A numerical and experimental study on dynamics of a towed low tension cable

Abstract

This paper presents a numerical and experimental investigation into the dynamic behaviour of a towed low tension cable applicable to a towed array sonar system for detecting submarines. In the numerical study, an implicit finite difference algorithm is employed for solving the three-dimensional cable equations. Bending stiffness is considered in order to cope with the low tension problem. Fluid and geometric non-linearities are solved by Newton-Raphson iteration. Block tri-diagonal matrix method is applied for the fast calculation of very large matrices. In order to verify the numerical results and to observe real physical phenomena, an experiment was carried out for a 6 m cable in a deep towing tank. In the experiment, the cable was towed in two different ways; one was towed at a constant speed and the other was towed at a constant speed with top end horizontal oscillations. Cable tension and shear forces were measured at the top end. Numerical results are compared with experimental data. Good agreements are only achieved by employing enhanced drag coefficients due to vortex induced vibrations. It is also found that in the numerical modelling, non-uniform element lengths are necessary to cope with the sharp variation of tension and shear forces near the top fixed end.