Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

CFD-Based Analysis of the Flow Field in Aquaculture Net Cages
Si-Yu Hou1, Wen-Yang Duan1, Ke-Xin Zhang1, Ji-Kang Chen1,*
1.College of Shipbuilding Engineering, Harbin Engineering University.
Harbin, Hei long Jiang Province, China

coefficient is overestimated because of the shielding effect. Balash et al.
(2009) proposed an expression to calculate the drag coefficient byintroducing a modified Reynolds number to determine the dragcoefficient of the net, the resistance coefficient was overestimatedprobably by the effect of surface roughness. Kristiansen and Faltinsen(2012) proposed a formula for calculating the resistance coefficient,which included Reynolds number and compactness, and fitted theresistance coefficient of the cylinder by a seventh-order polynomial.
The analysis of the calculated values compared with the physical model
results showed that the formula predicted values were overestimated.
Computational fluid dynamics (CFD) considers fluid viscosity. Bysolving the discrete equations, the velocity and pressure of the fluid canbe obtained. In the case of simulations for nets and net cages, they areusually simulated with porous media plates rather than cylindricalsystems. The feasibility of the porous media model depends mainly on
the selection of the drag coefficient. Patursson et al. (2010) used leastsquares to find the optimal porous media drag coefficient using a k-ε
turbulence model and the finite volume method to calculate the velocity
discount factor as 0.9. Zhao et al. (2013) investigated the flow fieldaround the porous media net and net cage models. To obtain the porous
media drag coefficients, Chen and Christensen (2016) proposed amethod based on a Morrison model. The results of this method yield alow drag coefficient for the porous media model net.
The screen model can only derive the force on the net cage without
describing the information of flow field, while the CFD method needs
the drag coefficient before calculating. Furthermore, the existing flowvelocity formula describe the flow velocity linear formula based ondrag coefficients for a measurement point. There is no quantitative
study of the flow field information after the net and the net cage. So,
this paper proposes a combination of the two methods and calculates
the force on the net cage, and then, summarizes empirical formulas forthe flow-velocity of flow field and the measurement points for the net
and the net cage.

ABSTRACT
Based on screen model and CFD method, this paper establishes areliable numerical tool to simulate and analyze the real aquaculture net
tank environment, numerically simulates and calculates the load for theflow field information of the net and the downstream of the net tank in
the flow field, and extracts the equations for the flow field of the net
and the downstream of the net tank and the measurement points.

KEY WORDS: culture net cage, CFD, screen model, flow fieldmeasurements, flow load.

INTRODUCTION
The world's food problem is getting worse, as a sustainable industry,aquaculture becomes more and more important. The aquaculture isestimated to reach a demand of about 1.8 billion tons of seafood by2030 (FAO, 2018).
Despite the considerable benefits of marine aquaculture, there aresome limitations. The biggest problem is the potentially harsh marine
environment. Harsh sea conditions can pose a significant challenge tostructural facilities as well as mooring system, and the flow field candirectly affect the dissolved oxygen levels and the accumulation of feedand excreta in the fishery, making the calculation of net tank loads andthe simulation of flow field information necessary.
At present, screen model and CFD method are the main method
applied to the simulation of net cage. Milne (1972) estimated the dragcoefficients for knotted and knotless nets based on empirical formulas
using solid ratio from 0.2 to 0.8 and operate a full-scale physicalexperiment within the Reynolds numbers of 389~5×103. The dragcoefficient of non-biofouling nets is from 1.260 to 1.416, while the dragcoefficients for biofouling nets is from 2.129 to 3.985. Aarsnes (1991)
established an empirical relationship for drag coefficients with respect
to solid ratio and incidence angle, but the equations did not consider theeffect of Reynolds number. To explain the discrepancy between theempirical formula and experimental results, the effect of Reynolds
number was introduced. Zhan et al. (2006) calculated the dragcoefficients of diamond-shaped net with considering the interaction
between the net lines. It was found that the drag coefficient was almost
constant for nets of low solid ratio, the prediction of the drag

NUMERICAL MODEL
Screen model
The Screen model uses a truss system to approximate the net. The
trusses define a number of planes in turn as shown in the Fig. 1(left),470