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.