Time-Resolved PIV investigations of the flow field around cod-end net structures

Abstract

Flow field measurements past a fixed rigid cod-end structure and past a porous fishing net structure are conducted using Time-Resolved PIV method. The rigid cod-end is first used to characterize finely the wake flow. Proper Orthogonal Decomposition (POD) is then applied in order to extract the large scale energetic vortices of the flow from the measured velocity field. It is then observed that the first POD modes are associated to the Karman's type flow structure of vortex shedding. It is shown that the characteristics of the wake flow behind the rigid cod-end flow configuration compare quite well with previous ones obtained from bluff cylinder or sphere wake analyzes. Second, PIV measurements are performed around a non-rigid bottom trawl which is free to move. Preliminary analyses show that the frequencies associated with the oscillatory motion of the porous structure are the same as the ones detected in the near wake flow demonstrating the lock-in regime. It is then expected that these preliminary results provide some interesting informations about future investigations on the drag force acting on fishing net structure.