LIAO, J.*; BEAL, D.N.*; LAUDER, G.V.*; TRIANTAFYLLOU, M.*: Novel body kinematics of trout swimming in a von Karman trail; can fish tune to vortices?

Flow past cylinders creates a downstream array of staggered, counter-rotating vortices known as the von Karman trail. By altering the combination of flow and cylinder diameter we can change the frequency at which these vortices are shed. Do rainbow trout (i) Oncorhynchus mykiss (/i) synchronize their swimming kinematics to these vortices or are they simply drafting in the velocity deficit? If trout are tuning themselves to the frequency of the shed vortices, we expect the tailbeat frequency to be close to the shedding frequency even as we change the flow speed and cylinder diameter. Preliminary analyses show that tailbeat frequencies track the shedding frequencies, while in the absence of a cylinder tailbeat frequencies are significantly higher. In addition, the body kinematics of trout swimming behind a cylinder are very different from those of free-swimming trout, suggesting a passive motion. Free-swimming trout do not translate their entire bodies laterally and their center of mass maintains a relatively constant velocity. They also have more than a half wavelength present on the body and undulate a relatively large proportion of their bodies with a small amplitude. For a trout behind a cylinder, the whole body undergoes large lateral translations (0.30-0.50 L) while the center of mass either maintains position or accelerates forward. As this is occurring only the posterior half of the body undulates and less than one half wavelength is present on the body. Novel body kinematics suggest that trout are tuning to the shed vortices, though a precise description of this interaction awaits simultaneous video of fish position and wake visualization.