2017 Volume 12 Issue 2 Pages JFST0016
This study examines experimentally the vortex-induced vibration (VIV) of a mechanical system with two eigenmodes. A previous experiment setup was refined to enable the experiment, and was placed in a circulating water channel to submerge a movable circular cylinder (cylinder A). This cylinder was subjected to fluid flow while being supported elastically, whereupon VIV occurred. A second movable cylinder (cylinder B) above the water was connected to cylinder A and supported elastically. The displacements of those two cylinders were measured. The underlying hypothesis was that the vortex shedding frequency would become locked to one or other of the two eigenfrequencies, and that which eigenmode the vortex shedding frequency became locked to would depend on the reduced velocity. To test this hypothesis, the experimental setup was refined to withstand the high flow speeds required to allow the vortex shedding frequency to become locked to either eigenfrequency. From the results obtained, the amplitude ratio (the ratio of the amplitude of cylinder B to that of cylinder A) and the frequencies of the two cylinders were determined. It was found that those amplitude ratio and frequencies were close in two ranges of reduced velocity to those calculated theoretically by solving the eigenvalue problem of the 2DOF system. This demonstrated that, depending on the reduced velocity, the vortex shedding frequency could become locked to either the first or second eigenmode to produce a definite vortex-induced vibration.