The University of Massachusetts Amherst
University of Massachusetts Amherst

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MIE Department Seminar

Observations on mode shape in the vortex-induced vibration of short, flexible cylinders


Monday, April 6, 2015 - 4:00pm


Jason Dahl – Assistant Professor, URI


Kellogg Conference Room, ELab II


Abstract:  Vortex-induced vibrations (VIV) are an inherent problem for many marine structures including towed cables, risers, mooring lines, and spar buoys.  The interaction of vortex shedding with the natural frequencies of the structure leads to non-linear self-limiting excitation of the structure that may cause fatigue damage or limit operational capabilities in offshore systems.  Since the phenomenon of VIV consists of a dynamic interaction between the motion of the structure and the resulting fluid wake, the motion of the structure plays an important role in the separation of vortex structures in the wake.  Typical industry practice treats the motion of the structure based on the excitation of linear modes based on the associated frequency of excitation, however the non-linear nature of VIV can lead to inherently different behavior in the modal excitation of the structure. The present study shows a series of experiments performed in a re-circulating flow channel and in a small flow visualization tank, where object tracking and digital particle image velocimetry (DPIV) is used to measure the excitation and wake of a series of flexible cylinders undergoing VIV.  The series of experiments attempts to isolate the excitation of specific structural mode shapes by tuning the natural frequencies of the cylinders to have a fixed natural frequency relation between the in-line and cross-flow directions.  It is found that the combination of an odd mode shape excited in the cross-flow direction with an even mode shape in the in-line direction results in an incompatible synchronization condition, where the dominant forcing frequency in-line is is twice the cross-flow frequency, an expected condition based on the nature of vortex-shedding, however the in-line mode shape more closely resembles an odd mode shape rather than the even mode associated with the excitation frequency.  It is shown that this condition occurs due to the distributed drag loading on the structure, where it is not possible to excite even mode shapes under the uniform distribution of the drag force in a uniform flow.  

Bio: Dr. Dahl has been an Assistant Professor in the Department of Ocean Engineering at the University of Rhode Island since 2011.  Dr. Dahl received the PhD degree in Ocean Engineering from MIT in 2008.  He has worked as a postdoctoral researcher at both the SMART Centre in Singapore and at MIT, focusing on dynamic fluid-structure interaction problems.  His research interests include flow-induced vibrations, offshore renewable energy systems, and experimental methods for studying fluid dynamics