As we try and understand more about the oceans and the creatures that inhabit them, the need for effective modes of aquatic transportation becomes abundantly clear. Taking a step back from traditional propeller-based systems, we look toward nature and the millions of years of natural selection to find inspiration. The successful designs that have prospered vary greatly from creature to creature depending on their lifestyle. From rays to jellyfish, the propulsion methods used are tailored for a specific purpose. Considering the vastness of the oceans and our desire to explore them, a quick and efficient mode of locomotion would be well suited for this task. A great example of this type of swimmer can be found within the genus Thunnus.
Tuna rely on a lift-based propulsion system classified as thunniform swimming. The majority of thrust from this propulsion method is derived from the caudal fin and part of the tail. As the tail sweeps through the water, interesting vortex structures are shed from the trailing edge of the lunate fin. Along with velocity components that travel parallel to the movement of the fish, two separate vortices are shed from the top and bottom inner surfaces of the caudal fin and meet at the lengthwise center axis of the fish. These can be best visualized from the flow velocity components analyzed within a plane just behind the caudal fin and perpendicular to the body length axis. Over time, a reverse Karman vortex street is formed from the combination of vortices from multiple tail beats. A robotic tuna and CFD model were created with the minimum number of joints to approximate thunniform swimming.
A modified scotch yoke mechanism was used to convert uniform rotation of a brushless DC motor to oscillatory motion that mimics the tail of a tuna. A servo is mounted on the tail to provide an adjustable angle of attack for the caudal fin. The dynamic CFD model of the tuna employs overset meshing techniques created in ICEM CFD 18.2 and is simulated within ANSYS Fluent 18.2. The model is actuated at the start of the tail and the base of the fin to represent thunniform swimming. The body of the tuna is held static as steady flow is passed around the model. The flow velocity was chosen as an approximation of the speed of a tuna of comparable size and tail-beat frequency.