The wake of a bluff body is a representative issue in vortex dynamics that plays a central role in civil engineering, ocean engineering and thermal engineering. In this work, a flowing soap film was used to investigate the wakes of multiple stationary circular cylinders and of a single oscillating cylinder. Corresponding computer simulations were also conducted. Vortex formation of a stationary circular cylinder was analyzed by proper orthogonal decomposition (POD). The POD analysis was used to define an unsteady vortex formation length, which suggests a relationship between the vortex formation length of a single cylinder and the critical spacing of two cylinders in a tandem arrangement. A systematic parametric study of the wake structure was conducted for a controlled transversely oscillating cylinder. Neural network and support vector machine codes assisted the wake classification procedure and the identification of boundaries between different wake regimes. The phase map of the vortex shedding regimes for the (quasi) two-dimensional experiment qualitatively agrees with previous three-dimensional experiments. The critical spacings of two identical tandem circular cylinders in a flowing soap film system were determined using visual inspections of the wake patterns and calculations of the Strouhal frequencies. The dimensionless spacing was both increased and decreased quasi-statically. Hysteresis was observed in the flow patterns and Strouhal numbers. This study appears to provide the first experimental evidence of critical spacing values that agree with published computational results. The wake interaction between a stationary upstream circular disk and a free downstream circular disk was also investigated. With the ability to tie together the wake structure and the object motion, the relationship between energy generation and flow structure in the simplified reduced order model system was studied. The research results find the optimal efficiency of the energy harvesting system by a parametric study. / PHD / The wake of a bluff body is a classic issue in vortex dynamics that has been the subject of much research in civil engineering, ocean engineering and thermal engineering. Bluff bodies, especially circular cylinders, can be found extensively in heat exchangers, cooling systems and offshore structures. Flow-induced vibration of a bluff body due to the formation of a wake is an important problem in many fields of engineering. Flow-induced vibration determines the oscillation of flexible pipes that transfer oil from the seabed to the surface of the ocean, for example [71]. In civil engineering, flow-induced vibration affects the design of bluff structures in wind such as bridges, chimneys and buildings [62]. Flow-induced vibration caused by vortices being shed from a bluff body is also a promising way to extract energy from geophysical flows [10]. FIV energy harvesting systems are especially suitable for slow flow speeds in the range 0.5-1.5m/s which cannot be efficiently harvested by traditional hydroelectric power technologies. When a pair of tandem cylinders is immersed in a flow, the downstream cylinder can be excited into wake-induced vibrations (WIV) due to the interaction with vortices coming from the upstream cylinder.
In this work, a flowing soap film was used to investigate the flow-induced vibration of the downstream cylinder of a tandem pair. With the ability to tie together the wake structure and the object motion, we investigate the relationship between energy generation and flow structure in the reduced order model system. The research results find the optimal efficiency of the energy harvesting system by a parametric study. To get deep physical understanding of the flow-induced vibration, wake structures of a circular cylinder undergoing controlled motion and the critical spacing of two identical tandem circular cylinders were also investigated in this research. These research results can help not only the optimization of energy harvesting systems based on flow-induced vibration of the circular-cylinder system, but also will benefit the understanding of wake interactions between multiple bluff bodies such as schooling fish, natural draft cooling towers and wind turbine farms.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/97563 |
| Date | 16 October 2018 |
| Creators | Yang, Wenchao |
| Contributors | Engineering Science and Mechanics, Stremler, Mark A., Hanna, James, Jung, Sunghwan, Ross, Shane D., Yue, Pengtao |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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