Unsteady turbulent flow developing inside and around different parts of fluttering trawl net in flume tank

Abstract

Fluttering motions of a trawl are affected by its geometrical shape, natural underwater flows, and unsteady turbulent flow inside and around it. Thus, knowledge of the interaction between fluttering motions and the turbulent flow developing inside and around the different parts of the trawl is essential for understanding the hydrodynamic performance and selectivity of fishing gear. This paper assesses the coupled dynamics of the fluttering motions of different parts of a trawl and their hydrodynamics behavior. The analysis is based on three-dimensional electromagnetic current velocity meter measurements carried out inside and around different parts of a 1/18 scaled bottom trawl model. Instantaneous flow velocity fields and turbulent flow parameters were analyzed from obtained flow data. Moreover, fast Fourier transform and wavelet transform analysis were conducted to analyze the time–frequency contents of instantaneous flow velocity fields and fluctuating parts. The motions of different parts of the trawl net were oscillatory quasi-periodic and related to low frequency activity. The mean velocity ratios inside and around trawl wing, square, trawl body, and codend were 0.92, 0.89, 0.86, and 0.84, respectively. These results demonstrate the existence of unsteady turbulent flow inside and around different parts of the trawl. This unsteady turbulent flow corresponds to turbulent boundary layer flow, turbulent flow because of trawl wake, and vortex shedding. The dominant frequencies of these unsteady turbulent flows showed low frequency activities and wavelet energy was transferred from small-scale to large-scale motions under unsteady shear turbulence. In addition, knowledge of unsteady turbulent flow is greatly important for better understanding the hydrodynamic forces acting on a trawl and its geometrical shape as well as for implementing various selective devices and mesh shapes to reduce juvenile by-catch.