Determination of the Response Amplitude Operator of a tidal turbine as a spectral transfer function

A transfer function determination method is proposed in this study to predict the unsteady fluctuations of the performance of a tidal turbine model. This method is derived from the Response Amplitude Operator (RAO) applied in the offshore industry to predict linear wave-induced loads on large structures. It is based on a spectral approach and requires the acquisition of a turbine parameter (e.g. torque, thrust, power or root-blade force) in synchronization with an upstream flow velocity measurement. On the frequency range where the causality between these two signals is proven, the transfer function is established using the ratio between the cross-spectral density and the spectral density of the incoming velocity.The linearity is verified using the coherence function, which shows validity for the turbine power in the lowest frequencies only. This transfer function is then used to reconstruct the power fluctuations which is compared to the recorded one for a particular flow condition with bathymetry generated turbulence. The result shows the dependence on the accurate location of the velocity measurement point used for the reconstruction. This point must exactly correspond to the expected turbine location, i.e. where the turbine response needs to be processed. Bearing in mind its limits, the method can be used to predict the loadings of extreme events on the turbine structure and the performance variations corresponding to the unsteady characteristics of a turbulent flow, for a better grid integration

Determination of the Response Amplitude Operator of a tidal turbine as a spectral transfer function

A transfer function determination method is proposed in this study to predict the unsteady fluctuations of the performance of a tidal turbine model. This method is derived from the Response Amplitude Operator (RAO) applied in the offshore industry to predict linear wave-induced loads on large structures. It is based on a spectral approach and requires the acquisition of a turbine parameter (e.g. torque, thrust, power or root-blade force) in synchronization with an upstream flow velocity measurement. On the frequency range where the causality between these two signals is proven, the transfer function is established using the ratio between the cross-spectral density and the spectral density of the incoming velocity. The linearity is verified using the coherence function, which shows validity for the turbine power in the lowest frequencies only. This transfer function is then used to reconstruct the power fluctuations which is compared to the recorded one for a particular flow condition with bathymetry generated turbulence. The result shows the dependence on the accurate location of the velocity measurement point used for the reconstruction. This point must exactly correspond to the expected turbine location, i.e. where the turbine response needs to be processed. Bearing in mind its limits, the method can be used to predict the loadings of extreme events on the turbine structure and the performance variations corresponding to the unsteady characteristics of a turbulent flow, for a better grid integration

PIV measurements combined with the motion tracking technique to analyze flow around a moving porous structure

To gain a better understanding of the fluid–structure interaction and especially when dealing with a flow around an arbitrarily moving body, it is essential to develop measurement tools enabling the instantaneous detection of moving deformable interface during the flow measurements. A particularly useful application is the determination of unsteady turbulent flow velocity field around a moving porous fishing net structure which is of great interest for selectivity and also for the numerical code validation which needs a realistic database. To do this, a representative piece of fishing net structure is used to investigate both the Turbulent Boundary Layer (TBL) developing over the horizontal porous moving fishing net structure and the turbulent flow passing through the moving porous structure. For such an investigation, Time Resolved PIV measurements are carried out and combined with a motion tracking technique allowing the measurement of the instantaneous motion of the deformable fishing net during PIV measurements. Once the two-dimensional motion of the porous structure is accessed, PIV velocity measurements are analyzed in connection with the detected motion. Finally, the TBL is characterized and the effect of the structure motion on the volumetric flow rate passing though the moving porous structure is clearly demonstrated

Experimental investigation of the influence of mast proximity on rotor loads for horizontal axis tidal turbines

While the world continues to grapple with the increasingly prominent impact on the planet of climate change, a shift towards greater reliance on renewable energy sources is observed. Wind, hydro and solar have seen a rise in uptake, however, tidal energy represents massive untapped potential. For tidal energy to become economically viable, focus must shift towards designing efficient yet structurally sound designs. The current study investigates the influence of the tower distance from the rotor plane on turbine performance, and on rotor loading. A test scale instrumented tidal stream turbine is studied in a water flume tank at the laboratory of IFREMER in Boulogne-sur-Mer, France. Experiments are carried out with 14 different tower positions and the turbine performance coefficients are compared. Both mean and values remain unaffected for these different positions. However, the structural rotor loading is found to fluctuate significantly as the distance between the tower and rotor is reduced. Load measurements are analysed in terms of coefficient of variation, through frequency analysis, in relation with the azimuthal position of the rotor and finally in terms of exceedance. All the experimental measurements associated with this study are available from: https://doi.org/10.17882/81077

Wave and current flume tank of IFREMER at Boulogne-sur-mer. Description of the facility and its equipment

This document presents the wave and current flume tank of IFREMER at Boulogne-sur-mer. This facility is unique in Europe because of its size and capabilities. The experimental working section is 4m wide by 2m deep and 18m long. Two pumps with a total power of 500kW enable the 700m3 of fresh water to move with a maximum velocity of 2.2m/s. Wave can be superimposed to the current using a dedicated wavemaker. It can be located at an upstream of downstream surface position in order to generate wave propagating with or against the current. Additional specific experimental devices are used in the laboratory as well, such as: Laser Doppler Velocimeter, Acoustic Doppler Velocimeter, Particles Image Velocimetry, underwater and aerial Qualysis Motion Tracking system or a 6 degrees of freedom motion generating system called hexapod. Many mono and multi-components waterproof transducers, with wide force measurement ranges, are available. Combined with an in-house and modular acquisition system, any kind of tension-based sensors can be recorded, with a 25kHz maximum sampling frequency. Because of its original characteristics and specific equipments, this experimental facility can be used to carry out various hydrodynamic studies in different fields: oil and gas, naval hydrodynamics, marine renewable energy or wave-current-structure interactions. Continuous improvements and equipment upgrades enable this facility to respond to a growing demand of experimental tests in all these hydrodynamic areas

Mise en évidence de l’importance de la turbulence ambiante sur les effets d’interaction entre hydroliennes

Les futures zones d’implantation des parcs hydroliens en mer sont caractérisées par des courants de forte intensité et des taux de turbulence importants. Cette turbulence de l’écoulement est susceptible d’altérer le comportement des hydroliennes, à la fois en termes de production d’énergie mais aussi de fatigue structurelle. Afin de déterminer l’influence de la turbulence ambiante sur les effets d’interaction entre hydroliennes, des essais expérimentaux ont été effectués dans un bassin à houle et courant, sur des maquettes d’hydroliennes tri-pales à axe horizontal, régulées en vitesse de rotation. Les interactions entre trois hydroliennes placées sur deux rangées (deux hydroliennes sur la rangée amont et une sur la rangée aval, ou inversement) ont été étudiées. Pour ce faire, les performances des machines ont été évaluées à l’aide d’une mesure de couple sur l’arbre de chaque turbine et le sillage de l’ensemble des trois machines à partir de mesures par vélocimétrie laser 2D. Ces mesures de comportement et de performances ont été effectuées à une vitesse de courant fixée à 0.8 m/s avec des taux de turbulence de l’écoulement amont de 3% et de 15%. L’impact de ces deux taux de turbulence sur les performances des machines et sur le développement de leurs sillages est donc mis en évidence dans ce papier. Des préconisations pour les futures modélisations numériques seront également établies

Etude expérimentale du phénomène d’entraînement de bulles à l’étrave des navires scientifiques

The bubble sweep-down phenomenon around the hull of scientific vessels leads to a disruption of the acoustic waves emitted by the sonars, and a loss of productivity during the acoustic surveys. Numerical or experimental tools used to study this phenomenon still have limitations, mainly due to scale effects on bubbles generation. An innovative method in a circulating tank, using numerical simulations and a motions generator, has been developed. Two mechanisms of air entrainment have been described, as well as the influence of waves characteristics and turbulence on bubbles generation.Le passage de bulles sous la coque des navires scientifiques peut provoquer une perturbation des ondes acoustiques émises par les sonars, et une perte de productivité lors des campagnes de mesure. Les outils numériques ou expérimentaux permettant d’étudier ce phénomène lors de la conception des navires montrent toujours des limites, notamment à cause des effets d’échelle agissant sur la génération et la dynamique des nuages de bulles. Une méthode d’essai innovante en bassin de circulation, couplant des calculs numériques et l’utilisation d’un générateur de mouvements, a été développée pour étudier ce phénomène. Deux mécanismes d’entrainement d’air ont ainsi pu être identifiés. Les effets de la houle et de la turbulence sur la génération des bulles ont également été étudiés

Caractérisation expérimentale du sillage généré par une hydrolienne. Influence du taux de turbulence ambiant

Le fonctionnement d’une hydrolienne est régi par les caractéristiques de l’écoulement incident. Cet article présente les résultats d’une étude expérimentale visant à déterminer les effets du taux de turbulence de l’écoulement ambiant sur le fonctionnement d’une hydrolienne et le sillage généré. Les essais expérimentaux ont été réalisés dans la veine hydrodynamique à surface libre de l’Ifremer, dont on peut faire varier le taux de turbulence de l’écoulement de 5 à 25%. Le prototype utilisé est une turbine tri-pales à axe horizontal de 0,7 m de diamètre (échelle 1/30). Des cartes de vitesses moyennes, de taux de turbulence, mais également d’énergie cinétique turbulente et de contrainte de cisaillement ont été obtenus à partir de mesures par vélocimétrie laser pour une vitesse de l’écoulement incident de 0,8m/s. L’analyse des sillages proche et lointain a été effectuée : le sillage proche est caractérisé par un fort gradient de cisaillement et une intensité de turbulence élevée, alors que le sillage lointain est caractérisé par son expansion. Une réduction du sillage d’un facteur 3 cumulée à d’importantes fluctuations d’efforts ont également pu être observées pour les taux de turbulence les plus élevés

Experimental study of the wall-mounted cylinder wake effects on a tidal turbine behaviour compared to free stream turbulence

International audienceTurbulence impacts significantly the performance and fatigue of tidal turbines. In this study, two cases of turbulence inflows are considered. At laboratory scale, in the wave and current circulating tank of Ifremer, experimental conditions are chosen to be representative of in-situ characteristics at a 1:20 scale, in Froude similitude (Fr = 0.23 and Re ∼ 10 5). The first inflow case is seabed variations, experimentally represented with a wall-mounted cylinder. Recent experiments have shown that a large aspect ratio bottom-mounted obstacle produces a very extended wake, with large velocity fluctuations. The second inflow case is a 14% ambient turbulence intensity flow. The main objective of this study is to characterize the effect of the turbulence present in the two inflow cases on a 3-bladed horizontal turbine. The turbine is positioned within the obstacle wake at a distance of 16H from the obstacle or within the ambient turbulent flow. To monitor the velocity fluctuations reaching the turbine, two types of measurement are performed in synchronisation with the turbine parameters: Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV). Measurements are performed far upstream and close to the turbine. In this work, both inflow conditions will be spatially and temporally described. Then, a full temporal and spectral analysis will be achieved on the turbine performance, in terms of thrust and torque. Finally, a comparison will be proposed, between the perturbation effects on the turbine induced by both inflow cases

Effet des concrétions marines sur le comportement hydrodynamique de structures circulaires

The presence of marine growth on underwater parts of jackets significantly increases diameter and roughness of tubular members of these structures. This paper escribes an experimental study carried out in the circulating tank of Boulogne-sur-mer which aim to quantify the hydrodynamical drag and added-mass coefficients of a circular cylinder encrusted with rigid and large-size biofouling, such as coral. Two different kinds of roughness are studied and compared to a reference cylinder. These coefficients are obtained from trials with current only, with imposed motions or with a combination of imposed motions and current. Results show that these coefficients are from 1.5 to 4 times higher 1 than those obtained for the reference cylinder. Nevertheless, differences between the two tested kinds of roughness are low and limited to the highest speeds and accelerations of motions.La présence de bio-salissures marines sur les parties immergées des jackets augmente significativement le diamètre et la rugosité des supports tubulaires de ces structures. Ce papier décrit l’étude expérimentale, réalisée au bassin d’essais de Boulogne-sur-mer, dans le but de quantifier les coefficients hydrodynamiques de trainée et de masse ajoutée s’appliquant sur un cylindre circulaire colonisé par des concrétions marines rigides de grandes dimensions de type corail. Deux types de rugosités sont testés et comparés au cas d’un cylindre de référence. Ces coefficients sont obtenus à partir d’essais en courant seul, en mouvements imposés et en mouvements imposés avec courant. Les résultats montrent des coefficients tous supérieurs d’un facteur compris entre 1,5 et 4 à ceux obtenus pour le cylindre de référence. Les différences observées entre les deux cas de rugosités testés sont, par contre, faibles et limitées aux plus grandes vitesses et accélérations de mouvement