Fluid-Structure Interactions in Pelagic Trawls and Probable Consequences for the Selectivity of the Fishing Gear

Abstract:

To study the phenomenon of fluid-net-structure interaction of a trawl the authors investigated effects of hydro-elasticity of stiff net panels (grids) by means of special experiments in a wind tunnel. The analyses were especially focused on how the grids affect the current. One of the targets was to observe the correlation between the actual distribution of velocity distant and close to the grid and the respective hydrodynamic load distribution namely under consideration of prevailing geometrical and kinematical parameters. Another aim of the tests was to get an idea of the fluid-dynamic interactions between neighbouring mesh bars which is implicitly expressed in the well known nomographic diagram by Koritzky (1973). And last but not least the third target was to analyse the impact of the grid on the fluid parameters in correlation with further investigations regarding the understanding of the phenomenon of selectivity of fishing gears. In context of these objectives the hydrodynamic loads as well as the current distributions close to and far from the grid surface were measured by a six-component balance, by hot-wire anemometers as well as by a 2d-PIV-system. The tests were focused on small angles of attack. To control the motion of fluid two end plates and a base plate were used. The consequences of a fluid blockage at the end of a grid were analysed. Results are presented and discussed.