1,720,965 research outputs found
Laboratory Experiments on Reflected Gravity Currents and Implications for Mixing
Full text linkWhen a gravity current encounters a barrier, it is reflected as a moving hydraulic jump or bore. These reflected flows, which play a significant role in estuarine mixing and sediment transport, are often simplified in theoretical models as purely advective processes with no mixing and dilution effects. This study explores the dynamics of gravity currents fully blocked by various inclined barriers, focusing on the resulting mixing behavior. Using an image analysis technique based on light attenuation to capture instantaneous density fields, we reveal how the presence of a barrier influences the current even before impact. By applying the Thorpe scale to assess turbulent mixing, we show that a barrier's geometry significantly affects mixing intensity. Notably, this study finds that barriers can increase the local turbulent mixing compared to horizontal surfaces
Non-intrusive density measurements in gravity currents interacting with an obstacle
No full textGravity currents are flows produced by a density difference between two fluids. Non-intrusive density measurements, based on image analysis, are employed to measure the instantaneous width averaged density fields of lock-release gravity currents. The developed technique uses dye concentration as a tracer. For each pixel of the acquired images, a calibration procedure is applied to relate the concentration of dye, uniformly distributed in the experimental tank, and the greyscale. Two calibration techniques with nine-image and two-image calibration curves, respectively, are employed to evaluate the instantaneous density fields of a gravity current with and without a bottom obstacle. The two different calibration curves affect the measurement results, i.e. instantaneous density fields, front propagation, current area and mixing. The image analysis using a nine-image calibration curve is found to give better results compared to the technique with a two-image calibration curve. The analysis performed shows how the image analysis based on a nine-image calibration curve is a more suitable technique for the study of the gravity currents dynamics and the associated mixing, which are strongly affected by the presence of a bottom obstacle
Gravity currents interacting with a bottom triangular obstacle and implications on entrainment
No full textLaboratory experiments investigating the interaction of a lock-exchange dense gravity current and a submerged obstacle with a triangular section have been conducted to determine the influence of the relative obstacle height, i.e. the ratio between the heights of the current and the obstacle, on the dynamics of the overflowing dense current. Experimental measurements were aimed to obtain detailed instantaneous density fields through an image analysis technique based on light attenuation. The dense current is deeply affected by the interaction, depending on the relative obstacle height. When the height of the current approaching the obstacle is small, a lower percentage of overflow propagates downstream and a large reduction in the gravity current velocity is observed. The analysis of the density fields highlights that the formation of a macro vortex downstream of the obstacle is the mechanism responsible for the enhanced dilution of the overflow. Nevertheless, the bulk entrainment parameter is slightly affected by the presence of a bottom obstacle. Results deepen our understanding of the entrainment processes related to the interaction with the obstacle and confirm the role of relevant non-dimensional numbers
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
PIV Investigation on Lock-Exchange Gravity Currents Propagating Over Roughness Elements
No full textLock-exchange gravity currents over a rough bottom are investigated through laboratory experiments; PIV technique is used to measure the instantaneous velocity field. The bottom roughness is realized by LEGO® bricks with a constant rectangular section. The main parameter varied is the relative height of the roughness elements to the gravity current depth. The analysis of the instantaneous velocity fields suggests that the presence of the bottom elements modifies the structure of the dense current introducing complexity to the flow dynamics. The streamwise velocity of the gravity current is lower than that observed in smooth surface experiments; the greater the height of the bottom elements, the greater the reduction of velocity. Furthermore, the flow appears dominated by the bottom-generated vorticity due to the presence of recirculation areas between the bricks
Gravity currents interacting with slopes and overhangs
Full text linkWhen a gravity current interacts with a barrier the flow is reflected back in the form of a moving hydraulic jump or bore. This latter phenomenon has important implications for estuarine mixing and sediment reworking in the oceanographic context. In order to examine those reflection processes two-dimensional gravity currents interacting with slopes and overhanging barriers are examined by laboratory experiments and theoretical analysis. Relevant parameters such as the initial volume of the dense fluid, the opening of the fixed gate used for the dense fluid release and the angle of the slope or overhang positioned inside the tank, were varied. An image analysis technique based on light attenuation was applied to evaluate the instantaneous density fields. The theoretical predictions, based on shallow-water theory, give reasonable agreement compared with the laboratory experiments about the spatial position of the dense current and the definition of the point where the reflection process starts. Moreover, the analysis of the density fields highlights the presence of areas within the dense current characterized by a greater dilution due to the reflection of the current. Nevertheless, the bulk entrainment parameter is only slightly affected by the presence of a steep slope or overhang
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