An investigation of the wakes of stranded cables using particle image velocimetry

Abstract

Stranded cables are used in a wide range of applications, but the flow over them has not been studied extensively. Instead, many applications assume circular cylinder behavior when modeling the wake dynamics. This paper provides a detailed study of the wakes of two stranded cables using two-dimensional particle image velocimetry in the streamwise-normal and streamwise-spanwise planes. The first cable had six outer strands and the second had three. A circular cylinder was also investigated experimentally as to provide a benchmark for comparison. The experiments were done in a water channel at a Reynolds number of ∼3100, based on the circumscribing diameter. Proper orthogonal decomposition and phase-averaging were used to investigate the coherent and incoherent fields. The results showed that 3 × 1 with the largest variations in the sectional width experiences a local stream of higher streamwise velocity along the span and that both cables have ∼20% higher mean spanwise velocity, relative to the cylinder. The stranded cable wakes are dominated by alternatively shed Kármán vortices, at a frequency similar to the circular cylinder. However, the Reynolds stresses, the shape factor, and the details of vortex shedding showed substantial alterations associated with the cable strands, including 58% variation, relative to the cylinder, in total, coherent, and incoherent Reynolds stresses for cables along the span. In addition, the cable strands resulted in the elongation and distortion of the mean spanwise vorticity, without changing their magnitude or thickness. Finally, the strands generally increase the magnitudes of turbulent transport and coherent diffusion, and production, especially at the shear layers.