## 4 Discussion
It means that the codend oscillates more violently in the z-direction than in the x-direction, the reason for this phenomenon may be that the vortex shedding pressure is the same in each direction, but there is the binding effect of the codend itself in the x-direction, while the binding effect of codend itself at z-direction is far less than that at x-direction. This conclusion was similar to those obtained by [PERSON] et al. [40] on the fishing net and [PERSON] and [PERSON] [15] on the codend. The quasi-periodic oscillations of the motions of the three codends obtained in this study showed two peak frequencies of 0.037 and 0.33 Hz for the 4P codends, and 0.025 and 0.19 Hz for the 2P codend both on the streamwise and transverse motions. These frequencies observed on the three codends showed flat spectra, which is consistent with the results reported by [PERSON] et al. [14] and [PERSON] and Germain [15]. These flat spectra occurred because the codend motions were caused by the presence of vortex shedding behind the codend related to the low frequencies [14; 41]. This vortex shedding can be justified by observing the Reynolds number as function of the codend diameter which was varied between 119,309.94-180,707.41, 107,519.87-164,040.63, and 102,829.49-153,736.04 for codends 1, 2, and 3, respectively. Indeed, according to the research of [PERSON] et al. [10] and that of [PERSON] and [PERSON] [15], the presence of the catch inside the trawl net results in a larger codend volume and blockage of the meshes, that limits the flow passage through the trawl structure and generates a greater transverse pressure on the codend and create the vortex shedding. However, the low-frequency observed on the codend motions seems to be directly linked to the [PERSON] variations in the three codend drag and associated to the elastic nature of the extended codend. Furthermore, the first peaks of these two motions can due to structure vibrations, while the second peaks could be due to the periodic oscillations induced by the vortex shedding behind these codends [15; 27]. Concerning the flow organization, this study analyzed the influence of the gear design on the development of the turbulent flow inside and behind the codend using ECVM measurements obtained at two points inside and outside the codends. This analysis showed that the turbulent intensity inside and behind the 4p codend were lower than that obtained inside and behind the 2P codend. Indeed, inside the codend, the turbulent intensity varied from 0.077-2.64% and 0.12-5.11% for the 4P codend and 2P codend, respectively. Behind the codend, it varied from 0.11-3.84% and 0.25-6.85% for the 4P codend and 2P codend, respectively. The reason of this grater turbulent intensity observed through the 2P codend compared to 4P codend was due to the fact that 2P codend motions were unstable and intense unlike those of the 4P codend. These unstable and intense motions decreased the mesh opening of the codend which limits the flow passage through the codend structure engendering flow disturbances. These disturbances engender the creation of the unsteady turbulent flow inside the codend and the vortex shedding behind the codend which was considerably important on the 2P coded compared to 4P coded making it less selective. This trend was confirmed by the experimental study carried out by [PERSON] et al. [14], [PERSON] and Germain [15], and [PERSON] et al. [13; 27] who demonstrated the existence of the vortex shedding behind the codend. The circumference of the standard codend (codend 1) was greater than that of codons 2 and 3 in the end-part of the coded, meaning that the mesh was more open in the standard codend. This greater mesh opening in the standard codend can be justified by the fact its motions are very lower compared to those of other codons. However, in codons 2 and 3, the mesh opening is not stable due to their intense motions. In this case, the mesh opening angle on these two cod-ends depends on the catch size and the codend position.