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. Highlights ►Trials on 3-bladed horizontal axis marine current turbine were run in a flume tank. ►Two ambient turbulence intensity rates are considered. ►The wake and performances of the turbine are characterised. ►The ambient turbulence intensity deeply influences the behaviour of the turbine.

The future implantation of second generation marine current turbine arrays depends on the understanding of the negative interaction effects that exist between turbines in close proximity. This is especially the case when the turbines are axially aligned one behind another in the flow. In order to highlight these interaction effects, experiments were performed in a flume tank on 3-bladed 1/30th scale prototypes of horizontal axis turbines. This work focuses on the interactions between two horizontal axis marine current turbines, axially aligned with the upstream flow. Thrust and power coefficients function of the rotation speed of the downstream device are presented. Besides, the wake of each turbine is characterised so as to explain their behaviour. A large range of inter-device distances is considered, as well as two upstream turbulence intensity conditions, namely 3% and 15%. This latter parameter deeply influences the behaviour of a marine current turbine and thus plays a preponderant role in the interactions effects between two devices. Indeed, this study points out that, for the considered turbine and blade geometry, higher ambient turbulence intensity rates (15%) reduce the wake effects, and thus allows a better compromise between inter-device spacing and individual performance. Highlights ► Interaction effects between two aligned 3-bladed horizontal axis marine current turbines are considered. ► Two ambient turbulence intensity rates are considered. ► The wake and performances of the turbine are characterised. ► A wide range of inter-device distances is considered. ► The ambient turbulence intensity deeply influences the interaction effects.

Abstract-The understanding of interaction effects between marine energy converters represents the next step in the research process that should eventually lead to the deployment of such devices. Although some a priori considerations have been suggested recently, very few real condition studies have been carried out concerning this issue.We therefore ran trials on 1/30 th scale models of three-bladed marine current turbine prototypes in a flume tank. Our work focuses on the case where a turbine is placed at different locations in the wake of a first device. The interaction effects in terms of performance and wake of the second turbine are examined and compared to the results obtained on single-device configurations. Besides, we are currently developing a three-dimensional code based on a vortex method, which will be used in the near future to model more complex layouts.The experimental study shows that the second turbine is deeply affected by the presence of an upstream device and that a compromise between individual device performance and inter-device spacing is necessary. Numerical results show good agreement with the experiment and are promising for the future modelling of turbine farms.

The long term reliability of tidal turbines is critical if these structures are to be cost-effective. Optimized design requires a combination of material durability models and structural analyses which must be based on realistic loading conditions. This paper presents results from a series of flume tank measurements on strain gauged scaled turbine blades, aimed at studying these conditions. A detailed series of tests on a 3-blade horizontal axis turbine with 400 mm long blades is presented. The influence of both current and wave-current interactions on measured strains is studied. These tests show that wave-current interactions can cause large additional loading amplitudes compared to currents alone, which must be considered in the fatigue analysis of these systems. Highlights ► This is one of the first papers to describe how wave and current conditions affect tidal turbine blade deformation. ► There are also very few published data from devices at sea so these results are very important. ► Results from flume tank tests are presented first, showing how blade deformation depends on current speed. ► Then results indicate that the wave currents combination will significantly enhance blade loads compared to currents alone.

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: Re ¼ 2:5 107. For that purpose, experiments are carried out in a flume tank with Re as high as achievable in Froude similitude (in the tank: Re ¼ 2:5 105 and Fr ¼ 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. Highlights ► Experimental study of coherent flow structures past a wall-mounted square cylinder. ► Tests are carried out on a wall-mounted cylinder representative of seabed elements, in Froude similitude with high Reynolds number. ► PIV measurements are performed in vertical measurement planes and spatial analyses are performed. ► POD analysis and center detection allow to study the vortices behaviour.

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.

The long-term reliability of tidal turbines is critical if these structures are to be cost effective. Optimized design requires a combination of material durability models and structural analyses. Composites are a natural choice for turbine blades, but there are few data available to predict material behaviour under coupled environmental and cycling loading. The present study addresses this problem, by introducing a multi-level framework for turbine blade qualification. At the material scale, static and cyclic tests have been performed, both in air and in sea water. The influence of ageing in sea water on fatigue performance is then quantified, and much lower fatigue lives are measured after ageing. At a higher level, flume tank tests have been performed on threeblade tidal turbines. Strain gauging of blades has provided data to compare with numerical models.

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.