Meeting Abstract
Tropical fish such as knifefish are commonly challenged by unsteady flows associated with natural and artificial structures. A classical example, broadly used in animal locomotion research, is the Kármán vortex street: a regular pattern of counter-rotating eddies downstream produced by a rigid body. However, natural structures in rivers (e.g., plants, submerged logs, rocks) are relatively loose, and tend to oscillate due to a phenomenon called vortex-induced vibration. These vibrations result in a more complex pattern of vortex shedding that can have pronounced effects on fish locomotion. We investigated the 3D kinematics and swimming behavior of the black ghost knifefish (Apteronotus albifrons, N=7) in response to the complex wake of a free oscillating cylinder, a fixed cylinder and laminar flow. Flow conditions were characterized using PIV. We found that knifefish maintain position with minimal movements in the recirculation zone of a fixed cylinder, using their ribbon and pectoral fins to occasionally regain stability. In contrast, for the oscillating treatment individuals actively swim using their ribbon and pectoral fins, as well as body bending to maintain position behind the moving cylinder. We observed that in some individuals, the body oscillated in or out-of-phase depending on downstream location. A model knifefish (reconstructed using photogrammetry and 3D printed) was placed at different downstream distances from an oscillating cylinder to verify passive movements. The fish model located just downstream from the cylinder oscillated out of phase with the moving cylinder, but oscillated in phase when placed one-cylinder diameter downstream. Thus, vortex-induced vibrations in bluff bodies create an unsteady flow environment that is more challenging for animals, than Kármán vortex streets.
