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.
Sebases schlegelii is an important economic reef fish in northern China. Its distribution in the reef area is affected by the reef structure and flow field. To evaluate the effects of differently shaped artificial reefs with distinct flow field properties on the distribution of adult S. schlegelii, four types of artificial reefs (cubic (CAR), pyramidal (PAR), star-shaped (SAR) and tubular (TAR) artificial reef) and the related flow regimes were experimentally examined. The mean gathering rate (MGR) and wavelet analysis were adopted to analyze the distribution patterns of S. schlegelii adults around reef bodies in three representative environmental conditions (static water (FE1), 40 Hz water flow environment (FE2), and 50 Hz water flow environment (FE3)). The significant gathering behaviors around the artificial reefs were depicted through the MGR of adult fish during physical observations. For FE1, the MGR of adult fish showed a decrease in the order PAR > CAR > SAR > TAR. With increasing incoming velocities, the MGR changed such that the quantity of adult fish in the CAR gradually increased more than that in the PAR, but no significant differences were observed between them (P>0.05). Wavelet analysis showed that the fluctuations in the gathering behaviors of adults decreased with increasing flow velocities. Based on the two-factor analysis of variance, the reef structural characteristics had a more significant impact than the flow velocities on the gathering behavior of adult fish. In summary, the PAR and CAR are recommended to restore the habitats of S. schlegelii in marine ranches given their dominant aggregation effects on S. schlegelii.
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