Sharif Digital Repository

Efficient location of medical services is an issue of paramount importance in healthcare strategic planning. In this research, a mathematical model is developed for the location of multi-service health centers, assuming probabilistic demand and service time. Since patients may be shifted to another service after receiving a service by doctors’ order, health system is considered a Jackson queueing network. We assume that patients have “probabilistic choice behavior” and the primary factors contributing to their choice of one center over another are their proximity to the center and the number of medical services offered by the center. The proposed mixed integer nonlinear programming model... 

Real-world, large-scale problems often have coefficient matrices with special structures that can be obtained by reordering their rows and columns. One fundamental problem in the field of combinatorial optimization is the Set Packing Problem (SPP), which takes on added significance in view of its far-reaching applications in routing and scheduling problems, scheduling surgical procedures, the winner determination problem, forestry, production management, etc. The coefficient matrix of this problem has low density (the number of 1’s is much less than the number of 0’s in the matrix); therefore, by reordering the matrix, one can aggregate its 1’s in order to exploit special structures like... 

This chapter at first proposes a framework to solve set covering problems (SCPs) with block-angular structures through solving their sub-problems and then develop the method for solving general SCPs without this structure. The proposed framework generates a guaranteed solution for the SCPs with block-angular structure based on a theorem which relates the optimal solution of the original problem to the optimal solutions of its sub-problems. Therefore, since sub-problems involve much fewer constraints and variables than the original problem, the complexity to solve the original SCP is much reduced. In addition, a method to exploit the block-angular structure of SCPs is proposed, using... 

This chapter at first proposes a framework to solve set covering problems (SCPs) with block-angular structures through solving their sub-problems and then develop the method for solving general SCPs without this structure. The proposed framework generates a guaranteed solution for the SCPs with block-angular structure based on a theorem which relates the optimal solution of the original problem to the optimal solutions of its sub-problems. Therefore, since sub-problems involve much fewer constraints and variables than the original problem, the complexity to solve the original SCP is much reduced. In addition, a method to exploit the block-angular structure of SCPs is proposed, using... 

The set covering problem (SCP) has quite a large coefficient matrix in practical problems, but due to its low density, most of its entries are zero. Drawing on this observation, some methods can be developed to exploit special structures of the coefficient matrix of an SCP in such a way that they contain smaller dense subproblems. Against this backdrop, in this paper, three structures, namely 'partitioned', 'semi-block angular', and 'block angular', are proposed. To begin with, some heuristic methods are presented to exploit these structures of the coefficient matrices of SCPs; then, by optimally solving the smaller subproblems of these structures, their solutions are used to solve the whole... 

In this paper, a decomposition technique based on constraint partitioning is developed to solve the crew pairing problem (CPP) which has an overriding importance in the airline industry as it determines the crew cost. The method is based on the observation that in large-scale problems, the constraints can be partitioned to some sub-problems which involve special subsets of variables. The resultant structure is called the 'partitioned structure'. Therefore, in the proposed method, first, a feasible solution is generated for a reduced CPP with a 'partitioned structure' through the optimal solutions of its sub-problems. Then, at each step, the feasible solution is improved through... 

A simple method has been demonstrated to grow silver nanoparticles on the surface of cellulose fibers. The preparation is based on photo-activation of surface by ultraviolet (UV) photons, followed by chemical reduction of silver nitrate. It is found that the concentration of silver nitrate in the solution is not a determining factor, while UV intensity affects the rate of initial growth and determines the final concentration of the loaded silver. We explain the phenomena based on a model including the number of reducing sites on the surface of cellulose fibers activated by UV photons, and a release mechanism that causes a slow rate of dissolution of silver back into the solution  

Abstract: The convection–diffusion process of carbon dioxide (CO2) dissolution in a saline reservoir is investigated to shed light on the effects of the permeability heterogeneity. Using sequential Gaussian simulation method, random permeability fields in two and three-dimension (2D and 3D) structures are generated. Quantitative (average amount of the dissolved CO2 and dissolution flux) and qualitative (pattern of the dissolved CO2 and velocity streamlines) measurements are used to investigate the results. A 3D structure shows a slightly higher dissolution flux than a 2D structure in the homogeneous condition. Results in the random permeability fields in 2D indicates an increase in the... 

This study aims to develop an equation for primary drainage capillary pressure as a function of water saturation based on the porous plate model. Several empirical and semiempirical equations are available to calculate Pc-Sw. The results of the previous equations are not ideal, so this is an attempt to present an accurate equation to calculate Pc-Sw based on governing theoretical physical analysis to describe two-phase immiscible displacement flow in porous media. Volumetric flow rates through the core plug and porous plate are applied to calculate the transport phenomena in porous media. The equation is derived from mass and momentum transfer through a core plug, and Darcy's law is applied... 

This study aims to improve rock-type classification by analyzing core data based on the water-oil primary drainage capillary pressure method. A modified empirical equation is proposed using permeability, porosity, and irreducible water saturation to classify rock types based on water-oil primary drainage capillary pressure. We used primary drainage capillary pressure data measured in carbonate and sandstone samples in the Ahvaz Asmari and Mansouri oilfields to evaluate the characterization number (Cn) method. This study consists of two main parts. First, the Cn method is introduced to rock typing, and the permeability is calculated from well log data. In the second part, we present rock-type... 

In this paper, the effects of flame retardant fillers mixture, zinc borate and aluminum hydroxide, on polypropylene including various ratios of their accumulated concentrations, have been studied. Maleic anhydrate grafted polypropylene and calcium stearate have been used to increase filler-matrix interactions and filler dispersion in matrix. Effects of filler types and their concentrations on fire resistibility of different composite samples have been determined via limiting oxygen index test method. Effects of foregoing parameters on the mechanical properties of prepared samples were also investigated. Recorded data shows that adding polypropylene, containing aluminum anhydrate lump,... 

Highlights: 1. What are the main findings? CO2–N2 mixture dissolution in brine is examined by considering the cross-diffusion effect for CO2 sequestration in a deep storage reservoir. Heterogeneity lowers the average dissolved CO2 and impedes the onset of convection. 2. What is the implication of the main finding? Correlations are developed to predict the transition time between the dissolution regimes. The possibility of impure carbon dioxide (CO2) sequestration can reduce the cost of these projects and facilitate their widespread adoption. Despite this, there are a limited number of studies that address impure CO2 sequestration aspects. In this study, we examine the convection–diffusion... 

The present paper deals with different effects of homogenization time and cooling environment on Ni-42.5wt%Ti-7.5wt%Cu alloy. The alloy was prepared by vacuum arc melting. Afterwards, three homogenization times (half, one and two hour) and three cooling environments (water, air and furnace) at 1373 K were selected. Optical and Scanning Electron Microscopic methods, EDX, DSC and hardness tests have been used to evaluate the microstructure, transformation temperatures and hardness. Results indicate that specimens that were cooled in air are super-saturated. Also, the microstructure from furnace cooling has many disparities with the other cooling environments' microstructure and two types of...