Numerical Modeling of Wake Effect on Aquaculture Nets

Abstract

Accurate modeling of drag forces on net cages due to water current is important when designing floating fish farm systems. These drag forces give a major contribution to the total environmental forces on a fish farm, especially mooring line forces. When subjected to current, the net cage will deform. High current velocities can result in large deformations and lead to collapse of the net cage. For circular fish farms with a flexible floating collar, large deformations may induce contact between the weighting system and the net, resulting in abrasion that can cause tearing of the net material and consequently failure that will lead to fish escape. The motivation for this paper is to obtain a better understanding and more accurate model for drag forces and corresponding deformations of circular net cages due to water current. Calculation of drag forces on a net cage is complicated due to the porous nature of the net, geometry and flexibility of the system. Adding to the complexity is the wake effect, or reduced velocity, behind each individual twine which will have a significant effect on the forces and deformations of the net cage. This wake effect will result in reduced inflow velocity on parts of the net being downstream. A method for estimating wake effects acting within an aquaculture net structure was developed and implemented in a numerical code taking net deformation into account. Numerical simulations of a cylindrical net cage were compared with experimental results. Comparison between simulations with and without wake effect revealed a reduction in total drag up to 22% when wake effect was applied. Although the model consistently overestimated drag forces on the net cage (average deviation of 25%), simulation results compared well with measurement data, particularly for low current velocities where deviations were as low as 7%. This indicates a consistent wake effect and drag model that produces conservative estimates of drag forces on net cages.