Numerical study on suppression of vortex-induced vibration of circular cylinder by helical wires Takeshi Ishihara, Tian Li * Department of Civil Engineering, School of Engineering, The University of Tokyo, Tokyo, 113-8656, Japan ARTICLE INFO Keywords: Suppression of VIV Circular cylinder Helical wire LES turbulence model ABSTRACT The mechanism of suppression for the vortex-induced vibration (VIV) of circular cylinder by the helical wires is investigated using LES turbulence model. Numerical models for the free-vibrating circular cylinders with and without the helical wires are proposed and validated by the experiments. Simulations are carried out for a large mass ratio of 248 and a small damping ratio of 0.00257 with the Reynolds numbers ranging from 16000 to 24500 and the reduced velocities ranging from 4.5 to 6.8. It is noticed that the helical wires of diameters ratio d/D ¼ 0.1 effectively suppress the amplitude of VIV by nearly 80% and avoid the “lock in”. The steady and unsteady aerodynamic forces of circular cylinders with and without the helical wires as well as the flow patterns are also examined to clarify the mechanism of VIV suppression. It is found that the fluctuating lift forces and their spanwise correlation for the wired cylinder are significantly reduced comparing with those for the bare cylinder, due to the enhancement of three-dimensional disturbance to the wake caused by helical wires. The aerodynamic damping for the oscillated bare cylinder is negative, while that for the wired cylinder is positive with a vibration amplitude A/D ¼ 0.1 at the resonance velocity of bare cylinder. 1. Introduction When a circular cylinder is immersed into a steady flow, vortices are shed from alternating sides of the cylinder. The latter is then subjected to the unsteady drag and lift forces. These forces may induce vibration of the cylinder, namely vortex-induced vibration, which affects the shed- ding of the vortices in turn. This interaction between the flow and the cylinder forms a highly nonlinear and complex phenomenon. It will not only increase the dynamic load on the structures but also influence the structural stability. The vibrations may accelerate the fatigue failure and increase the expenses for maintenance and replacement. During the last decades, the VIV has been extensively investigated through the canonical problem of a rigid circular cylinder elastically mounted in the cross-flow direction by experiments. Feng (1968) firstly conducted an experiment of an elastically mounted cylinder in air at high mass ratios defined as m* ¼ 4m/(πρD 2 L) and demonstrated that the resonance of the cylinder occurs over a regime of reduced velocity U r defined by U r ¼ U/f n D, where m, L and D are the mass, length and diameter of the cylinder respectively. ρ is the fluid density, U is the free-stream velocity and f n is the natural frequency of structure. More studies focused on the VIV in water, where the mass ratios are generally 1 to 3 orders of magnitude smaller than those in air. The VIV in water happens for a wider regime of reduced velocity and the branches of response are different compared with the phenomena in air (Khalak and Williamson, 1997). Comprehensive reviews of various aspects of VIV can be found in the publications of Sarpkaya (2004), Williamson and Govardhan (2008) and Bearman (2011). The research for VIV of other shapes of cylinder can be found in Zhao et al. (2013) and Singh and Biswas (2013). A wide variety of aerodynamic and hydrodynamic countermeasures for suppressing vortex shedding was used and divided into three cate- gories (Zdravkovich, 1981): (1) surface protrusions (helical strakes, wires, fins, studs or spheres, etc.), which affect separation lines and/or separated shear layers; (2) shrouds (perforated, gauze, axial-rods, axial slats, etc.), which affect the entrainment layers; (3) nearwake stabilisers (splitter plates, guiding vanes, base-bleed, slits cut across the cylinder, etc.), which prevent interaction of entrainment layers. More recently, suppression of vortex shedding by thermal effects was mentioned by Kakade et al. (2010), Arif and Hasan (2019) and Zafar and Alam (2019). Among these devices, helical strakes have been proven effective and widely used in various industrial and offshore applications to suppress fluid-induced vibration. * Corresponding author. E-mail address: li@bridge.t.u-tokyo.ac.jp (T. Li). Contents lists available at ScienceDirect Journal of Wind Engineering & Industrial Aerodynamics journal homepage: www.elsevier.com/locate/jweia https://doi.org/10.1016/j.jweia.2019.104081 Received 25 September 2019; Received in revised form 25 December 2019; Accepted 25 December 2019 Available online xxxx 0167-6105/© 2019 Elsevier Ltd. All rights reserved. Journal of Wind Engineering & Industrial Aerodynamics 197 (2020) 104081