Analysis of Fluid Induced Transverse Forces on Multi-Strand Twisted Ropes

Abstract

Twisted multi-strand steel cables and ropes made of natural or synthetic fibres are the technical base for the manufacture of extended mechanically flexible structures. Examples can be found in a wide range of applications and subsystems throughout the maritime sector, e.g. experimental marine research, aquaculture, fishery and more. Here, one of the main concerns is the generated drag, a considerable quantity for flown through flexible structures. Furthermore, cables and ropes experience significant lift acting in the plain spanned by the cord vector, perpendicular to flow. In addition to that a transverse force occurs acting perpendicular to the plain spanned by the cord and inflow vector. This transversal force can generate significant deflections causing problems for example during towing processes. However, although of considerable impact this effect is often neglected with the underlying mechanisms not well understood. This work presents an approach to derive these transverse forces examining the pressure distribution as well as the three-dimensional circulation around a multi-stranded rope. Following from that, two working theses are formulated: 1. The transverse force acting on multi-stranded ropes is a result of the circulation induced on the fluid by the geometry and its orientation to the incident flow. 2. The transverse force acting on multi-stranded ropes is comparable to the flow-induced hydrodynamic lift acting on an aerofoil. With the use of experimentally determined pressure distributions across the surface of three-strand twisted rope segments at different angles of attack, cross-section-dependent circulation is demonstrated. The experimental data is compared with numerical calculations showing the limitations of a Reynolds-based approaches.