Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering ConferenceShanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Numerical Simulation of Vortex-induced Motion Characteristics of Four Rigidly ConnectedCylinders
Ning Liang1, Yanghong Xiao2, Haishan Xia2, Lei Li2*
1.POWERCHINA Huadong Engineering Corporation Limited, Hangzhou, China
2.School of Naval Architecture and Maritime, Zhejiang Ocean University ，Zhoushan, China

a given velocity, as the boundary layer produces unstable separation,
the downstream sides of the cylinder bring about
periodic andanti-symmetrically aligned vortex shedding. This periodic
symmetrical or asymmetrical vortex alternately shedding can excite
a drag force at in-line direction (IL) and a lift force in the cross-flow
direction (CF). The cylinder vibrates under this periodic excitation
force, which in turn to changes the shape of its own wake field.
The VIM of a floating cylinder is a highly complex fluid-solid
coupling problem which involves boundary layer simulation,
turbulence model selection, the discrete solutions of governing
equations, and dynamic mesh processing. Model testing andnumerical simulation are currently the primary research methods.
Blevins and Coughran et al.(2009) conducted an experimental study
on the VIV of a bare cylinder with 1 DOF elastically supported. The
maximum transverse response amplitude was found to increase with
the increase of the Reynolds number from 200 to 10 5 when the mass
ratio is equal to 5.3. The same phenomenon also appeared in theresearch of Govardhan and Willianmson (2006) , King (1974) , Chenand Jendrzejczyk (1979), who studied the two degrees of freedom (2DOF) vibration of a cylinder. The resonance interval in the IL isbetween the reduced velocity 2.5 ≤ Ur ≤ 4.5, while the transverse
resonance interval is in Ur ≥ 4. Sarpkaya(1995) conducted anexperimental comparison between 1 DOF and 2 DOF vibration of thecylinder. The freely vibrating cylinder in the plane has obvious
coupled motion in two directions, and the motion is in a
“figure-eight” shape. Furthermore, the amplitude of transverse
motion in the case of 2 DOF is 19% larger than that in the case of thesingle degree counterpart. The above conclusion is directly related to
the natural period of the structure in the in-line and transverse
motion.
Williamson and Roshko (1988) designed an experiment to study
the wake vortex structure, which they observed the vortex structureof symmetric or asymmetric structure through a large number ofexperiments, and the wake vortex pattern can be classified into 2S, 2Pand P+S according to the relationship between the motion response
amplitude and the reduced velocity. This symmetrical structure of thevortex has attracted the interest of many researchers. Khan (2020)

ABSTRACT
Floating offshore platforms mostly consist of a single cylinder ormulti-cylinder, which is susceptible to vortex-induced motions (VIM)
in the ocean current. In this paper, numerical simulations areperformed to study the VIM of four rigidly connected cylinder at alarge reduced velocity range ， which represent the four-column
structure of a Tension Leg Platform (TLP). The flow field around thecylinders was calculated by the Fluent software, and the differential
motion equation was solved by fourth order Runge-Kutta method
which was manually written into the user-defined functions (UDF),
then dynamic mesh technique was adopted to update the flow field.
The range of Reynolds number covered is 5000<Re<15000, and themass ratio (m*) are fixed as 0.96 which is the same as the TLP model
test..The results showed that the numerical simulation andexperimental results of VIM were in good agreement. It can bedivided into three typical phases: initial branch, lock-in region andunlocked lower branch. The results also found that the lift coefficientof downstream column are much greater than the upstream column.

KEY WORDS : vortex-induced motions ; cylinder; fluid-structureinteraction; response characteristics; dynamic mesh；

INTRODUCTION
Bluff body structures can be used in a broad variety of industrial
applications, especially in the field of offshore engineering.
Vortex-induced vibration (VIV) is one of the major concerns indesign-problems of offshore structures because of the damage it will
be caused. Blevins (1979), Jauvtis et al. (2004) and Sun et al (2019)
discovered VIV phenomenon can be traced back to the last century,and many investigations. For large offshore structures such as
offshore platforms which have long periods and large amplitudes ofmotion under the action of currents, we refer to the excitation motion
generated as a vortex-induced motion (VIM).
When a non-streamlined cylinder located in an incoming flow at2556