186,327 research outputs found
Abiotic and biotic factors in the nutrient solution and filter skin (Schmutzdecke) of slow filters integrated to closed hydroponic greenhouse systems
No full textRapid pathogen dispersal by circulating nutrient solutions in closed hydroponic systems poses a major threat to this production method. Slow filtration is very effective against important fungal pathogens such as Phytophthora cryptogea and Pythium aphanidermatum. Filter efficacy against pathogens can partly be predicted by observation of autochthonous microbial communities in slow filters and filter efficacy can be improved by manipulation of these communities. However, data about factors influencing filter efficacy are scarce. Generally, slow filter efficacy is the result of complex interactions between abiotic and biotic attributes of nutrient solution and filter body. Selected abiotic (electric conductivity, pH, oxygen content, chemical oxygen demand, dissolved organic carbon) and biotic factors (general bacterial and fungal microflora, fluorescent pseudomonads, filamentous actinomycetes, Fusariumoxysporum, P. aphanidermatum, biochemical oxygen demand, enzyme activity) were monitored in a two-year study of two distinct commercial hydroponic systems (tomatoes and ornamentals) with integrated slow filters in supernatant, filter skin and effluent. The same filter skin parameters were monitored in six small-scaleexperimental systems with slow filters to establish possible correlations with filter efficacy against F. oxysporum f.sp. cyclaminis. In half these systems, filter skins were enriched with fungal cell wall preparation to enhance biological activity of microfauna. Total polysaccharide and extracellular biofilm polysaccharide content of the filter skin were determined and a method for extracellular biofilm polymer extraction devised. The hydroponic systems studied differed in terms of chemical oxygen demand, dissolved organic carbon, oxygen consumption of slow filters by aerobic autochthonous microorganisms and fungal densities before and after slow filtration. A seasonal influence on xylanase enzyme activity in filter skins was also observed. However, as filter efficacy and other parameters in the experimental systems exhibited no significant differences, no firm conclusions could be drawn. Total polysaccharide content within filter skins increased over time and was a good parameter for demonstrating the effects of organic amendments on filter skin formation. Extracellular biofilm polysaccharide content of filter skins was not influenced by organic matter addition but more research is needed to reveal the influence of this parameter on filter efficacy
A critical review on the vulnerability assessment of natural gas pipelines subjected to seismic wave propagation. Part 1: Fragility relations and implemented seismic intensity measures
No full textNatural gas (NG) pipeline networks constitute a critical means of energy transportation, playing a vital role in the economic development of modern societies. The associated socio-economic and environmental impact, in case of seismically-induced severe damage, highlights the importance of a rational assessment of the structural integrity of this infrastructure against seismic hazards. Up to date, this assessment is mainly performed by implementing empirical fragility relations, which associate the repair rate, i.e. the number of repairs/damages per unit length of the pipeline, with a seismic intensity measure. A limited number of analytical fragility curves that compute probabilities of failure for various levels of predefined damage states have also been proposed, recently. In the first part of this paper, a thorough critical review of available fragility relations for the vulnerability assessment of buried NG pipelines is presented. The paper focuses on the assessment against seismically-induced transient ground deformations, which, under certain circumstances, may induce non-negligible deformations and strains on buried NG pipelines, especially in cases of pipelines crossing heterogeneous soil sites. Particular emphasis is placed on the efficiency of implemented seismic intensity measures to be evaluated or measured in the field and, more importantly, to correlate with observed structural damage on buried NG pipelines. In the second part of this paper, alternative methods for the analytical evaluation of the fragility of steel NG pipelines under seismically-induced transient ground deformations are presented. Through the discussion, recent advancements in the field are highlighted, whilst acknowledged gaps are identified, providing recommendations for future research
Optimal intensity measures for the structural assessment of buried steel natural gas pipelines due to seismically-induced axial compression at geotechnical discontinuities
No full textThis paper investigates the efficiency and sufficiency of various seismic intensity measures for the structural assessment of buried steel natural gas (NG) pipelines subjected to axial compression caused by transient seismic ground deformations. The study focuses on buried NG pipelines crossing perpendicularly a vertical geotechnical discontinuity with an abrupt change on the soil properties, where the potential of high compression strain is expected to be increased under seismic wave propagation. A detailed analytical framework is developed for this purpose, which includes a 3D finite element model of the pipe-trench system, to evaluate rigorously the pipe-soil interaction phenomena, and 1D soil response analyses that are employed to determine critical ground deformation patterns at the geotechnical discontinuity, caused by seismic wave propagation. A comprehensive numerical parametric study is conducted by employing the analytical methodology in a number of soil-pipeline configurations, considering salient parameters that control the axial response of buried steel NG pipelines, i.e. diameter, wall thickness and internal pressure of the pipeline, wall imperfections of the pipeline, soil properties and backfill compaction level and friction characteristics of the backfill-pipe interface. Using the peak compression strain of the pipeline as engineering demand parameter and a number of regression analyses relative to the examined seismic intensity measures, it is shown that the peak ground velocity PGV at ground surface constitutes the optimum intensity measure for the structural assessment of the examined infrastructure
Seismic fragility of buried steel natural gas pipelines due to axial compression at geotechnical discontinuities
No full textThis paper presents an extended set of numerical fragility functions for the structural assessment of buried steel natural gas (NG) pipelines subjected to axial compression caused by transient seismic ground deformations. The study focuses on NG pipelines crossing sites with a vertical geotechnical discontinuity, where high compression straining of a buried pipeline is expected to occur under seismic transient ground deformations. A de-coupled numerical framework is developed for this purpose, which includes a 3D finite element model of the pipe–trench system employed to evaluate rigorously the soil–pipe interaction effects on the pipeline axial response in a quasi-static manner. One-dimensional soil response analyses are used to determine critical ground deformation patterns at the vicinity of the geotechnical discontinuity, caused by the ground shaking. A comprehensive parametric analysis is performed by implementing the proposed analytical framework for an ensemble of 40 recorded earthquake ground motions. Crucial parameters that affect the seismic response and therefore the seismic vulnerability of buried steel NG pipelines namely, the diameter, wall thickness, burial depth and internal pressure of the pipeline, the backfill compaction level, the pipe–soil interface friction characteristics, the soil deposits characteristics, as well as initial geometric imperfections of the walls of the pipeline, are systematically considered. The analytical fragility functions are developed in terms of peak ground velocity at the ground surface, for four performance limit states, considering all the associated uncertainties. The study contributes towards a reliable quantitative risk assessment of buried steel NG pipelines, crossing similar sites, subjected to seismically-induced transient ground deformations
A critical review on the vulnerability assessment of natural gas pipelines subjected to seismic wave propagation. Part 2: Pipe analysis aspects
No full textThe socio-economic and environmental impact, in case of severe damage on Natural Gas (NG) pipeline networks, highlights the importance of a rational assessment of the structural integrity of this infrastructure against seismic hazards. Up to date, this assessment is mainly performed by employing empirical fragility relations, while a limited number of analytical fragility curves have also been proposed recently. The critical review of available fragility relations for the assessment of buried pipelines under seismically-induced transient ground deformations, presented in the first part of this paper, highlighted the need for further investigation of the seismic vulnerability of NG pipeline networks, by employing analytical methodologies, capable of simulating effectively distinct damage modes of this infrastructure. In this part of the paper, alternative methods for the analytical evaluation of the seismic vulnerability of buried steel NG pipelines are presented. The discussion focuses on methods that may appropriately simulate buckling failures of buried steel NG pipelines since these constitute critical damage modes for the structural integrity of this infrastructure, when subjected to seismically-induced transient ground deformations. Salient parameters that control the seismic response and vulnerability of buried pressurized steel pipelines and therefore should be considered by the relevant analytical methods, such as the operational pressure of the pipeline, the geometric imperfections of the pipeline walls, the trench backfill properties, the site characteristics and the spatial variability of the seismic ground motion along the pipeline axis, are thoroughly discussed. Finally, a new approach for the assessment of buried steel NG pipelines against seismically-induced buckling failures is introduced. Through the discussion, recent advancements in the field are highlighted, whilst acknowledged gaps are identified, providing recommendations for future research
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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