The Flow Dynamics of Stranded Cables

Stranded cables are used in many applications, e.g. power transmission lines, suspension bridges, guy supports, underwater towing, and others. Despite being widely used, stranded cables have been investigated only in a few studies, of which the most well-known are by Batill et. al. (1989) and Nebres et. al. (1993). Past studies were not sufficiently detailed to show the effect of cable strands on flow structures. Stranded cables are usually compared to circular cables, due to the similar geometry. Even more, the codes for stranded cable applications such as the rating of power transmission lines, are based on circular conductors instead of stranded ones. Such approximation may not be accurate, as cable strands are expected to significantly affect the flow dynamics and heat transfer. A stranded cable is described by two numbers, the first is the number of strands while the second is the number of wires per strand. The aim of this study is to investigate experimentally in details the effects of two common geometries, 3x1 and 6x1 cables, on wake flow dynamics, and to compare the results to the circular cable. Two Component Planar Particle Image Velocimetry (2D-PIV) was used to study the wake flow dynamics of 3x1 and 6x1 cables at Reynolds, Re, of ~1,500 and ~3,000. The University of Calgary Laboratory for Turbulence Research in Aerodynamics and flow Control (LTRAC) water tunnel was used in this study. The cables were 3D printed and reinforced using stainless-steel rods at their core, to increase strength and natural frequency. Endplates were used to reduce flow end effects. The cables’ aspect ratio was 17.5 and the blockage was 5%. The experiment used a Phantom Miro LAB340 camera and a pulsed Nd:YLF photonic laser (Photonics DM20-527). One cross-stream, mid-span, plane was recorded for the circular cable, and four cross-stream planes were recorded along the stranded cable span. More than 300 shedding cycles were recorded for Re ~1,500 and ~500 shedding cycles for Re ~3,000. First and second order statistics were obtained for all geometries. Besides, Proper Orthogonal Decomposition (POD) of the velocity and vorticity fields were used to determine the effect of the strands on the coherent structures. Results showed that wake flow dynamics is significantly affected by cable strands, specially as the ratio of cable overall diameter over strand diameter, d/D, increases. Unlike the circular cable case, mean flow fields and Reynolds stress contours were asymmetrical in the cross-stream direction for the stranded cables cases. Nevertheless, the Strouhal number for stranded cables was almost the same as for the circular cable. Besides, the high energy content first and second velocity and vorticity POD modes, were dominated by the shedding frequency, indicating that strands do not significantly alter the vortex shedding process and that the first harmonic frequency dominates the asymmetric coherent structures of the stranded cables. For vorticity, the fourth and fifth POD modes were directly affected by the second and third harmonics, and the contours of these modes were clearly asymmetric relative to the circular cable case.  Finally, this study provides detailed stranded cables wake flow dynamics. Such understanding could be used in optimizing cable design for different applications, e.g. overhead transmission lines cables could be better designed to passively increase their current carrying capacity.