Numerical Simulation of Scalar Mixing and Transport through a Fishing Net Panel

Abstract

With the rapid development of commercial aquaculture in recent decades, large numbers of submerged cages or pens are clustered in fish farms that are commonly located within inland lakes, reservoirs, and coastal embayments around the world. The submerged structures have significant influence on both the flow fields and mass transport processes in surrounding water bodies. While existing studies have concentrated mainly on the flow blockage effects produced by fish cages, the associated effect on near-field mass transport processes, important for pollution transport and dispersal, remains largely unclear. To address this knowledge gap, a CFD (computational fluid dynamics) model was established using OpenFOAM to investigate the wake characteristics and scalar transport processes through a fishing net panel, as representative of a key component of the fish cage or pen. In this model, the net panel was represented as porous media, and the finite volume method was applied to solve the governing flow equations with the standard k-ε model used for turbulence closure. Experimental data from previous studies were used to calibrate and validate the numerical model, which was applied to different scenarios over a range of net solidities and incoming flow velocities. Overall, the numerical model results demonstrated that porous media schematization could adequately reproduce the blocking effect from the net panel on the mean flow field, as well as the induced changes to scalar transport, with satisfactory accuracy. The flow velocity reduction across the net panel was found to strengthen with increasing net solidity and decreasing incoming velocity, while the scalar concentration decay tended to become enhanced when the incoming velocity was decreased. The lateral profile of the scalar concentration exhibited a self-similar Gaussian distribution with the spreading width of the plume reduced by increasing the incoming velocity. This lateral concentration distribution was minimally affected by the upstream scalar source location relative to the net panel, when adopting the current RANS and porous media modelling approach. The model results provide useful references for the assessment of the environmental impacts and carrying capacity of cage-based fish farming.

Uncertainty Assessment of the Remaining Volume of an Offshore Gravity Fish Cage

“…Their results can be used to check whether the water exchange is enough for the fish school inside the cage. Zeng et al [17][18][19] compared flow field and mass-transport in the near-wake region of a rigid and of a flexible net panel for aquaculture in their experiments. They found that the rigid panel created a lower flow blockage and larger wake eddies, which contributed to generating favorable environments for fish growth.…”

Effects of Inclination Angles on the Hydrodynamics of Knotless Net Panels in Currents

“…In addition to experimental techniques, numerical methods, including fluid-structure interaction (FSI) and computational fluid dynamics (CFD) methodologies, are also capable of analysing flow patterns. Patursson et al [11], Bi et al [12] and Yang et al [13] studied downstream velocity reductions of fixed net panels through the equivalence of porous media, and the accuracies of the numerical method were validated against the published experimental data. Chen and Christensen [14] and Cheng et al [15] incorporated porous media modelling with the lumped mass method and Morison force model, enabling a two-way coupling scheme and expanding its applicability to flexible nets and cages.…”