Scombrid fish lunate caudal fins are characterized by a wide range of sweep angles. Scombrid that have small sweep-angle caudal fins move at higher swimming speeds, suggesting that smaller angles produce more thrust. Furthermore, scombrids occasionally use high angles of attack (AoA) suggesting this also has some thrust benefit. This work examined the hypothesis that a smaller sweep angle and higher AoA improved thrust in swimmers by experimentally analyzing a robophysical model. The robophysical model was tested in a water tunnel at speeds between 0.35 and 0.7 body lengths per second. Three swept caudal fins were analyzed at three different AoA, three different freestream velocities, and four different Strouhal numbers, for a total of 108 cases. Results demonstrated that the fin with the largest sweep angle of 50° resulted in lower thrust production than the 40° and 30° fins, especially at higher Strouhal numbers. Larger AoA up to 25° increased thrust production at the higher Strouhal numbers, but at lower Strouhal numbers, produced less thrust. Differences in thrust production due to fin sweep angle and AoA were attributed to the variation in spanwise flow and leading edge vortex dynamics.
Thunniform swimmers are known to travel at high speeds for long periods of time and at high hydrodynamic efficiency. Thus, there is a great deal of interest in their swimming physics. In order to better understand these physics, a newly designed robotic tuna was constructed that allows for interchangeable caudal fins. This robot was put in a water tunnel and tested at tail beat frequencies ranging from 0.5 to 1.0 Hz and at freestreams of 0, 0.2, and 0.4 m/s. A lever assembly was used to transmit thrust force to a load cell, and power was calculated using data from current sensors. Preliminary results suggest that swept caudal fins produce more thrust and are more efficient than trapezoidal fins at higher freestreams while the opposite is true at lower freestreams. However, several induction factors need to be resolved before more confident assertions can be made.
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.
The powered flight of bats is unique in nature because of the agility that it allows them to achieve in comparison to other flying animals of equivalent size. One example of this is a bat’s ability to take off with no initial freestream velocity and transition to cruising flight over the duration of relatively few wing beat cycles. Bat’s wings are highly complex and have 20+ degrees of freedom (DOFS) per wing. Adjustments to several of these DOFS occur to allow for quick transition to cruising flight. In order to capture this transition in wing motion, video of Great Himalayan Leaf-Nosed Bats (Hipposideros Armiger) was captured over a period that included both take off and cruising flight. Images captured using a multi-camera setup, containing three rings of 10 RGB cameras each, were used in conjunction with triangulation techniques to capture the 3D coordinates of marker points on the wing. This setup eliminated the point dropout that can occur due to occlusion in traditional 2 camera systems due to its 360 degree coverage. Furthermore, the redundancy caused by collecting 3 or more 2D marker point projections from some of the 10 cameras per ring improved accuracy of the 3D coordinates. In order to capture the transition in wing kinematics, changes in flap amplitude, velocity, and frequency, were recorded. The data suggests that this species of bat utilize a change in flap amplitude as the primary means to transition from takeoff to cruising flight while keeping other parameters such as angle of attack and flap frequency constant.
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