Date of Thesis

Winter 2021

Description

Flexible bluff bodies exposed to a uniform fluid flow undergo vibration due to vortex-shedding from the body. This phenomenon is known as vortex-induced vibration (VIV). Structural members, such as cables, conduits, and pipes, are susceptible to VIV. Vortex-induced vibrations result in structural stresses due to elastic structural deformation from the flow-induced loading. Cylindrical structures in proximity to planar surfaces can undergo vortex-induced vibrations when the gap between the cylinder and the planar surfaces is above a critical value. This laboratory study investigated the mechanism of vibration and planar wall proximity effect on the cylinder response to the vortex shedding process. The vibration mechanism at subcritical gap ratios is hypothesized to be movement-induced vibrations (MIV) caused by unsteady fluid flow interactions. On the other hand, vortex-induced vibration has been established as the vibration mechanism at large gap ratios in uniform flow (Blevins, 1990). A circular cylinder, mounted on two degree-of-freedom (DOF) leaf springs at each end, was positioned in a tolerant, open return subsonic wind tunnel at Bucknell University. At a reduced velocity of Ur = 5 and a Reynolds number, using the cylinder’s diameter, of 1.73 x 104, a series measurement of two-DOF structural accelerations along with the fluctuating wake velocity at a fixed position relative to the cylinder was recorded. This study provides data on the relationship between two DOF vibrations and the proximity of a circular structure to a planar surface. For gap ratios of G/D > 1.0 and δ/G ≈ 0.30, the alternate shedding of vortices (the von Kármán vortex street) produces a fluctuating y-direction acceleration of the body at the same frequency as that of vortex shedding. Furthermore, the transverse y-direction acceleration has an associated streamwise x-direction acceleration at twice the vortex shedding frequency, indicating a fluid-structure interaction due to von Kármán vortex shedding as expected for a cylinder in a uniform flow. For G/D < 0.5 and δ/G ≈ 0.61, it is theorized that the bistable upstream wall boundary layer separation bubble periodically detaches and reattaches to the outer front top quarter surface of the cylinder as the cylinder moves upstream and towards the wall. As a result, coherent vortices are shed only from the outer side of the cylinder. The cylinder’s resulting MIV from the fluid forces has an oblong acceleration trajectory, reinforcing the single-sided vortex shedding from the cylinder. The bistable wall boundary layer is suggested as a possible mechanism for the near-wall region.

Keywords

Fluid Structure Interactions, Vibration Mechanism Profile, Bluff Body Fluid Interaction

Access Type

Masters Thesis

Degree Type

Master of Science in Mechanical Engineering

Major

Mechanical Engineering

First Advisor

M. Laura Beninati

Second Advisor

Andrew Sloboda

Third Advisor

Charles W. Knisley

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