41.2 Saturday, Jan. 5 Knifefish surge like eels while heaving like trout NEVELN, ID*; BALE, R; BHALLA, APS; CURET, OM; LAUDER, GV; PATANKAR, NA; MACIVER, MA; Northwestern University; Northwestern University; Northwestern University; Brown University; Harvard University; Northwestern University; Northwestern University email@example.com
Knifefish generate thrust with their elongated ventral anal fin. This ribbon fin has over 100 bony rays, which oscillate laterally around the fulcrum at the base of the fin. When viewing a transverse section of the fin, the oscillatory kinematics of one ray resemble the flapping motions of the caudal fin of a trout. However, the rays oscillate with certain relative phases to one another, creating a traveling wave along the longitudinal axis of the fin similar to the undulatory body motions of a swimming eel. Using a robotic ribbon fin with 32 oscillating rays connected with a fabric fin, we investigate how the flow structure generated by the ribbon fin compares with flow structures generated by eel-like and trout-like swimming. We use particle imaging velocimetry to visualize orthogonal planes transecting the wake of the fin. A propulsive jet emanates at an angle ventral to the ribbon fin, with the strongest part of the jet occurring just posterior of the fin. A horizontal slice through the jet shows a reverse Von Kármán vortex street, the same vortex pattern that is shed off of the oscillating caudal fin of a trout. 3D simulation of the fin-fluid interaction confirms that this vortex pattern originates from trout-like flow structures which are shed off of the fin ventrally along the heave axis. The eel-like traveling wave of the fin adds momentum to the fluid longitudinally along the surge axis, reorienting the ventrally shed trout-like structures along the angled axis of the jet. This reorientation could be an accidental side effect with negative consequences for thrust, or it could actually accentuate the thrust. Regardless, the resulting flow structure combines features evident in the wake of both the trout and the eel.