Sea Butterfly Swimming: Time-resolved Tomographic PIV measurements

The planktonic sea butterfly \textit{Limacina helicina} swims by flapping its flexible, wing-like parapodia. The appendage stroke kinematics of this shell-bearing pteropod are three-dimensional and likely contain elements of both drag-based (rowing) and lift-based (flapping) propulsion. Unsteady lift-generating mechanisms such as clap-and-fling may also be present. Upstroke and downstroke motions both propel the animal upward and roll it forwards and backwards, resulting in a sawtooth trajectory. We present time-resolved, tomographic PIV measurements of flow generated by free-swimming pteropods (\textit{Limacina helicina}) moving upwards with average swimming speeds of 5 -- 17 mm/s. The pteropods beat their appendages with a stroke frequency of 4 -- 5 Hz. With a size range of 1 -- 2 mm, the animals filmed in this study operate in a viscous environment with a Reynolds number of 5 to 20. The volumetric flow measurements provide insight into the three dimensional nature of the flow and into the relative importance of drag- and lift-based propulsion at this low Reynolds number. Preliminary results from Navier-Stokes simulations of the flow associated with the~swimming of this organism will also be presented.