Toward understanding galloping: Near-wake study of oscillating smooth and stranded circular cylinders in forced motion

Low-frequency high-amplitude oscillation of electrical transmission lines, known as galloping, usually occurs after freezing-rain storms due to ice accretion on the conductors. The resulting damage to transmission lines and the associated service costs prompted the present study. Comparison of oscillating stranded and unstranded circular cylinders is made in investigating the effects of stranding, transverse oscillations, and Reynolds number of the near-wake flow field. A series of forced-oscillation wind tunnele experiments were performed at amplitude ratios, A/d, of 2 and 7, for 2000 ≤ Re < 40,000. Several subharmonic vortex lock-ins are reported in addition to that of the fundamental mode. For relative reduced velocities of about Ur/(A/d) ≥ 20, turbulence intensity in the near wake remains constant (quasi-steady region). However, most actual on-site galloping cases of single conductors have occurred in the region where turbulence intensity begins to rise dramatically, as Ur/(A/d) moves below a value of 20 (multiple lock-in region) indicating that onset and/or perpetuation of galloping should be studied with vortex synchronization in mind, limiting the use of the quasi-steady theory.