Drag reduction of an elastic fish model

Investigations into unsteady fish-like locomotion have shown that it is a highly efficient method of marine propulsion. Recent experimental work argued that the power needed to propel a swimming fish-like body is significantly less that the power needed to tow an identical, but non-swimming body. This experimental work to prove drag reduction has involved complex robotic systems with many moving parts and actuation devices. Often, the complexity of these systems overshadows their purpose, which is to understand the interaction between the fluid and the body. The purpose of this project is to experimentally obtain drag reduction using the simplest experimental setup possible; a solid urethane rubber fish with a single actuator. This simple model does not allow for precise control of the body movement. However, in nature, there are broad ranges of species that are all able to swim efficiently; and inside each species, each individual has a different size, shape, and swimming style. Therefore to fulfill the goal of the project, it should only be necessary to make a model that looks and moves like an "average" fish. A slight change in the model\´s form or motion should not drastically change the efficiency results. We chose to base the physical and kinematic characteristics of the model off of a rainbow trout. Trout are a common laboratory fish, and extensive data on their swimming behavior is available. The model was tested at a range of different actuation amplitudes and frequencies, with a range of Strouhal numbers between 0.1 and 0.5. The highest efficiencies of 30% for a self-propelled fish were measured at a Strouhal number of 0.2. Drag reduction was not shown, because the hydrodynamic efficiency of the fish was not high enough. However, the results show that by adjusting the swimming parameters of the fish, a wide range of efficiencies can be achieved. These results suggest that efficient, evolving fish models will have to be used to maximize efficiency and show drag reduction. Fish was more efficient and maneuverable than any existing manmade underwater vehicle. A better understanding of the fluid dynamics of fish swimming combined with the development of new technologies such as artificial muscles will allow for the application of unsteady fish-like propulsion to underwater vehicles.