Experimental on hydrodynamic characteristics of the hydroid-fouled net

The massive attachment of fouling organisms will clog the net of fish cages, reduce the water exchange within the cages, and increase the structural load and deformation; which will lead to an increased risk of damage to the cage under severe sea conditions, and may cause significant economic losses. To study the effects of fouling organisms on the hydrodynamic characteristics of fish cage nets, this study conducted field test by deploying nets in marine cage aquaculture areas, obtaining nets with varying levels of biofouling (S_n=0.213-0.442). The composition of the fouling organisms was analyzed, and flume test were conducted to explore the hydrodynamic characteristics of hydroid-fouled nets for different flow velocities (u=0.2-0.5 m/s) and angles of attack (θ=0°-90°). The results indicate that attachment of fouling organisms initially increases and then decreases with the duration of immersion time, with the maximum attachment occurring at a depth of 4.5 meters during the fourth week. There is a positive correlation between the wet weight of the net and the degree of attachment. Notably, the attachment of fouling organisms like hydroids significantly alters the hydrodynamic characteristics of the nets. The net with the most severe fouling (S_n=0.442) exhibited a 6.09-fold increase in maximum drag force and a 5.99-fold increase in maximum lift force compared to a clean net (S_n=0.146). The influence of hydroid-fouled nets on their hydrodynamic coefficients under varying angles of attack exhibits notable differences, the drag coefficient can increase by as much as 2.1 times, and correspondingly, the lift coefficient undergoes a 2.0-fold enhancement, in comparison to clean nets. At an angle of attack of 90°, the relationship between the drag coefficient (C_d) and solidity ratio (S_n) of the hydroid-fouled nets is well-fitted by the equation C_d =0.42+8.98S_n-7.78S_n² (R²=0.803, S_n=0.145-0.442). Our research shows fouling organisms, like hydroids, significantly affect the hydrodynamic loads on fish cage nets, worsening stress distribution, and increasing the risk of damage to the net. Therefore, in the design and safety assessment of cages, it is essential to comprehensively consider the impact of changes in net drag force and lift force on the structural integrity of the fish cage. Moreover, during the production process of cage aquaculture, fouling organisms should be removed from the nets in a timely manner to ensure structural integrity and safety. This study provides valuable insights for the design and optimization of fish cages, as well as for the scheduling of net cleaning practices. By addressing these aspects, we can enhance the durability of cage net systems, ultimately contributing to more sustainable and efficient aquaculture.