External vortex pumping by oscillating plate arrays of mayfly nymphs

Mayfly nymphs are aquatic insects, many of which can generate ventilation currents by beating two linear arrays of external plate-like gills. The oscillation Reynolds number associated with the gill motion changes with animal size, varying from $Re \sim$ 2 to 50 depending on age and species. Thus mayflies provide a novel system model for studying ontogenetic changes in pumping mechanisms associated with transitions from a more viscous- to inertia-dominated flow. Observation of the 3-D kinematics of the gill motion of the species \textit{C. triangulifer} reveal that the mayfly makes a transition in stroke motion when $Re>$5, with a corresponding shift in mean flow from the ventral to the dorsal direction. Time-resolved PIV measurements within the inter-gill space reveal the basic elements of the flow consist of vortex rings generated by the strokes of the individual gills. For the larger $Re$ case, the phasing of the plate motion generates a complex array of small vortices that interact to produce an intermittent dorsally directed jet. For $Re<$5, distinct vortices are still observed, but increased diffusion creates vortices that simultaneously envelope several gills, forcing a new flow pattern to emerge and preventing the effective use of the high $Re$ stroke kinematics. Thus we argue the transition in the kinematics is a reflection of a single mechanism adapted over the traversed $Re$ range, rather than a shift to a completely new mechanism. This work is supported by the NSF under grant CBET-0730907.