Sharif Digital Repository

Comprehensive numerical analyses are conducted to simulate and capture “Buzz” phenomenon in a supersonic mixed compression air inlet. The buzz is an unsteady self-sustained feature that occurs in supersonic inlets, especially when operating in their subcritical condition. In such a situation, the shock waves oscillate along the inlet and cause mass flow fluctuations inside the inlet that will deteriorate the engine performance significantly. An axisymmetric unsteady numerical simulation was used to solve the Navier–Stokes equations combined with the URANS SST k–ω turbulence model. The simulations for two different free-stream Mach numbers of M∞=2.0 and 2.2 and at two specific subcritical... 

We derive exact expressions for the forces and torques between biaxial molecules interacting via the RE-squared potential, a recent variant of the Gay-Berne potential. Moreover, efficient routines have been provided for rigid body MD simulations, resulting in 1.6 times speedup compared to the two-point finite difference approach. It has also been shown that the time cost of a MD simulation will be almost equal to a similar MC simulation, making use of the provided routines. © 2006 Elsevier Inc. All rights reserved  

Gas sparging and back-flushing treatments were compared as a means to tackle the problem of fouling in yeast microfiltration. Based on the feed and membrane characteristics, it was shown that either of these techniques could be superior to the other in order to overcome the problem. At more concentrated feed streams the main cause of flux decline was cake formation. In this condition gas sparging showed greater efficiency in flux enhancement. On the other hand at lower feed concentration the relative importance of internal fouling due to pore blockage, increased. In this case back-flushing was more effective. © 2007  

For large and complex reacting systems, computational efficiency becomes a critical issue in process simulation, optimization and model-based control. Mechanism simplification is often a necessity to improve computational speed. We present a novel approach to simplification of reaction networks that formulates the model reduction problem as an optimization problem and solves it using genetic algorithm (GA).The aim of simplification kinetics modeling is to derive the simplest reaction system, which retains the essential features of the full system. Numerical results for H2/O2 combustion reaction mechanism illustrate the potential and proficiency of this approach  

With the growing rate of network attacks, intelligent methods for detecting new attacks have attracted increasing interest. The RT-UNNID system, introduced in this paper, is one such system, capable of intelligent real-time intrusion detection using unsupervised neural networks. Unsupervised neural nets can improve their analysis of new data over time without retraining. In previous work, we evaluated Adaptive Resonance Theory (ART) and Self-Organizing Map (SOM) neural networks using offline data. In this paper, we present a real-time solution using unsupervised neural nets to detect known and new attacks in network traffic. We evaluated our approach using 27 types of attack, and observed... 

We have proposed an efficient parametrization method for a recent variant of the Gay Berne potential for dissimilar and biaxial particles [Phys. Rev. E 67, 041710 (2003)] and demonstrated it for a set of small organic molecules. Compared with the previously proposed coarse-grained models, the new potential exhibits a superior performance in close contact and large distant interactions. The repercussions of thermal vibrations and elasticity have been studied through a statistical method. The study justifies that the potential of mean force is representable with the same functional form, extending the application of this coarse-grained description to a broader range of molecules. Moreover, the... 

A very efficient and mild procedure for the trimethylsilylation of a wide variety of alcohols, including primary, allylic, benzylic, secondary, hindered secondary, tertiary, and phenols with hexamethyldisilazane on the surface of silica gel dispersed with LiClO4 in room temperature at few minutes in excellent yields under neutral conditions is reported. This procedure also allows the excellent selectivity under LP-SiO2 system for silylation of alcohols in the presence of amine and phenolic hydroxy groups. © 2005 Elsevier B.V. All rights reserved  

A comprehensive model for pervaporation in thin membrane channels was developed to study the effect of changing temperature and concentration on mass flux. This model consists of momentum, energy and species mass balances along and across the membrane in the flow channel. A computational fluid dynamics (CFD) code was written in C++ programming language to solve the coupled non-linear transport equations in the channel by finite volume method. The Semi-Implicit Pressure Link Equation (SIMPLE) CFD algorithm is used to modify the dependent variables in each of the iterations. The effect of variation of temperature and concentration on transport and thermodynamic properties were considered by... 

The employment of tissue engineering scaffolds in the reconstruction of the damaged bone tissues has shown remarkable promise since they significantly facilitate the healing process. Fabrication of highly porous biocompatible scaffolds with sufficient mechanical strength is still challenging. In this regard, polymers have been widely utilized to construct three-dimensional (3D) porous scaffolds due to their excellent processability and biocompatibility. However, insufficient mechanical strength and inappropriate degradation rate of the monophasic polymer scaffolds in the bone regeneration process, as the main challenges, limit their extensive clinical application. The incorporation of... 

Flow through porous media depends strongly on the spatial distribution of the geological heterogeneities which appear on all length scales. We lack precise information about heterogeneity distribution on various scales, from pore level to reservoir scale. However, some sources provide suitable information. At pore scale, for example, the micro-CT images show considerable insights into pore space structures and play valuable role in porous media characterization. The consequence of all geological heterogeneities is a great deal of uncertainty in dynamic performance of porous media which can be investigated using percolation theory. The main percolation quantities include the connected pore... 

Laminated composites are made using different routes, including adhesive bonding. In this investigation, Aluminum layers are bonded together using epoxy adhesive (neat, rubber-modified and filled-epoxy). In phase one of this investigation, the influence of number of layers and adhesive ductility have been studied on dynamic behavior of composites. The results of impact behavior of laminates indicate that the impact energy increases with the number of layers. Also, it can be concluded that there is much more sensitivity of the impact energy of the composite to the number of the layers than that to the adhesive composition. © 2004 Elsevier B.V. All rights reserved  

Iron oxide nanoparticles supported on mesoporous silica-type materials have been successfully utilized in the aqueous selective oxidation of alkenes under mild conditions using hydrogen peroxide as green oxidant. The supported catalyst could be easily recovered after completion of the reaction and reused several times without any loss in activity (no metal leaching observed during the reaction), constituting a facile and straightforward example of aqueous oxidation chemistry promoted by iron-based heterogeneous systems. Copyright  

The present investigation introduces a new series of cycloplatinated(II) complexes, with the general formula Pt(O-bpy)(Me)(CN-R)] (R = benzyl, 2-naphtyl and tert-butyl), which are able to generate the stable trans-Pt(IV) product in the solution after the reaction with iodomethane. In fact, the trans product is both the kinetic and thermodynamic product of the reaction; this observation was supported by DFT calculations. These Pt(II) complexes are supported by 2,2'-bipyridine N-oxide (O-bpy) and one of several isocyanides as the cyclometalated and ancillary ligands, respectively. These new Pt(II) complexes undergo oxidative addition with MeI to give the corresponding trans-Pt(IV) complexes.... 

Zeolite-based Ag/AgBr and Ag/AgBr/TiO 2 photocatalysts were prepared by sol-gel and deposition methods and were characterized. Their photocatalytic activities were evaluated by inactivation of Escherichia (E.) coli and the photodegradation of Acid Blue 92 and potassium permanganate. The composites containing Ag/AgBr showed the antibacterial activity in the dark by releasing Ag ions into the medium. The results for inactivation of E. coli indicated that Ag/AgBr/TiO 2 modified photocatalyst had better antibacterial activity than Ag/AgBr/zeolite, while zeolite and TiO 2/zeolite did not show any antibacterial activity under visible light and dark conditions. Photodecolarization rate was affected... 

CeO 2-TiO 2 nanoparticles were prepared by the sol-gel process using 2-hydroxylethylammonium formate as room-temperature ionic liquid and calcined at different temperatures (500-700°C). CeO 2-TiO 2-graphene nanocomposites were prepared by hydrothermal reaction of graphene oxide with CeO 2-TiO 2 nanoparticles in aqueous solution of ethanol. The photocatalysts were characterized by X-ray diffraction, BET surface area, diffuse reflectance spectroscopy, scanning electron microscopy, and Fourier transformed infrared techniques. The results demonstrate that the room-temperature ionic liquid inhibits the anatase-rutile phase transformation. This effect was promoted by addition of CeO 2 to TiO 2.... 

Serious and challengeable role in markets of low power supplies for cellular phones and other small portable devices is expected for single fuel cell in future. Single fuel cell voltage level is extremely low in comparison with its output current. According to the extremely low level of the voltage, a suitable dc/dc boost converter must be used for increasing the voltage. In this paper, a model for a single fuel cell is presented and the effect of external load variation and changing of reactant pressure on cell performance is investigated. Moreover, a predictive voltage controller for boost convertor of PEM fuel cell is presented, simulated and validated using MATLAB/Simulink. The results... 

This paper addresses the problem of identifying central nodes from the information flow standpoint in a social network. Betweenness centrality is the most prominent measure that shows the node importance from the information flow standpoint in the network. High betweenness centrality nodes play crucial roles in the spread of propaganda, ideologies, or gossips in social networks, the bottlenecks in communication networks, and the connector hubs in biological systems. In this paper, we introduce DICeNod, a new approach to efficiently identify central nodes in social networks without direct measurement of each individual node using compressive sensing, which is a well-known paradigm in sparse... 

Betweenness centrality is a prominent centrality measure expressing importance of a node within a network, in terms of the fraction of shortest paths passing through that node. Nodes with high betweenness centrality have significant impacts on the spread of influence and idea in social networks, the user activity in mobile phone networks, the contagion process in biological networks, and the bottlenecks in communication networks. Thus, identifying k-highest betweenness centrality nodes in networks will be of great interest in many applications. In this paper, we introduce CS-HiBet, a new method to efficiently detect top-k betweenness centrality nodes in networks, using compressive sensing.... 

We describe a general method for analyzing a nonstationary stochastic process X (t) which, unlike many of the previous analysis methods, does not require X (t) to have any scaling feature. The method is used to study the fluctuations in the daily price of oil. It is shown that the returns time series, y (t) =ln [X (t+1) X (t)], is a stationary and Markov process, characterized by a Markov time scale tM. The coefficients of the Kramers-Moyal expansion for the probability density function P (y,t y0, t0) are computed. P (y,t, y0, t0) satisfies a Fokker-Planck equation, which is equivalent to a Langevin equation for y (t) that provides quantitative predictions for the oil price over times that... 

Modern power systems have been faced with a rising appeal for the upgrade to a highly intelligent generation of electricity networks known as the smart grid. Thus, security for the smart grid has emerged as an important issue. Recently, Hur proposed an attribute based data sharing for smart grid which unfortunately is vulnerable to the denial of service (DoS) attack. Moreover, it does not support the user revocation property and the grid system manager cannot prevent the revoked user of having access to the shared data in the storage center. For these weaknesses, we suggest an efficient revocable data sharing scheme which is immune against DoS attack. In addition, we present the security...