Reconfiguration refers to the ability of a flexible structure to change its shape, allowing it to reduce its area perpendicular to the flow, to reduce drag. Decreasing the flexural rigidity of human-made structures can lead to improved designs that operate at higher propulsive efficiencies. The work presented in this thesis examines the physics surrounding a flexible plate under prescribed oscillatory heaving motions. White light movies were recorded at constant frequency and varying proximity to the free surface to investigate the change in reconfiguration as the plate approaches the free surface. Results, analyzed in terms of deformed plate shape, deflection, and plate tip kinematics, found that free surface effects increase the deflection of the plate as the plate approaches the free surface. Expanding on the initial experiments, a variety of frequencies were tested. The results show that each heaving frequency has a different critical height to the free surface in which deep water behavior is distinguished from shallow water behavior. At the critical depth, the plate deflection becomes asymmetric due to free surface effects. The second stage of experiments focused on measuring the fluid loading and fluid flow surrounding the flexible plate. The fluid loading, or drag force, acting on the plate was estimated by using a strain gauge load cell. Results of these experiments found that the drag force is equivalent on plates with lower heaving frequencies when compared to the highest heaving frequency tested due to increased reconfiguration at the higher frequency. The fluid moved from the keel to the edge of the plate as seen in the particle image velocimetry experiments. Higher heaving frequencies created faster fluid flow off the plate and stronger tip vortices being shed from the plate.
When the flexible plate operated at large distances from the free surface, the fluid dynamics showed the same behavior for the upstroke and downstroke of the plate. Whereas, when the plate operated close to the free surface, a vortex only forms on the upstroke, leading to asymmetric loading and deformations. / Master of Science / The ability for a structure to bend under loading and return to its original shape after the load is removed presents a desirable characteristic for structural design. The flexibility of the structure can lead to significant weight loss in contrast to rigid structures. In nature, almost all structures are able to bend when faced with fluid forces which decreases the loading the structure has to handle. Decreasing the stiffness of human-made structures can lead to improved designs that operate at higher propulsive efficiencies. The work presented here examines the physics surrounding a flexible plate under prescribed oscillatory heaving motions, which are motions that are purely vertical. White light movies were recorded at constant frequency and varying proximity to the free surface to investigate the change in plate shape as the plate approaches the free surface. Results, analyzed in terms of deformed plate shape, deflection, and plate tip kinematics, found that free surface effects increase the deflection of the plate as the plate approaches the free surface. Expanding on the initial experiments, a variety of frequencies were tested. The results show that each heaving frequency has a different critical height to the free surface in which deep water behavior is distinguished from shallow water behavior. At the critical depth, the plate deflection becomes asymmetric due to free surface effects. The second stage of experiments focused on describing the fluid loading and fluid flow surrounding the flexible plate. The fluid loading, or drag force, acting on the plate was estimated by using a strain gauge load cell.
Results of these experiments found that the drag force is equivalent on plates with lower heaving frequencies when compared to the highest heaving frequency tested due to increased reconfiguration at the higher frequency. The fluid moved from the center of the plate to the edge of the plate as seen in the particle image velocimetry experiments, which track the movement of particles in the fluid. Higher heaving frequencies created faster fluid flow off the plate. When the flexible plate operated at large distances from the free surface, the fluid flow showed the same behavior for the upstroke and downstroke of the plate. Whereas, when the plate operates close to the free surface, the fluid flow behaves differently leading to asymmetric loading and deformations.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110351 |
| Date | 26 May 2022 |
| Creators | Scianna, Nicholas Alexander |
| Contributors | Aerospace and Ocean Engineering, Gilbert, Christine Marie, Philen, Michael Keith, Coutier-Delgosha, Olivier |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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