THE EFFECT OF UNSTEADY MOTION ON THE DRAG FORCES AND FLOW-INDUCED VIBRATIONS OF A LONG VERTICAL TOW CABLE

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 2.2-2.5. 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.

• Supplemental Notes:
  • European Offshore Mechanics Symposium, 1st 20-22 Aug 1990, Trondheim, Norway. Sponsored and publ by the International Soc of Offshore & Polar Engrs (ISOPE), Proc, p 464 [10 p, 12 ref, 2 tab, 17 fig]
• Authors:
  • Grosenbaugh, M A
• Publication Date: 1990

Language

• English

Subject/Index Terms

• TRT Terms: Cables; Coefficients; Drag forces; Motion; Towing devices; Vibration
• Old TRIS Terms: Drag coefficients; Towing cables
• Subject Areas: Data and Information Technology; Marine Transportation;

Filing Info

• Accession Number: 00699770
• Record Type: Publication
• Source Agency: British Maritime Technology
• Files: TRIS
• Created Date: Aug 14 1995 12:00AM