On the role of a centrifugal instability in the vortex-induced vibration of a two-degrees of freedom circular cylinder at moderate Reynolds numbers

Abstract

In this work, the formation and role of a centrifugal instability in the VIV phenomenon at moderate Reynolds numbers are investigated for the ¿rst time. Large eddy simulations of a low mass ratio (m = 2.6) two-degree-of-freedom circular cylinder at subcritical Reynolds (Re = 3900, 5300, 11000, 50000) numbers andatzerodamping(¿ = 0)havebeenperformed. Results for three different values of the reduced velocity U* = 4.3, 5.5 and 6.5 at Re = 5300 are shown. This valuesof thereduced velocitymark thebeginning, the middleandtheendofthestablepartofthesuper-upper (SU) branch. Results show that the xy oscillations trigger a centrifugal instability at the cylinder boundary layer. G¨ortler-like vortices appear at U* ˜ 4.5 with Ay/D ˜ 0.902 and Ax/D = 0.156 when there is an important change in the slope of the increment of the maximum amplitude trend as a function of the reduced velocity. The G¨ortler like vortices become irregular as the reduced velocity increases and cease at thesuper-upperlowertransition. Also,itisshownthat the formation of the counter-rotating pair of vortices is modulated by a frequency of twice the vortex shedding (the frequency of the cylinder oscillation in the in-linedirection)whenthecylinderstartstoaccelerate. Finally, some important characteristics of the Görtler like vortices and their interaction with the vortex formation are described.