Sharif Digital Repository

In this paper, we present a randomized LOCAL constant approximation factor algorithm for minimum dominating set (MDS) problem and minimum total dominating set (MTDS) problem in graphs. The approximation factor of this algorithm for planar graphs with no 4-cycles is 18 and 9 for MDS and MTDS problems, respectively. © 2020 Owner/Author  

Surface registration is a necessary step and widely used in medical image-aided surgery. It's relevance to medical imaging is that there is much useful anatomical information in the form of collected surface points which originate from complimentary modalities. In this study, the kinematic relations between two point clouds with different coordinate definitions have been generated. Using Influence Method of surface modeling for extracting point clouds functions, the transformation matrix would be resulted. The proposed method was applied for an experimental femur data points(651 points) using the MRI images. These data points were transformed in a 30 degrees flexion of knee. This... 

Given a set S of n disjoint line segments in ℝ2, the visibility counting problem (VCP) is to preprocess S such that the number of segments in S visible from any query point p can be computed quickly. This problem can trivially be solved in logarithmic query time using O(n4) preprocessing time and space. Gudmundsson and Morin proposed a 2-approximation algorithm for this problem with a tradeoff between the space and the query time. They answer any query in Oε(n1−α) with Oε(n2+2α) of preprocessing time and space, where α is a constant 0 ≤ α ≤ 1, ε > 0 is another constant that can be made arbitrarily small, and Oε(f(n)) = O(f(n)nε). In this paper, we propose a randomized approximation algorithm... 

This study proposes a new technique for registration of complicated freeform surfaces. The relationship between the initial and transferred location/orientation of a points cloud is formulated and then generalized within the framework of the influence surface modeling approach using least squares method. Results of case studies for estimation of the transformation matrix of articular surface of the knee joint in two scenes of MR images indicated high accuracies of ±1 mm and ±1 degree  

In real applications, there are situations where we need to model some problems based on uncertain data. This leads us to define an uncertain model for some classical geometric optimization problems and propose algorithms to solve them. The assigned version of the k-center problem for n uncertain points in a metric space is studied in this paper. The main approach is to replace each uncertain point with a clever choice of a certain point. We argue that the k-center solution for these certain replacements of our uncertain points, is a good constant approximation factor for the original uncertain k-center problem. This approach enables us to present fast and simple algorithms that give... 

Given a set S of n disjoint line segments in R2, the visibility counting problem (VCP) is to preprocess S such that the number of segments in S visible from any query point p can be computed quickly. This problem can be solved trivially in O(log⁡n) query time using O(n4log⁡n) preprocessing time and O(n4) space. Gudmundsson and Morin (2010) [10] proposed a 2-approximation algorithm for this problem with a tradeoff between the space and the query time. For any constant 0≤α≤1, their algorithm answers any query in Oϵ(m(1−α)/2) time with Oϵ(m1+α) of preprocessing time and space, where ϵ>0 is a constant that can be made arbitrarily small and Oϵ(f(n))=O(f(n)nϵ) and m=O(n2) is a number that depends... 

Beenakker noticed that the peculiar band structure of Dirac fermions in two-dimensional (2D) solids allows for the specular Andreev reflection in these systems, which has no analogue in other 2D electron systems. An interesting deformation of the Dirac equation in the solid state is to tilt it, which has now been realized in materials. In this paper we report another peculiar feature of the Andreev reflection in tilted 2D Dirac cone systems. The tilt deformation of the Dirac equation is characterized by two parameters ζ=ζ(cosθ,sinθ). We show that when the tilt parameter is tuned to its "horizon value" ζ=1, irrespective of the incidence angle of electrons, the Andreev reflected hole is always... 

The electrodynamics of Weyl semimetals is an extension of Maxwell's theory where in addition to field strength tensor Fμν, an axion field enters the theory which is parametrized by a four-vector bμ=(b0,b). In tilted Weyl materials (TWMs) an additional set of parameters ζ=(ζx,ζy,ζz) enters the theory that can be encoded into the metric of the spacetime felt by electrons in TWMs. This allows an extension of Maxwell's electrodynamics that describes electric and magnetic fields in TWMs, and tilted Dirac materials (TDMs) that correspond to bμ=0. The tilt parameter ζ appearing as an off-diagonal metric matrix element, mixing time and space components, which mingles E and B fields, whereby the... 

Traditionally, solid-liquid mixing has always been regarded as an empirical technology with many aspects of mixing, dispersing and contacting were related to power draw. One important application of solid-liquid mixing is the preparation of brine from sodium formate. This material has been widely used as a drilling and completion fluid in challenging environments such as the Barents Sea. In this paper, large-eddy simulations of a turbulent flow in a solid-liquid baffled cylindrical mixing vessel with large number of solid particles are performed to obtain insight into the fundamental aspects of a mixing tank. The impeller-induced flow at the blade tip radius is modeled by using the... 

The oxidation of ferrous iron in solution using Acidithiobacillus ferrooxidans has industrial applications in the regeneration of ferric iron as an oxidant agent for the removal of hydrogen sulfide from waste gases, desulphurization of coal, leaching of non-ferrous metallic sulfides and treatment of acid mine drainage. The aim of this attempt was to increase the biooxidation rate of ferrous sulfate by using immobilized cells. Rate of ferrous iron oxidation was determined in a packed-bed reactor configuration with low density polyethylene (LDPE) particles as support material in order to find the most practical system for scale-up. The present work studies the influence of basic parameters on... 

Katsnelson has shown that, within the Fermi-liquid approach, the optical conductivity of Dirac electrons in graphene is not affected by many-body interactions [M. I. Katsnelson, Eur. Phys. Lett. 84, 37001 (2008)EULEEJ0295-507510.1209/0295-5075/84/37001]. We show that, when the Dirac cone is tilted, the Fermi-liquid corrections arise in the optical conductivity in a manner that the correction depends on the angle between the electric field of the incident light and the tilt direction. Therefore the mapping of the optical conductivity for various directions of the incident light enables a determination of the many-body effect in the optical conductivity spectrum of the two-dimensional tilted... 

In this paper, a novel structure for MOSFET like CNTFETs (MOSCNTs) is proposed, combining the advantages of both high and low dielectrics to improve output characteristics. In this structure, the gate dielectric at the drain side is selected from a material with low dielectric constant to form smaller capacitances, while a material with high dielectric constant is selected at the source side to improve on current and reduce leakage current. The new structure is simulated based on the Schrödinger-Poisson formulation. Obtained results show that the proposed configuration has lower off and higher on current in comparison with low-k MOSCNTs. Also, using a two-dimensional model, a wide range of... 

Vibration analysis of simply supported rotating cross-ply laminated stiffened cylindrical shell is performed using an energy approach which includes variational and averaging method. The stiffeners include rings and stringers. The equations are obtained by Rayleigh-Ritz method and Sander's relations. To validate the present method, the results are compared to the results available in other literatures. A good adoption is observed in different type of results including isotropic shells, rotating laminated shells, stiffened isotropic shells and stiffened laminated shells. Then, the optimization of parameters due to shell and stiffeners is conducted by genetic algorithm (GA) method under weight... 

This paper presents the results of diagonal compression tests conducted on a series of un-reinforced and reinforced masonry specimens. Reinforced specimens were strengthened by using three types of mortar coating (with and without inner reinforcement), mesh reinforcement, and PP-band reinforcement. In particular, four different reinforcement methods with mortar coating (bare mortar coating, coating with steel mesh, coating with polymer reinforcement, and coating with GFRP mesh reinforcement), three different mesh-only reinforcement (steel mesh, polymer mesh with larger grids, and polymer mesh with smaller grids), and 2 different arrangements of PP-bands (4 × 4 and 6 × 6) were tested. Also,... 

This study investigates the tensile properties of various GFRP laminates after exposure to elevated temperatures. Fiber configuration, exposure temperature and laminate thickness were considered as the test variables. A total number of 180 specimens were tested in tension to obtain the mechanical properties of GFRP laminates. Alongside the mechanical tests, SEM analyses were conducted on selected samples before testing to investigate the resin, fiber, and their interface damages. Regardless of the sample type, it was generally observed that the reduction rate in the tensile strength increased with an increase in the exposure time and a decrease in the laminates’ thickness. The results of... 

In this paper, the necessary similarity conditions, or scaling laws, for free vibrations of orthogonally stiffened cylindrical shells are developed using similitude theory. The Donnelltype nonlinear strain-displacement relations along with smearing theory are used to model the structure. Then the principle of virtual work is used to analyze the free vibration of the stiffened shell. After nondimensionalizing the derived formulations, the scaling laws are developed, using similitude theory. Then, different examples are solved to validate the scaling laws numerically and experimentally. The obtained results show the effectiveness of the derived formulations  

A mesophilic iron oxidizing bacterium, Acidithiobacillus ferrooxidans, has been isolated (33°C) from a typical chalcopyrite copper concentrate of the Sarcheshmeh Copper Mine in the region of Kerman located in the south of Iran. In addition, a thermophilic iron oxidizing bacterium, Sulfobacillus spp., has been isolated (60°C) from the Kooshk Lead and Zinc Mine near the city of Yazd in the mid-west region of Iran. The variation of pH, ferrous and ferric concentration and cell growth on the time and effects of some factors, such as temperature and initial ferrous concentration, on the bioleaching of chalcopyrite concentrate obtained from Sarcheshmeh Copper Mine, were investigated. Bioleaching... 

Biological oxidation of ferrous sulfate by Acidithiobacillus ferrooxidans has proved to be a significant step in the bioleaching of sulfide minerals and the treatment of acid mine drainage. The same bioreaction also has beneficial applications in the desulphurization of coal and removal of hydrogen sulfide from gaseous effluents. In this research, the effects of some process variables such as pH, temperature, elemental sulfur, amount of initial ferrous and magnesium ions on oxidation of ferrous sulfate by a native A. ferrooxidans, which was isolated from a chalcopyrite concentrate, were investigated. All experiments carried out in shake flasks at 33 °C that was obtained as optimum... 

In this study, effects of solid concentration, temperature, and initial Fe2+ concentration on bioleaching of sulfide mineral (chalcopyrite) obtained from Sarcheshmeh Copper Mine in the region of Kerman located in the south of Iran were investigated. A mesophilic iron oxidizing bacterium, Acidithiobacillus ferrooxidans has been isolated from a typical chalcopyrite copper concentrate of the mentioned mine. Bioleaching experiments were carried out in two batch air-lift bioreactors with recycling stream. One reactor contained 2 liters of medium and 10% (v/v) inoculum while in the other reactor, control bioleaching tests were carried out with sterilized concentrate without inoculum by the...