LIAO, James. C.; Harvard University: How trout interact with K�rm�n vortices behind a cylinder: insights from kinematics, electromyography, and flow visualization.

Trout match their swimming kinematics to the shed vortices behind cylinders (i.e. adopt the K�rm�n gait) at relatively high flow speeds when cylinder diameter approaches half the total body length. The K�rm�n gait is characterized by the tail-beat frequency matching the vortex shedding frequency, a propulsive body wavelength that is longer than the cylinder wake wavelength, and large body amplitudes. Electromyograms recorded along the axial body show very little regular red muscle activity and no white muscle activity during the K�rm�n gait, indicating a largely passive method of station holding at high swimming speeds. When present, red muscle activity is most prevalent in the anterior myomeres, suggesting that corrective movements are established by the head when necessary. Simultaneous imaging of swimming trout and the flow behind a cylinder (using digital particle image velocimetry) reveals that trout slalom around, rather than pass through, the core of each oncoming vortex. By positioning themselves at a specific distance downstream from the cylinder, trout use these counter-rotating vortical flows to passively alternate their body attack angle. Trout are able to hold station because this passive alternation of body angle, on average, tilts the lift vector upstream over the course of one K�rm�n gait cycle. This mechanism is analogous to tacking in sailboats, whereby the lift force is resolved into thrust by the angle of the sail relative to the incident wind. Preliminary fieldwork suggests that trout may take advantage of the K�rm�n gait only in situations when saving locomotory costs is at a premium and flow regimes are favorable.