## 1 Introduction  
Understanding the engineering performance of the trawl codend in terms of hydrodynamic characteristics and selectivity is essential for the improvement, development, and operation of bottom trawl and midwater trawl effectiveness with successful retention of target catch, high fuel efficiency, and low collateral environmental impact [1; 2; 3; 4]. Indeed, as one of the main components of the trawl net, the codons are the rear part of the trawls, which collect the catch and where most selectivity process occurs [5; 6]. In addition, the codend drag has a particularity that varies during the towing in the fact that it increases as the catch builds up, which must be taken into account [7; 8; 9; 10]. This codend drag varies considerably depending on the trawl motions, which will influence the codend geometry and reduce the mesh opening, restricting the flow passage through the codend. Therefore, taking into account the codend drag linked to the codend design and oscillatory motions is essential for the better knowledge and understanding of the trawl selectivity process, fish behavior, and the state of health of fish escaping into the trawl mesh [10; 11; 12; 13]. However, the interaction between fluid and fluttering codend motions is an extremely complex and important issue that can be used to understand the development of unsteady turbulent flow such as vortex shedding inside and outside the codend and its impact on the trawl performance. Indeed, the interaction between the catches and water motions will create a pressure gradient on the codend surface that will create intense codend motions. These codend motions produce the vortex shedding and the wake of the gear directly related to elastic, kinematic, and dynamic instabilities of the drag force acting on it and may affect the selectivity via the fish response, such as swimming speed and maneuverability [14; 15; 16]. The fact that the study of the interaction between fluid and fluttering codend motions greatly affects the gear selectivity makes it a hot research topic in many fisheries. This is because the improvement in gear selectivity has increased in recent years due to the decline in fish stocks influenced by several discards at sea and bycatches of undesirable species [10; 15; 17]. However, much research has improved the gear selectivity, largely by examining the mesh size selection relative to the fish size without considering this complex interaction [18; 19; 20; 21]. Therefore, the development and the innovation of the codend design to improve the selective properties, the physical performance of the codend, and increasing the fisheries sustainability are some of the great challenges that have attracted more attention and studies in scientific research in order to maintain the marine ecosystem. In the past decades, some experimental studies on the analysis of the codend behavior have been reported in several researches. [PERSON] et al. [10; 22] examined the effect of flow velocity and codend catch on the codend drag associated with the drag coefficient of each codend. They concluded that the towing speed and the maximum frontal area of the codend were the predominant components of the codend drag. [PERSON] and [PERSON] [18] analyzed the effect of catch weight on trawl behavior by using the real measurements at sea and found that the ratio of the volume behind the front on the catch volume could significantly influence the netting surface area affected by the catch. [PERSON] et al. [17] evaluated the behavior of six different codends designed with different mesh shapes to evaluate the impact of codend design (four- and two-pieces using diamond, T0, and T90 meshes) on the codend drag and movement of the codend, and found that each codend oscillates considerably when loaded with the catch. [PERSON] et al.