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 takeoff 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. Highlights ► An experimental model of undulating membrane tidal energy converter at 1/20th scale is presented. ► Power TakeOff simulation using hydraulic dampers is proposed and described. ► Two methods of power conversion estimation have been described and evaluated. ► Membrane motion and forces are measured in function of current speed and damping adjustment ► Power conversion scales with the square product of undulation main frequency and motion amplitude.

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Bubble sweep-down is a significant issue for the oceanographic vessels, which affect the acoustic sur- veys. Experimental trials, carried out in the Ifremerwave and current circulating tank on a 1/30 model of the Pourquoi pas?, are presented. The results demonstrate that this kind of experimental facility is well suited to study the phenomenon of bubble sweep-down encountered around the bow of a ship under specific conditions. From these results, two kinds of bubble clouds formation have been observed and analysed: bubble clouds generated by vortex shedding and breaking waves. The vortex shedding bubble clouds appear randomly in all the configurations tested, even without waves or motions. This phe- nomenon is due to the interaction between the turbulent flow and the specific bow shape of the Pourquoi pas?. On the other hand, the breaking wave clouds appear in the presence of relative motions between the free surface and the bow ship and more significantly under wave sollicitations. A complementary paper presents a parametric study carried out to quantify the influence of the test conditions

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 Highlights ► Characterization of sub-surface bubbles occurrence conditions from acoustic data analysis. ►Correlation between sailing conditions and acoustic perturbations. ► Bubbles distribution comparisons demonstrate the effect of bubble sweep-down.

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. Highlights► Undulating membrane first wake characterization from PIV measurements, ► Phase-averaged velocity deficit and vorticity are measured and presented, ► Vorticity dissipation rate result can be used to calibrate numerical model.