging. Smoke extraction in subway station depends on the duct laid above the ceiling, so vents are situated in the same level of platform layer's ceiling. If subway station catches fire, smoke will cumulate in smoke reservoirs at the beginning and mechanical fan cannot exhaust any smoke in this process. In This paper, FDS 5.0 is used to simulate smoke's movement in a side platform of an actual subway station in case of a fire. Simulations are carried out... 

Aximum coverage location problem is considered in this study. Extension of this problem is investigated for situations that coverage areas are elliptical; these ellipses can locate anywhere on the plane with any angle. Mixed integer nonlinear programming (MINLP) is applied for formulation. This problem can be used in many practical situations such as locating wireless transmitter towers. A heuristic algorithm named MCLPEA for solving this problem was designed. This algorithm can produce very good results in efficient running time. Finally, the comparison of the results for this study was carried done  

Any channelized receiver suffers from a phenomenon referred to as the rabbit ear effect. This phenomenon appears when an out-of-band pulse generates a distorted response at the output of the selected filter. In this paper, the rabbit ear effect is analyzed, and a triband filter structure is proposed to identify and suppress the rabbit ear effect. The average likelihood detection strategy is adopted to optimize the parameters of the triband filter, as well as the detector structure. It is shown that although competing methods reject all short-duration pulses, the proposed system preserves all in-band pulses, while eliminating most of the out-of-band pulses (i.e., rabbit ear pulses)  

A very simple and computationally low cost numerical algorithm is developed to generate a quasi-structured data structure for an unstructured grid. To achieve this purpose, the data structure in the matrices of an unstructured grid is classified to address the element layers and node lines in the computational domain. In this regard, elements and nodes of the unstructured grid are renumbered in a directional ordering-based strategy. The elements and nodes arrangement in each layer and line is accomplished in a unique direction around an interior object of the grid either clockwise or counterclockwise. Furthermore, a new searching scheme is introduced which guarantees a quick search inside... 

Reference trajectory management is a method to modify reference trajectories for the faulty system. The modified reference trajectories define new maneuvers for the system to retain its pre-fault dynamic performance. Controller reconfiguration is another method to handle faults in the system, for instance by adjusting the controller parameters (coefficients). Both of these two methods have been considered in the literature and are proven to be capable of handling various faults. However, the comparison of these two methods has not been considered sufficiently. In this paper, a controller reconfiguration mechanism and a reference trajectory management are proposed for the spacecraft attitude... 

The use of air-cooled condenser (ACC) has become very popular in erecting thermal powerplants around the world since two or three decades ago. The advantages of forced convection heat cooling system, instead of the classical natural draught convection heat transfer cooling systems, promote the thermal powerplant designers and users to benefit more from such systems in their thermodynamics cycles. However, such forced convection heat transfer mechanisms, can lose their cooling efficiency in off-design ambient conditions, i.e., in high wind velocity and high ambient temperature conditions. There have already been some efforts to analyze the reduction of ACC System performance in some critical... 

In this work, a hybrid finite volume element FVE method is extended to simulate the evolution of soot nanoparticles in a turbulent axisymmetric confined diffusion flame. The FVE method can handle irregular-shaped solution domains and maintain the underlying physical conservation principles. To consider the evolutionary process of soot nanoparticles including nucleation, coagulation, surface growth, and oxidation, a two-variable approach is employed. In this approach, the soot mass fraction and soot number density transport equations are solved using an extended detailed chemical kinetics. Considering the phenyl route to describe the nucleation process, soot inception is based on the... 

Increasing rate of demanding biodiesel as alternative energy resource, persuade researchers to investigate engine performance of biodiesel-fueled engines, which are highly influenced by ignition delay (ID) and combustion characteristics of such a fuel. This review article introduces a literature review on ignition delay (ID) and combustion characteristics of diesel engine fueled with biodiesel. Slightly difference between combustion characteristics of bio fueled engine and petroleum diesel one recognized as result of carried out investigations. Early start of combustion (SOC) and shorter ID of biodiesel comparing to diesel is reported by most of investigations. Lower compressibility, higher... 

In this work, a hybrid finite-volume-element method is used to solve turbulent reacting flow in a combustion chamber considering detailed chemical kinetics. The hybrid unification enables the solver to treat the reacting flow in very complex geometries and to respect the required physical considerations fully. We employ two-equations standard κ-ω turbulence model incorporated with suitable wall functions to model the turbulence behavior. Assuming optically-thin gases, radiation effects are taken into account in our simulations. A flamelet combustion model is applied to consider the large detailed chemical kinetics, which can normally occur within combustion processes. Turbulence-chemistry... 

Oil analysis technique is used as predictive and proactive tools to identify the wear modes of rubbing parts and to diagnose the faults in machinery. In this paper, the wear behavior of diesel engines, especially on oil analysis, is studied based on condition data. In terms of analyzing historical data, descriptive statistics is used as data mining tools to find the relationship between condition factors of the machine and its final status. The equipments have been monitored in two different environments which are: plantation-forestry, and general construction. Based on this relationship a specific baseline is achieved for selected sets of equipment in their specific conditions. A striking... 

Stress distribution in Carbon Nanotube (CNT) reinforced composites is studied using nonlocal theory of elasticity. Two nearby CNTs are modeled as two circular inclusions embedded in an infinite elastic medium, and classical stresses are obtained using the complex stress potential method. Nonlocal stresses are calculated using nonlocal integral elasticity equation. Effects of the distance between CNTs as well as effects of the nonlocal parameters on the stress distribution and stress concentration are studied. For unit normal stress at infinity, stress at the interface of the CNT and matrix increases from 0.1 for classical analysis to 0.85 for nonlocal analysis. Furthermore, when two CNTs... 

A Volume of Fluid (VOF) method is used to stdy the breakup of droplets in T-junction geometries. Symmetric T-junctions, which are used to produce equal size droplets and have many applications in pharmacy and chemical industries, are considered. Two important factors namely "breakup time" and "breakup length" that can improve the performance of these systems have been introduced. In addition a novel system which consists of an asymmetric T-junction is proposed to produce unequal size droplets. The effects of the channel width ratio and the capillary number on the size and length of the generated droplets and also the time of the generation have been studied and discussed. For simulation the... 

There are hundreds models of reticulated structures including the squared reticulated cylindrical shells. It is considered as comprising of a number of circumferential and longitudinal rods. Analytical governing equation for natural frequencies has been derived for this type of structures and to verify the validity of solutions, Finite Element Method (FEM) is used. The comparison of results demonstrate close agreement between analytical and FE solutions. Also a comparison is preformed between a reticulated and equivalent solid hollow cylinder shell. The equivalent solid hollow cylinder has equal weight, length and outer diameter with the squared reticulated cylindrical shell. This comparison... 

This Paper presents the simulation of electroosmotic flow in nanochannels using the dissipative particle dynamics (DPD) method. Most of the past electroosmotic phenomenon studies have been carried out using the continuum flow assumptions. However, there are many electroosmotic applications in nanoscales NEMS and microscales MEMS, which need to be treated using non-continuum flow assumptions. We simulate the electroosmotic flow within the mesoscopic scale using the DPD method. Contrary to the ordinary molecular dynamics method, the DPD method provides less computational costs. We will show that the current DPD results are in very good agreement with other available non-DPD results. To expand... 

The flight of unmanned aerial vehicles is often associated with model uncertainties, measurement noises, and environmental disturbances such as wind gust. To mitigate these challenges, the accurate estimation of states is vital. Moreover, the wind model and its parameters should also be estimated and compensated during the flight. In this paper, a multiple model filter is implemented for this purpose. To investigate the performance of the multiple model filter, three different models including constant wind, '1-cosine' model and wind shear model are considered. The multiple model filter utilizes three extended Kalman filter to simultaneously estimate the model of wind, the parameters of the... 

Cat free fall righting maneuverer has inspired many aerial, space and legged robotic research. Conservation of angular momentum principle is used to derive the inverse differential kinematic and TMT vector form dynamics of the motion for three robotic models: a two-link model, a three-link model with tail, and a comprehensive eight-link model with the addition of legs. The path planning problem in the presence of geometric and kinematic constraints is addressed using a novel singularity free single shooting optimization method. While a 2 DOF torso is sufficient to perform a full maneuver, the addition of the tail reduces the time and increases the maneuverability despite the leg motions that... 

The accuracy of Reynolds averaged Navier-Stokes (RANS) turbulence models to predict the behavior of two-dimensional (2-D) density current has been examined. In this work, a steady density current is simulated by the κ -ε , κ -ε RNG, two-layer κ -ε and modified v 2̄ - f models. All models are compared with available experimental data. Density current with uniform velocity and concentration enters a channel via a sluice gate into a lighter ambient fluid and moves forward down-slope. The eddy-viscosity concept cannot accurately simulate this flow because of two stress production structures in it. Results show that all isotropic models have a weak outcome for this current, but with improving the...