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

Abstract

Knotless nets have been extensively used in sea cages and offshore fish farms; the explicit illustrations of local flow patterns and subjected hydrodynamic loads have practical implications for the safe operations of aquaculture pens in open oceans. However, the effects of inclination angles of knotless nets on local flow patterns have not yet been fully discussed. In this paper, the effects of inclination angles on local time-averaged and instantaneous flow fields, velocity reductions and force coefficients are investigated using a k-ω shear stress transport-improved delayed detached eddy simulation (SST-IDDES) high-fidelity turbulence model. It is demonstrated that the inclination angles have dominant effects on the time-averaged velocity magnitudes around the net meshes. Secondly, the considerable flow interactions amongst the transverse and longitudinal net meshes, as well as cross-flow effects, are observed but weakened with the inclination angles. Thirdly, the profiles of velocity decelerations behind the net panels for 0∘–45∘ are not as trivial as those for 45∘–90∘. The associations between the embedded mechanism of subjected loads and pressure fields are established to analyse the current loads on nets in small inclination angles. The inclination angles are finally elaborated into the area-averaged velocity reduction and screen force models, then the improved characteristics are validated against the experimental data.

Flow Field Pattern and Hydrodynamic Characteristics of a Grid Device Made with Various Grid Bar Spacings at Different Inclination Angles

“…Thierry et al [23] (2021) Used Numerical modeling based on fluid-structure interaction in two-way coupling to identify the turbulent flow that develops around different parts of the bottom trawl. Wang et al [24] (2023) numerically analyses the effects of inclination angles on local time-averaged and instantaneous flow fields, velocity reductions, and force coefficients and found that the inclination angles have dominant effects on the time-averaged velocity magnitudes around the net meshes and the hydrodynamic characteristic of netting.…”

On the Digital Twin of The Ocean Cleanup Systems—Part I: Calibration of the Drag Coefficients of a Netted Screen in OrcaFlex Using CFD and Full-Scale Experiments

“…Since a fishing net can change orientation with regard to the incoming flow, it is important to understand the effect of its angle of attack. For example, Simonsen et al [15], Zhao et al [16], Gansel et al [17], and Thierry et al [18], Tang et al [19], Mi et al [20], Yu et al [21], Endresen and Moe Føre [22], Wang et al [23] studied the effects of the flow speed and angle of attack on the drag and lift forces on nets through numerical simulation, empirical formulation, and/or physical model experiments. As a matter of fact, as the angle between the flow direction and the plane net is getting smaller, a shielding effect can drastically impact the drag coefficient.…”