Experimental investigation of the reduction in flow velocity downstream from a fishing net

A series of laboratory experiments was conducted to investigate the reduction in flow velocity downstream from a fishing net in a current. The flow-velocity distribution downstream from the plane net(s) was obtained using both the particle image velocimetry (PIV) technique and the acoustic Doppler velocimeter (ADV). The reduction in flow velocity was investigated with different net solidities, plane-net inclination angles, spacing distances between two plane nets and plane net numbers. The experimental data show that there was an obvious reduction in flow velocity downstream from the plane net and that the reduction increased with increasing net solidity. The reduction in flow velocity tended to increase with increasing inclination angle between the plane net and the vertical direction. For two plane nets with different spacing distances, the average reduction factor was 0.90 between the two nets and 0.83 downstream from the two nets. As the net number increased from 1 to 4, the minimum reduction factor downstream from the plane nets decreased from 0.90 to 0.68. The study will contribute to understanding of the flow characteristics around a net cage.