Date of Award

2014

Degree Type

Thesis

Degree Name

Master of Science in Ocean Engineering

Department

Ocean Engineering

First Advisor

Jason M. Dahl

Abstract

The excitation of three low-mode number, flexible cylinders in uniform-flow is investigated to determine effects of structural mode shape on vortex-induced vibrations. Experiments are performed in a re-circulating flow channel and in a small flow visualization tank using object tracking and digital particle image velocimetry (DPIV) to measure the excitation of the cylinder, to estimate forces acting on the structure, and to observe the wake of the structure under the observed body motions. Previous research has focused on understanding the effect of in-line to cross-flow natural frequency ratio on the excitation of the structure in an attempt to model the excitation of multiple structural modes on long, flexible bodies. The current research investigates the impact of structural mode shape on this relationship by holding the inline to cross-flow natural frequency constant and attempting to excite a specific structural mode shape. Understanding the effects of mode excitation will improve the ability to predict fatigue life in structures which may exhibit combined irregular and regular oscillations by understanding how mode shape affects the vibrating response. In this series of experiments, three cylinders are tested. The in-line to cross-flow natural frequency is kept constant with a ratio of 2:1, while the modes associated with these frequencies are varied: first mode inline to first mode cross-flow, second mode in-line to first mode cross-flow, and third mode in-line to first mode cross-flow.

It is found that it is difficult to isolate the effect of mode shape on the vibration of the structure as the cylinder can easily be excited with different combinations of modes. It is shown that for the low-mode number beams tested, there are three primary responses possible for the cylinder when trying to excite the first mode in the cross-flow direction and first mode in the in-line direction (cylinder 1), the first mode in the cross-flow direction and the second mode in the in-line direction (cylinder 2); and the first mode in the cross-flow direction and third mode in the in-line direction (cylinder 3).

Cylinder 1 response consists of an expected excitation of the first mode cross-flow with an excitation of the first mode frequency in in-line direction. Regular and periodic cylinder oscillations obtained along the length of the cylinder. Cylinder 2 response consists of an expected excitation of the first mode cross-flow with an excitation of the second mode frequency in in-line direction; however, the structural shape resembles the first mode shape. This response is attributed to the structural mode being an insufficient description of the phenomenon as the wake must be included as part of the mode shape. This response is characteristic of a figure eight response of the cylinder. In cylinder 3 experiments, two different responses as crescent and figure eight patterns were obtained. In this case, higher flow speeds could not be reached so third mode shape in the in-line direction could not be obtained.

The wake is observed using DPIV on a rigid cylinder with forced motions equivalent to the flexible body. A case of mode switching is also observed where the even in-line mode exhibits an excitation at twice the cross-flow frequency; however, the spatial mode shape in-line appears similar to the first structural mode shape. It is hypothesized that this situation is possible due to variation in the effective added mass along the length of the cylinder. Odd mode shapes in both in-line and cross-flow directions show similar motions to those observed for rigid cylinders.

Share

COinS