1,720,960 research outputs found
Detached-eddy simulation of flow around a circular cylinder in ground effect
No full textIn this study we performed a detached-eddy simulation (DES) of flow around a circular cylinder placed near and parallel to a moving ground, on which no boundary layer developed to interfere with the cylinder. The results were compared with experiments previously reported by the authors, and also with two-dimensional unsteady RANS computations. The DES correctly predicted the cessation of the Kármán-type vortex shedding behind the cylinder, whereas the unsteady RANS also predicted it but at a much smaller ‘gap ratio’ (i.e. the ratio of the gap between the cylinder and the ground, h, to the cylinder diameter d) compared with the experiments. Time-averaged force coefficients, separation angles and velocity profiles in the near wake region predicted by the DES were in good agreement with the experiments. The major features of instantaneous wake structures were well reproduced in both large- and small-gap regimes, and also in the intermediate-gap regime, where the DES predicted a temporary formation of a small dead-fluid zone behind the cylinder
Absolute and convective instabilities of two-dimensional bluff body wakes in ground effect
No full textLocal and global instabilities are investigated of wakes of general two-dimensional bluff bodies placed near and parallel to a plane boundary or ground. A spatio-temporal linear stability analysis is first applied to a four-parameter family of local wake profiles to investigate the fundamental local stability characteristics of the wake in ground effect. The analysis shows significant dependencies of the stability characteristics of the wake on the distance from the wake centreline to the ground (normalised by the wake width), and also on the velocity ratio of the near- and far-ground sides of the wake. The analysis is then compared with earlier experiments on a circular cylinder to examine, according to the transition scenario of the steep global modes, the streamwise variation of the local stability characteristics of the wake in ground effect. The comparison indicates that the near wake region of the cylinder changes from being absolutely unstable to being convectively unstable when the cylinder comes down into the near-ground range in which the von Kármán-type vortex shedding from the cylinder is suppressed, being qualitatively consistent with the transition scenario for general wake-type flows. A possible explanation is also given for the counter-intuitive relation between the thickness of the boundary layer on the ground and the critical gap distance for the cessation of the von Kármán-type vortex shedding in ground effect
An experimental study of flow around a circular cylinder with and without end plates in ground effect
No full textFlow and force characteristics have been experimentally studied for a circular cylinder, with and without end plates, placed near and parallel to a moving ground, over which no boundary layer developed to interfere with the cylinder. Mean drag and lift measurements, surface oil-flow visualisation, and particle image velocimetry (PIV) measurements were carried out at two upper-subcritical Reynolds numbers of 0.4 and 1.0×105 (based on the cylinder diameter d) to investigate the effects of the gap ratio h/d, where h is the gap between the cylinder and the ground. For the cylinder with end plates, the drag rapidly decreased ash/d decreased to less than 0.5, but became constant for h/d of less than 0.35. This distinctive drag behaviour was explained by a global change in the near wake: the Kármán-type vortex shedding was found to be intermittent at h/d = 0.4, and then totally ceased (and instead two nearly parallel shear layers were formed) at h/d = 0.3 and below. Meanwhile, for the cylinder without end plates, no such a critical change in drag was observed as the Kármán-type vortices were not generated in the near wake region for all h/d investigated
A one-dimensional multicomponent two-fluid model of a reacting packed bed including mass, momentum and energy interphase transfer
No full textIn this paper a multicomponent two-phase flow numerical model for reacting packed beds is presented, where one phase is compressible. The model is applied to a catalyst bed which decomposes highly concentrated hydrogen peroxide into oxygen and water vapour. Using dimensional analysis a number of simplifications to the governing equations are made. A pressure based finite volume approach is used to solve the resulting equations where the pressure–velocity–density coupling is established with a SIMPLE algorithm for two-fluid flows. The model is validated against experimental data and shows in general a reasonable agreement given the complexity of the physics. It is shown that with the two-fluid formulation characteristics are predicted that cannot be reproduced with mixture models. Simulations reveal that the largest part of the total mass transfer takes place in a very small section of the catalyst bed at the point where the liquid reaches the boiling point and that most of this transfer is due to evaporation. It is further shown that a thermal description of the packing of the catalyst bed would improve the prediction of temperature distribution in the two-phase regime
Performance evaluation of A40N hybrid thruster using green propellants for space applications
No full textThis paper introduces the work performed at the University of Southampton on the analysis, performance estimation, and testing of a 40 N green hybrid thruster. It uses high density polyethylene (HDPE) as fuel, and high concentration hydrogen peroxide (or high-test peroxide, HTP) as the oxidiser, both of them considered as “green” (i.e. non-toxic) propellants. The results of the tests for different configurations of the fuel grain are presented. They include the evaluation of the thrust profile, mass flow rate, and specific impulse, whilst also analysing the regression rate of the fuel grains, the time of burning and the variation of pressure during the burning
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Auto-ignition of fuels using highly stabilised hydrogen peroxide
No full textDue to the toxic nature of common high-performance rocket propellant combinations (such
as monomethyl-hydrazine/di-nitrogen tetroxide), there is now growing interest in so-called
'green' propellants. In response to this, and in the continuation of existing research, a small
demonstration bi-propellant rocket-engine was designed, built and tested using
homogeneous catalysis of highly stabilised hydrogen peroxide. An investigation into
suitable fuels was conducted, and candidates selected. Auto-ignition was successfully
achieved, demonstrating the robustness of the propellant combination
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
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