1,720,973 research outputs found
Investigation into noise emitted by bluff bodies with large roughness
No full textA set of wind tunnel experiments were performed to study the effect of large surface roughness on circular cylinder noise, with the goal of improving landing gear noise predictions. Roughness increases vortex shedding noise levels, and shifts the peak to a lower Strouhal number. The noise levels in the fall-off range also increase, but no significant change in the fall-off rate is observed. The decrease of the vortex shedding peak frequency has been associated with early detachment caused by the effect of roughness on the TBLs, which is in agreement with previous experimental studies with smaller roughness. The high frequency range of the spectrum revealed a broadband, Strouhal-based peak, which is caused by roughness noise generated on the upstream face of the cylinder. The peak Strouhal number is well predicted by Howe's model using the maximum outer velocity around the cylinder. Cylindrical roughness presents a weaker roughness noise peak, but higher noise levels for higher frequencies, and is thought to be caused by sharp edge separation. A bluff body roughness noise model has been developed based on the model of Howe and a Green's function tailored to the bluff body geometry, calculated using the Boundary Element Method. The application to rough circular cylinders using a at wall (ZPG) TBL model shows good agreement with experiments for downstream observers, but the model overpredicts the levels in over-head observers. The disagreement is thought to be due to inaccuracy of the at wall TBL model. The transition from smooth regime to rough regime was studied experimentally by partially covering the cylinder with distributed roughness in spanwise uniform configurations. Transition regarding vortex shedding happens mainly when roughness is added or removed around the separation region. The results agree with the fact that roughness changes the separation location by perturbing the TBL close to separation. Sparse and dense two-dimensional roughness on a circular cylinder, studied using CFD, have similar effects than distributed roughness regarding the vortex shedding peak level and frequency
Experimental study of noise emitted by circular cylinders with large roughness
No full textThe aerodynamic noise generated by high Reynolds number flow around a bluff body with large surface roughness was investigated. This is a relevant problem in many applications, in particular aircraft landing gear noise. A circular cylinder in cross-flow and a zero-pressure-gradient turbulent boundary layer with various types of roughness was tested in a series of wind tunnel experiments. It has been shown that distributed roughness covering a circular cylinder affects the spectra over the entire frequency range. Roughness noise is dominant at high frequencies, and the peak frequency is well described by Howe's roughness noise model when scaled with the maximum outer velocity. There are differences between hemispherical and cylindrical roughness elements for both the circular cylinder and the zero-pressure-gradient turbulent boundary layer cases, indicating a dependence on roughness shape, not described by the considered roughness noise models. Cylindrical roughness generates higher noise levels at the highest frequencies, especially for the zero-pressure-gradient turbulent boundary layer case. Cable-type roughness aligned with the mean flow does not generate roughness noise, and its spectrum has been found to collapse with the smooth cylinder at medium and high frequencies. At low and medium frequencies the noise spectra have the same features as the smooth cylinder, but with higher shedding peak levels and fall-off levels, despite the decrease in spanwise correlation length. Roughness induces early separation, and thus a shift of the spectra to lower frequencies
Experimental Study of Noise Emitted by Circular Cylinders with Large Roughness
No full textA set of experiments of rough circular cylinder flow as well as rough wall turbulent boundary layer (TBL) flow were performed with the goal of studying the effect of large roughness on the aerodynamic noise emitted by single bluff bodies. Roughness consisted of uniformly distributed hemispherical and cylindrical elements with normalised heights of 0.031 and 0.035, respectively, as well as cable helicoidally wrapped with normalised heights of 0.031 and 0.047. The Reynolds numbers in the circular cylinder tests ranged between 1.6×105 and 3.2×105. A broadband spectral peak has been identified mainly in the hemispherical roughness configurations, centred at frequencies well predicted by Howe's roughness noise prediction model that was extended to the case of a circular cylinder. The peak level was overpredicted and for higher frequencies the difference increased. The helicoidal cable configurations didn't generate significant roughness noise and, furthermore, its far field noise spectra have been found to collapse with the smooth cylinder spectra for Strouhal numbers greater than 0.7. The observed effect of roughness at lower frequencies was to increase the vortex shedding peak level and to reduce the peak Strouhal number, which is in accordance with the observations of previous studies
Extension of roughness noise to bluff bodies using the boundary element method
No full textA prediction model of roughness noise generated by bluff body flow at high Reynolds numbers is proposed. Howe's roughness noise theory extended by Liu and Dowling is used, and the boundary layer inputs to the theory have been modified for a bluff body. The scattering due to the bluff body has been accounted for by the boundary element method. The procedure to couple the roughness noise sources to the tailored Green's function is detailed for the case where the boundary element method mesh is orthogonal and aligned with the boundary layer outer velocity. The proposed method has been implemented and compared to experimental results for the particular case of a circular cylinder with large roughness. Two different estimations of the skin friction, which is an input to the roughness noise theory, are considered. One is a zero-pressure gradient model, and the second is based on published experimental data of the skin friction on a rough circular cylinder, but with smaller roughness than was used in the experiments. The zero-pressure gradient skin friction estimate leads to a better prediction of the effect of changes in the area covered by roughness elements. The success of the zero-pressure gradient skin friction estimate is encouraging as the only modifications that need to be made to the boundary layer model to account for a bluff body are the boundary layer outer velocity distribution and the location of separation
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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