Dynamic response of a freely movable downstream cylinder in the upstream wake by an oscillating fin

Abstract

Extensive studies for the dynamic response of downstream structures to von Karman wakes generated by upstream structures have been performed thus far. However, a critical gap remains in how a downstream structure responds to thrust-generating wake patterns typically produced by swimming fish in nature. To bridge this crucial gap, numerical simulations are performed on a system consisting of an upstream flexible fin and a downstream rigid cylinder vibrating freely in the cross-stream direction to investigate the interaction of the cylinder with various upstream wake patterns. The various wake patterns are generated by the heaving motions of the fin as the cylinder-diameter-based dimensionless heaving amplitude and frequency are varied. When the von Karman wake is generated behind the fin, not only the slaloming motion of the cylinder and constructive vortex-to-vortex interaction shed from the fin and the cylinder (hereafter, SC mode) but also the interception motion of the cylinder and mixed constructive and destructive vortex-to-vortex interaction (IM mode) dominate. For a reversevon Karman wake, both the slaloming motion of the cylinder and destructive vortex-to-vortex interaction (SD mode) and IM mode are prominent. When vortices consisting of the von Karman wake or reverse von Karman wake are aligned near the centerline, the SC þ SD þ IM mode arises. Because the induced velocity created by the destructive vortex-to-vortex interaction plays an important role in the generation of a large pressure difference around the cylinder, the dynamic response of the cylinder to the upstream wake is rank-ordered as SD, IM, and SC modes.