Numerical study on the hydrodynamic characteristics of biofouled full-scale net cage

Abstract

The effect of biofouling on the hydrodynamic characteristics of the net cage is of particular interest as biofouled nettings can significantly reduce flow of well-oxygenated water reaching the stocked fish. For computational efficiency, the porous-media fluid model is proposed to simulate flow through the biofouled plane net and full-scale net cage. The porous coefficients of the porous-media fluid model can be determined from the quadratic-function relationship between the hydrodynamic forces on a plane net and the flow velocity using the least squares method. In this study, drag forces on and flow fields around five plane nets with different levels of biofouling are calculated by use of the proposed model. The numerical results are compared with the experimental data of Swift et al. (2006) and the effectiveness of the numerical model is presented. On that basis, flow through full-scale net cages with the same level of biofouling as the tested plane nets are modeled. The flow fields inside and around biofouled net cages are analyzed and the drag force acting on a net cage is estimated by a control volume analysis method. According to the numerical results, empirical formulas of reduction in flow velocity and load on a net cage are derived as function of drag coefficient of the corresponding biofouled netting.

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“…Governing equations review In the present numerical model, the governing equations comprises the Navier-Stokes equations coupled with the realizable k-ε model, wh ich have been described in details in our previous studies (Bi, Zhao, & Dong, 2015;Zhao et al, 2013), thus a brief review is given here. Navier-Stokes equation are…”

“…As the most part of the float collar is above the free surface of the water and the sinkers and mooring system are relatively small-scale co mponents. Thus, only the netting is simu lated in this numerical model and the cylindrical netting is divided into 16 plane nets around the circu mference and one bottom net (Bi et al, 2015). Both the clean and biofouled netting of the net cage were simu lated using the porous -media flu id model and the corresponding porous coefficients were determined fro m the hydrodynamic forces acting on the plane-net sheet which are correlated with the features of the biofouled netting (Bi et al, 2015;Zhao et al, 2013).…”

“…Tab le 1 shows the porous coefficients for the netting model with a thickness of 1 m corresponding to both clean and biofouled nettings. The porous coefficients were mainly determined by the field measurements of Swift et al (2006) together with the empirical formu las by Aarsnes et al (1990), which has been described in details in our previous work (Bi et al, 2015). To analyze the effect of incidence angle on the flow field, t idal currents through the fish farm at 0°, 22.5°, 45°, 67.5° and 90° were investigated numerically without considering the effect of biofouling.…”

Wave diffraction of a hybrid wind turbine foundation with a double-layer aquaculture cage

“…They created models of twines with artificial hydroid fouling and tested drag forces in a towing tank. Bi et al (2015); Bi et al (2017) studied the hydrodynamic characteristics of a single offshore net cage and a square array of offshore net cages with biofouling using a three-dimensional CFD model. They found that the damping effects of the cage array increased with increasing biofouling.…”

Soft computing method for predicting pressure drop reduction in crude oil pipelines based on machine learning methods

“…Then, the cutoff would suppress the small eddies and, presumably, lead to drag reduction. Several researchers conducted a comprehensive study on drag reduction for water flow and proposed relationships for the Fanning friction factor, which helps other researchers to analyze their results [16][17][18][19]. Virk [20] investigated the performance of different polymer solutions and found a trend to a maximum drag reduction (MDR) asymptote in all cases.…”