1,720,958 research outputs found
Performance quantification of tidal turbines subjected to dynamic loading
No full textThe behaviour of Tidal Stream Turbines (TST) in the dynamic flow field caused by waves and rotor misalignment to the incoming flow (yaw) is currently poorly understood. The dynamic loading applied to the turbine could drive the structural design of the power capture and support subsystems, device size and its proximity to the water surface and sea bed. In addition, the strongly bi directional nature of the flow encountered at many tidal energy sites may lead to devices omitting yaw drives; accepting the additional dynamic loading associated with rotor misalignment and reduced power production in return for a reduction in capital cost. For such a design strategy it is imperative to quantify potential unsteady rotor loads so that the TST device design accommodates the inflow conditions and avoids an unacceptable increase in maintenance action or, more seriously, suffers sudden structural failure. The experiments presented as part of this work were conducted using a 1:20th scale 3-bladed horizontal axis TST at a large towing tank facility. The turbine had the capability to measure rotor thrust and torque, blade root strain, azimuthal position and speed. The maximum outof- plane bending moment was found to be as much as 9.5 times the in-plane bending moment, within the range of experiments conducted. A maximum loading range of 175% of the median out-of-plane bending moment and 100% of the median in-plane bending moment was observed for a turbine test case with zero yaw, scaled wave height of 2m and intrinsic wave period of 12.8s. A Blade Element Momentum (BEM) numerical model has been developed and modified to account for wave motion and yawed flow effects. This model includes a new dynamic inflow correction which is shown to be in close agreement with the measured experimental loads. The gravitational component was significant to the experimental in-plane blade bending moment and was included in the BEM model. Steady yaw loading on an individual blade was found to be negligible in comparison to wave loading (for the range of experiments conducted), but becomes important for the turbine rotor as a whole, reducing power capture and rotor thrust
Design, commissioning and performance of a device to vary the turbulence in a recirculating flume
No full textAmbient turbulent flow structures are one of the key drivers that will determine the rate of wake recovery downstream of tidal turbines. For second and third generation arrays or farms such a parameter is critical for the determination of inter-device spacing and the optimisation of energy extraction per unit surface area. At present offshore flow characterisation is dominated by seabed or surface-mounted diverging-beam acoustic Doppler profilers that whilst having a good spatial capture cannot characterise turbulent flow structures to the same accuracy as single point converging laboratory-scale velocimeters. So a paradox presently exists: We can measure the (mean) flow characteristics at real tidal energy sites but lack the ability to accuracy ascertain high-frequency flow characteristic at discrete spatial locations. This is possible at laboratory-scale with convergent-beam devices but as we do not know the real site conditions replication at small-scale can only be approximated. To date there has been few laboratory studies where the ambient flow turbulence has been varied. The standard method is to generate turbulence from a static structure such as a grid. Here we have developed an articulated rig that has the ability to oscillate cylindrical members along two axes in the flow upstream of tidal turbine models. Initial results presented in this paper show the effect upon the ambient flow that the turbulence-generating rig can impose and the effects upon wake dissipation for varying levels of turbulent length and time scales. Also the formation and insistence of turbulent structures shed from the device are reported. As expected increasing ambient turbulence intensity serves to dissipate the turbine wake more rapidly and whilst we cannot directly relate these laboratory flow characteristics to full-scale tidal energy sites at present it is hoped that offshore measurement technology and that of laboratory replication can converge so that device performance prediction can be performed at smaller-scale and at a corresponding lower cost to the technology
Studies of a scale tidal turbine in close proximity to waves
No full textThe understanding of wave-current interaction is of increasing interest in the field of coastal and offshore engineering. An area where wave-current interaction is likely to be an important factor is in the development of Marine Current Energy Converters (MCECs). Little has been done to investigate the behaviour of MCECs in unsteady flow caused by wave motion and close proximity of the device to the structure. The additional forces applied to the blades through the action of the waves could very well dictate the structural design of blades as well as the proximity to the water surface. The forces applied to the blades from waves will not be perpendicular to the blades as with marine currents. The forces will be composed of a more complex circular motion. This repeating oscillation will place additional strain on blades, but if the magnitude of this can be quantified it may permit optimised MCEC devices to be placed nearer to the sea surface allowing manufactures to use larger blade diameters to maximise energy extractio
Quantifying wave and yaw effects on a scale tidal stream turbine
No full textThe behaviour of Tidal Stream Turbines (TST) in the dynamic flow field caused by waves and rotor misalignment to the incoming flow (yaw) is currently unclear. The dynamic loading applied to the turbine could drive the structural design of the power capture and support subsystems, device size and its proximity to the water surface and sea bed. In addition, the strongly bi-directional nature of the flow encountered at many tidal energy sites may lead to devices omitting yaw drives; accepting the additional dynamic loading associated with rotor misalignment and reduced power production in return for a reduction in device capital cost. Therefore it is imperative to quantify potential unsteady rotor loads so that the TST device design accommodates the inflow conditions and avoids an unacceptable increase in maintenance action or, more seriously, suffers sudden structural failure. The experiments presented in this paper were conducted using a 1:20th scale 3-bladed horizontal axis TST at a large towing tank facility. The turbine had the capability to measure rotor thrust and torque whilst one blade was instrumented to acquire blade root strain, azimuthal position and rotational speed all at high frequency. The maximum out-of-plane bending moment was found to be as much as 9.5 times the in-plane bending moment. A maximum loading range of 175% of the median out-of-plane bending moment and 100% of the median in-plane bending moment was observed for a turbine test case with zero rotor yaw, scaled wave height of 2 m and intrinsic wave period of 12.8 s. A new tidal turbine-specific Blade-Element Momentum (BEM) numerical model has been developed to account for wave motion and yawed flow effects. This model includes a new dynamic inflow correction which is shown to be in close agreement with the measured experimental loads. The gravitational component was significant to the experimental in-plane blade bending moment and was also included in the BEM model. Steady loading on an individual blade at positive yaw angles was found to be negligible in comparison to wave loading (for the range of experiments conducted), but becomes important for the turbine rotor as a whole, reducing power capture and rotor thrust. The inclusion of steady yaw effects (using the often-applied skewed axial inflow correction) in a BEM model should be neglected when waves are present or will result in poor load prediction reflected by increased loading amplitude in the 1P (once per revolution) phase
Experimental and numerical results of rotor power and thrust of a tidal turbine operating at yaw and in waves
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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
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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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