Caractérisation de l’écoulement autour de structures souples et poreuses : Application aux engins de pêche

The objective of this study is to experimentally determine the flow characteristics governing the hydrodynamic behaviour of fishing gear and in particular of a bottom trawl. This porous structure is characterized by horizontal and curved parts, but also a final part of spherical shape named codend. In order to characterize the different types of flow observed over this porous structure, Particle Image Velocimetry (PIV) measurements are carried around several kind of devices, representative of all or part bottom trawl. In particular, a complete 1/10 scaled bottom trawl and a sheet of net are used to characterize the development of the boundary layer above the horizontal part of this porous structure. The sheet of net is also used to characterize the wake of the curved part. The flow is also measured around rigid and porous cod end in order to investigate the vortex shedding governing the dynamic of this specific part. To determine the frequency of the large scale vortex structures, a spectral analysis of the longitudinal velocity is applied. Using the Proper Orthogonal Decomposition (POD) for such investigation, it is demonstrated the effectiveness of this procedure to extract the large scale spatial flow structure and their associated frequencies in the wake of the cod-end and to develop a phase averaged allowing to extract the mean velocity associated to each position of the moving porous structure. It is demonstrated that this new phase averaged obtained by POD procedure allows the determination of the spatial and temporal evolution of the boundary layer over moving porous structure. The experimental results obtained are useful for the improvement of the numerical models aiming at computing the drag of the fishing gear. Indeed, to enhance the overall drag computation of a fishing gear, one has to take into account velocity information in the close vicinity of the porous structure. Furthermore, the present work provides benchmark PIV data of the unsteady flow developing on fishing net porous structures allowing the advancement of unsteady numerical codes.