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Experimental investigations of wind turbine blades having NACA airfoils 0021 and 4412 with and without tubercles on the leading edge have been performed in a wind tunnel. It was found that the lift coefficient of the airfoil 0021 with tubercles was higher at Re = 1.2×105 and 1.69×105 in post critical region (at higher angle of attach) than airfoils without tubercles but this difference relatively diminished at higher Reynolds numbers and beyond indicating that there is no effect on the lift coefficients of airfoils with tubercles at higher Reynolds numbers whereas drag coefficient remains unchanged. It is noted that at Re = 1.69×105, the lift coefficient of airfoil without tubercles drops from 0.96 to 0.42 as the angle of attack increases from 15° to 20° which is about 56% and the corresponding values of lift coefficient for airfoil with tubercles are 0.86 and 0.7 at respective angles with18% drop.

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.

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