The profile and fluttering characteristics of a codend with different mesh sizes and catch by fast Fourier transform and Morlet wavelet methods

The codend is the end and an essential part of the trawl, which plays a vital role in the selectivity and the storage of fish catches. Thus, to improve the trawl selectivity and control the probability of the fish escaping, it is essential to understand its hydrodynamic performance, fluttering motions, and spatial form of the codend structure. This study investigates the effects of the mesh size and simulated catch on hydrodynamic characteristics, net cross-section, and fluttering motions of the codend in the flume tank. Thus, three codends models were designed using polyethylene material with three different mesh sizes (60 mm, 80 mm, and 100 mm). Fourier analysis was performed via the Morlet wavelet analysis and the fast Fourier transform (FFT) method to observe the content of the wavelet coefficient variance and amplitude of the codend fluttering motions and time evolution of the drag forces. The results showed that at the empty stage, the decrease in mesh size led to an increase in drag force by 6.6%− 25.1%, an increase of the codend length by 2.6%− 4.3%, and a decrease in cross-sectional area by 14.9%− 51.7% of the codend. In addition, the time evolution was oscillatory quasi-periodic and related to the low-frequency activity. The codend drag oscillation frequency was mainly 0.30–0.45 Hz and decreased with the mesh size, and the amplitude and wavelet coefficient of the drag force increased with decreasing mesh size. For the codend with simulated catch, the shape of each codend was closed at the front of the mesh and bulging atthe rear contour, with the middle part of each codend being the most closed, and the cross-sectional area was about 15.1% of the empty codend (X = 104 cm). The codend position oscillates frequency was 0.30–0.45 Hz in X and Z directions. The drag oscillation frequency was mainly 0.30–0.45 Hz and synchronized with the position motion. The dominant frequencies of these codend motions and the associated drag showed low-frequency activities, with a small wavelet spectrum shifting from small-scale to large-scale motions. In addition, the codend with a greater mesh size showed a more stable behavior with a lower frequency and oscillatory motions than the codend with a small mesh size led to a decrease in drag on this codend. In addition, we recommend that the FFT can be used to obtain the amplitude and frequency directly for codend steady oscillation.