Investigations of eddy coherence in jet flows

In turbulent shear flow the term coherent structures refers to eddies which are both spatially coherent, i.e., large eddies, aand also temporally coherent, i.e., they retain their identities for times which are long compared with their time scales in fixed point measurements. In transitional flows, the existence of such structures is evident from flow visualizations. In many other flows, such structures are not so evident. The reasons for the existence of these two classes of flows are discussed and attention is focused upon the more difficult flows, where coherent structures are not so evident. Techniques by which the existence (or nonexistence) of such structures in these flows can be established from point measurements, are also discussed. A major problem is shown to be the need to discriminate between real losses in eddy coherence and apparent losses in coherence introduced by phase scrambling effects which 'smear' multipoint correlations. The analysis of multiprobe time dependent data in cold and reacting round turbulent jets is described and it is shown how evidence of strong eddy coherence can be extracted from data

Vaporization of droplets in premixing chambers

Detailed measurements were made of the structures of turbulent fuel sprays vaporizing in heated airstreams. The measurements show the size dependent vaporization and dispersion of the droplets and the important influence of the large eddies in the turbulence. The measurements form a data base for the development of models of fuel spray vaporization. Two laser techniques were specially developed for the investigation. A laser tomography technique converts line-of-sight light scattering measurements into time averaged 'point' measurements of droplet size distribution and volume concentration. A laser anemometer particle sizing technique was further developed to permit accurate measurements of individual particle sizes and velocities, with backscatter collection of light. The experiments are combined with heat transfer models to analyze the performance of miniature thermocouples in liquid sprays

Statistics of Lead Changes in Popularity-Driven Systems

We study statistical properties of the highest degree, or most popular, nodes in growing networks. We show that the number of lead changes increases logarithmically with network size N, independent of the details of the growth mechanism. The probability that the first node retains the lead approaches a finite constant for popularity-driven growth, and decays as N^{-phi}(ln N)^{-1/2}, with phi=0.08607..., for growth with no popularity bias.Comment: 4 pages, 4 figures, 2 column revtex format. Minor changes in response to referee comments. For publication in PR

Detailed investigation of a vaporising fuel spray. Part 1: Experimental investigation of time averaged spray

A laser tomographic light scattering technique provides rapid and accurate high resolution measurements of droplet sizes, concentrations, and vaporization. Measurements using a computer interfaced thermocouple are presented and it is found that the potential exists for separating gas and liquid temperature measurements and diagnosing local spray density by in situ analysis of the response characteristics of the thermocouple. The thermocouple technique provides a convenient means for measuring mean gas velocity in both hot and cold two phase flows. The experimental spray is axisymmetric and has carefully controlled initial and boundary conditions. The flow is designed to give relatively insignificant transfer of momentum and mass from spray to air flow. The effects of (1) size-dependent droplet dispersion by the turbulence, (2) the initial spatial segregation of droplet sizes during atomization, and (3) the interaction between droplets and coherent large eddies are diagnosed

Do South-Eastern Arabia's Earliest Extant Copper-Alloy Arrowheads date to the Wadi Suq Period?

Die Untersuchung der prähistorischen metallischen Artefakte Südostarabiens entwickelt sich schnell und fast konvulsiv. Es leidet jedoch an einem Mangel an Forschungs- und Veröffentlichungsstandards und einer Diskussionskultur. Nach heutigem Kenntnisstand argumentieren die Autoren, die frühesten erhaltenen metallischen Pfeilspitzen Südost-Arabiens der Wadi Suq-Zeit zuzuordnen. Die zahlreichen Pfeilspitzen sind ein wichtiger Bestandteil der prähistorischen Fundinventar Südostarabiens, die für die arabische Chronologie insgesamt von wesentlicher Bedeutung ist. Dennoch sind die Kontexte des 3. und 2. Jahrtausends unerklärlicherweise frei von metallischen Pfeilspitzen und daher chronologisch verzerrt. Die südostarabische Archäologie muss die Klassifizierung von Artefakten zum Zwecke der Datierung stärker nutzen. Pfeilspitzen, die früher auf die Mitte des 2. Jahrtausends datiert waren, sind jetzt eindeutig teilweise in der frühen Eisenzeit datiert.On current knowledge the authors argue to assign the earliest extant metallic arrowheads of south-eastern Arabia to the Wadi Suq period. Arrowheads are an important component of the south-eastern Arabia's prehistoric find assemblage, integral for chronology on the whole. Nonetheless, 3rd and 2nd millennium contexts are depleted of metallic arrowheads and are chronologically skewed. South-eastern Arabian archaeology must make greater use of artefact classification and typology for the purposes of dating not only to arrowheads, but also in general

The investigation of time dependent flame structure by ionization probes

Ionization probes were used to measure mean ionization current and frequency spectra, auto-correlations and cross-correlations in jet flames with variation in the initial Reynolds numbers and equivalence ratios. Special attention was paid to the transitional region between the burner exit plane and the plane of onset of turbulence

Nonuniversal power law scaling in the probability distribution of scientific citations

We develop a model for the distribution of scientific citations. The model involves a dual mechanism: in the direct mechanism, the author of a new paper finds an old paper A and cites it. In the indirect mechanism, the author of a new paper finds an old paper A only via the reference list of a newer intermediary paper B, which has previously cited A. By comparison to citation databases, we find that papers having few citations are cited mainly by the direct mechanism. Papers already having many citations ('classics') are cited mainly by the indirect mechanism. The indirect mechanism gives a power-law tail. The 'tipping point' at which a paper becomes a classic is about 21 citations for papers published in the Institute for Scientific Information (ISI) Web of Science database in 1981, 29 for Physical Review D papers published from 1975-1994, and 39 for all publications from a list of high h-index chemists assembled in 2007. The power-law exponent is not universal. Individuals who are highly cited have a systematically smaller exponent than individuals who are less cited.Comment: 7 pages, 3 figures, 2 table

Protein kinase A regulation of P2X4 receptors: Requirement for a specific motif in the C-terminus

AbstractThe P2X purinergic receptor sub-family of ligand-gated ion channels are subject to protein kinase modulation. We have previously demonstrated that P2X4R signaling can be positively regulated by increasing intracellular cAMP levels. The molecular mechanism underlying this effect was, however, unknown. The present study initially addressed whether protein kinase A (PKA) activation was required. Subsequently a mutational approach was utilized to determine which region of the receptor was required for this potentiation. In both DT-40 3KO and HEK-293 cells transiently expressing P2X4R, forskolin treatment enhanced ATP-mediated signaling. Specific PKA inhibitors prevented the forskolin-induced enhancement of ATP-mediated inward currents in P2X4R expressing HEK-293 cells. To define which region of the P2X4R was required for the potentiation, mutations were generated in the cytoplasmic C-terminal tail. It was determined that a limited region of the C-terminus, consisting of a non-canonical tyrosine based sorting motif, was required for the effects of PKA. Of note, this region does not harbor any recognizable PKA phosphorylation motifs, and no direct phosphorylation of P2X4R was detected, suggesting that PKA phosphorylation of an accessory protein interacts with the endocytosis motif in the C-terminus of the P2X4R. In support of this notion, using Total Internal Reflection Fluorescence Microscopy (TIRF)\ P2X4-EGFP was shown to accumulate at/near the plasma membrane following forskolin treatment. In addition, disrupting the endocytosis machinery using a dominant-negative dynamin construct also prevented the PKA-mediated enhancement of ATP-stimulated Ca2+ signals. Our results are consistent with a novel mechanism of P2XR regulation, whereby PKA activity, without directly phosphorylating P2X4R, markedly enhances ATP-stimulated P2X4R currents and hence cytosolic Ca2+ signals. This may occur at least in part, by altering the trafficking of a population of P2X4R present at the plasma membrane