2,044 research outputs found
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
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
Analyzing X-ray variability by State Space Models
In recent years, autoregressive models have had a profound impact on the
description of astronomical time series as the observation of a stochastic
process. These methods have advantages compared with common Fourier techniques
concerning their inherent stationarity and physical background. If
autoregressive models are used, however, it has to be taken into account that
real data always contain observational noise often obscuring the intrinsic time
series of the object. We apply the technique of a Linear State Space Model
which explicitly models the noise of astronomical data and allows to estimate
the hidden autoregressive process. As an example, we have analysed a sample of
Active Galactic Nuclei (AGN) observed with EXOSAT and found evidence for a
relationship between the relaxation timescale and the spectral hardness.Comment: 4 pages, Latex, uses Kluwer Style file crckapb.cls To appear in Proc.
of Astronomical Time Series, Tel Aviv, 199
Abiotic and biotic factors affecting Rhagoletis mendax [Diptera :Tephritidae] populations in eastern Canadian lowbush blueberry fields
Une étude a été effectuée en Nouvelle-Ecosse et au Nouveau-Brunswick au Canada, afin d'identifier des facteurs naturels de régulation, tels que l'acidité du sol et la présence de mauvaises herbes, qui pourraient influencer l'occurrence de populations de la mouche de l'airelle (Rhagoletis mendax) dans les bleuetières naines commerciales (Vaccinium angustifolium). Les niveaux observés d'acidité du sol n'ont pas été identifiés comme un facteur régissant la distribution et la mortalité des larves et des pupes du R. mendax. Les mouvements migratoires des adultes ont été affectés par la pratique culturale du brûlage des champs de bleuets aux deux ans et il en résulte que les adultes, émergeant de la portion brûlée des champs, migrent vers les zones en production. Une corrélation positive entre l'intensité de la pousse des mauvaises herbes indigènes et les niveaux d'infestation de la mouche de l'airelle a été démontrée.An investigation was carried out in Nova Scotia and New Brunswick, Canada, to identify natural regulatory factors, such as soil acidity and weed incidence, that could influence the occurrence of blueberry maggot (Rhagoletis mendax) populations in commercial lowbush blueberry fields (Vaccinium angustifolium). Over the range observed, the soil acidity had no detectable influence on the distribution and mortality of R. mendax larvae and pupae. Adult migrational patterns were affected by the cultural practice of burning on alternate years causing the emerging adults of burned fields to move toward sareasin production. There was also a positive correlation between the growth intensity of indigenous weeds and blueberry maggot infestation levels
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
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