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Areas with strong tidal currents that can be exploited with tidal turbines often have complex bathymetries where the turbulent intensity of these currents can reach very high values. To study the effects of bathymetric variations on the operation of a tidal turbine, an experimental device has been developed in the Ifremer circulation basin. The wake of two kinds of bathymetries, a cylinder with a square base and a cube followed by this same cylinder, are characterized and compared from PIV measurement results. The results show that the wake of the cylinder is more energetic than that of the tandem configuration. The vortex structures that it emits rise to the surface unlike those resulting from the tandem, the cube preventing their development. The effects of these wakes on a tidal turbine are then observed. To this aim, a turbine is positioned successively at different positions in the wake of obstacles placed at the bottom of the basin. The results obtained show that the characteristics of the wake in terms of organization, integral length scale and vortex detachment frequency play a major role in the operation of a machine. It is therefore essential to know the characteristics of the operating conditions of tidal turbines in order to guarantee their long-term operation., Les zones à forts courants de marée exploitables avec des hydroliennes présentent souvent des bathymétries complexes où l’intensité turbulente des courants peut atteindre des valeurs très élevées. Afin d’étudier les effets des variations de courants induites par les variations bathymétriques sur le fonctionnement d’une hydrolienne, un dispositif expérimental est développé dans le bassin à circulation de l’Ifremer. Le sillage de deux bathymétries, un cylindre à base carrée et un cube suivi de ce même cylindre, est caractérisé à partir de résultats de mesures PIV. Les résultats montrent que le sillage du cylindre est plus énergétique que celui de la configuration en tandem. Les structures tourbillonnaires qu’il émet remontent vers la surface contrairement à celles issues du tandem, le cube empêchant leur développement. Les effets de ces sillages sur une hydrolienne sont ensuite observés. A cette fin, une turbine est positionnée successivement à différentes positions dans le sillage des deux types d’obstacles étudiés précédemment. Les résultats obtenus montrent que les caractéristiques du sillage en terme d’organisation, d’échelle intégrale et de fréquence de détachement tourbillonnaire jouent un rôle majeur sur le fonctionnement d’une machine. Il est donc primordial de bien connaitre les caractéristiques des conditions de fonctionnement des hydroliennes pour garantir leur fonctionnement sur le long terme.

International audience; Turbulence 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.

International audience; There is a strong tidal energy potential in France, especially in the Alderney Race. In the area of study, where turbines will be installed, bathymetry variations are causing velocity fluctuations with a high turbulence rate in the water column: large coherent turbulent structures can be observed at the sea surface. Such events can have a major impact on the marine tidal turbines behaviour and structural fatigue. To reproduce and analyse these turbulent events, tests are carried out in the wave and current circulating flume tank of IFREMER in Boulogne-sur-Mer. Before trying to reproduce a complex bathymetry, we chose to introduce the topic by studying elementary obstacles representative of real seabed elements (with an aspect ratio of the magnitude of the mean bathymetry variations): a wide square cylinder and an inclined floor. Experiments are carried out with Reynolds number as high as achievable in Froude similitude: Re = 2.5 × 10 5 and Fr = 0.23. The impact of the aspect ratio is studied by comparing results obtained with PIV and LDV measurements on the cube and cylinder cases. The addition of an inclined floor is also investigated. Results show a significant increase of the wake with the aspect ratio. The inclined floor induces a reduction of the shear layer created by the obstacle and modifications on the shedding frequency.

International audience; The pressure field in the vicinity of a shear layer developing over the circular cavity installed in a landing gear wheel is investigated numerically by means of wavelet-based filtering algorithms originally developed for the analysis of turbulent jet noise by Mancinelli et al. (J. Fluid Mech. (2017), vol. 813, pp. 716-749). First, the well-known wavenumber-frequency filtering in the Fourier space is performed on a linear array of microphones over the shear layer. Then, four separation algorithms are successively tested at a given observer location on this array. We show that in the specific case of CFD signals, when the mesh cut-o_ frequency is lower than the Nyquist frequency, the recursive filtering method only extracts the background numerical noise of the simulation due to the band-pass filter behavior of the wavelet daughters at the smallest scales. Resampling the data yields more physical results by suppressing the Gaussian but nonphysical noise of the signal. Overall, important scatter is found between the methods in the low frequency range. An analysis of the filtered pressure decay with distance is finally carried out on the basis of expected near and far-field behavior of the acoustic field.; Le champ de pression au voisinage de la couche de cisaillement se développant au dessus de la cavité circulaire installée dans une roue de train d’atterrissage est étudiée numériquement à l’aide d’algorithmes de filtrage basés sur la transformée en ondelette initialement développés pour l’analyse du bruit de jet turbulent par Mancinelli et al. (J. Fluid Mech. (2017), vol. 813, pp. 716-749). D’abord, le traditionnel filtrage en fréquence-nombre d’onde dans l’espace de Fourier est réalisé sur une antenne de microphone au dessus de la couche de cisaillement. Ensuite, quatre algorithmes de filtrage sont successivement mis à l’épreuve pour un microphone donné sur cette antenne. On montre que dans le cas spécifique de signaux numériques, lorsque la fréquence de coupure du maillage est très faible devant la fréquence de Nyquist des signaux, la méthode de filtrage récursive ne retient que le bruit numérique de la simulation du fait du comportement passe-bande des ondelettes filles aux plus petites échelles. Le rééchantillonnage des données amène des résultats plus consistants avec la physique attendue en supprimant le bruit gaussien mais non physique du signal. Globalement, d’importants écarts sont obtenus entre les méthodes dans la gamme des basses fréquences. Une analyse de la décroissance spatiale du champ de pression filtré est finalement menée sur la base des comportements attendus pour les champs acoustiques respectivement proches et lointains.

In high flow velocity areas like those suitable for marine energy application, bathymetry variations create strong velocity fluctuations in the water column. Experiments showed that wide cylindrical obstacles generate large velocity fluctuations, like those measured at sea. In order to study the effects of these fluctuations on the turbine behaviour, an experimental set-up has been developed in the circulating tank of Ifremer in Boulogne-sur-mer by positioning a cylinder and a turbine in its wake. Simultaneous PIV measurements give access to the flow velocity upstream of the turbine. Velocity and efforts are compared in terms of cross-correlation, coherence and their spectral content is studied. Results show that the turbine loads follow velocity fluctuations until a frequency of 1H z and that low frequency velocity fluctuations have an higher impact when the turbine is in function. Coherence between fluid velocity and the rotation speed differs from coherence to the loads, probably due to the rotation control system., Dans les zones à forts courants telles que celles adaptées à l’exploitation des énergies marines renouvelables, les variations de bathymétrie creent de fortes fluctuations de vitesse. Des expériences ont montré que de larges obstacles cylindriques généraient de fortes fluctuations de vitesses, telles que celles observables en mer. Afin d’étudier les effets de ces fluctuations de chargement sur le comportement d’une hydrolienne, un dispositif expérimental a été e développé dans le bassin à circulation de l’Ifremer à Boulogne-sur-mer, en positionnant un cylindre et une turbine dans son sillage. Des mesures PIV simultanées donnent accès à la vitesse du fluide en amont de la turbine. La vitesse et les efforts sont comparés en terme de corrélation croisées, de cohérence et leur contenu spectral est étudié. Les résultats montrent que les efforts suivent les fluctuations de vitesses jusqu’à une fréquence de 1H z et les fluctuations de vitesse à basse fréquence ont plus d’impact lorsque la turbine est en fonctionnement. La cohérence entre la vitesse du fluide et la vitesse de rotation diffère de celle des chargements, probablement dû au système de contrôle de rotation de la machine.

International audience; This study aims at completing the lack of knowledge on turbulence characteristics in high flow velocity areas like those suitable for marine energy application. We present in this paper the methodology used to recreate the turbulent phenomenon existing in the Raz Blanchard by representing significant sea-bed elements in a circulating tank. The first set of experiment was achieved for the flow measurement results obtained around a wall-mounted cube at Re = 2.5 × 10 5 and F r = 0.23, in representative in-situ conditions. These measurements have been done for different kinds of flow from laminar to highly turbulent (turbulence intensity rate from 3 to 15%) in both horizontal and vertical planes, using LDV and PIV measurements. The study shows similar results for the low turbulence intensity cases in terms of flow topology (stagnation point, recirculation length and energetic considerations). It is also shown that the wall-mounted cube does not create the structures observed in-situ. Indeed, the structures of interest here are very persistent macroturbulences that may be generated at the sea floor and erupt at the free surface. The possibility that these structures could be Kolk vortices is considered. In order to recreate such phenomena, the idea of placing an inclined floor downstream of the cube is investigated. The results show that turbulent structures are significantly impulsed by a 6.5 • inclined floor and rise up to 13% higher in the water column. The question of the aspect ratio of the obstacle is also investigated. Furthermore, numerical simulations on dune with similar incoming flow conditions show encouraging results and indicate the path for further experimental testing.