Simultaneous Wake and Vortex Induced Vibrations of a Cylinder With Two Degrees of Freedom in Each Direction

Abstract

The flowinduced vibration of one cylinder in the wake of another is the subject of continuing interest in connection with interactions between vertical tension risers in deep water. When one riser is downstream of another, it is likely to be subject to wakeinduced and vortexinduced excitations at different frequencies simultaneously. Both are complex mechanisms, and it is reasonable to assume that they interact. To begin to understand this complicated process, it is desirable that any modeling should incorporate some features of a multidegreeoffreedom structural response. With this aim, this paper describes experiments in which one cylinder was free to undergo simultaneous wakeand vortexinduced vibrations downstream of a similar but stationary cylinder in a steady flow. The downstream cylinder was mounted on an elastic system that had two natural frequencies in both the inline and crossflow directions. Mass ratios were almost the same in all four modes. Measurements are presented of simultaneous wakeand vortexinduced vibrations for cylinder separations of 5 and 10 diameters in the inline direction, and up to 4 diameters transversely. At a reduced velocity of 83 (based on the cylinder's lower submerged natural frequency) and a separation of 5 diameters, excursions of wakeinduced vibrations peaked at almost 5 diameters, when the downstream cylinder was near the edge of the upstream cylinder's wake.