Date of Award

2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Ocean Engineering

Department

Ocean Engineering

First Advisor

Jason Dahl

Abstract

Vortex-induced vibrations (VIV) has a large impact on solid structures such as marine risers, offshore and deep water structures which subject to strong ocean currents. This dissertation primarily presents investigation of vortex-induced vibration (VIV) of a rigid circular cylinder by doing experiment in towing tank to form database and better understand basic mechanism of VIV. Forced motions of a rigid cylinder with two degree of system are performed for building a database of force coefficients and wake measurements to be used for better understand basic principles of VIV. In this study, the combined effect of in-line and cross-flow of prescribed motion of the circular cylinder undergoing VIV is investigated for potentially having large impact on the development of predictive models for flow-induced vibration which is very important concept for the oil, gas and offshore industry.

In towing tank experiments, controlled motion experiment of a rigid cylinder with two degree of freedom system is performed for small amplitude region. We presented hydrodynamics coefficients varied in defined parameter space. In addition, we observe the presence of large mean lift forces for some motion parameters. In some cases, the mean lift is very large and comparable to oscillating lift forces at other motion parameter combinations. The presence of a mean lift is significant as it implies an asymmetry to the wake that may occur due to forced motions. On long, slender structures, if this type of forced motion were to occur at locations along the length of the structure, this could lead to kiting or mean deflections of the structure perpendicular to the direction of the flow. One case is presented for large lift case. We extended our parameter space for the combined in-line and cross-flow motion of a rigid circular cylinder with force measurements and flow visualizations with digital particle image velocimetry (DPIV) technique. Decomposed forces are mapped over the range of parameters to investigate how these quantities change as a function of the combined motion. Interestingly, we show multi-branch regions detected in force database by analyzing decomposed hydrodynamic coefficients depending on motion parameters. It appears that the multi branch region is expanding by including in line motion over the parameter space. We show how the rigid cylinder forces on the body and its wake visualization measurements are changed in this multi branch regions based on different force responses and vortex formation modes. Flow visualization of the wake behind the circular cylinder with combined in-line and cross-flow motion illustrates the complexity of the wake behind the cylinder with combined IL and CF motion and how wake structure is depended on motion parameters. This gives us a reference point for all entire parameter region. New combination of vortex modes are observed in this study. We are going to show these vortex modes with observed samples from experiment parameters and give their definitions.

Available for download on Saturday, December 17, 2022

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