Unsteady flow dynamic response in the cylinder-netting structure for the design of offshore fish farm systems using an SST-IDDES turbulence model

Abstract

An improved comprehension of the flow dynamics and hydrodynamic features of the interaction between the cylinder and nettings, crucial elements of offshore aquaculture systems, can optimize the development of environmentally friendly and sustainable large-scale aquaculture pens. This study investigates the instantaneous flow fields and hydrodynamic response in the interaction between the cylinder and netting structures using a high-fidelity Computational Fluid Dynamics approach based on the k-ω shear stress transport-improved delayed detached eddy simulation turbulence model within the Large Eddy Simulation region. The Richardson Extrapolation method was employed to assess the convergence of the numerical solutions across various levels of mesh refinement. The conducted research revealed that the inclusion of the cylinder resulted in an increase in the drag coefficient ranging from 2.25 % to 36.78 % due to the complex interaction between the combined netting-cylinder, shear layer instabilities, and vortex shedding, when compared to the two individual nettings. As Reynolds number and cylinder diameter increased, cylinder drag coefficients decreased, causing large-scale vortex shedding and unsteady turbulent wake flow. Furthermore, the wake interactions of the netting are relatively strong with low flow velocity when the solidity ratio and twine diameter increased due to the disruption caused by the netting-twine, mesh shape, and netting position in the flow passage. The drag and lift coefficients of the combined cylinder-netting structure increased with increasing solidity ratio, inclination angle, and decreasing flow velocity. The Fourier analysis showed that the hydrodynamic coefficients of only the cylinder and single netting are primarily low-Strouhal numbers activities, while the hydrodynamic forces of the combined cylinder-netting structure are primarily low-frequency activities connected to unsteady turbulent flow streets. The understanding of the hydrodynamic and flow instabilities that occur close to the wake of the contact between the cylinder and the nettings is of utmost significance for the purpose of optimizing the design of offshore aquacultural structures, particularly for situations that are characterized by biofouling.