Flow Turbulence Characteristics and Mass Transport in the Near-Wake Region of an Aquaculture Cage Net Panel

Abstract

Cage-based aquaculture has been growing rapidly in recent years. In some locations, cage-based aquaculture has resulted in the clustering of large quantities of cages in fish farms located in inland lakes or reservoirs and coastal embayments or fjords, significantly affecting flow and mass transport in the surrounding waters. Existing studies have focused primarily on the macro-scale flow blockage effects of fish cages, and the complex wake flow and associated near-field mass transport in the presence of the cages remain largely unclear. As a first step toward resolving this knowledge gap, this study employed the combined Particle Image Velocimetry and Planar Laser Induced Fluorescence (PIV-PLIF) flow imaging technique to measure turbulence characteristics and associated mass transport in the near wake of a steady current through an aquaculture cage net panel in parametric flume experiments. In the near-wake region, defined as ~3M (mesh size) downstream of the net, the flow turbulence was observed to be highly inhomogeneous and anisotropic in nature. Further downstream, the turbulent intensity followed a power-law decay after the turbulence production region, albeit with a decay exponent much smaller than reported values for analogous grid-generated turbulence. Overall, the presence of the net panel slightly enhanced the lateral spreading of the scalar plume, but the lateral distribution of the scalar concentration, concentration fluctuation and transverse turbulent scalar flux exhibited self-similarity from the near-wake region where the flow was still strongly inhomogeneous. The apparent turbulent diffusivity estimated from the gross plume parameters was found to be in reasonable agreement with the Taylor diffusivity calculated as the product of the transverse velocity fluctuation and integral length scale, even when the plume development was still transitioning from a turbulent-convective to turbulent-diffusive regime. The findings of this study provide references to the near-field scalar transport of fish cages, which has important implications in the assessment of the environmental impacts and environmental carrying capacity of cage-based aquaculture.

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

“…away from wall effects). We compare these experimental results directly with our previous flexible net panel results (Shao et al, 2021) and other relevant studies from literature, discussing the potential implications of our experimental findings for cagebased aquaculture.…”

“…Specifically, Bi et al (2013) examined the wake flow field for a steady current through flexible polyethylene and polyamide net panels, observing a significant velocity reduction that depended on net solidity and the net inclination angle to the current. More recently, Shao et al (2021) studied turbulence characteristics and scalar transport for a steady current through a flexible polyethylene net panel using a combined Particle Image Velocimetry (PIV) -Planar Laser Induced Fluorescence (PLIF) technique. They found that the flow turbulence 5 exhibited high inhomogeneity and anisotropy in the near wake region downstream of the panel, with the turbulent intensity decay following a power-law relationship.…”

“…Our study aims to examine both the flow and mass transport characteristics in the near-wake region of a rigid planar net panel. As for our previous experimental study on flexible net panels (Shao et al, 2021), rigid panels with different geometric properties (i.e. twine diameters and mesh sizes) were secured vertically within an open channel water flume, occupying the entire channel cross-section perpendicular to the mean flow direction.…”

Advances in Environmental Hydraulics

“…One review paper [10] features the historical evolution of EH and the observed shift in its research trends through the bibliometric analysis of the latest EH literature. The second review paper [11] deals with the environmental impacts associated with the rapidly growing mariculture industry, while one research paper [12] investigated the flow and concentration patterns downstream an aquaculture cage net panel in parametric flume experiments. Four studies approached the stream and river systems from a variety of perspectives, ranging from the physical modelling of flow with vegetation [13] and the numerical simulation of solute transport in river with dead zones [14], to the laboratory study of flood discharge atomisation [15] and the geomorphic characterisation and classification of a large river [16].…”

“…Cage-based aquaculture has been growing rapidly in recent years, and the clustering of large quantities of cages in fish farms located in inland lakes or reservoirs and coastal embayments or fjords significantly affects flow and mass transport in the surrounding waters. To address this issue, Shao et al [12] employed the combined particle image velocimetry and planar laser induced fluorescence (PIV-PLIF) flow imaging technique to measure turbulence characteristics and associated mass transport in the near wake of a steady current through an aquaculture cage net panel in parametric flume experiments. In the near wake region, defined as~3 M (mesh size) downstream of the net, the flow turbulence was observed to be highly inhomogeneous and anisotropic in nature.…”