1,720,963 research outputs found
EFFECT OF THE SEPARATION WALL IN THE MIXING PROPERTIES OF COAXIAL JETS
No full textThe velocity and mixing field of two coaxial jet configurations has been experimentally characterized by means of hot- and cold-wire anemometry to investigate the effect of the initial conditions on the flow development, and to determine the leading processes in the ``mixing transition'' of the two streams.
Amongst the possible operating conditions, four configurations with different inner and outer jet bulk velocity pairs have been spatially characterized in terms of velocity and mixing field covering a region where with two different separation wall geometries, namely a sharp and a thick separating wall. For high velocity ratios the difference between the two geometries appears small and inside the experimental accuracy. On the other hand, for nearly unitary velocity ratio and with the thick wall configuration, the presence of a strong wake instability increases the interpenetration between the two streams enhancing the mixing process
A method to correct third and fourth order moments in turbulent flows
No full textIt is well known that spatial averaging, resulting from the finite size of a hot-wire probe, significantly affects the accuracy of such measurements in turbulent flows close to the wall. Here, a theoretical model which describes the effect of the spatial filtering of hot-wire probes on the third and fourth order moments of the streamwise velocity is presented. The model, which is based on the three (four) point velocity correlation function for the third (fourth) order moment, shows that the filtering can be related to a characteristic length scale which is an equivalent of the Taylor transverse micro-scale for the second order moment. The capacity of the model to accurately describe the attenuation is validated against direct numerical simulation (DNS) data of a zero pressure-gradient turbulent boundary layer. The DNS data allow the filtering effect to be appraised for different wire lengths and for the different moments. A procedure, based on the developed model, to correct the measured moments in turbulent flows is finally presented. The method is applied by combining the response of two single hot-wire sensors with different wire lengths. The technique has also been validated against spatially averaged DNS data showing a good capacity to reconstruct the actual profiles over the entire height of the boundary layer except, for the third order moment, in the region where the latter is close to zero
Challenges in Hot Wire Measurements in Wall-Bounded Turbulent Flows
No full textDespite the rapid development of optical velocimetry methods (like LDV, PIV, etc.) the hot-wire anemometer remains the main instrument used in wind tunnel studies of turbulence. To obtain precise results close to walls in turbulent boundary layers, requires the user to have accurate procedures for a good calibration at low velocities, knowledge of effects of blockage and heat conduction to the wall, and how spatial resolution influences the results. We have carried out measurements in three different wind tunnels (at KTH, Univ. Melbourne and IIT) with various hot-wire probes (stubbed and stubless, as well as straight and boundary layer type) operated with commercially available and home-made anemometer systems. The use of different facilities enabled measurements at similar Reynolds numbers, but with different free stream velocities, resulting in a wide range of viscous scales for the hot-wire sensor lengths. The results indicate that poor spatial resolution influences the measured fluctuating velocity distribution well into the overlap region and clarifies controversial aspects regarding the scaling of the near-wall peak and the apparent existence of an outer peak in the rms distribution. The mean velocity within the buffer region has been found to be affected by probe geometry and size, an influence that is especially important when correcting for the absolute wall position by means of common correction methods
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
Correcting hot-wire spatial resolution effects in third- and fourth-order velocity moments in wall-bounded turbulence
No full textSpatial averaging, resulting from the finite size of a hot-wire probe, significantly affects the accuracy of velocity measurements in turbulent flows close to walls. Here, we extend the theoretical model, introduced in Segalini et al. (Meas Sci Technol 22:104508, 2011) quantifying the effect of a linear spatial filter of hot-wire probes on the mean and the variance of the streamwise velocity in turbulent wall-bounded flows, to describe the effect of the spatial filtering on the third- and fourth-order moments of the same velocity component. The model, based on the three-(four) point velocity-correlation function for the third-(fourth-) order moment, shows that the filtering can be related to a characteristic length scale which is an equivalent of the Taylor transverse microscale for the second-order moment. The capacity of the model to accurately describe the attenuation is validated against direct numerical simulation (DNS) data of a zero pressure-gradient turbulent boundary layer. The DNS data allow the filtering effect to be appraised for different wire lengths and for the different moments. The model shows good accuracy except for the third-order moment in the region where a zero-crossing of the third-order function is observed and where the equations become ill-conditioned. An “a posteriori” correction procedure, based on the developed model, to correct the measured third- and fourth-order velocity moments is also presented. This procedure, based on combining the measured data by two single hot-wire sensors with different wire lengths, is a natural extension of the one introduced by Segalini et al. (Exp Fluids 51:693–700, 2011) to evaluate both the turbulence intensity and the transverse Taylor microscale in turbulent flows. The technique is validated against spatially averaged simulation data showing a good capacity to correct the actual profiles over the entire height of the boundary layer except, as expected, for the third-order moment in the region where the latter exhibits a zero-crossing. Moreover, the proposed method has been tested on experimental data from turbulent pipe flow experiments
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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