1,720,965 research outputs found
Neutral stability of the flow in a toroidal pipe
No full textThis work is concerned with the numerical investigation of the linear stability properties of the viscous, incompressible flow inside a toroidal pipe. A Hopf bifurcation is found and tracked in phase space, showing that the flow is modally unstable even at extremely low curvatures. The bifurcation and the eigenfunctions associated with it are analysed as a function of the two parameters governing the flow, i.e. the Reynolds number, Re, and the curvature, δ. For all curvatures, the critical Reynolds number is found to be about 3000
The influence of temperature fluctuations on hot-wire measurements in wall-bounded turbulence
No full textThere are no measurement techniques for turbulent flows capable of reaching the versatility of hot-wire probes and their frequency response. Nevertheless, the issue of their spatial resolution is still a matter of debate when it comes to high Reynolds number near-wall turbulence. Another, so far unattended, issue is the effect of temperature fluctuations - as they are, e.g. encountered in non-isothermal flows - on the low and higher-order moments in wall-bounded turbulent flows obtained through hot-wire anemometry. The present investigation is dedicated to document, understand, and ultimately correct these effects. For this purpose, the response of a hot-wire is simulated through the use of velocity and temperature data from a turbulent channel flow generated by means of direct numerical simulations. Results show that ignoring the effect of temperature fluctuations, caused by temperature gradients along the wall-normal direction, introduces - despite a local mean temperature compensation of the velocity reading - significant errors. The results serve as a note of caution for hot-wire measurements in wall-bounded turbulence, and also where temperature gradients are more prevalent, such as heat transfer measurements or high Mach number flows. A simple correction scheme involving only mean temperature quantities (besides the streamwise velocity information) is finally proposed that leads to a substantial bias error reduction. © 2014 Springer-Verlag Berlin Heidelberg
Generalized diagnostic scaling for high-order moments in turbulent boundary layers
No full textThe present work builds upon the diagnostic plot for the streamwise turbulence intensity [Alfredsson & Örlü, 2010] and generalises it for higher-order (even and odd) moments providing a general description of the probability density distribution of streamwise velocity fluctuations. Turbulent boundary layers (up to a friction Reynolds number of 20'000) are employed and demonstrate its feasibility to scale data throughout the overlap and outer region
Appraisal of cavity hot-wire probes for wall-shear-stress measurements
Full text linkFlush-mounted cavity hot-wire probes have emerged as an alternative to classical hot-wire probes mounted several diameters above the surface for wall-shear stress measurements. They aim at increasing the frequency response and accuracy by circumventing the well-known issue of heat transfer to the substrate that hot-wire and hot-film probes possess. Their use, however, depends on the assumption that the cavity does not influence the flow field. In this study, we show that this assumption does not hold, and that turbulence statistics are modified by the presence of the cavity with sizes that are practically in use. The mean velocity and fluctuations increase near the cavity while the shear stress decreases in its surroundings, all seemingly stemming from the fact that the no-slip condition is not present anymore and that flow reversal occurs. Overall, the energy spectra and the probability density function of the wall shear stress fluctuations indicate a change of nature of turbulence by the presence of the cavity
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
Experimental evaluation of the mean momentum and kinetic energy balance equations in turbulent pipe flows at high Reynolds number
No full textIn light of recent data from hot-wire anemometry and laser Doppler velocimetry, this article explores experimentally the momentum balance and kinetic energy production in fully developed turbulent pipe flow for shear Reynolds numbers in the range from two pipe facilities. It has become common practice to indirectly deduce the Reynolds shear stress via the mean flow data and the mean-momentum balance whenever the simultaneous measurements of the streamwise and wall-normal velocity fluctuations can not be performed precisely. The current assessment underlines, however, the importance of measuring the Reynolds shear stress directly, and the friction velocity independently from the mean-velocity profile to ascertain the quality of the data when utilising the momentum balance. The present analysis also reinforces the universality of the viscous stress gradient to the Reynolds shear stress gradient in the wall vicinity up to the inner limit of the logarithmic layer. The new set of the experimental data shows that Panton's stress function reproduces the measured Reynolds shear stress and kinetic energy production in turbulent pipe flows over a wide Reynolds number range to a high degree
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