Cable fairings are routinely used on towed instruments where tow speeds are in excess of 3 knots and here good depth performance is desirable In order to minimize the cable fairing drag the correct choice of fairing for the application is essential. Typical oceanographic or survey applications utilize wires of 8 to 20 mm diameter at tow speeds of 3 to 10 knots. This corresponds to a Reynolds number range of 10000 to 100000 In terms of aircraft aerodynamics these Reynolds numbers are low. Most of our knowledge of foil section performance comes from model studies of aircraft wings at Reynolds numbers of 1 to 10 million At these higher Re values boundary layer transition occurs close to the leading edge, and consequently most of the boundary layer on the surface is turbulent. At the Reynolds numbers typical of oceanographic fairings the laminar boundary may persist longer, and laminar separation bubbles are more likely to occur. Furthermore early trailing edge flow separation may be precipitated on poor designs which significantly increases the drag.
The laminar boundary layer is actually very beneficial in that it exerts much less skin friction drag than the turbulent boundary layer. It is possible to design section profiles which make use of this by encouraging the laminar boundary-layer to remain attached and delaying transition. The secret lies in the form of the pressure distribution on the surface Figure 1 shows the inviscid theoretical pressure distribution calculated for one of the best commercially available cable fairings. The flow visualization sketch for a Reynolds number of 210 000 is after Henderson (ref 1) who reported the wind-tunnel test results. The rounded nose positions the suction peak close to the leading edge, and the remainder of the boundary layer is subjected to a destablizing adverse pressure gradient. The laminar boundary layer separates near the suction peak, forming a separation bubble.
Following reattachment the turbulent boundary layer does not survive the long run to the trailing edge and separates early. The resulting drag coefficient is 0.04-approximately four times the drag that might be expected for a section like this at Reynolds numbers in excess of 1 million. Henderson also demonstrated that the fairing stalled at an incidence of 4°, and he calculated that the section aerodynamic centre was located at 15.4 % chord. The importance of the aerodynamic centre location is in determining the weathercock stability of the freely pivoted fairing, i.e how well it aligns itself with the flow. The distance between the wire centre and aerodynamic centre on a fairing like this is a direct measure of the weathercock stability In this case Henderson determined the separation to be only about 0 03 c where c is the chord length of the section. The poor performance of this section at RE = 210 000 is entirely due to the behaviour of the boundary layer.
Fig. 1 Inviscid pressure distribution for the commercial fairing section and flow visualization sketch at Re = 210 000 (after Henderson (ref 1) (available in full paper)