Date of Award

4-22-2020

Document Type

Thesis

Degree Name

Master of Science in Coastal Marine and Wetland Studies

Department

Coastal and Marine Systems Science

College

College of Science

First Advisor

Erin E. Hackett

Second Advisor

Roi Gurka

Third Advisor

Daniel C. Abel

Abstract

Many sharks are considered highly efficient swimmers. Their swimming efficiency is partially governed by their morphological features such as dorsal fins, which play a role in their hydrodynamics. Most sharks feature two dorsal fins that can vary in size and location over the body. The first dorsal fin has been shown to improve stabilization, maneuverability, and increase thrust production during swimming, whilst the hydrodynamic role of the second dorsal fin is largely unknown. An understanding of the hydrodynamic function of the dorsal fins can be utilized in engineering applications, e.g., to replicate fins on underwater autonomous vehicles to increase energy efficiency during locomotion. To explore the hydrodynamics of the second dorsal fin and seek solutions applicable to the biomimetic world, we selected a species with a second dorsal fin that is almost as large as its first dorsal fin: the Lemon Shark. An enlarged second dorsal fin is an uncommon characteristic among sharks. Here, we performed a comparative study between the Lemon Shark and another shark species that has a small 2nd dorsal fin compared to the first: the Spinner Shark. We experimentally investigated the hydrodynamic role of the 2nd dorsal fin of the Lemon Shark and used particle image velocimetry to measure the fluid dynamics in its wake. Measurements were collected in the streamwise-spanwise plane behind the 1st and 2nd dorsal fins, as well as the caudal fin for deceased sharks of both species (Spinner Shark and Lemon Shark). In addition, a 3D flexible Lemon Shark model was also used to compare with the deceased specimens. Using the measured data, we: i) evaluated the characteristics of the specimens' and model's wake and ii) examined what effect these characteristics have on the hydrodynamic forces acting on the sharks. The presence of a vortex street in the wake was identified using proper orthogonal decomposition (POD). Based on the POD, the vortex street characteristics such as: wavelength, cross-stream distance, spacing ratio, Kronauer stability, von-Karman stability, and Strouhal number were computed. These wake characteristics were used to compute the thrust, and the drag was computed based on the momentum deficit in the wake. Results showed a stable vortex street developed in the wake behind the 1st dorsal fin for both species. Although the Lemon Shark had two dorsal fins similar in size, the Lemon Shark's 2nd dorsal fin did not feature a vortex street in the wake. The size of the wake was larger than that of the 1st dorsal fin, which resulted in higher drag behind the second dorsal fin compared to the first dorsal fin. It was also found that the flow behind the 2nd dorsal fin did not fully recover prior to reaching the caudal fin, which indicated some interaction with the wake formation behind the caudal fin. It appeared that this interaction reduced the size of the overall wake for the Lemon Shark. Ultimately, this smaller wake resulted in lower drag behind the caudal fin compared to a species with only one large dorsal fin, like the Spinner Shark.

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