Analysis of hydrodynamics of a moving trawl codend and its fluttering motions in flume tank

Abstract

The coupled dynamics of the codend fluttering motion and its hydrodynamics behavior are investigated based on PIV measurements carried out around a 1/10 scaled model codend. The mean wake flow characteristics are firstly analyzed demonstrating the great effect of the codend motion. The differences in flow contour between fluttering motion and no-fluttering codend are highlighted by analyzing the classical mean characteristics of a body wake flow. In presence of fluttering motion, the classical results are recovered only if one considers the vertical extent of the codends oscillations as a reference length scale. Secondly, the frequency content of codend oscillations is analyzed and it is linked to the physical mechanisms. It is observed that the motion of the codend is mainly of a low-frequency activity and of another component related to the vortex shedding street. It is then demonstrated a complex fluidstructure interaction where the near wake hydrodynamics of the moving codend structure are an intricate interplay between the codend fluttering motion and the shear layer instabilities and large scale vortex shedding. The knowledge of such flow instabilities is of great importance to improve the understanding of the force acting on a trawl and for the implementation of some selected device to drive fish near the codend and then to reduce juvenile by-catch.

Effect of Cutting Ratio and Catch on Drag Characteristics and Fluttering Motions of Midwater Trawl Codend

“…The experimental apparatus effect of model nets is a common problem, especially in codend model tests. This study and previous studies by Pichot et al [24], Bouhoubeiny et al [14], and Druault and Germain [13] used a self-designed configuration of this rigid frame to accurately measure the hydrodynamic forces of the codend. One of the principal objectives in fishing gear hydrodynamic measurements is to design frames with very lower drag forces unlike those of fishing equipment in general, and nettings or codends in particular.…”

“…A codend with catch has obvious oscillations in position or drag force. Previously, Bouhoubeiny et al [14], Madsen et al [18], and Druault and Germain [13] reported that codend oscillations can be caused by the vortices behind the codend. When dragging the trawl, vortex shedding generates vertical pressure on the codend and makes it oscillate.…”

“…Single large-volume objects mainly include a water-injected balloon or a spherical cap. When Druault and Germain [13] used the PIV technology to study the effect of catch on shadowing and flow field around the codend, a spherical cap and water-injected balloon were used in the place of catches. Such a surrogate also plays a role in blocking the flow directly through the mesh, but it completely blocks the flow.…”

“…Moreover, as the codend is connected to the end of trawl, the effect of shadowing on the codend cannot be ignored [12]. Druault and Germain [13] reported that to optimize the codend efficiency in terms of catchability and its effect on energy consumption, it must have a high static stability, which is difficult due to the significant influence of hydrodynamic turbulence flow. Furthermore, they mentioned that the knowledge of flow instability is important to better understand the force acting on a codend and to implement a selected device.…”

“…These preliminary analyses showed that a symmetrical vortex exists behind the rigid codend and possible turbulent flow interactions with the fluttering codend structure without a complete study of the PIV (Particle Image Velocimetry) database. Recently, Druault and Germain [13] used the PIV method to evaluate the flow field distribution around a codend and found that, while dragging the trawl, vortices are alternately generated behind the codend. The shedding of the vortex generates vertical pressure on the codend and causes it to oscillate.…”