Abstract
AbstractMeasurements are presented of the structural response and wake of a two-degree-of-freedom (2-DOF) pivoted cylinder undergoing streamwise vortex-induced vibrations (VIV), which were carried out using particle-image velocimetry (PIV). The results are compared with those of previous studies performed in the same experimental facility examining a cylinder free to move only in the streamwise direction (1-DOF). The aim of this study is to examine to what extent the results of previous work on streamwise-only VIV can be extrapolated to the more practical, multi-DOF case. The response regimes measured for the 1- and 2-DOF cases are similar, containing two response branches separated by a low-amplitude region. The first branch is characterised by negligible transverse motion and the appearance of both alternate and symmetric vortex shedding. The two wake modes compete in an unsteady manner; however, the competition does not appear to have a significant effect on either the streamwise or transverse motion. Comparison of the phase-averaged vorticity fields acquired in the second response branch also indicates that the additional DOF does not alter the vortex-shedding process. However, the additional DOF affects the cylinder-wake system in other ways; for the 1-DOF case the second branch can appear in three different forms (each associated with a different wake mode), while for the 2-DOF case the second branch only exists in one form, and does not exhibit hysteresis. The cylinder follows a figure-of-eight trajectory throughout the lock-in range. The phase angle between the streamwise and transverse motion decreases linearly with reduced velocity. This work highlights the similarities and differences between the fluid–structure interaction and wake dynamics associated with 1- and 2-DOF cylinders throughout the streamwise response regime, which has not received attention to date.
Highlights
Vortex-induced vibrations (VIV) is a classical fluid–structure interaction problem, affecting a wide range of industrial applications such as marine risers, off-shoreVortex-induced vibrations of cylinders with 1 and 2 degrees of freedom structures, tall chimneys, bridges and heat exchangers
This paper aims to address these questions, and to extend the knowledge of the wake and structural dynamics associated with streamwise-only vortex-induced vibrations (VIV) to the more practical 2 (1-DOF) Current study (2-DOF) case
The peak of the first response branch occurs at Ur/f ∗ ≈ 2.3, at which point the frequency of the velocity fluctuations is equal to half the streamwise response frequency, indicating the onset of lock-in
Summary
Vortex-induced vibrations (VIV) is a classical fluid–structure interaction problem, affecting a wide range of industrial applications such as marine risers, off-shore. The first branch occurs in the range Ur/f ∗ ≈ 1.5–2.5, and is typically associated with ‘wake breathing’ or symmetric vortex shedding (Naudascher 1987; Jauvtis & Williamson 2003), in which the streamwise motion of the cylinder causes the shear layers to roll up simultaneously at the same frequency as the cylinder motion, fvs = fx This vortex pattern is referred to as the S-I mode (following the notation of Ongoren & Rockwell (1988)) and is depicted in figure 1. Forced oscillation studies have shown that the wake can exhibit the A-IV mode (in which two pairs of counter-rotating vortices are shed per cycle; figure 1) at similar ranges of reduced velocity and vibration amplitude to that which are experienced by a cylinder in this region of the response regime (Ongoren & Rockwell 1988; Nishihara, Kaneko & Watanabe 2005; Konstantinidis, Balabani & Yianneskis 2007). It should be noted that their study did not examine the variation in the wake mode throughout either branch, and it is unclear whether the A-II mode occurred in the first branch, as in streamwise-only VIV
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