Sharif Digital Repository

In this study, dissimilar friction stir welding was performed between AA5052 aluminium and AZ31 magnesium alloys using a variety of parameters. The results revealed that the optimal range of welding parameter is between 20 and 38 rpm/mm.min−1 (the ratio of rotation speed to travel speed). The findings of the X-ray diffraction and Micro-hardness tests on the stir zone indicated that when the rotation speed is increased, it leads to the formation of intermetallic compounds such as Al12Mg17 and Al3Mg2. The polarization test was used to assess the corrosion rate in various zones. The corrosion rate in the stir zone is higher than in other zones due to macro-galvanic couple between Al and Mg.... 

Discrete ordinates method relies on approximating the integral term of the transport equation with the aid of quadrature summation rules. These quadratures are usually based on certain assumptions which assure specific symmetry rules and transport/diffusion limits. Generally, these assumptions are not problem-dependent which results in inaccuracies in some instances. Here, various methods have been developed for more accurate estimation of the independent angle in S2 approximation, as it is tightly related to valid estimation of the diffusion coefficient/length. We proposed and examined a method to reduce a complicated problem that usually is consisting many energy groups and discrete... 

Discrete ordinates method relies on approximating the integral term of the transport equation with the aid of quadrature summation rules. These quadratures are usually based on certain assumptions which assure specific symmetry rules and transport/diffusion limits. Generally, these assumptions are not problem-dependent which results in inaccuracies in some instances. Here, various methods have been developed for more accurate estimation of the independent angle in S2 approximation, as it is tightly related to valid estimation of the diffusion coefficient/length. We proposed and examined a method to reduce a complicated problem that usually is consisting many energy groups and discrete... 

We study the problem of maximizing constrained non-monotone submodular functions and provide approximation algorithms that improve existing algorithms in terms of either the approximation factor or simplicity. Different constraints that we study are exact cardinality and multiple knapsack constraints for which we achieve (0.25-)-factor algorithms. We also show, as our main contribution, how to use the continuous greedy process for non-monotone functions and, as a result, obtain a 0.13-factor approximation algorithm for maximization over any solvable down-monotone polytope  

Manta ray swimming or bio-inspiration propulsion system, as a special type of marine propulsion system, is used for submersible vehicles that require high-speed maneuverability and stability, such as glider and AUV. In a manta ray swimming, the thrust force is generated by a couple of undulation and oscillation of wing, so that the direction of undulation wave and oscillation is upright and perpendicular to the direction of thrust force, respectively. It is possible to combine these two movement modes (flapping motion) on the three-dimensional model without considering the effects of wing twisting and flexibility to simplify and better understand the physical behaviors or special study of... 

In the (k, λ)-subgraph problem, we are given an undirected graph G = (V,E) with edge costs and two positive integers k and λ, and the goal is to find a minimum cost simple λ-edge-connected subgraph of G with at least k nodes. This generalizes several classical problems, such as the minimum cost k-spanning tree problem, or k-MST (which is a (k, 1)-subgraph), and the minimum cost λ-edge-connected spanning subgraph (which is a (|V(G)|, λ)-subgraph). The only previously known results on this problem [L. C. Lau, J. S. Naor, M. R. Salavatipour, and M. Singh, SIAM J. Comput., 39 (2009), pp. 1062-1087], [C. Chekuri and N. Korula, in Proceedings of the IARCS Annual Conference on Foundations of... 

Assume that we are given a set of points some of which are black and the rest are white. The goal is to find a set of convex polygons with maximum total area that cover all white points and exclude all black points. We study the problem on three different settings (based on overlapping between different convex polygons): (1) In case convex polygons are permitted to have common area, we present a polynomial algorithm. (2) In case convex polygons are not allowed to have common area but are allowed to have common vertices, we prove the NP-hardness of the problem and propose an algorithm whose output is at least (Formula presented.). (3) Finally, in case convex polygons are not allowed to have... 

Many technological networks can experience random and/or systematic failures in their components. More destructive situations can happen if the components have limited capacity, where the failure in one of them might lead to a cascade of failures in other components, and consequently break down the structure of the network. In this paper, the tolerance of cascaded failures was investigated in weighted networks. Three weighting strategies were considered including the betweenness centrality of the edges, the product of the degrees of the end nodes, and the product of their betweenness centralities. Then, the effect of the cascaded attack was investigated by considering the local weighted flow... 

DFT method (B3LYP) with 6-31G* basis set was utilized in the computation of a fully optimized structure, net atomic charges and spin densities of the intermediate of cytochrome P-450-oxoiron(IV) porphyrin cation radical, compound I - in the presence of axial ligand such as thiolate (SMe-) imidazole (IM), phenoxide (OPh-), methoxide (OMe-) and chloride (Cl-). The results show doublet states in compound I are about 2-4 kcal/mol more stable than quartet states for all aforementioned ligands, and the doublet state is the ground state in all cases. However, electron donor ability of the ligands are in the order of SMe- > IM > OMe- > OPh- > Cl-. Also the active oxidant intermediate of cytochrome... 

The evolution of primary and secondary γ′ precipitates in the high γ′ volume fraction Rene 80 Ni based superalloy has been examined during aging at elevated temperatures for periods up to 1750 h. While the increase in average dimension of particles followed the cube rate Lifshitz, Slypzof and Wagner (LSW) law, rt-3-r0 -3=kt, there were significant discrepancies between the experimental and theoretical particle size distributions (PSDs) and inconsistency with the kinetic constants associated with the two populations of particles. These differences are attributed to the influence of elastic coherency strains which have not been considered in conventional capillarity driven coarsening models.... 

We consider a class of optimization problems called movement minimization on euclidean plane. Given a set of m nodes on the plane, the aim is to achieve some specific property by minimum movement of the nodes. We consider two specific properties, namely the connectivity (Con) and realization of a given topology (Topol). By minimum movement, we mean either the sum of all movements (Sum) or the maximum movement (Max). We obtain several approximation algorithms and some hardness results for these four problems. We obtain an O(m) -factor approximation for ConMax and ConSum and extend some known result on graphical grounds and obtain inapproximability results on the geometrical grounds. For the... 

Assume that we are given a set of points some of which are black and the rest are white. The goal is to find a set of convex polygons with maximum total area that cover all white points and exclude all black points. We study the problem on three different settings (based on overlapping between different convex polygons): (1) In case convex polygons are permitted to have common area, we present a polynomial algorithm. (2) In case convex polygons are not allowed to have common area but are allowed to have common vertices, we prove the NP-hardness of the problem and propose an algorithm whose output is at least (OPTlog(2n/OPT)+2log(n))1/4. (3) Finally, in case convex polygons are not allowed to... 

We study a classical problem in communication and wireless networks called Finding White Space Regions. In this problem, we are given a set of antennas (points) some of which are noisy (black) and the rest are working fine (white). The goal is to find a set of convex hulls with maximum total area that cover all white points and exclude all black points. In other words, these convex hulls make it safe for white antennas to communicate with each other without any interference with black antennas. We study the problem on three different settings (based on overlapping between different convex hulls) and find hardness results and good approximation algorithms  

Distribution companies try to meet different goals such as increasing profitability, reducing investment and improving reliability indices. Therefore, distribution network planning has converted into complex multi-objective optimization problems. Mathematicians and operation research experts have developed many methods which can be used for solving these problems. Despite all efforts to develop applicable approaches for these problems, there is still no general consensus on project selection policy. Evaluating the strengths and weaknesses of each network optimization strategy selection in pragmatic manner is the objective of current research. In this paper, many methods used in network... 

We consider a market in which two competing sellers offer two similar products on a social network. In this market, each agent chooses iteratively between the products based on her neighbors reactions and prices. This introduces two games: one between the agents and one between the sellers. We show that the first game is a full potential game and provide an algorithm to compute its convergence point. We also study various properties of the second game such as its equilibrium points and convergence  

One of the underlying factors in the conservation and development of urban green spaces is the available water resources use for green areas in each region. The primary purpose of this study is to introduce the integration of a proper simulation method for runoff along with AHP-PROMETHEE decision-making method for water resources management in urban basins. The application of this approach was investigated in the city Buin Zahra of Qazvin Province of Iran. In the first step by using the analytic hierarchy process (AHP), the available water resources in the case study were weighted. In the second step, the water resources of the municipal and residential wells and water collected by... 

One of the underlying factors in the conservation and development of urban green spaces is the available water resources use for green areas in each region. The primary purpose of this study is to introduce the integration of a proper simulation method for runoff along with AHP-PROMETHEE decision-making method for water resources management in urban basins. The application of this approach was investigated in the city Buin Zahra of Qazvin Province of Iran. In the first step by using the analytic hierarchy process (AHP), the available water resources in the case study were weighted. In the second step, the water resources of the municipal and residential wells and water collected by... 

We consider a class of optimization problems called movement minimization on euclidean plane. Given a set of nodes on the plane, the aim is to achieve some spe- cific property by minimum movement of the nodes. We consider two specific properties, namely the connectiv- ity (Con) and realization of a given topology (Topol). By minimum movement, we mean either the sum of all movements (Sum) or the maximum movement (Max). We obtain several approximation algorithms and some hardness results for these four problems. We obtain an O(m)-factor approximation for ConMax and ConSum and an O( p m=OPT)-factor approximation for Con- Max. We also extend some known result on graphical grounds in [1, 2] and... 

Hardware/Software partitioning is one of the most important issues of codesign of embedded systems, since the costs and delays of the final results of a design will strongly depend on partitioning. We present an algorithm based on Particle Swarm Optimization to perform the hardware/software partitioning of a given task graph for minimum cost subject to timing constraint. By novel evolving strategy, we enhance the efficiency and result's quality of our partitioning algorithm in an acceptable run-time. Also, we compare our results with those of Genetic Algorithm on different task graphs. Experimental results show the algorithm's effectiveness in achieving the optimal solution of the HW/SW... 

We introduce hyper-D-width and hyper-T-width as the first stable (see Definition 3) measures of connectivity for hypergraphs. After studying some of their properties and, in particular, proposing an algorithm for computing nearly optimal hyper-T-decomposition when hyper-T-width is constant, we introduce some applications of hyper-D-width and hyper-T-width in solving hard problems such as minimum vertex cover, minimum dominating set, and multicut