Experimental study of flow and mass transport in the near-wake region of a rigid planar metal net panel

Abstract

Aquaculture cages constructed from rigid material have become increasingly popular in recent years due in part to the continuous expansion of the aquaculture industry offshore, where cages need to withstand much harsher ocean conditions. Most previous studies of hydrodynamic interactions with aquaculture cages have explored the impacts of more traditional flexible cage net panels on wake flow fields and mass transport properties. In the current experimental study, Particle Image Velocimetry and Planar Laser Induced Fluorescence techniques were used to measure flow and concentration fields in the near wake region of a steady current through a rigid metal net panel to analyze both the flow patterns and mass transport processes. The results indicated that, similar to a flexible polyethylene net panel, the time-average velocities and turbulence characteristics in the near wake region were highly inhomogeneous and anisotropic, with the turbulent intensity following a short-range, power-law decay downstream of the panel. In direct comparison with a flexible net panel of similar net solidity, the rigid net panel was found to slightly enhance the lateral spreading of the plume, with cross-sectional profiles of the mean concentration and concentration fluctuation shown to exhibit self-similar Gaussian distributions. The rigid net panel also resulted in a lower flow blockage, as measured by a velocity reduction coefficient, with the corresponding turbulent intensity also reduced compared to the flexible panel due to an overall reduction in the drag force. In addition, the rigid panel tended to create larger wake eddies than the flexible panel, leading to enhanced scalar mixing and, hence, faster rates of concentration decay in the near-wake region. These experimental findings have potential implications for cage-based aquaculture operations, where rigid fish cages are comparatively more likely to generate favorable environments for fish growth, as well as helping to reduce negative impacts on the surrounding aquatic environment.