## 1 Introduction
[6] analyzed the effect of cutting ratio and catch on drag characteristics and fluttering codend motions and found that the unfolding degree of codend netting and the height of codends was directly proportional to the current speed and inversely proportional to the cutting ratio. Some researchers have studied the physical performance and the codend selectivity by analyzing the flow field around the codend linked to their motions. For example, [PERSON] [16; 23] performed one-point measurements inside realistic codons of a shrimp beam and bottom trawls using Acoustic Doppler Velocimeter (ADV) and confirmed that turbulence flow inside the codend structure impacted the selective properties of the codend by impacting the likelihood of escape. [PERSON] and [PERSON] [15] characterized the flow in the unsteady wake developed behind the fluttering codend structure and found that the local hydrodynamiceffects that occur in the wake zone, and the motion of the fluttering codend can cause the coded oscillations. [PERSON] et al. (2018) analyzed the hydrodynamic performance of one full-scale T0 and T90 codends and compared using flume tank testing, with and without a small-mesh cover. They discovered that T90 codends kept meshes that were comparatively open and had much more drag than T0 codends. [PERSON] et al. (2018) developed a full-scale shaking codend (T90 codend with canvas) to reduce the retention of undersized fish in the reddish. They discovered that the total acceleration and drag forces estimated for the shaking codend were significantly higher than the T90 codend across all flow velocities[PERSON]  
However, despite the existing work on the subject and the available data, there is no relevant conclusion on the experimental studies evaluating the effect of the catch weight and codend design on the fluid-structure interaction of the codend. The existing experimental and numerical research lacks a detailed analysis of the simultaneous effect of turbulent flow and the fluttering codend motions on the hydrodynamic behavior of the codend, although it is important for the trawl design because of its effect on hydrodynamic forces, selectivity, and fish behavior. In addition, the previous studies have other limitations, such as: (i) the different size of catches was not taken into account; and (ii) the effect of the codend oscillations and shape on the analysis of the hydrodynamic forces and unsteady turbulent flow in order to reliably improve the engineering performance and understand the catch process and fish selectivity were not discussed. In this study, three scaled flexible codend structure models were designed and tested in a flume tank with different catch weights to evaluate the basic behavior of different codend designs mainly used in the midwater fisheries to discuss the relevant information that could be used to improve codend selectivity, reducing energy consumption, and understand the fish behavior. More precisely, the analysis was focused on the hydrodynamic force's measurements, codon motions, gear shape, and velocity measurements inside and behind the codend on the determination of the instantaneous velocity fields and turbulence indicator such as turbulent kinetic energy. Thereby, this study analyzes the influence of gear design parameters, catch weights, and inflow velocities on the fluid structure interaction of the flexible coded structure. The power spectra density (PSD) method was applied to analyze the frequency of temporal measured parameters. The findings are expected to contribute to the improvement of the trawl performance and selectivity control.