| Vortex-induced vibration (VIV) of elastically supported cylinders in cross-flow is studied experimentally for cylinder mass ratios (average cylinder density/fluid density), 2.1 < m* < 72. For small mass ratios below 10, a new VIV mode is discovered which does not involve a lock-in behavior. The oscillation and the shedding frequencies coalesce and deviate slightly from the nominal Strouhal frequency of St = 0.2 to smaller values with increasing free stream velocity U. With increasing mass ratio above 10 ( m* > 10), the frequency growth with free stream velocity U appears to approach the lock-in limit while the amplitude and the frequency range of oscillations diminish. Additionally, a novel technique is employed to deduce the unsteady lift coefficient on the body using VIV time traces of the cylinder displacement and their numerical derivatives.; An analytical study of the dynamical equation shows that the oscillation amplitude (A/D) is inversely proportional to effective stiffness, k*eff = (m*/U2)(1 − ( f/fn)2), where U represents the non-dimensional flow speed and f/ fn, the ratio of the oscillation to natural frequencies. It is hence maintained that at high mass ratio cases studied previously ( m* > 100), lock-in behavior (f/fn ∼ 1 for U ∼ 1) is a prerequisite for nominal vibration amplitudes. At low values of mass ratio, however, k* eff is minimized naturally without a need for lock-in.; Through a detailed study of a large number of cases with low to medium mass ratios in different experimental settings, it is additionally argued that lock-in is a sporadic phenomenon that appears at various mass ratios. The few occurrences of lock-in at low mass ratios with nominal damping and the unexpected absence of any oscillations at medium mass ratios ( m* ∼ 30), except for a few cases exhibiting lock-in tendencies indicate that lock-in is not as common as classically believed. |
