Elsevier

Applied Ocean Research

Volume 64, March 2017, Pages 258-280
Applied Ocean Research

Numerical simulation of the effects of fish behavior on flow dynamics around net cage

https://doi.org/10.1016/j.apor.2017.03.006Get rights and content

Highlights

  • A precise numerical model is developed to analyze the flow field around net cage.

  • The structural model of net cage is build using nonlinear finite element method.

  • Effects of fish behavior on flow field and deformation of net cage are analyzed.

Abstract

The computational fluid dynamics study is performed to analyze the impact of the cultured fish on the flow field through net cage and the deformation of net cage. The shear stress turbulent k-omega model is applied to simulate the flow field through the net cage, and the large deformation nonlinear structure model is adopted to conduct the structural analysis of the flexible net cage. To validate the net-fluid interaction model of the net cage in current, a series of physical model tests are conducted, which indicate that the numerical model can accurately simulate the flow field around the net cage and the deformation of the net cage. A fish model is used to simulate the effect of fish behavior on the flow pattern around the net cage and the deformation of the net cage. In addition, the flow fields around the net cage in current are investigated considering different fish group structures, fish swimming speeds, fish distributions and fish stocking densities. The results indicate that the circular movement of fish in the still water leads to a low pressure zone at the center of net cage, which causes a strong vertical flow along the center line of the net cage. The drag force on the net cage is significantly decreased with the increasing fish stocking density, but the most severe deformation of net cage occurred in the case of medium fish stocking density.

Introduction

Due to the shrinking availability of near-shore region, it is difficult for the expansion of the near-shore aquaculture. Thus, the fish farm is forced to move into the offshore area, however, the fish farm at the offshore area has to withstand large loads due to waves and current, which calls for a new challenge for the design of net cage and mooring system in the open sea. Numerous studies have shown that the force on the net cage is proportional to the square of the flow velocity. A small velocity differences may lead to great force differences, therefore, the predictability of the flow field inside and outside a net structure is important in order to accurately simulate the hydrodynamic loads and deformation of the net structure.

The flow regimes around the net cage mainly depend on the incoming current, the net cage structure and the effect of fishes and their behavior. To our knowledge, the researches on the interaction between the net cage and the environment have focused on the effects of current and waves on the net cage. Fredheim and Faltinsen [1] proposed a three-dimensional numerical model of the fluid flow around the net cage, in which each of the net elements including the twines and knots is modeled as a set of source distribution to describe the disturbance of the net structure on the fluid flow. Lader et al. [2] conducted a series of physical model test to analyze the force and geometry of a net cage in uniform flow field, and an average of 20% velocity reduction was measured inside the net cage. Lader and Fredheim [3] investigated dynamic properties of a flexible net sheet exposed to waves and current by numerical simulation, in which the net was modeled by dividing it into a series of super elements. Harendza et al. [4] measured the flow field around the net cages in current by PIV, and a strong attenuation effect on the current occurred with the increasing solidity. Shim et al. [5] investigated flows through and around farms by numerical simulation and observed a complex flow pattern with maximum drag at the highest net solidity. Gansel et al. [6] described the different wake flow regimes for low and high values of net solidity. Kristiansen and Faltinsen [7] proposed a screen type of force model for the viscous hydrodynamic load on nets, in which the net is divided into a slew of flat net panels, or screens. Xu et al. [8] investigated the hydrodynamic behavior of a submersible gravity cage with grid mooring system in the surface and submergence conditions using numerical simulation and physical model test. Levy et al. [9] experimentally investigated the impact of the twine/mesh ratio on the flow through a porous hollow cylinder. Bi et al. [10] conducted numerous physical model tests to investigate the damping performance of net cages in waves, and the result indicated that the damping of net cage has a close relationship with the number of net cages, the net solidity and the geometrical shape.

Few studies have focused on describing the hydrodynamic influence of the fish school in net cage. Chacon-Torres et al. [11] observed that the fish swimming in the small-scale net cage may cause the vertical or horizontal currents through a dye dispersion experiment. Johansson et al. [12] performed the field measurements at four fish farms in Norway and analyzed the effect of the fish behavior and the biomass on the vertical distribution of the oxygen and the flow pattern. The salmon switched from the traditional circular polarized group structure to a group structure where all fishes kept stations at fixed positions swimming against the current. Gansel et al. [13] analyzed the effects of the biofouling and the fish behavior on the flow patterns around net cages through the physical model tests and the field measurements. On the interaction between the current and the fish school, some experiments were conducted and the current attenuation and redirection were observed, while the quantitative effects of the fish swimming speed and the fish stocking density are still largely unknown. Due to its complexity in simulating the fish behavior, there has been a rather large disconnection between the observations and the simulations.

Due to the fact that the net can be approximated by a huge number of small cylinders connected with knots, an approach to simulate the net as a thin volume of porous media was proposed for analyzing the flow field around the net cage structure by Patursson et al. [14] and Zhao et al. [15]. In their approach, the net cage is modeled as a porous membrane with finite thickness over which the total pressure drop in the incoming current is defined by the combination of Darcy's law and an additional term accounting for inertial effects. Although a series of researches on the flow field inside and outside of the net structure were analyzed, the details of the flow field around each net mesh remain to be misunderstood. The numerical simulation of the flow field around the stocked net cage is complex due to the innumerable meshes and other properties like boundary conditions of the fish skin, and the structure flexibility. In this study, a precise net-fluid interaction model is developed, in which the details of microscale hydrodynamic processes can be captured by simulating each of the filaments that compose the net meshes. For this net-fluid interaction model, the kω SST model is applied to simulate the flow filed around the net cage structure, and the large deformation nonlinear structure (LDNS) model is used to simulate the net structure. The pressure on the net mesh is adopted to directly calculate the hydrodynamic loads on the net mesh other than the modified Morison equation, in which the increase of current velocity through the net mesh can be considered in this numerical model. A series of physical model tests were conducted to validate this net-fluid interaction model. Furthermore, the flow patterns are closely related to the oxygen supply and wastes removing for aquaculture production inside net cages. However, there is scarce information on the flow patterns inside the stocked net cages, therefore, it is necessary to analyze the effect of fish behavior on the flow patterns inside the net cage and the netting deformation. In this study, a simplified fish model was proposed to analyze the effect of the fish behavior on the flow patterns around the net cage and the deformation of net cage.

The current study is organized as follows: in Section 2; an elaborate fluid-net one-way coupling model and a fish model are introduced; and then the numerical model is validated by comparing the numerical results with the experimental data in Section 3; after that, the impact of the fish behavior on the flow pattern around the net cage and the deformation of the net cage is analyzed in Section 4; Finally, some conclusions are drawn.

Section snippets

Numerical modeling approach

A numerical model for analyzing the flow field through the stocked net cage and the deformation of net cage is developed. The main concept of the numerical approach is to combine the kω Shear Stress Turbulent (SST) model and the large deformation nonlinear finite element method to simulate the interaction between flow and stocked net cage with one-way coupling techniques, as shown in Fig. 1. A more detailed calculation procedure is given as follows.

The calculation procedure includes three

Physical model test

A series of physical model tests for the net in current were conducted. The flow field around a plane net and the deformation of a flexible net were measured, and the net-fluid interaction model was validated by comparing the numerical results with the experimental data. The physical model tests were carried out in a wave-current flume at the State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian, China. The flume is 69 m long, 2 m wide and 1.8 m deep,

Results and discussion

The previous observations [20] of the swimming behavior of Atlantic salmon at a commercial farm indicate that at high current velocities, the salmon switched from the traditional circular polarized group structure, seen at low and moderate current velocities, to a group structure where all fishes kept stations at fixed positions swimming against the current. In this study, two kinds of fish group structures are considered here: Circular (Fig. 19) = polarized swimming in a circular movement;

Conclusions

A net-fluid interaction model and a simplified fish model were proposed for analyzing the effects of fish behavior on the flow field around the net cage and the deformation of the net cage. A series of physical model tests were conducted to validate the numerical model. The results indicate that the net-fluid interaction model can simulate the flow field around the net cage and the deformation of the net cage accurately. The following conclusions can be drawn from the case study:

  • (1)

    The fish

Acknowledgements

This work was financially supported by the National Natural Science Foundation (NSFC) Projects No. 51239002, 51409037, 51579037, and 51221961, China Postdoctoral Science Foundation (No. 2014M560211 and No. 2015T80254), the Fundamental Research Funds for the Central Universities No. DUT16RC(4)25 and Cultivation plan for young agriculture science and technology innovation talents of Liaoning Province (No. 2014008).

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