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Tidal turbine arrays are planed to be installed in areas with strong currents where the flow can often be sheared throughout the water column. To study the shear flow effects on tidal turbine, four vertical velocity profiles are generated in a flume tank and are imposed to a three-bladed horizontal axis turbine model. Results show that the sheared velocity profiles do not impact the turbine average performance but are responsible for an increase of blade root streamwise load variations. Blade root streamwise load is moreover linked to the turbine rotational frequency and its harmonics. The velocity perceived by the blades during their rotation is estimated over the rotor area and is compared to the angular phase average of the streamwise load measured on the blades. The phase average of the load and the velocity perceived by the blades are highly correlated even if a varying phase lag has been noticed between these two quantities. This phase lag is dependent on the rotational speed of the turbine, on the incoming flow shear, and is probably caused by the turbine induction effects. This experimental study is a first step to understand the effect of shear velocity profiles on tidal turbines better.

The bubble sweep-down phenomenon around oceanographic research vessels generates acoustic disturbances. The phenomenon can be reproduced on a 1/30 scale ship model in a wave and current circulating flume tank taking into account ship motions. Optical probe can then be used to measure locally the size and the velocity of the bubbles generated by breaking waves. A comparison of the obtained results with more classical results obtained from bubble detection by image tracking is discussed. The two methods provide complementary results for bubble characteristics. However, bubble detection by optical probe has the advantage of knowing the bubble position in the three dimensions of space. The overall results of this paper provide new elements for the study and the understanding of the bubble sweep-down phenomenon in addition to previous 2D and 3D studies, where the dynamics of bubble clouds have been characterized.

International audience; To improve the turbine operational life, the interaction between flow properties and turbine performance needs to be elucidated. We then propose to examine the physical origin of the power-law scaling in the inertial range of turbine power outputs by experimentally exploring the spectral content of a 1:20 scaled model of a three-bladed horizontal-axis turbine positioned in a 3D turbulent flow. First, measurements confirm that the turbine power frequency spectra exhibit a power law decay proportional to −11/3 in the inertial range. Knowing that the turbine power fluctuations are linearly dependent on the incoming velocity fluctuations, PIV measurements are carried out to study the effect of the spatially integrated velocity onto its resulted spectrum. It is demonstrated that in inhomogeneous anisotropic turbulent flow, the velocity spectrum of its spatial average along N direction(s) has an inertial slope of −5/3 − 2N/3. This information is used to physically interpret the power-law scaling in the inertial range of the turbine power spectra. The previously observed f −11/3 scaling results from a 2D-spatial average velocity field coupled with a spectral average over blades. This physical explanation confirms previous works in which a transfer function was developed between incoming turbulence and the turbine power outputs.

Tidal devices are likely to faced with shear flows and subjected to various wave climates. The paper presents an experimental study of the combined impacts of shear profile and irregular waves on the loading of a 1/20th scale device operating at peak power extraction. The experiments presented were conducted at various depths to facilitate analysis of the effects of the shear flow and wave impact on the device at various positions in the water column. The fluid field was measured at three different upstream positions and at three depths (top, middle and bottom of the rotor) for each experiment, in doing so, data from the device were captured three times. The fluid measurements were of a high quality and were analysed to present the structure flow upstream of the device, which contained velocity and turbulence profiles. The upstream measurement was utilised to understand the development of flow structures in the approach to the device, and the impact of the flow structures measured was confirmed via cross-covariance calculations. The long datasets gathered were used to produce full rotational probability density functions for the blade-root-bending moments for three blades. The spectral characteristics were also considered, and showed that rotor loading quantities are less reactive to smaller scale flow structures.

After a few weeks, underwater components of offshore structures are colonized by marine species and after few years this marine growth can be significant. It has been shown that it affects the hydrodynamic loading of cylinder components such as legs and braces for jackets, risers and mooring lines for floating units. Over a decade, the development of Floating Offshore Wind Turbines highlighted specific effects due to the smaller size of their components. The effect of the roughness of hard marine growth on cylinders with smaller diameter increased and the shape should be representative of a real pattern. This paper first describes the two realistic shapes of a mature colonization by mussels and then presents the tests of these roughnesses in a hydrodynamic tank where three conditions are analyzed: current, wave and current with wave. Results are compared to the literature with a similar roughness and other shapes. The results highlight the fact that, for these realistic roughnesses, the behavior of the rough cylinders is mainly governed by the flow and not by their motions.

International audience; The development of Marine Renewable Energy based on tidal currents at a selected area needs to first analyze the flow characteristics and thus the bathymetry induced turbulence in this area. The present study aims to investigate turbulence characteristics around two wall mounted obstacles in tandem, which represent seabed obstacles in a high Reynolds number flow. 2D planar PIV measurements are performed in multiple planes to access the main flow organization. Based on successive analyses of mean flow characteristics as well as of the flow organization, it is demonstrated that the presence of an upstream cube clearly impacts the cylinder wake development by preventing the rising of the large scale flow structures observed for an isolated wall-mounted cylinder in the same flow conditions. The TKE budget analysis shows that the energy is mainly produced in the upstream cube shear layer, and TKE exchanges are very small in the far wake field, in comparison with a single cylinder flow configuration. A Fourier analysis shows that the cube–cylinder interaction generates a low frequency peak in the far wake field which persists for long downstream distances. This particular frequency is the signature of the cylinder wake coherent structures. A statistical analysis based on the two-point correlation velocity tensor confirms that energetic flow structures present in the far wake are mainly of 2D nature and develop horizontally. Elongated structures are identified in the wake and seem to correspond to hairpin-like structures. Instantaneous vortex structures are also characterized with local vortex identification swirling strength criterion, confirming previous FFT and statistical results. The impact of the upstream cube is strongest in the near wake in the symmetrical transverse plane and prevents the large scale flow structures from rising in the water column.

This thesis examines the impact that turbulence and turbine operating condition have on the wake of a horizontal axis tidal turbine, and the implications of this impact for array design. This is done via the use of computer simulations using a scale-resolving hybrid turbulence model known as detached eddy simulation, which combines a Reynolds-averaged Navier-Stokes model in the near-wall region with large eddy simulation-like behaviour in other regions. This allows turbulent uctuations to be resolved in the wake region whilst reducing the computational expense when compared to a pure large eddy simulation model. These computer simulations were supported by extensive ume measurements at three tip-speed ratios and three different turbulence conditions, which were used for both validation of the computational model, as well as data in their own right. It was shown that turbulence intensity has a significant impact on the wake; increasing wake recovery, reducing wake swirl and increasing the width of the downstream region impacted by the wake, whilst simultaneously reducing the magnitude of this impact. In addition, high turbulence intensities appear to have some impact on the mean performance characteristics of the turbine and cause large increases in the magnitude of the uctuations of these performance characteristics. These increases in uctuations become larger still as the turbulence length scale increases. Turbulence length scale was not found to have a significant impact on the wake, however, an impact could not be ruled out. The turbine operating condition was also found to have a profound impact on the resulting wakes. In the near wake, centreline velocity recovery was found to increase with increasing turbine thrust due to ow being diverted towards the turbine nacelle. For a volumetric averaged wake, greater power extraction was found to cause the greatest near-wake deficit. Wake swirl was found to be at a maximum at the point of peak turbine torque, whilst wake width was found to increase with increasing tip-speed ratio (and therefore turbine thrust). It was found that the detached eddy simulation turbulence model produced good agreement with the experimental results across a wide variety of turbulence conditions. This is expected to be of interest to array designers wishing to optimise array configurations due to its ability to accurately predict both wakes and turbine performance, including transient effects.

A comparison of a tidal turbine's performance and structural loads is conducted using lab-scale numerical models and experimental testing under multiple current-only and wave-current conditions at the IFREMER wave-current flume. Experimental testing, used to validate CFD models, was accomplished using a 0.9 m diameter, 3-bladed tidal turbine and had a blockage ratio of 8% while the turbine was submerged. Initial investigations analysed the performance and loads on the turbine under uniform and profiled current-only conditions. The presence of a profiled velocity gradient was found to have a negligible effect on the average performance characteristics; however, transient thrust, torque and out of plane bending moment loads experienced much greater variations. These load fluctuations were further increased with increasing levels of shear in the velocity profile, while peaks in the turbine loads coincided with its rotational frequency. The addition of regular, Stokes 2nd Order Theory waves added to the complexity of the flow conditions experienced by the turbine. The effect on the average performance characteristics were negligible while the total turbine thrust and torque fluctuations increased by 35 times that of the current-only cases. Peaks in the loads aligned with the wave surface elevation, indicating the importance of transient analyses of dynamic loads.

International audience; This paper describes a detailed implementation of the Synthetic Eddy Method (SEM) initially presented in Jarrin et al. (2006) applied to the Lagrangian Vortex simulation. While the treatment of turbulent diffusion is already extensively covered in scientific literature, this is one of the first attempts to represent ambient turbulence in a fully Lagrangian framework. This implementation is well suited to the integration of PSE (Particle Strength Exchange) or DVM (Diffusion Velocity Method), often used to account for molecular and turbulent diffusion in Lagrangian simulations. The adaptation and implementation of the SEM into a Lagrangian method using the PSE diffusion model is presented, and the turbulent velocity fields produced by this method are then analysed. In this adaptation, SEM turbulent structures are simply advected, without stretching or diffusion of their own, over the flow domain. This implementation proves its ability to produce turbulent velocity fields in accordance with any desired turbulent flow parameters. As the SEM is a purely mathematical and stochastic model, turbulent spectra and turbulent length scales are also investigated. With the addition of variation in the turbulent structures sizes, a satisfying representation of turbulent spectra is recovered, and a linear relation is obtained between the turbulent structures sizes and the Taylor macroscale. Lastly, the model is applied to the computation of a tidal turbine wake for different ambient turbulence levels, demonstrating the ability of this new implementation to emulate experimentally observed tendencies.

International audience; The undulating membrane tidal energy converter is a device that uses the flutter instabilities occurring from the interaction between a slender body and a fluid flow. A new numerical model has been developed using a 2D corotational finite element method to represent the structure and the unsteady point-vortex method to compute the flow. These methods as well as the interaction process are presented. Trajectory and frequency of the undulating motion, hydrodynamic forces on the structure and velocity field in the wake are presented. Comparison shows a good agreement with experimental results obtained from a 1/20 th scale prototype without power take off tested in flume tank.

After positioning a 1:20 scaled model of a three-bladed horizontal-axis turbine in the wake of a wall-mounted cylinder, synchronized turbine performance and flow measurements are carried out to investigate the relationship between the incoming flow field and the turbine power fluctuations. The Linear Stochastic Estimation (LSE) is used to predict the turbine output fluctuations from the knowledge of the Large Scale flow Structures (LSS) embedded in the incoming turbulent flow. LSS extraction by Fourier analysis or Proper Orthogonal Decomposition shows that LSS are responsible for the main unsteady variations of the power fluctuations, especially their highest amplitudes. The RMS of turbine output fluctuations are entirely due to the LSS. It is also demonstrated that whatever the nature of the incoming turbulent flow is, the low frequency filtering process coupled with the LSE method allows the recovering of at least 90% of the turbine power RMS. Furthermore, the low-frequency spectral content of the turbine power fluctuations is very well predicted, especially the frequency peaks. A preliminary LSE application is performed in order to predict the instantaneous turbine output fluctuations at more than 85% confidence level, from only three velocity signals measured in front of the turbine.

In the main tidal energy sites like Alderney Race, turbulence intensity is high and velocity fluctuations may have a significant impact on marine turbines. To understand such phenomena better, a three-bladed turbine model is positioned in the wake of a generic wall-mounted obstacle, representative of in situ bathymetric variation. From two-dimensional Particle Image Velocimetry planes, the time-averaged velocity in the wake of the obstacle is reconstructed in the three-dimensional space. The reconstruction method is based on Proper Orthogonal Decomposition and enables access to a representation of the mean flow field and the associated shear. Then, the effect of the velocity gradient is observed on the turbine blade root force, for four turbine locations in the wake of the obstacle. The blade root force average decreases whereas its standard deviation increases when the distance to the obstacle increases. The angular distribution of this phase-averaged force is shown to be non-homogeneous, with variation of about 20% of its time-average during a turbine rotation cycle. Such force variations due to velocity shear will have significant consequences in terms of blade fatigue. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

International audience; In high flow velocity areas like those suitable for tidal applications, turbulence intensity is high and flow variations may have a major impact on tidal turbine behaviour. A three-bladed horizontal axis turbine model (scale 1:20) is positioned in the wake of a square wall-mounted cylinder, representative of specific in situ bathymetric variation, to experimentally study these effects in a current flume tank. Local and global loads are acquired in synchronisation with velocity measurements to study the turbine response to flow fluctuations. Velocity measurements need to be obtained close to the turbine, contrary to what is commonly considered, to properly correlate velocity and loads fluctuations. Results show that the loads phase average and their dispersion evolve according to the sheared velocity profile. We conclude that the turbine load fluctuations directly respond to the low frequency velocity fluctuations and are dominated by the turbulent structures shed from the cylinder. It is then possible to compare the effects of large coherent turbulent structures on the turbine behaviour to cases with more classical free stream turbulence commonly studied. These results provide a substantive database in high Reynolds number flows for further fatigue analysis or recommendations for turbine positioning in such flows.

Most of the research done on tidal energy focuses on marine current turbines. Therefore, tidal turbine’s wake is well-documented. The tidal energy converter studied here is based on the fluid-structure interactions that occur between a flexible membrane and an axial flow, resulting in an undulating motion that can be used to harvest energy. This device’s performance had been studied but it is the first time its wake is experimentally characterized from two-dimensional Particle Image Velocimetry (PIV) measurements. PIV is synchronized with a motion tracking system that gives information on trajectory and power conversion. Wake measurement gives access to velocity deficit, turbulence intensity and vorticity. Three configurations are tested in order to identify the influence of the main adjustment parameters. Pre-stress increases the membrane vertical speed, leading to a more important vertical expansion of the wake. The power extraction slows down the membrane’s motion, thus limiting the wake’s length and intensity. The flume tank measurements suggest that the best location for a downstream device in a tidal farm would be in the same horizontal position, at 5 membrane’s length. In the open sea, the membrane interaction effects should not predominant.

The dynamic behaviour of an undulating tidal energy converter under wave and current loadings is studied. Therefore, a small scale prototype of a damped pre-stressed undulating membrane is tested in a wave and current recirculatory tank. The membrane’s dynamic profile and hydrodynamic forces are measured synchronously with the surface elevation for both regular and irregular waves in current direction. Spectrum analysis reveals that the response to wave loading depends on the wave frequency and is reduced by damping-type power take-off. Coupling between the wave frequency and the main undulation frequency or its harmonics is also observed. An increase of 30% of the forces must be foreseen in case of wave condition in the tested range.

This Round Robin Test program aims to establish the influence of the combined wave and current effect on the power capture and performance of a generic tidal turbine prototype. Three facilities offering similar range of experimental conditions have been selected on the basis that their dimensions along with the rotor diameter of the turbine translate into low blockage ratio conditions. The performance of the turbine shows differences between the facilities up to 25% in terms of average power coefficient, depending on the wave and current cases. To prevent the flow velocity increasing these differences, the turbine performance coefficients have been systematically normalized using a time-average disc-integrated velocity, accounting for vertical gradients over the turbine swept area. Differences linked to blockage effects and turbulence characteristics between facilities are both responsible for 5 to 10% of the power coefficient gaps. The intrinsic differences between the tanks play a significant role as well. A first attempt is given to show how the wave-current interaction effects can be responsible for differences in the turbine performance. In these tanks, the simultaneous generation of wave and current is a key part often producing disruptions in both of these flow characteristics.

The development of marine current turbine arrays depends on the understanding of the interaction effects that exist between turbines in close proximity. Moreover, the ambient turbulence intensity also plays a major role in the behaviour of tidal turbines. Thus it is necessary to take ambient turbulence into account when studying interaction effects between several turbines. In order to highlight these interaction effects, experiments have been carried out in the IFREMER flume tank. These experiments focus on interactions between three horizontal axis turbines. This paper presents the experimental results obtained for three configurations with two ambient turbulence intensity rates. The results are presented in terms of turbine wakes and performance. The wake characterisation presents complex features for the three configurations and the lowest ambient turbulence rate: upstream turbines wakes are still present at the location of the downstream turbine and their wakes can interact or merge, depending on the tested configurations. On the contrary, for the highest turbulence rate, the downstream turbine wake is not affected in his shape by the two upstream ones which are not visible any more. In fact, as already observed in the previous studies of Mycek et al. [1, 2], the wake shape rapidly spreads out in the stream-wise direction behind the turbines. However, the velocity deficit and the turbulence intensity are higher for the downstream turbine comparing to the upstream ones. In terms of performance, one tested case presents an increase of the downstream turbine power production: when this turbine is exactly in the centre of the two upstream turbines and for the lowest turbulence rate only. A small misalignment of the layout axis with respect to the tidal current may result in a decrease of performance at the end. An analysis of the power spectral density functions of the downstream turbine torque and thrust shows that no signature of the upstream turbines can be found in these answers. Furthermore, the same spectral analysis carried out on the velocity measurements shows no signature of the upstream turbines either, from 3 diameters distance. This result is noticeable for the highest and the lowest tested turbulence cases and whatever the turbines configuration is.

An experimental study of a 1/20th scale undulating membrane tidal energy converter (UMTEC) based on EEL Energy’s device is presented to assess the impact of the membrane's aspect ratio on its behaviour. The aim of this work is to study 3D effects on the machine to optimise the energy extraction. Five aspect ratios from 0.26 to 0.57 are tested at constant length, keeping other geometric parameters constant too. The mechanical energy of the membrane is conveyed above the free surface by a vertical cogwheel/chain assembly to be converted by an electric generator. After describing the experimental set-up, results are presented according to the reduced flow speed. Undulation dynamics and power conversion are analysed. This study highlights the effects of an undulation mode change on the undulation amplitude and frequency, independently from the aspect ratio. In addition, it shows that the UMTEC performances increase with the aspect ratio up to 0.5. Over this aspect ratio value, the efficiency of the device seems to stagnate at its highest value, suggesting that 3D effects become negligible.

In this paper the turbulence effects are studied for three rotors mounted on the same instrumented hub. Two scaled models of industrial turbines and one open-geometry turbine are considered. The turbulence characteristics are obtained from 2D Laser Doppler Velocimeter measurements and the turbine behaviour is analysed from thrust and torque measurements. Three turbulence intensities and a large range of tip speed ratios and flow velocities are considered. The results are anonymised in order to ensure confidentiality. The rotors have different blade profiles, blade numbers and solidity. The rotor design largely modifies the mean power and thrust coefficients. The turbulence intensity only slightly changes these results but has a larger influence on the fluctuating loads than the different rotor designs. The spectral analysis of the rotor torque and thrust shows that, at low frequencies the load variations are correlated to those of the flow velocity with some differences due to the turbulence intensity levels. The coherences between the loads and the velocity seem to be not affected by the rotor type. At high frequencies, the load variations are correlated to the speed control unit of the scaled model and the rotor design has an impact on the rotational speed and loads coherences. info:eu-repo/semantics/publishedVersion

In high flow velocity areas like those suitable for tidal applications, turbulence intensity is high and flow variations may have a major impact on tidal turbines behaviour. Large boils that can be observed at the sea surface are emitted from the sea floor and may interact with the tidal turbines. These boils have then to be characterized. The Reynolds number, based on the rugosity height and the mean flow velocity, is rather high in this context: R e = 2.5 × 10 7 . For that purpose, experiments are carried out in a flume tank with R e as high as achievable in Froude similitude (in the tank: R e = 2 . 5 × 10 5 and F r = 0 . 23 ) in order to study coherent flow structures emitted behind seabed obstacles. The obstacle is here a canonical square wall-mounted cylinder chosen to be representative of specific in-situ bathymetric variations. Using PIV and LDV measurements, the flow past the cylinder is investigated. Using a POD filter, large coherent structures are identified and their trajectories are analysed. By means of a Lamb-Oseen profile approximation, properties of these structures are determined. The formation mechanism of such structures is discussed in this paper and their behaviour is characterized. It is assumed that vortices periodically shed from the obstacle interact and generate hairpin structures.

This paper presents an analytical linear model developed to study the behaviour of a buckled membrane tidal energy converter. The Euler beam theory and the elongated body theory are used for the fluid structure interaction formulation. The effect of electromechanical converters used to convert the undulating motion into electrical energy is reproduced by adding a term equivalent to viscous material damping. The influence of compression force, flaps and hanging conditions is studied, as well as the effects of simulated power take-off through internal damping. The system's behaviour is characterized by undulating mode, critical flow velocity, motion frequency and amplitude. The model shows good agreement in terms of frequency and satisfactory results for the amplitude compared to experimental data. The linear assumptions were validated on fluid and structure models as a good start for a first analytical model describing the system's physic. The obtained results confirmed the benefits of initial stress and optimized damping to the tidal converter for energy harnessing.

Bubble sweep-down on oceanographic vessels generates acoustic perturbations. We propose in this work to characterize the sub-surface bubbles occurrence conditions from acoustic data analysis acquired during surveys in relatively shallow water with the IFREMER research vessels Thalassa and Pourquoi Pas?. The methodology of data analysis used in this work allows us to characterize the sailing conditions influence on bubble sweep-down occurrence. The correlation between sailing conditions and acoustic perturbations tends to demonstrate that the presence of bubbles under the hull is clearly related to the wind speed and natural aeration, and that surface bubbles are advected differently in the water column by the two vessels.

One key step of the industrial development of a tidal energy device is the testing of scale prototype devices within a controlled laboratory environment. At present, there is no available experimental protocol which addresses in a quantitative manner the differences which can be expected between results obtained from the different types of facilities currently employed for this type of testing. As a consequence, where differences between results are found it has been difficult to confirm the extent to which these differences relate to the device performance or to the test facility type. In the present study, a comparative “Round Robin” testing programme has been conducted as part of the EC FP VII MaRINET program in order to evaluate the impact of different experimental facilities on the test results. The aim of the trials was to test the same model tidal turbine in four different test facilities to explore the sensitivity of the results to the choice of facility. The facilities comprised two towing tanks, of very different size, and two circulating water channels. Performance assessments in terms of torque, drag and inflow speed showed very similar results in all facilities. However, expected differences between the different tank types (circulating and towing) were observed in the fluctuations of torque and drag measurements. The main facility parameters which can influence the behaviour of the turbine were identified; in particular the effect of blockage was shown to be significant in cases yielding for high thrust coefficients, even at relatively small blockage ratios.

This paper presents possible techniques for modelling ambient turbulence in the Lagrangian Vortex Method formalism. Due to the fact that regular Synthetic Eddy Method (SEM) already presented in previous studies is not divergence free by definition; improvements were necessary to develop a similar SEM method with such a divergence free property. The recent improvements formulated by R. Poletto give the way to such a possibility. This new Divergence Free Synthetic Eddy Method (DFSEM) is presented here in comparison with the regular SEM. Obtained numerical velocity fields are compared in terms of convergence properties, Power Spectral Density and also Taylor macro-scale. Finally, turbine wakes are computed with both the recent Poletto’s DFSEM and the regular Jarrin’s SEM to highlight differences. At this stage of development, the DFSEM seems very promising even though some improvements are still necessary.

International audience; In high flow velocity areas, turbulence intensity is high and flow variations may have a major impact on tidal turbines behaviour. Previous studies show that a square wall-mounted cylinder produces a very extended and energetic wake. In this work, two-component PIV measurements are conducted in the symmetry plane of the flow in order to evaluate the Turbulent Kinetic Energy (TKE) budget. This analysis enables to show how the TKE, produced in the shear-layer region, is redistributed and dissipated within the flow. The Large-Eddy PIV method enables to obtain full spatial maps of dissipation and to assess the validity of the constant involved in the spectral method. Results show that the production occurs when the Reynolds shear stress is the more intense, so is the dissipation. Energy is then transported through turbulent motion into the outer flow and swept to the bottom due to the pressure gradient effects. Production is directly due to the periodic vortex shedding unlike for other terms. A better description of the energy exchanges in the turbulent wake flow is obtained. This knowledge is important for the performance determination of a tidal turbine positioned in such an obstacle wake which affects the flow-induced dynamic load on turbine.

Flows at tidal-stream energy sites are characterised by high turbulence intensities and by the occurrence of highly energetic large and coherent flow structures. The interaction of the flow with seabed roughness is suspected to play a major role in the generation of such coherent flow structures. The problem is introduced with canonical wall-mounted square obstacles representing abrupt changes of bathymetry, with high Reynolds number flow (Re = 250000). Two methods are used: a numerical model, based on the LBM (Lattice Boltzmann Method) combined with LES (Large Eddy Simulation) and an experimental set-up in a circulating tank. The numerical model is validated by comparison with experimental data. In the case of a wall-mounted square cylinder, large-scale turbulent structures are identified in experiments where boils at the free surface can be observed. LBM simulation allows their three-dimensional characterisation. The dynamic of such large-scale events is investigated by temporal, spatial and spectral numerical analysis. Results show that periodical Kelvin–Helmholtz vortices are emitted in the cylinder wake. Then, they merge to form larger and more coherent structures that rise up to the surface. A wavelet study shows that the emission frequency of the Kelvin–Helmholtz vortices is not constant over time.

The bubble sweep-down phenomenon around oceanographic research vessels generates acoustic disturbances. A specific experimental protocol in a wave and current circulating tank is used to study this phenomenon around a 1/30 scale ship model in a configuration with current, waves and ship motions. 3D visualizations of the bubble clouds and Stereo-PIV (Particle Image Velocimetry) measurements results, both obtained in the bow vicinity of the ship model, are presented in this paper. The behaviour of the breaking wave with bubble generation is studied and the spatial evolution of the bubble clouds is investigated. It is demonstrated that the bubble cloud generation is mainly conditioned by the ascending vertical velocity of the flow near the bow. It is observed that the bubble clouds are propagated in the flow direction with a horizontal expansion which is non negligeable, reaching a close position to the hull. The overall results provide new elements for the study and the understanding of the phenomenon compared with previous 2D studies.

International audience; The bubble sweep-down phenomenon around the oceanographic research vessels generates acoustic perturbations. A specific experimental protocol has been developed in a wave and circulating tank to study this phenomenon. This protocol is used to carry out trials on three different ship models in order to study the influence of the bow geometry on the bubble generation. For different test configurations, bubble clouds are described and compared in terms of area, maximal depth and vertical velocity to highlight bubble cloud dynamics surrounding the three ship models. The relation between the hydrodynamic flow field and the bubble generation is studied by means of Particle Image Velocimetry (PIV) measurements to study the phenomenon by the use of phase averaged velocity fields. The overall results enable us to characterize the bubble sweep-down phenomenon from the air bubble generation and propagation to the frequency of occurrence and the clouds behaviour.

An experimental study of an undulating membrane based on the Eel Energy device is presented in this paper. This system uses interaction between a semi-rigid plate and an axial flow to generate undulation. At full scale, the deformation of the structure is then converted into electrical power using linear electromagnetic generators. In order to simulate the power take-off on a 1/20th scale prototype, hydraulic dampers are located all over the length of the device. The dampers have non-linear behaviour. A representative damping parameter have been introduced to study their impact. Results are presented in function of fluid velocity and damping adjustment. Undulation mode, frequency and forces on the structure are described. Results show that fluid velocity has a destabilizing effect on the membrane, increasing undulation frequency and lowering its amplitude, while damping has the opposite effect. Furthermore, two methods used to evaluate the power conversion are described and evaluated from trajectory and forces data analysis. Power estimation shows an evolution proportional to cubic current speed at low flow speed, as for classical tidal turbines. Power also seems to scale with the square of the product between undulation frequency and amplitude.

The present study aims at investigating turbulence characteristics in high flow velocity areas like those suitable for marine energy application. The Reynolds number, based on the rugosity height and mean flow velocity, is rather high: R e = 2 . 5 × 1 0 7 . For that purpose, experiments are carried out in a flume tank with R e as high as achievable in Froude similitude (in the tank: R e = 2 . 5 × 1 0 5 and F r = 0 . 23 ). Obstacles are canonical wall-mounted elements chosen to be representative of averaged bathymetric variations: a cube and a cube followed by an inclined floor. First, the wake topology past a canonical wall-mounted cube is illustrated from PIV measurements. Results show a flow behaviour already observed in the literature but for different upstream conditions ( R e and turbulence intensity). Second, the impact of the addition of an inclined floor is studied. It is shown that the inclination causes a squeezing of the cube wake that strongly impacts the shape and intensity of the shear layer (up to 10% more intense with the inclined floor). To fully grasp the turbulence organization in the wake for both test cases, an analysis using both complementary Proper Orthogonal Decomposition and quadrant method is performed. POD acts as a turbulent noise filter and quadrant method decomposes turbulent events. Results show the predominance of ejection (Q2) and sweep (Q4) events in the flow Reynolds shear stress. Q2 events are more energetic although Q4 events prevail. It is observed that the inclined floor causes a persistence of Q2 and Q4 events higher into the water column, more than the impulsion given by the floor altitude variations. The rise of the cube wake due to the inclined floor is thus illustrated using Q4 predominance area.

International audience; 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.

International audience; The correlation between the flow turbulence and the performances of a ma- rine current turbine is studied. First, the incoming flow encountered in the flume tank is characterized in the framework of fully developed turbulent cascades in the inertial range. The Reynolds number, the Kolmogorov dissipation scale and the integral scale, are estimated from flow measurements. The intermittency of the turbulence is characterized in the lognormal multifractal framework, and the influence of the turbulent flow on the turbine power is assessed. The rotor speed control unit characteristics used for the turbine regulation induces non-negligible effects on the turbine behavior under fluctuations loads. Even if the power spectrum does not reveal any scale invariance, a multiscale analysis allows us to show the correlations between the turbulence time se- ries and the power produced. The classical Mean Square Coherency function shows that for scales larger than 10 seconds, the upstream velocity and power have large correlations. In the framework of the Empirical Mode Decomposition method, such correlations are studied using the time-dependence intrinsic corre- lation analysis method. This method allows to zoom into time-frequency scales where the flow perturbations induced some modifications in power production

The present paper aims at describing the use of a Synthetic-Eddy-Method (SEM), initially proposed by Jarrin et al. [12], in the 3D Lagrangian Vortex method framework. The SEM method is used here in order to generate a far-field incoming flow with a prescribed ambient turbulence intensity. However, for the account of the diffusive term in the Navier-Stokes equations, a classical Particle Strength Exchange model with a LES eddy viscosity is used. Firstly, the general characteristics of the Synthetic-Eddy-Method will be presented together with its integration in the framework of the developed 3D unsteady Lagrangian Vortex software [27]. The capability of the ambient turbulence model to reproduce a perturbed flow that verifies any turbulence intensity I∞ and any anisotropic ratio (σu :σv :σw ) will be discussed and validated. Then, the capability of the presented ambient turbulence model to compute turbine wakes will also be presented together with first results. Finally, comparisons will be made between the obtained numerical results against experimental data [22, 23] for two levels of ambient turbulence, namely I∞ = 3% and I∞ = 15%. Although the present study was initially performed in the framework of tidal energy, its application to wind energy is straightforward.

International audience; The ambient turbulence intensity in the upstream flow plays a decisive role in the behaviour of horizontal axis marine current turbines. Experimental trials, run in the IFREMER flume tank in Boulogne-Sur-Mer (France) for two different turbulence intensity rates, namely 3% and 15%, are presented. They show, for the studied turbine configuration, that while the wake of the turbine is deeply influenced by the ambient turbulence conditions, its mean performances turn out to be slightly modified. The presented conclusions are crucial in the view of implanting second generation turbines arrays. In addition, complete and detailed data sets (wake profiles and performance graphs) are made available to the scientific community in order to encourage further comparisons.

International audience; Tidal turbine will be installed in area with high current and high turbulence level. A characterisation of this last is required. The aim of the project THYMOTE is to characterize and understand the generation of eddies from smaller to several tens of meters. Three technics are used: Numerical modelling, Physical modelling, field measurements. Physical and numerical modelling show clearly the appearance of the eddies close to the bottom in presence of dunes or rocks and their motion towards the free surface.

International audience; The modelling of diffusive terms in particle methods is a delicate matter and several models have been proposed in the literature. The Diffusion Velocity Method (DVM) consists in rewriting these terms in an advective way, thus defining a so-called diffusion velocity. In addition to the actual velocity, it is used to compute the particles displacement. On the other hand, the well-known and commonly used Particle Strength Exchange method (PSE) uses an approximation of the Laplacian operator in order to model diffusion. This approximation is based on an exchange of particles strength. Although DVM is particularly well suited to particle methods since it preserves their Lagrangian aspect, its major drawback stems in the fact that it suffers from severe singular behaviours. This paper intends to give insights and ideas for coping with these issues, based on an exact decomposition of the diffusion coefficient allowing a hybrid DVM-PSE treatment of diffusive terms.

In this second part, images acquired from the specific trials developed in order to study the phenomenon of bubble sweep-down are analysed. A post-processing method has been developed to analyse the two air entrainment mechanisms described in the first part, for several test configurations. Bubble clouds are described in terms of depth, area and velocity for both vortex shedding and breakingwave bubble clouds. A parametric study is also performed to calculate the influence of each test parameter on the frequency of bubble generation. It is demonstrated that the occurrence of bubble clouds is proportional to the wave height, with a considerable influence of the phase shift between waves and motions. The overall results provide new elements for the understanding and the study of the phenomenon, with the final objective of obtaining a reliable tool that facilitates the design of research vessels. The authors gratefully acknowledge the DGA (the French Government Agency for Defense) and Ifremer for the financial support of this co-financed Ph.D. thesis. We also would like to thank Thomas Bacchetti and Jean-Valery Facq for their assistance in the design and set-up of these experiments, and Alcino Ferreira for the proof-reading of the final paper.

International audience; Managing ecological systems, which operate over large spatial scales, is inherently difficult and often requires sourcing data from different countries and organizations. The assumption might be made that data collected using similar methodologies are comparable, but this is rarely tested. Here, benthic video data recorded using different towed underwater video systems (TUVSs) were experimentally compared. Three technically different TUVSs were compared on different seabed types (rocky, mixed ground and sandy) in Kingmere Marine Conservation Zone, off the south coast of England. For each TUVS, species metrics (forward facing camera), seabed impact (backward facing camera) and operational performance (strengths and limitations of equipment and video footage) were compared with the aim of providing recommendations on their future use and comparability of data between different systems. Statistically significant differences between species richness, density, cover and assemblage composition were detected amongst devices and were believed to be mostly due to their optical specifications. As a result of their high image definition and large field of vision both the benthic contacting heavy and benthic tending TUVS provided good quality footage and ecological measurements. However, the heaviest TUVS proved difficult to operate on irregular ground and was found to cause the most impact to the seabed. The lightest TUVS (benthic contacting light) struggled to maintain contact with the seabed. The benthic tending TUVS was able to fly over variable seabed relief and was comparably the least destructive. Results from this study highlight that particular care should be given to sled and optic specifications when developing a medium- or long-term marine protected area monitoring programme. Furthermore, when using data gathered from multiple sources to test ecological questions, different equipment specifications may confound observed ecological differences. A benthic tending TUVS is recommended for benthic surveys over variable habitat types, particularly in sensitive areas, such as marine protected areas.

International audience; The modelling of diffusive terms in particle methods is a delicate mat-ter and several models were proposed in the literature to take such terms into account. The Diffusion Velocity Method (DVM), originally designed for the dif-fusion of passive scalars, turns diffusive terms into convective ones by expressing them as a divergence involving a so-called diffusion velocity. In this paper, DVM is extended to the diffusion of vectorial quantities in the three-dimensional Navier-Stokes equations, in their incompressible, velocity-vorticity formulation. The inte-gration of a Large Eddy Simulation (LES) turbulence model is investigated and a DVM general formulation is proposed. Either with or without LES, a novel expres-sion of the diffusion velocity is derived, which makes it easier to approximate and which highlights the analogy with the original formulation for scalar transport. From this statement, DVM is then analysed in one dimension, both analytically and numerically on test cases to point out its good behaviour.

International audience; The coupled dynamics of the codend fluttering motion and its hydrodynamics behavior are investigated based on PIV measurements carried out around a 1/10 scaled model codend. The mean wake flow characteristics are firstly analyzed demonstrating the great effect of the codend motion. The differences in flow contour between fluttering motion and no-fluttering codend are highlighted by analyzing the classical mean characteristics of a body wake flow. In presence of fluttering motion, the classical results are recovered only if one considers the vertical extent of the codends oscillations as a reference length scale. Secondly, the frequency content of codend oscillations is analyzed and it is linked to the physical mechanisms. It is observed that the motion of the codend is mainly of a low-frequency activity and of another component related to the vortex shedding street. It is then demonstrated a complex fluid-structure interaction where the near wake hydrodynamics of the moving codend structure are an intricate interplay between the codend fluttering motion and the shear layer instabilities and large scale vortex shedding. The knowledge of such flow instabilities is of great importance to improve the understanding of the force acting on a trawl and for the implementation of some selected device to drive fish near the codend and then to reduce juvenile by-catch.

International audience; When performing Particle Image Velocimetry (PIV) measurements in complex fluid flows with moving interfaces and a two-phase flow, it is necessary to develop a mask to remove non-physical measurements. This is the case when studying, for example, the complex bubble sweep-down phenomenon observed in oceanographic research vessels. Indeed, in such a configuration, the presence of an unsteady free surface, of a solid– liquid interface and of bubbles in the PIV frame, leads to generate numerous laser reflections and therefore spurious velocity vectors. In this note, an image masking process is developed to successively identify the boundaries of the ship and the free surface interface. As the presence of the solid hull surface induces laser reflections, the hull edge contours are simply detected in the first PIV frame and dynamically estimated for consecutive ones. As for the unsteady surface determination, a specific process is implemented like the following: i) the edge detection of the gradient magnitude in the PIV frame, ii) the extraction of the particles by filtering high-intensity large areas related to the bubbles and/or hull reflections, iii) the extraction of the rough region containing these particles and their reflections, iv) the removal of these reflections. The unsteady surface is finally obtained with a fifth-order polynomial interpolation. The resulted free surface is successfully validated from the Fourier analysis and by visualizing selected PIV images containing numerous spurious high intensity areas. This paper demonstrates how this data analysis process leads to PIV images database without reflections and an automatic detection of both the free surface and the rigid body. An application of this new mask is finally detailed, allowing a preliminary analysis of the hydrodynamic flow.