Showing papers by "Tunis University published in 2017"

Direct Remaining Useful Life Estimation Based on Support Vector Regression

Abstract: Prognostics is a major activity in the field of prognostics and health management It aims at increasing the reliability and safety of systems while reducing the maintenance cost by providing an estimate of the current health status and remaining useful life (RUL) Classical RUL estimation techniques are usually composed of different steps: estimations of a health indicator, degradation states, a failure threshold, and finally the RUL In this work, a procedure that is able to estimate the RUL of equipment directly from sensor values without the need for estimating degradation states or a failure threshold is developed A direct relation between sensor values or health indicators is modeled using a support vector regression Using this procedure, the RUL can be estimated at any time instant of the degradation process In addition, an offline wrapper variable selection is applied before training the prediction model This step has a positive impact on the accuracy of the prediction while reducing the computational time compared to existing indirect RUL prediction methods To assess the performance of the proposed approach, the Turbofan dataset, widely considered in the literature, is used Experimental results show that the performance of the proposed method is competitive with other existing approaches

Renewable and non-renewable energy use - economic growth nexus: The case of MENA Net Oil Importing Countries

Abstract: This study examines the energy use – economic growth nexus by disaggregating energy use into two types of energy, renewable and non-renewable energy use. Our sample consists of eleven MENA Net Oil Importing Countries (NOICs) during the period 1980–2012. A multivariate panel framework was used to estimate the long run relationship and the panel Granger causality tests was employed to assess the causality direction among variables. The empirical results provide evidence for long-term equilibrium relationship between real Gross Domestic Product (GDP), renewable energy use, non-renewable energy use, real gross fixed capital formation and labor force. The results provide evidence also for positive and statistically significant elasticities. Moreover, the empirical findings from panel Error Correction Model confirm the existence of bidirectional causality between renewable energy use and economic growth, and between non-renewable energy use and economic growth, results that support the feedback hypothesis. Moreover, our empirical findings provide evidence for two way (bidirectional) causal association in both the short and long-run between renewable and non-renewable energy use which proves the substitutability and interdependence between these two types of energy sources. The policies implications of these results are also proposed and discussed.

Modeling systemic risk and dependence structure between oil and stock markets using a variational mode decomposition-based copula method

Abstract: This study combines the variational mode decomposition (VMD) method and static and time-varying symmetric and asymmetric copula functions to examine the dependence structure between crude oil prices and major regional developed stock markets (S&P500, stoxx600, DJPI and TSX indexes) during bear, normal and bull markets under different investment horizons. Furthermore, it analyzes the upside and downside short- and long-run risk spillovers between oil and stock markets by quantifying three market risk measures, namely the value at risk (VaR), conditional VaR (CoVaR) and the delta CoVaR (∆CoVaR). The results show that there is a tail dependence between oil and all stock markets for the raw return series. By considering time horizons, we show that there is an average dependence between the considered markets for the short-run horizons. However, the tail dependence is also found for the long-run horizons between the oil and stock markets, with the exception of the S&P500 index which exhibits average dependence with the oil market. Moreover, we find strong evidence of up and down risk asymmetric spillovers from oil to stock markets and vice versa in the short-and long run horizons. Finally, the market risk spillovers are asymmetric over the time and investment horizons.

A Robust Observer-Based Method for IGBTs and Current Sensors Fault Diagnosis in Voltage-Source Inverters of PMSM Drives

Abstract: Permanent magnet synchronous motors (PMSMs) drives using three-phase voltage-source inverters (VSIs) are currently used in many industrial applications. The reliability of VSIs is one of the most important factors to improve the reliability and availability levels of the drive. Accordingly, this paper presents a robust fault diagnostic method for multiple insulated gate bipolar transistors (IGBTs) open-circuit faults and current sensor faults in three-phase PMSM drives. The proposed observer-based algorithm relies on an adaptive threshold for fault diagnosis. Current sensor and open-circuit faults can be distinguished and the faulty sensors and/or power semiconductors are effectively isolated. The proposed technique is robust to machine parameters and load variations. Several simulation and experimental results using a vector-controlled PMSM drive are presented, showing the diagnostic algorithm robustness against false alarms and its effectiveness in both IGBTs and current sensors fault diagnosis.

Renewable energy consumption, International trade, oil price and economic growth inter-linkages: The case of Tunisia

Abstract: This paper contributes to the small but growing literature on the linkages between renewable energy consumption, international trade, oil price and economic growth. It aims to investigate such dynamic relationships using the bounds testing approach to cointegration and the ARDL methodology for Tunisia over the period 1980–2011. The main empirical findings reveal the presence of a bidirectional relationship between renewable energy consumption and international trade in the short-run. Indeed, an increase in oil price may imply an increase of renewable energy consumption. Furthermore, a unidirectional relationship between renewable energy and oil price is proven in the short-run.

Resource theory of coherence: Beyond states

Abstract: We generalize the recently proposed resource theory of coherence (or superposition) [T. Baumgratz et al., Phys. Rev. Lett. 113, 140401 (2014); A. Winter and D. Yang, Phys. Rev. Lett. 116, 120404 (2016)] to the setting where not just the free (``incoherent'') resources, but also the manipulated objects, are quantum operations rather than states. In particular, we discuss an information theoretic notion of the coherence capacity of a quantum channel and prove a single-letter formula for it in the case of unitaries. Then we move to the coherence cost of simulating a channel and prove achievability results for unitaries and general channels acting on a $d$-dimensional system; we show that a maximally coherent state of rank $d$ is always sufficient as a resource if incoherent operations are allowed, and one of rank ${d}^{2}$ for ``strictly incoherent'' operations. We also show lower bounds on the simulation cost of channels that allow us to conclude that there exists bound coherence in operations, i.e., maps with nonzero cost of implementing them but zero coherence capacity; this is in contrast to states, which do not exhibit bound coherence.

Antibiotic resistance in Escherichia coli in husbandry animals: the African perspective.

Abstract: In the last few years, different surveillances have been published in Africa, especially in northern countries, regarding antimicrobial resistance among husbandry animals. Information is still scarce, but the available data show a worrying picture. Although the highest resistance rates have been described against tetracycline, penicillins and sulphonamides, prevalence of plasmid-mediated quinolone resistance genes and extended spectrum β-lactamase (ESBL) are being increasingly reported. Among ESBLs, the CTX-M-1 group was dominant in most African surveys. Within this group, CTX-M-15 was the main variant both in animals and humans, except in Tunisia where CTX-M-1 was more frequently detected among Escherichia coli from poultry. Certain blaCTX-M-15-harbouring clones (ST131/B2 or ST405/D) are mainly identified in humans, but they have also been reported in livestock species from Tanzania, Nigeria or Tunisia. Moreover, several reports suggest an inter-host circulation of specific plasmids (e.g. blaCTX-M-1-carrying IncI1/ST3 in Tunisia, IncY- and Inc-untypeable replicons co-harbouring qnrS1 and blaCTX-M-15 in Tanzania and the worldwide distributed blaCTX-M-15-carrying IncF-type plasmids). International trade of poultry meat seems to have contributed to the spread of other ESBL variants, such as CTX-M-14, and clones. Furthermore, first descriptions of OXA-48- and OXA-181-producing E. coli have been recently documented in cattle from Egypt, and the emergent plasmid-mediated colistin resistance mcr-1 gene has been also identified in chickens from Algeria, Tunisia and South Africa. These data reflect the urgent need of a larger regulation in the use of veterinary drugs and the implementation of surveillance programmes in order to decelerate the advance of antimicrobial resistance in this continent.

The nexus between economic growth, financial development, trade openness, and CO2 emissions in European countries

Abstract: In this paper, we empirically investigate the causal nexus between economic growth (GDP), CO2 emissions (environmental degradation), financial development, and trade openness using the ordinary least squares technique for a yearly panel data of 40 European economies, during the period of study from 1985 to 2014. To examine this causal link, we utilize the Cobb–Douglas production function. The empirical findings point to a bidirectional Granger causal linkage among GDP and pollution, GDP and financial sector development, GDP and trade openness, financial sector development and trade openness, and trade openness and pollution in the case of European economies. From the causal link between GDP and environmental pollutants, we validate the existence/confirm the validity of the environmental Kuznets curve hypothesis. Also, we confirm/bear out the feedback suggestion of the bidirectional causality among trade openness and financial sector development. Besides, we find the neutrality hypothesis linking c...

Lipid Biomarkers in Alzheimer's Disease

Abstract: Alzheimer’s disease (AD) is the most frequent form of dementia. It is characterized by three successive stages (preclinical, mild cognitive impairment (MCI), and dementia), which are determined with a panel of complementary tests including behavioral and biochemical tests and imaging methods. Currently accepted biomarkers of AD include levels of brain chemicals related to amyloid or tau, and imaging-derived estimates of the size and metabolic activity of specific brain regions. At the moment, the pathogenesis of AD is still poorly understood but some studies support a link between lipid metabolism disorders and AD. Consequently, there has been a stronger focus on lipidomic studies to identify lipid biomarkers of AD. The strongest genetic risk factor for AD is the e4 variant of apolipoprotein E. Abnormal levels of 24S-hydroxycholesterol resulting from alterations in brain cholesterol metabolism are also observed in the cerebrospinal fluid and plasma of AD patients. Some fatty acids alterations concerning monounsaturated fatty acids, docosahexaenoic acid (DHA, C22:6 n-3), very long chain fatty acids and plasmalogens support the hypothesis of peroxisomal dysfunctions and/or of abnormal desaturase and elongase activities. It is also suggested that levels of some plasma phospholipids may be biomarkers of AD and that reductions in these levels may enable the accurate prediction that a cognitively normal individual will go on to develop MCI or AD. Several of these lipid biomarkers, especially those identified in the plasma, could constitute convenient and potent diagnostic and prognostic biomarkers of AD in routine clinical practice, and contribute to the development of new and effective treatments.

Robust Active Damping Methods for LCL Filter-Based Grid-Connected Converters

Abstract: Grid-connected converters (GcCs) employ LCL filters instead of simple L filters in order to meet new grid codes and their on-going changes in the near future. Active damping methods, without power losses, are preferred to passive ones for solving the resonance problems of LCL -filter-based GcCs ( LCL -GcCs). However, large changes in the grid inductance (typically under weak grid conditions and in rural areas) may compromise the system stability. Moreover, the delay of digital controllers will change system phase-frequency characteristics and consequently will affect the system stability. In view of this, this paper proposes a systematic design procedure for the active damping of voltage-oriented PI control for LCL- GcCs. The procedure considers active damping methods based on capacitor current feedback and is aimed to ensure stable operation under severe grid inductance variations while taking into account the influence of digital control delay and LCL filter parameters changes on the system stability. Simulation and experimental results are presented and discussed in order to validate the proposed design procedure.

A dynamic multi-objective evolutionary algorithm using a change severity-based adaptive population management strategy

Abstract: In addition to the need for simultaneously optimizing several competing objectives, many real-world problems are also dynamic in nature. These problems are called dynamic multi-objective optimization problems. Applying evolutionary algorithms to solve dynamic optimization problems has obtained great attention among many researchers. However, most of works are restricted to the single-objective case. In this work, we propose an adaptive hybrid population management strategy using memory, local search and random strategies, to effectively handle environment dynamicity for the multi-objective case where objective functions change over time. Moreover, the proposed strategy is based on a new technique that detects the change severity, according to which it adjusts the number of memory and random solutions to be used. This ensures, on the one hand, a high level of convergence and on the other hand, the required diversity. We propose a dynamic version of the Non dominated Sorting Genetic Algorithm II, within which we integrate the above-mentioned strategies. Empirical results show that our proposal based on the use of the adaptive strategy is able to handle dynamic environments and to track the Pareto front as it changes over time. Moreover, when confronted with several recently proposed dynamic algorithms, it has presented competitive and better results on most problems.

Isolation and Characterization of Thermophilic Bacteria from Jordanian Hot Springs: Bacillus licheniformis and Thermomonas hydrothermalis Isolates as Potential Producers of Thermostable Enzymes

Abstract: The aim of this study was the isolation and characterization of thermophilic bacteria from hot springs in Jordan. Ten isolates were characterized by morphological, microscopic, biochemical, molecular, and physiological characteristics. Sequencing of the 16S rDNA of the isolates followed by BLAST search revealed that nine strains could be identified as Bacillus licheniformis and one isolate as Thermomonas hydrothermalis. This is the first report on the isolation of Thermomonas species from Jordanian hot springs. The isolates showed an ability to produce some thermostable enzymes such as amylase, protease, cellulose, gelatins, and lecithin. Moreover, the UPGMA dendrogram of the enzymatic characteristics of the ten isolates was constructed; results indicated a high phenotypic diversity, which encourages future studies to explore further industrial and environmental applications.

Characterization of two polyvalent phages infecting Enterobacteriaceae.

Abstract: Bacteriophages display remarkable genetic diversity and host specificity. In this study, we explore phages infecting bacterial strains of the Enterobacteriaceae family because of their ability to infect related but distinct hosts. We isolated and characterized two novel virulent phages, SH6 and SH7, using a strain of Shigella flexneri as host bacterium. Morphological and genomic analyses revealed that phage SH6 belongs to the T1virus genus of the Siphoviridae family. Conversely, phage SH7 was classified in the T4virus genus of the Myoviridae family. Phage SH6 had a short latent period of 16 min and a burst size of 103 ± 16 PFU/infected cell while the phage SH7 latent period was 23 min with a much lower burst size of 26 ± 5 PFU/infected cell. Moreover, phage SH6 was sensitive to acidic conditions (pH < 5) while phage SH7 was stable from pH 3 to 11 for 1 hour. Of the 35 bacterial strains tested, SH6 infected its S. flexneri host strain and 8 strains of E. coli. Phage SH7 lysed additionally strains of E. coli O157:H7, Salmonella Paratyphi, and Shigella dysenteriae. The broader host ranges of these two phages as well as their microbiological properties suggest that they may be useful for controlling bacterial populations.