1,721,159 research outputs found
Blast wave attenuation using liquid sheets
No full textStrong blast waves, such as those associated with gun fire or rocket exhausts, can cause serious physiological and/or structural damage. It is necessary, therefore, to develop ways in which to minimise this damage whilst still allowing the system that produces the blast waves to function normally. In an effort to develop such a system this study has examined, both theoretically and experimentally, the interaction that occurs when a shock wave is passed along a free unconstrained liquid tube before emerging into the surrounding atmosphere. In the theoretical analysis the problem was confined to the two-dimensional case and involved dividing the liquid sheet up into infinitesimal sections that were then regarded as small piston-cylinder systems, which were driven by the high pressure shocked gas behind the shock wave. The results from these one-dimensional systems were then used as input into a two-dimensional solution of the wave equation which predicted the gross changes in the remainder of the space. The experimental investigation involved laboratory experiments that examined, visually and with pressure transducers, the result of the shock/liquid interactions in two-dimensional and axi-symmetric cases, both between and external to the liquid sheets. The experimental investigation also included field trials that examined the pressure profiles of blast waves that were produced when a shock wave, which resulted from the ignition of a rocket motor, was passed along a liquid tube. From this work it was found that the high pressure gas, behind the shock wave, caused the liquid sheets to move perpendicularly from the line of travel of the shock wave which in turn caused expansion waves and compression waves to propagate out from the face of the water sheets, into the shocked gas and the surrounding atmosphere, respectively. These compression waves were then found to interact with the weak blast (produced when the shock, which had been weakened by the expansion waves, emerged into the atmosphere) in such a way that they produced a weaker blast field than would have been the case had the shock wave emerged directly into the atmosphere; the maximum observed reduction in the strength of the blast wave was 16.4 dB. Experiments were also performed that examined the effect of using rigid sheets in place of liquid sheets. From these experiments it was found that the differences between the liquid and rigid sheet cases was a function of the size of the inertial barrier (i.e. the mass of the water sheet) that the water presented to the shocked gas. Consequently, it was noted that, in terms of attenuating the blast wave, the rigid sheets proved to be inferior to the thicker water sheets and superior to the thinner water sheets. However, when the spectra of the pressure disturbances were examined it was found that, with regard to the attenuation of the 2-4 kHz region of the spectra, all the liquid sheet results showed an improvement in relation to the rigid sheet results
Biological Cost of Pyocin Production during the SOS Response in Pseudomonas aeruginosa
Full text linkLexA and two structurally related regulators, PrtR and PA0906, coordinate the Pseudomonas aeruginosa SOS response. RecA-mediated autocleavage of LexA induces the expression of a protective set of genes that increase DNA damage repair and tolerance. In contrast, RecA-mediated autocleavage of PrtR induces antimicrobial pyocin production and a program that lyses cells to release the newly synthesized pyocin. Recently, PrtR-regulated genes were shown to sensitize P. aeruginosa to quinolones, antibiotics that elicit a strong SOS response. Here, we investigated the mechanisms by which PrtR-regulated genes determine antimicrobial resistance and genotoxic stress survival. We found that induction of PrtR-regulated genes lowers resistance to clinically important antibiotics and impairs the survival of bacteria exposed to one of several genotoxic agents. Two distinct mechanisms mediated these effects. Cell lysis genes that are induced following PrtR autocleavage reduced resistance to bactericidal levels of ciprofloxacin, and production of extracellular R2 pyocin was lethal to cells that initially survived UV light treatment. Although typically resistant to R2 pyocin, P. aeruginosa becomes transiently sensitive to R2 pyocin following UV light treatment, likely because of the strong downregulation of lipopolysaccharide synthesis genes that are required for resistance to R2 pyocin. Our results demonstrate that pyocin production during the P. aeruginosa SOS response carries both expected and unexpected costs.National Institutes of Health (U.S.) (GM31010)National Institutes of Health (U.S.) (National Research Service Award
SnapShot: DNA Polymerases II Mammals
No full textDNA polymerases ensure the faithful duplication of genetic information inside the nuclease and mitochondria of eukaryotic cells and the nucleoid of prokaryotic cells. These remarkable enzymes synthesize polynucleotide chains based on the complementarity of an incoming nucleotide to an existing DNA template. DNA polymerases are grouped into six families (A, B, C, D, X, and Y). The previous SnapShot (DNA Polymerases I Prokaryotes) described the structural and functional characteristics conserved across the families, using the DNA polymerases from the bacterium Escherichia coli as an example. In this SnapShot, we now highlight DNA polymerases from humans (Homo sapiens) and their relationship to human diseases. However, this list is certainly not exhaustive, and the number of putative links between DNA polymerases and diseases continues to grow
SnapShot: DNA Polymerases I Prokaryotes
No full textThe nucleus and mitochondria of eukaryote cells and the nucleoid of prokaryote cells contain remarkable enzymes, called DNA polymerases, which ensure the faithful duplication of genetic material. These enzymatic machines incorporate the building blocks of DNA, deoxyribonucleotide triphosphates (dNTPs), into growing polynucleotide chains. The error rate of these enzymes is astonishingly low with only ~1 error for every 10 [superscript 9]–10[superscript 10] bases replicated. The first safeguard contributing to this low error rate is the ability of the DNA polymerase to discriminate among incoming dNTPs based on their complementarity to a parental DNA template. However, in the event of misincorporation, many DNA polymerases also have associated “proof-reading” activities that remove an inappropriately added dNTP, thus providing a second safeguard to protect the integrity of the genome. Moreover, cells use a variety of DNA polymerases, called translesion DNA polymerases, whose sole function is to enable recovery from specific genetic insults by endogenous and exogenous mutagens
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Full text linkWe 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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