Experimental and Numerical Investigations of the Hydrodynamic Characteristics, Twine Deformation, and Flow Field Around the Netting Structure Composed of Two Types of Twine Materials for Midwater Trawls

Abstract

Nettings are complex flexible structures used in various fisheries. Understanding the hydrodynamic characteristics, deformation, and the flow field around nettings is important to design successful fishing gear. This study investigated the hydrodynamic characteristics and deformation of five nettings made of polyethylene and nylon materials in different attack angles through numerical simulation and physical model experiment. The numerical model was based on the one-way coupling between computational fluid dynamics (CFD) and large deflection nonlinear structural models. Navier-Stokes equations were solved using the finite volume approach, the flow was described using the k-ω shear stress turbulent model, and the large deflection structural dynamic equation was derived using a finite element approach to understand the netting deformation and nodal displacement. The porous media model was chosen to model the nettings in the CFD solver. Numerical data were compared with the experimental results of the physical model to validate the numerical models. Results showed that the numerical data were compatible with the experimental data with an average relative error of 2.34%, 3.40%, 6.50%, and 5.80% in the normal drag coefficients, parallel drag coefficients, inclined drag coefficients, and inclined lift coefficients, respectively. The hydrodynamic forces of the polyethylene and nylon nettings decreased by approximately 52.56% and 66.66%, respectively, with decreasing net solidity. The drag and lift coefficients of the nylon netting were approximately 17.15% and 6.72% lower than those of the polyethylene netting. A spatial development of turbulent flow occurred around the netting because of the netting wake. However, the flow velocity reduction downstream from the netting in the wake region increased with increasing attack angle and net solidity. In addition, the deformation, stress, and strain on each netting increased with increasing solidity ratio.