Characterization of vortex-induced vibration of a flexible cylinder

In this study, the phenomena of 3D vortex-induced vibration (VIV) of a flexible cylinder (diameter D) is shown to be distinct from 2D VIV. We seek to identify correlations between wake regimes and vibration responses for a low mass-ratio ($m^*$ = 1.2), flexible ($E =1.2$ MPa, natural frequency in water $f_N = 0.37$ Hz) cantilevered cylinder undergoing cross-flow for reduced velocity $U^*$ = 20-120 ($U^* = U / f_N D$). A P+S wake mode appears for a range of $U^*$; the onset of this range may be correlated with a hysteretic jump to an upper branch in the transverse amplitude response ($A^*_Y = A_Y / D$) at several locations along the midspan. This asymmetric wake mode does not present a unique transverse frequency response ($f^*_Y = f_Y / f_N$) in the cylinder. The upper branch in the amplitude response gives way to an abrupt decrease in $A^*_Y$ to a lower branch, accompanied by a bifurcation in $f^*_Y$. The bifurcation takes place over a narrow range of $U^*$ where the lower $f^*_Y$ gradually transfers power to a higher $f^*_Y$, and may demarcate a wake transition regime between laminar and turbulence states.