## 2 Material and Methods
### The Measurement of the Hydrodynamic Forces in the Flume Tank  
A series of experiments measuring the drag of the three codons for a range flow velocities and range catch sizes were carried out in the flume tank at Tokyo University of Marine Sciences and Technology (TUMSAT). The flume tank is 9.0 m long by 2.2 m wide and 1.95 m deep with a side observation window of 9.0 m \(\times\) 1.6 m. A front and bottom view window of the flume tank allows users to observe the codend behavior during testing and to take video footage during drag measurements. The volume of the water in the flume tank was \(\sim\)150 m\({}^{3}\). The flow was circulated with four contra-rotating impellers via constant-speed hydraulic delivery pumps with a diameter of 1.6 m in diameter. The flow speed can be varied from 0.1 to 2.0 m/s, the acceleration varied from 1.0 to 5.0 cm/s\({}^{2}\), and the reciprocating oscillation flow of velocity amplitude was \(\pm\)0.5 m/s. The turbulent level in the flume tank is 4.63% and can be increased to 18% when removing the flow straighteners [26; 27]. The codend models were mounted on a 0.45 m circular rigid frame and submerged in water at a depth of 0.15 m that were attached at a vertical bar to a sensor recording the drag forces of the codend (see Figure 3). A current meter was installed at approximately 2.0 m upstream of the codend model to detect the flow velocity. The drag force signals, as measured using the load cell, were amplified using a dynamic strain amplifier (DPM-6H). These signals and the flow velocity signals were then sent to an A/D converter and subsequently to a computer. The drag forces were measured using a six-component load cell with a capacity of 5 kg each and a specified accuracy of 2% (TLP-5 KS, Tokyo Measuring Instruments Co., Ltd.; Tokyo, Japan). These load cells were calibrated and zeroed at both the beginning and the end of each test, and their linearity was confirmed. The measurements were taken using five (5) flow velocities varying from 0.5 to 0.9 m/s with an interval of 0.1m/s. The flow velocity in the flume is relatively even (\(\pm\)4%). The data rate was 50 samples per second and an average value was estimated based on 3000 individual measurements obtained over 60 s. The data were sampled at the frequency of 50 Hz. As shown in Figure 3, the codend models located at the middle of the flume tank were free to move in the water flow. The measuring methods and steps of hydrodynamic forces are as follows (see Figure 3):  
1. The voltage signals of hydrodynamic forces are transmitted to the strain meter using the sensor during the flow passage thought the combining codend and circular frame system;
2. The strain meter amplifies and integrates sensor signals, producing analog signal information that can be displayed continuously online;
3. The analog signals are changed to digital signals by an A/D converter;
4. Hydrodynamic forces are represented using computer calibration coefficients. In the above manner, flow velocity signals are also obtained from the propeller tachometer sensor. The displacement of the catch was not observed during the test. The end-part of the codend was assumed to be a sphere, and the diameter of this sphere (\(d\)) was measured during the experiments and the values are reported in Table 2. Thus, to evaluate the effect of twine area and catch size on the drag of different codends, the relationship between drag coefficient and Reynolds number was established. Reynolds number represents the ratio of inertial and viscous forces:  
\[R_{e}\ =\ \frac{u_{0}\ L}{\
u}, \tag{1}\]  
where \(u_{0}\) is the flow velocity, \(L\) is the length in m, \(\
u\) is the kinematic viscosity, and \(R_{e}\) is the Reynolds number. In this study, twine thickness was used to evaluate the effect of twine material and twine diameter on the drag, and codend diameter was used to evaluate the effect of catch weight on the drag.