On the importance of angle-dependent hydrodynamic coefficients in the equilibrium configuration analysis of synthetic fiber towing ropes

The underwater configuration analysis of a towing rope is crucial for estimating the rope tension and the state of the towed body. In the case where the rope inclination angle significantly increases, the partial hydrodynamic force coefficients of a rope segment should display apparent variations with the local inclination angle. However, constant drag coefficients ignoring the angle-dependent drag effect are widely used in numerical studies. To address the importance of the angle-dependent force coefficients for analyzing the underwater configuration of a towing rope, Large Eddy Simulations are carried out to obtain the drag forces of a cylindrical rope segment with varying inclination angles and Reynolds numbers. To model the rope deformation, a tensile experiment is also conducted to establish the nonlinear tension-strain relationship of the fibre rope. Parametric studies are performed to investigate the difference in the results calculated by both constant and angle-dependent hydrodynamic force coefficients. It was found that the application of constant drag coefficients leads to significant underestimation of the rope tension near the towed body, especially when the rope experiences large inclination angles. Therefore, the present study highlights the importance of using the angle-dependent hydrodynamic coefficients to predict the configuration and tension of towing ropes.