Determining the drag coefficient of a cylinder perpendicular to water flow by numerical simulation and field measurement

• Finite element analysis was used to develop a 3D numerical longline model.
• 3D current velocity in different water layers and hook depths were used.
• Numerical modeling and at-sea measurement can be used for determining CN90.
• Drag coefficient CN90 between 1.08 and 1.16 is a reasonable value for a cylinder.

This study aimed to improve the estimation of the cylinder drag coefficient, and its accuracy for fishing gears. Five surveys on tuna longline fishing grounds were conducted between 2005 and 2010. Collected data include three dimensional current velocities in different water layers and hook depth. Hook depths were calculated by drag coefficients of the cylinder with a perpendicular flow (CN90) of 1.04 to 1.40 (interval of 0.02), and compared with measured depths. The coefficient of variation (CV) was calculated for 70% of the hooks at their measured depths. Additionally, a t-test was conducted comparing depths predicted by the model against measured depths of the remaining sites, totaling 30%. The results showed that the hydrodynamic forces on longline gear have a CN90 between 1.08 and 1.16, and the drag coefficient decreased with increasing Reynolds number (Re, Re<103). These results suggest that numerical modeling and at-sea measurement can be used to determine the drag coefficient (CN90) of cylinders such as ropes and lines, and that a drag coefficient of 1.08 to 1.16 are reasonable values for cylinder components of longlines.