Drag coefficients and flow-induced vibrations of a long vertical tow cable are measured under steady and unsteady towing conditions. The steady-state drag coefficients range from 2.2 to 2.5. For unsteady towing conditions, the drag coefficient was lower by as much as 40%, depending on the frequency content of the planar ship motion. For purely oscillatory motion, the drag coefficient decreased as the frequency of motion increased. The reduction in the drag coefficient may be related to the amplitude modulation of the flow-induced vibrations of the cable which are magnified during unsteady operations. When the surface ship changes speed, differences in the normal component of the velocity along the cable are present because of time delays in the response of the bottom of the cable to inputs at the top. The longer the cable, the greater are the delays. This creates large velocity gradients in the oncoming flow which are responsible for the intensification of the amplitude-modulation above the level that is observed during steady-state towing conditions. The overall effect of the amplitude modulation is a reduction in the hydrodynamic drag forces.
In this paper, we present the results from full-scale experiments of the quasi-static and dynamic behavior of a long vertical tow cable during steady and unsteady towing operations. The overall motivation for the experiments was to improve performance in the positioning of towed vehicles, for example, ARGO - the Deep Submergence Laboratory's search and survey system. The data was collected in two separate experiments. The first experiment [1,2] took place in the Spring of 1987 at the US Navy's Atlantic Underwater Test and Evaluation Center (AUTEC) in the Bahamas. The studies show that the drag coefficient of the cable is lower during unsteady towing operations (by as much as 40%) than duringsteady-state operations.