1,720,978 research outputs found
Offshore multi-rotor wind turbines: Blade interactions under surging conditions
No full textThe need to reduce the levelized energy cost of floating offshore wind turbines has spurred interest in scaling up rotor blades. However, this approach poses challenges related to material strength, manufacturing, and logistical hurdles in transportation. An emerging alternative is multi-rotor configurations on a single platform and tower structure. The present study concerns floating multi-rotor wind turbines operating under surge conditions. A comprehensive characterization of rotor and blade loads, power generation dynamics, and a thorough frequency domain analysis are performed within an actuator line model coupled with the OpenFOAM® Navier–Stokes solver. Results show that global loads and power generation associated with a multi-rotor configuration exhibit variability contingent upon blade azimuth and surge displacement, but heightened rotor power and loads compared to single-rotor configurations. Frequency analysis unveils the dominance of the surge frequency alongside the presence of secondary peaks attributed to rotor-to-rotor interactions. The angle of attack is shown to be primarily influenced by surge motion rather than inter-rotor interactions, underscoring the impact of platform motions on load dynamics. The outcomes of this study have significant implications for designing and optimizing multi-rotor wind turbines, particularly in mitigating load-induced fatigue effects, rendering this work a basis for further efforts in this field
The impact of platform motion phase differences on the power and load performance of tandem floating offshore wind turbines
No full textThe interactions between tandem floating offshore wind turbines remain an essential research topic with a view to optimizing future large-scale offshore wind farms. The focus of past floating offshore wind turbine aerodynamic research was centred on platform motion impact on rotor aerodynamics for standalone or in-phase tandem turbines. The question of how the differences in the surge motion phase of tandem turbines influence the performance of the downstream turbine remains open. Recent investigations reveal that these motions might not invariably be in-phase. This study seeks to determine the repercussions of platform motion phase differences on power, thrust, and wake recovery. Notably, it postulates that phase differences could detrimentally affect fatigue. Using a validated Navier–Stokes actuator line model rooted in the OpenFOAM® toolbox, the study aims to address this hypothesis. It was found that the out-of-plane root bending moment amplitude increase by 100% when the rotors are out of phase. A 72% increase in amplitude was also observed for CT. Both of these observations have also been linked to an increase in angle of attack amplitude over the blade span. This work stimulates future research in floating offshore turbine control strategies and provides guidance to minimize turbine failures and downtime
Offshore multi-rotor wind turbines: Tandem turbine interactions
No full textMulti-rotor turbines are emerging as a promising upscaling solution for offshore wind farms, yet optimization of these systems, particularly in floating configurations, has received limited attention in the literature. This study examines the wake recovery and performance of tandem, floating multi-rotor turbines under surge motion, focusing on wake velocities and dynamic loads experienced by a quad-rotor system. Using a validated actuator line model, multiple realistic surge conditions are simulated to assess the effects of motion kinematics on wake behavior and rotor performance. It is demonstrated that tandem quad-rotor systems exhibit superior wake recovery compared to single rotors, due to enhanced mixing and faster wake diffusion. While surge motion has minimal effect on average power and thrust coefficients, particularly at low frequencies, it significantly impacts the wake structure and increases dynamic loads. Findings also indicate that quad-rotor configurations can mitigate wake effects, improving wind farm efficiency and reducing the risk of dynamic load-induced fatigue. Thus, insights into wake dynamics and rotor interactions from this study can practically guide the design and operation of future offshore wind farms providing new insights on how surge motion influences wake recovery and blade loads in such environments
Modelling of two tandem floating offshore wind turbines using an actuator line model
No full textThe aerodynamic and wake recovery dynamics of floating offshore wind turbines differ from fixed turbines due to the platform motions. Understanding tandem rotor interactions is essential for both turbines as well as wind farm design. This paper investigates the wake interactions in offshore wind farms by studying the effect of the upstream turbine motion on the downstream wind turbine loads and performance. A previously developed and validated Navier-Stokes actuator line model is used and implemented in the OpenFOAM® solver. The NREL 5 MW turbine is selected as a reference, and the upstream turbine is prescribed both surging and pitching motions (of different amplitude) while the downstream turbine is maintained fixed. Results for the turbine loading, wake and flow development are presented. It was found that the peak-to-peak thrust and power variations depend on modelling the discrete nature of the blades. Although the discrete tip vortices in fixed conditions diffuse within the first two diameters, downstream of the rotor, the platform motion can transform them into a new wake topology form with discrete ring shapes. The frequency spectra of the parameters showed a significant impact from these motion-induced discrete rings. The results indicate the need for higher fidelity modelling approaches when studying floating wind turbine interactions
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
3D flows near a HAWT rotor: A dissection of blade and wake contributions
No full textInvestigating the flow physics in the vicinity of the wind turbine blade is a challenging endeavour. In the past, focus was placed on the understanding of near wake flows arising from wake vorticity and the rotor loading. In this work, a different approach is taken by considering the flow field in the blade vicinity as a consequence of the separate effects of bound and wake vorticity. This enables new insight regarding the role of the blade as having a direct influence on the three-dimensional flow. The approach is applied for the reference axial flow condition and hence for the yawed flow condition where the issue of flow three-dimensionality takes a new level of complexity. Three research hypotheses are investigated in this work: 1. Radial flow components especially close to the wind turbine blade are not negligible. This contradicts the classical momentum approach which treats the flow as two-dimensional. The situation for yawed flow is even more important since wake vorticity not only exhibits an expansion but also a skewness. A fundamental understanding of the behaviour of the radial flow component is hence of paramount importance. 2. The three-dimensional flow field close to a Horizontal Axis Wind Turbine (HAWT) rotor is due to the effects of body and wake vorticity. The blade tip shape plays a fundamental role on the behaviour of the flow field near the blade. 3. The tip vorticity for axial and yawed flow results in a different tip flow behaviour. The hypotheses are linked by a common goal; to establish new insight into three-dimensional flows in the proximity of the rotor in yawed flow, using axial flow as a baseline investigation. Both numerical and experimental approaches have been used to investigate these hypotheses. A 3D unsteady potential flow panel model is used for the numerical computations. The model permits to decompose flow due to diff erent vorticity components. Stereo Particle Image Velocimetry (SPIV) is used for the experimental measurements. This enables measurement of all velocity components in a 2D plane and can then be used to construct a 3D volume of data. Flow data from three different rotors is used: SPIV measurements from the Model Experiments in Controlled Conditions (MEXICO) rotor in the German-Dutch DNW wind tunnel and experiments performed in the Open Jet Facility of TU Delft on two different 2m diameter rotors. The thesis is structured into six parts as follows: Part I - Literature review to support and contextualize the research Part II - Analysis of the hypotheses on ow three-dimensionality Part III - Decomposition of velocities in the rotor proximity Part IV - Origins and dynamics of vorticity Part V - Conclusions Part VI - Appendices The results presented in this thesis challenge the current understanding of flow three-dimensionality in the rotor plane particularly for the yawed flow case. The blade's role as a vorticity generator as well as its active role in disturbing the flow due to its vorticity distribution are both supported. The creation of a HAWT tip vortex over the blade thickness is studied leading to important implications about the induced flow field at the tip. The details of flow three-dimensionality due to the complex behaviour of the tip vortex upon release are presented and the implications of this discussed. The outcome of this research bridges the gap between existing knowledge of the flow on the rotor scale to future lines of research which will be directed to the study of boundary layer flows of rotating blades. By extensively analyzing the rotor blade scale outer flow (outside of the boundary layer) this research gives impetus to a necessary revision of tip corrections in the application to the industry standard BEM design codes which to this day rely on models which are not based on the detailed knowledge of rotor blade flow which this research provides.DUWINDAerospace Engineerin
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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