Sharif Digital Repository

Performance degradation remains as one of the primary limitations for practical applications of proton exchange membrane (PEM) fuel cells. The performance of a PEM fuel cell stack is affected by many internal and external factors, such as fuel cell design and assembly, degradation of materials, operational conditions, and impurities or contaminants. Performance degradation is unavoidable, but the degradation rate can be minimized through a comprehensive understanding of degradation and failure mechanisms. In present work, a single PEM fuel cell for stationary applications is investigated. Membrane and catalyst layers (anode and cathode electrodes) are considered as critical components that... 

Simplicity and validity are recognized as important attributes of an effective simulation model. However, some disagreement exists both on the definitions of these concepts and on quantitative measures for them. This paper introduces Enrichment Level (EL) for (simulation) models. The EL is a quantitative measure to compare the effectiveness of alternative models when used in a particular project. While there is no conceptual restriction on expanding the EL, it currently captures the bias, speed, variance, and scope of a model as the main contributing enrichment factors. This paper explores these factors and suggests a utility-based method to combine them according to the relative importance... 

This paper presents a new set of reliability sensitivity measures. The purpose is to identify the optimal manner in which to mitigate risk to civil infrastructure, and reduce model uncertainty in order to improve risk estimates. Three measures are presented. One identifies the infrastructure components that should be prioritized for retrofit. Another measure identifies the infrastructure that should be prioritized for more refined modeling. The third measure identifies the models that should be prioritized in research to improve models, for example by gathering new data. The developments are presented in the context of a region with 622 buildings that are subjected to seismicity from several... 

A library of probabilistic models for prediction of seismic risk is presented. The models are specifically intended for use with reliability methods to compute event probabilities, such as seismic loss probabilities. Several models are presented here for the first time. In particular, new and generic models are proposed for earthquake location, regional loss, building response, building damage, and building loss. Each model is presented with an explanation of its development and a discussion of its predictions. In addition, models from the literature are " smoothed" to make them amenable to reliability analysis. The models are implemented in a new computer program that is tailored for... 

Depletion of fossil fuels has increased the pressure on energy systems to operate in the most efficient and economical mode. This tendency promotes energy systems to operate at optimum operating conditions, which maximizes the system profit over lifetime. Recently, there have been many attempts to maximize lifetime profit. Most of them concentrate on the power generation aspect without incorporating further aspects such as system degradation and profitability through lifetime. However, the main intention of the system operators is to optimize the profitability of system at the moment of operation and not the total profitability through the system lifetime. In this study a novel approach is... 

Share of leakage in total power consumption of static RAM (SRAM) memories is increasing with technology scaling. Reverse body biasing increases threshold voltage (Vth), which exponentially reduces subthreshold leakage, but it increases SRAM access delay. Traditionally, when all cells of an SRAM block used to have almost the same delay, within-die variations are increasingly widening the delay distribution of cells even within a single SRAM block, and hence, most of these cells are substantially faster than the delay set for the entire block. Consequently, after the reverse body biasing and the resulting delay rise, only a small number of cells violate the original delay of the SRAM block; we... 

Guess-and-determine (GD) attacks are general attacks on stream ciphers, which have often been implemented in an ad hoc manner. The authors introduce a heuristic approach to the design of GD attacks, that is a dynamic programming method using a Viterbi-like algorithm which is a well-known decoding algorithm for convolutional codes. The authors also show that with this method, the resulting GD attacks, named heuristic GD (HGD) attacks, on TIPSY, SNOW1 and SNOW2 lead to less computational complexity than the previously known GD attacks. The main advantage of HGD attacks, over ad hoc GD attacks, is that while being powerful, they can be designed algorithmically for classes of stream ciphers,... 

Epoxy resins have been used as structural materials since the late 1940s. Despite their desirable properties such as high strength, excellent creep resistance, and good adhesion, they suffer from low fracture energy. Rubber modification as a major toughening approach to overcome the inherent brittleness of epoxy polymers was introduced during the early 1970s. Since then, a large number of investigations have been conducted to elucidate different aspects of rubber-toughened epoxies. The present work is a critical review of the field focusing on the important parameters affecting rubber-toughening. The studies reviewed are classified in five categories including roles of matrix ductility,... 

The Next Bit Test briefly states that a sequence is random if and only if, given any i bits of the sequence, it is not possible to predict the next bit of the sequence with a probability of success significantly greater than 1/2. In 1996, Sadeghiyan and Mohajeri proposed a so-called "new universal test for bit strings", based on the theoretical next bit test. In this paper, we study different aspects of this test and show its weakness. Then, we improve it both theoretically and practically for better classification of the sequences. As a result, a Practical Next Bit (PNB) test is introduced in two Global and Local versions and a histogram, which gives an impression of the global evaluation... 

In this paper, we propose a threshold increasing algorithm for a (t; n) latticebased Threshold Multi-Stage Secret Sharing (TMSSS) scheme. To realize the changeability feature, we use the zero addition protocol to construct a new (t0; n) TMSSS scheme. Therefore, the new scheme enjoys the significant feature of threshold changeability along with the inherited features of being multi-stage, multi-use, and verifiable derived from our previously proposed lattice-based TMSSS scheme. Furthermore, we use the improved TMSSS scheme to propose a threshold decryption algorithm for the Learning With Error (LWE) based public key encryption scheme based on the study of Lindner and Peikert. For threshold... 

In this paper, we construct a lattice based (t, n) threshold multi-stage secret sharing (MSSS) scheme according to Ajtai's construction for one-way functions. In an MSSS scheme, the authorized subsets of participants can recover a subset of secrets at each stage while other secrets remain undisclosed. In this paper, each secret is a vector from a t-dimensional lattice and the basis of each lattice is kept private. A t-subset of n participants can recover the secret(s) using their assigned shares. Using a lattice based one-way function, even after some secrets are revealed, the computational security of the unrecovered secrets is provided against quantum computers. The scheme is multi-use in... 

Due to the simple structure and low production cost of Joule-Thomson (JT) refrigeration cycles, these cryogenic systems have widely been used in many food or medicine industries. However, in many scenarios, two or even three cryogenic temperatures are needed to simultaneously supply different cryogenic cooling loads since each product has its own comfort low-temperature range. In this case, multi-evaporator JT systems can be a promising solution to address several cryogenic temperatures via employing different evaporators. Following this objective, an innovative triple-evaporator JT cycle is proposed, where the pressure difference between evaporators is created via a pumping mechanism by... 

A Publicly Verifiable Secret Sharing (PVSS) scheme, as introduced by Stadler, has a feature where anyone, besides the participants, can verify the validity of the shares distributed by the dealer. Schoenmakers added a new feature, by providing a proof of correctness of the shares released by the players in the reconstruction process. This protocol is claimed to be an improvement on Stadler's and Fujisaki-Okamoto's, both in efficiency and in the type of intractability assumptions. However, Young-Yung improved Schoenmakers' PVSS, using a Discrete-Log instead of a Decision Diffie-Hellman. In this paper, a new PVSS is presented, having an intrinsic difference with its predecessors, that is, the... 

Primal-dual interior-point methods (IPMs) have shown their power in solving large classes of optimization problems. However, there is still a discrepancy between the practical behavior of these algorithms and their theoretical worst-case complexity results with respect to the update strategies of the duality gap parameter in the algorithm. Recently, this discrepancy was significantly reduced by Peng, J., Roos, C. and Terlaky, T., 2002, Self-Regularity: A New Paradigm for Primal-Dual Interior-Point Algorithms (Princeton, NJ: Princeton University Press) who introduced a new family of self-regular (SR)-proximity functions based IPMs. In this paper, based on a class of SR proximities, we propose... 

Growing nanocrystals in a matrix is a way to have separate, non-agglomerated nanoparticles, which are surface passivated and stabilized against environmental attacks. To create nanocrystals of the desired size and properties in a matrix, it is required that the growth parameters are properly tuned. In this work, we study the growth parameters for ZnS nanocrystals in an SiO2 matrix. Samples were prepared by cosputtering of SiO 2 and ZnS on a silica substrate, followed by annealing at different temperatures. Two categories of samples with low-content ZnS (4 mol%) and high-content ZnS (15 mol%) were examined. In the low-content samples, ZnS nanocrystals are not formed below 900°C annealing... 

Fluid particle diffusion through blood flow within a capillary tube is an important phenomenon to understand, especially for studies in mass transport in the microcirculation as well as in solving technical issues involved in mixing in biomedical microdevices. In this paper, the spreading of tracer particles through up to 20% hematocrit blood, flowing in a capillary tube, was studied using a confocal micro-PTV system. We tracked hundreds of particles in high-hematocrit blood and measured the radial dispersion coefficient. Results yielded significant enhancement of the particle diffusion, due to a micron-scale flow-field generated by red blood cell motions. By increasing the flow rate, the... 

We introduce a dynamical picture, referred to as correlation picture, which connects a correlated bipartite state to its uncorrelated counterpart. This picture allows us to derive an exact dynamical equation for a general open-system dynamics with system-environment correlations included. This exact dynamics is in the form of a Lindblad-like equation even in the presence of initial system-environment correlations. For explicit calculations, we also develop a weak-correlation expansion that allows us to introduce systematic perturbative approximations. This expansion provides approximate master equations that can feature advantages over existing weak-coupling techniques. As a special case, we... 

The insulation lifetime of power cables is determined by several factors. One of the more important of these is the occurrence of partial discharge (PD) at the dielectric. The ability to detect and locate a PD source is limited by attenuation of the high frequency PD pulses as they propagate through the cable. Therefore it is necessary to understand the high frequency response of such cables. Further, to enable reconstruction of PD signals as emitted a viable high frequency model for simulation is needed. This paper presents results of measurements of PD calibration pulse and high frequency sinusoid propagation in HV XLPE cables. In addition to the tests a cable model was developed using the... 

Asphaltene deposition in the early stage of the oil reservoir life and later during any stimulation process emerges critical problems to the petroleum industry. Deposition of asphaltene aggregates raises strict problems in industries and demands markedly a practical and scientific knowledge of the mechanisms of aggregation and precipitation. Fluorescence emission spectroscopy has been widely used to illuminate the fundamental properties of crude oils and asphaltenes. It proposes analysis of some details of equilibrium, dynamic behavior, and aggregation composition of crude oil under specific condition. In this work, the fluorescence spectra of crude-oil extracted asphaltene samples were...