1,721,032 research outputs found
Modification of two-equation models to account for plant drag
No full textA modification of the most popular two-equation (E-phi) models, taking into account the plant drag, is proposed. Here E is the turbulent kinetic energy (TKE) and phi is any of the following variables: El (product of E and the mixing length l), epsilon (dissipation rate of TKE), and omega (specific dissipation of TKE, omega = epsilon/E). The proposed modification is due to the fact that the model constants estimated experimentally for 'free-air' flow do not allow for adequate reconstruction of the ratio between the production and dissipation rates of TKE in the vegetation canopy and have to be adjusted. The modification is universal, i.e. of the same type for all E-phi models considered. The numerical experiments carried out for both homogeneous and heterogeneous plant canopies with E-phi models (and with the E-l model taken as a kind of reference) show that the modification performs well. They also suggest that E-epsilon and E-omega schemes are more promising than the E-El scheme for canopy flow simulation since they are not limited by the need to use a wall function. In addition, a new parameterization for enhanced dissipation within the plant canopy is derived. It minimizes the model sensitivity to C mu, the key parameter for two-equation schemes, and whose estimates unfortunately vary considerably from experiment to experiment. The comparison of results of new modified E-epsilon and E-omega models with observations from both field and wind-tunnel experiments shows that the proposed parameterization is quite robust. However, because of uncertainties with the turbulence Prandtl and Schmidt numbers for the E-epsilon model within the canopy, the E-omega model is recommended for future implementation, with the suggested modifications
Influence of gap size on wind damage variables in a forest
No full textWindstorms are the major disturbance factor in tropical and European forest ecosystems. An airflow model can provide the basis to interpret spatial patterns of wind damage on trees, and guide strategies with respect to that concern. In contrast to recent advances on modelling of perturbed canopy flows, few studies have considered the effects of canopy inhomogeneity on the pattern of flow statistics, which in its turn describes the relative risk of wind damage on trees. An atmospheric boundary-layer two-equation closure model SCADIS based on transport equations for turbulent kinetic energy (E) and specific dissipation (omega = epsilon/E, where E is the dissipation of E) (E-omega model), which accounts for the flow dynamics within a plant canopy [Sogachev, A., Panferov, O., 2006. Modification of two-equation models to account for plant drag. Bound. Lay. Meteorol. 121, 229-266] was used to carry out a series of numerical experiments with gap sizes from 3 to 75 tree heights, h, in a modelled forest. Spatial variations of integral wind loading presented as a sum of static and dynamic (gust) components on trees around the gaps were estimated from modelled data. To quantify the changes of wind load characteristics due to gap growth relatively to the undisturbed forest they were normalized by the correspondent values for that forest. The results show that for round gaps the maximal static wind loading on trees surrounding the gap as large as 75h increases up to 14 times of that for undisturbed forest. The maximal static load is located on the exposed (or downwind) gap edge independently of gap size. The maximal value of the gust component increases with the gap diameter up to the gap size of 20h only, where it is 2.6-3.0 times higher than for undisturbed forest, and remains constant for larger gaps. With the growth of gap size the area of maximal values of E shifts from downwind gap edge to the lateral borders of the gap increasing the contribution of gust loading there. Thus, the integral wind loading increases nonlinearly with gap size and for the gap size of 75h it can be up to seven times higher than that for undisturbed forest. The spatial distribution of maximal values of integral loading is similar to that of static loading up to gap size of 20h. For larger gaps the location of integral loading maximum shifts gradually towards lateral borders with increasing of gap diameter. (c) 2008 Elsevier B.V. All rights reserved
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
A Note on Two-Equation Closure Modelling of Canopy Flow
No full textThe note presents a rational approach to modelling the source/sink due to vegetation or buoyancy effects that appear in the turbulent kinetic energy, E, equation and a supplementary equation for a length-scale determining variable, φ, when two-equation closure is applied to canopy and atmospheric boundary-layer flows. The approach implements only standard model coefficients C φ1 and C φ2 in the production and destruction terms of the φ equation, respectively. Numerical tests illustrate the practical applicability of the method, where, for example, simulations with the E–ω model (where is the specific dissipation and is the dissipation rate of E) properly reproduce both the surface-layer wind profile estimated from the Monin-Obukhov similarity theory and the mixing-height evolution observed above forested terrain in Southern Finland
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
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