1,720,956 research outputs found
Analysis of atmospheric wind and gust prediction
No full textThis thesis examines how atmospheric wind varies across a broad range of spatial and temporal scales, from fluctuations over seconds to patterns extending hundreds of kilometers. The analysis is based on long-term measurements from the offshore research platform FINO1 in the North Sea. A detailed investigation of scaling features in energy spectra and structure functions is carried out, using the local Taylor hypothesis to relate temporal observations to spatial flow structures.
Despite varying weather conditions, the smallest scales below the measurement height consistently exhibit signatures of classical three-dimensional turbulence. At intermediate scales between the measurement height and the typical height of the planetary boundary layer, the influence of the Earth's surface becomes apparent through patterns characteristic of wall-bounded turbulence. On horizontal scales of tens or hundreds of kilometers, buoyancy-driven internal gravity waves and quasi-two-dimensional geostrophic turbulence shaped by Earth's rotation dominate the wind field. The transition between the gravity-wave and the geostrophic regime manifests itself in a remarkably abrupt sign change of the third-order structure function. It occurs at a scale of 500 km, which can be explained by the maximum horizontal length scale permitted by gravity wave dynamics. In both regimes, the scaling of structure functions agrees well with aircraft measurements reported in the literature. The analysis demonstrates that applying the Taylor hypothesis locally is essential for obtaining the correct scaling behavior.
Distributions of velocity increments reveal an additional, secondary peak at scales corresponding to the gravity-wave regime. These secondary maxima play a decisive role in producing the observed scaling of structure functions. Moreover, anticorrelations in increment time series are linked to the constraint of finite turbulent kinetic energy.
The final part of the thesis focuses on short-term gust prediction. Existing wind gust definitions in the literature are critically reviewed, and gusts are defined on a physical basis as sudden changes in drag force or wind power. Autoregressive models may be used to predict single gust events, but they tend to forecast gusts persistently in gusty phases. To address this problem, ARCH-type models are employed to predict the variance of wind speed increments, which correlates with the number of gusts within a given time window. This approach offers a complementary perspective on short-term gust predictability relevant for wind energy and aviation
Signatures of geostrophic turbulence in power spectra and third-order structure function of offshore wind speed fluctuations
No full textWe analyze offshore wind speeds with a time resolution of one second over a long period of 20 months for different heights above the sea level. Energy spectra extending over more than seven decades give a comprehensive picture of wind fluctuations, including intermittency effects at small length scales and synoptic weather phenomena at large scales. The spectra S(f) show a scaling behavior consistent with three-dimensional turbulence at high frequencies f, followed by a regime at lower frequencies, where fS(f) varies weakly. Lowering the frequency below a crossover frequency , a rapid rise of fS(f) occurs. An analysis of the third-order structure function of wind speed differences for a given time lag shows a rapid change from negative to positive values of at . Remarkably, after applying Taylor’s hypothesis locally, we find the third-order structure function to exhibit a behavior very similar to that obtained previously from aircraft measurements at much higher altitudes in the atmosphere. In particular, the third-order structure function grows linearly with the separation distance for negative , and with the third power for positive . This allows us to estimate energy and enstrophy dissipation rates for offshore wind. The crossover from negative to positive values occurs at about the same separation distance of 400 km as found from the aircraft measurements, suggesting that this length is independent of the altitude in the atmosphere
Signatures of geostrophic turbulence in power spectra and third-order-structure function of offshore wind speed fluctuation
Full text linkWe analyze offshore wind speeds with a time resolution of one second over a
long period of 20 months for different heights above the sea level. Energy
spectra extending over more than seven decades give a comprehensive picture of
wind fluctuations, including intermittency effects at small length scales and
synoptic weather phenomena at large scales. The spectra show a scaling
behavior consistent with three-dimensional turbulence at high frequencies ,
followed by a regime at lower frequencies, where varies weakly.
Lowering the frequency below a crossover frequency , a rapid rise of occurs. An analysis of the third-order structure
function of wind speed differences for a given time lag
shows a rapid change from negative to positive values of at
. Remarkably, after applying
Taylor's hypothesis locally, we find the third-order structure function to
exhibit a behavior very similar to that obtained previously from aircraft
measurements at much higher altitudes in the atmosphere. In particular, the
third-order structure function grows linearly with the separation distance for
negative , and with the third power for positive . This allows us to
estimate energy and enstrophy dissipation rates for offshore wind. The
crossover from negative to positive values occurs at about the same separation
distance of 400 km as found from the aircraft measurements, suggesting that
this length is independent of the altitude in the atmosphere.Comment: 11 pages, 4 figure
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
Distributions and Correlation Properties of Offshore Wind Speeds and Wind Speed Increments
No full textDispelling 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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