alinik
۱۴ مهر ۱۳۸۸, ۰۰:۲۵
Journal of Fluids and Structures ] (]]]]) ]]]–]]]
Free-stream turbulence effects on vortex-induced vibration and
flow-induced force of an elastic cylinder
R.M.C. Soa,, X.Q. Wanga, W.-C. Xieb, J. Zhub
aDepartment of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
bDepartment of Civil and Environmental Engineering, Faculty of Engineering, University of Waterloo, Waterloo, Ont., Canada
Received 15 February 2007; accepted 22 October 2007
Abstract
The effect of free-stream turbulence on vortex-induced vibration of an elastic cylinder in a cross-flow and the
associated fluid forces was investigated experimentally. The range of Reynolds number investigated is 5000–41 000.
A turbulence generation grid was used to generate turbulence intensity around 10% in the upstream flow. Cylinder
displacements in the transverse direction at cylinder mid-span were measured in the reduced velocity range
Ur0 ¼ 1.45–12.08. The cylinder vibration has two dominant components, one at the frequency of vortex shedding and
another at the natural frequency of the fluid-cylinder system. These are represented by their root-mean-square (r.m.s)
values, w0yV and w0yM, respectively. For a nonturbulent uniform flow, w0yV is dominant in the Ur0 range of
1.45oUr0o7.25, where lock-in occurs at Ur0 ¼ 5.31, while w0yM is dominant in the post-lock-in Ur0 range of
8.21oUr0o12.08. Free-stream turbulence increases the vortex shedding frequency in the post-lock-in region but does
not change the system natural frequency. It also substantially increases w0yV in the Ur0 range of 1.45oUr0o8.21 range
and w0yM in the Ur0 range of 9.18oUr0o12.08. These features are further studied using flow-induced forces deduced
from vibration data. The overall effect of free-stream turbulence can thus be interpreted from the ratio of energy
increment; free-stream turbulence feeds energy to the cylinder in general, and this energy transmission reaches a
maximum at the lock-in point, and drops rapidly in the off-lock-in region. Therefore, the lock-in region is of main
concern when free-stream turbulence is present.
r 2007 Elsevier Ltd. All rights reserved.
1. Introduction
It is well known that flow-induced vibrations of bluff bodies in a cross-flow is affected by a host of excitations; chief
among them are instability-induced excitation, movement-induced excitation and extraneously induced excitation
(Naudascher and Rockwell, 1994). These excitations are associated with the vortex-induced force, the motiondependent
force and the turbulence-induced buffeting force, respectively. The buffeting force is present as long as there
is free-stream turbulence in the oncoming flow and is not dependent on the motion of the bluff bodies (So and Savkar,
1981). This effect is derived from the flow around the bluff bodies and the behavior in the wakes. For example, in the
case of a flow past a single cylinder, a strong effect of free-stream turbulence is observed in the states of transition of the
wake, the shear layers, and the boundary layers (Zdravkovich, 1997). Consequently, free-stream turbulence promotes
ARTICLE IN PRESS
www.elsevier.com/locate/jfs
0889-9746/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfluidstructs.2007.10.013
Corresponding author.
E-mail address: mmmcso@polyu.edu.hk (R.M.C. So).
Please cite this article as: So, R.M.C., et al., Free-stream turbulence effects on vortex-induced vibration and flow-induced force of
an elastic cylinder. Journal of Fluids and Structures (2007), doi:10.1016/j.jfluidstructs.2007.10.013
Free-stream turbulence effects on vortex-induced vibration and
flow-induced force of an elastic cylinder
R.M.C. Soa,, X.Q. Wanga, W.-C. Xieb, J. Zhub
aDepartment of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
bDepartment of Civil and Environmental Engineering, Faculty of Engineering, University of Waterloo, Waterloo, Ont., Canada
Received 15 February 2007; accepted 22 October 2007
Abstract
The effect of free-stream turbulence on vortex-induced vibration of an elastic cylinder in a cross-flow and the
associated fluid forces was investigated experimentally. The range of Reynolds number investigated is 5000–41 000.
A turbulence generation grid was used to generate turbulence intensity around 10% in the upstream flow. Cylinder
displacements in the transverse direction at cylinder mid-span were measured in the reduced velocity range
Ur0 ¼ 1.45–12.08. The cylinder vibration has two dominant components, one at the frequency of vortex shedding and
another at the natural frequency of the fluid-cylinder system. These are represented by their root-mean-square (r.m.s)
values, w0yV and w0yM, respectively. For a nonturbulent uniform flow, w0yV is dominant in the Ur0 range of
1.45oUr0o7.25, where lock-in occurs at Ur0 ¼ 5.31, while w0yM is dominant in the post-lock-in Ur0 range of
8.21oUr0o12.08. Free-stream turbulence increases the vortex shedding frequency in the post-lock-in region but does
not change the system natural frequency. It also substantially increases w0yV in the Ur0 range of 1.45oUr0o8.21 range
and w0yM in the Ur0 range of 9.18oUr0o12.08. These features are further studied using flow-induced forces deduced
from vibration data. The overall effect of free-stream turbulence can thus be interpreted from the ratio of energy
increment; free-stream turbulence feeds energy to the cylinder in general, and this energy transmission reaches a
maximum at the lock-in point, and drops rapidly in the off-lock-in region. Therefore, the lock-in region is of main
concern when free-stream turbulence is present.
r 2007 Elsevier Ltd. All rights reserved.
1. Introduction
It is well known that flow-induced vibrations of bluff bodies in a cross-flow is affected by a host of excitations; chief
among them are instability-induced excitation, movement-induced excitation and extraneously induced excitation
(Naudascher and Rockwell, 1994). These excitations are associated with the vortex-induced force, the motiondependent
force and the turbulence-induced buffeting force, respectively. The buffeting force is present as long as there
is free-stream turbulence in the oncoming flow and is not dependent on the motion of the bluff bodies (So and Savkar,
1981). This effect is derived from the flow around the bluff bodies and the behavior in the wakes. For example, in the
case of a flow past a single cylinder, a strong effect of free-stream turbulence is observed in the states of transition of the
wake, the shear layers, and the boundary layers (Zdravkovich, 1997). Consequently, free-stream turbulence promotes
ARTICLE IN PRESS
www.elsevier.com/locate/jfs
0889-9746/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfluidstructs.2007.10.013
Corresponding author.
E-mail address: mmmcso@polyu.edu.hk (R.M.C. So).
Please cite this article as: So, R.M.C., et al., Free-stream turbulence effects on vortex-induced vibration and flow-induced force of
an elastic cylinder. Journal of Fluids and Structures (2007), doi:10.1016/j.jfluidstructs.2007.10.013