Experimental Investigation on the Mechanism of VIV Reduction using Helical Strakes

Siti Fatin Mohd. Razali, Tongming Zhou, Liang Cheng School of Civil & Resource Engineering, The University of Western Australia,

Perth, WA Australia.

ABSTRACT In this study, the mechanism of vortex-induced vibration (VIV) mitigation using helical strakes is further investigated complying with the recent literatures. A rigid circular cylinder of diameter d = 80 mm attached with three-strand helical strakes of a dimension of 10d in pitch and 0.12d in height was tested in a wind tunnel. A bare cylinder was also tested to provide a benchmark for the straked cylinder system. It was found that the helical strakes reduce VIV by about 98%. Unlike the bare cylinder which experiences lock-in over the reduced velocity of 5 ~ 9, the straked cylinder does not show any lock-in region. In exploring the mechanism of VIV reduction by helical strakes, it was found that vortices shed from the straked cylinder are weakened significantly. The cross-correlation coefficients in the bare cylinder wake were much larger than that obtained in the straked cylinder wake, indicating that the correlation length of the former is larger than that of the latter. As a result, the straked cylinder wake agrees more favorably with isotropy than the bare cylinder wake. INTRODUCTION Time-dependent drag and lift forces induced by vortex shedding on the bluff structures become important subjects when vortices are shed from a bluff body. The lift force oscillates at the vortex shedding frequency while the drag force oscillates at twice the vortex shedding frequency (Sumer & Fredsøe, 1997). If the bluff structure is flexibly-mounted, these forces may induce vibration of the former. This phenomenon is called vortex-induced vibration (VIV) (Blevins, 2001). VIV is one of the main issues in the design of bluff structures such as heat exchanger tubes, marine risers and pipelines, bridges and tethered structures in the ocean. This is because VIV will increase not only the dynamic load to the structures but will also influence the structural stability. The VIV may cause structural failure or accelerate the fatigue failure. The above factors will influence the capital investment of the structures and the expenses for maintenance and replacement. Therefore, suppression of vortex shedding and VIV of a bluff body has been one of the most active topics of research and patenting in fluid dynamics for many decades due to its significance in engineering applications. To prevent VIV lock-in, one can either change the natural frequency of the

structure by structural modification or to inhibit the formation of vortices or to disrupt their structural formation, through the application of the suppression devices. Helical strake is one of the most proven and widely used control measures in various industrial and offshore applications to suppress VIV. Basically, it is believed that they adversely affect the shear layer roll up and disrupt the spanwise vortex formation and shedding process (Scruton, 1963). It is believed that as fluid flows past a cylinder with helical strakes, the strake chops up the flow and creates vortices at various places along the cylinder. These vortices are out of phase with one another and produce destructive interference to the dominant vortex shedding. Due to partial cancellation of the out-of-phase lift forces at different spanwise positions, the lift coefficient for the straked cylinder is much smaller than that of a bare cylinder, resulting in a significant reduction in the amplitude of VIV. Although helical strakes may cause drag increment to the pipe, it is still within the acceptable range (Chung et al., 1994). While the effect of helical strakes on suppression of VIV has been studied extensively, the mechanism of VIV suppression has not yet been fully investigated (Constantinides & Jr. Oakley, 2006). Therefore the present study aims to shed some light about the vortex characteristics and to enhance our understanding on the effect and mechanism of helical strakes has on VIV suppression. In this context, experiments using two X-wire probes have been conducted in the wakes generated by a bare cylinder and a straked cylinder in a wind tunnel at x/d = 5 for various probe separation ∆z along the cylinder length direction (∆z = 10 – 250 mm), where the origin of the z-axis is defined at a location when one of the strake is just located at the top of the cylinder. One of the X-probe is fixed at z = 0 while the other X-probe is moved along the cylinder axial direction. Three-start strakes with a pitch length of 10d and a strake height of 0.12d is used, which is in the range of the most effective dimension in VIV suppression (e.g. Kumar et al., 2008). Using the measured velocity signals, the characteristics of vortex shedding in the spanwise and streamwise directions can be examined. EXPERIMENTAL DETAILS The experiments were conducted in the low speed section of the

Proceedings of the Twentieth (2010) International Offshore and Polar Engineering Conference Beijing, China, June 2025, 2010 Copyright © 2010 by The International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1-880653-77-7 (Set); ISSN 1098-6189 (Set); www.isope.org

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