Over the last 10 years, our understanding of the biomechanics of swimming has been revolutionised by the development of particle imaging velocimetry(PIV), which has revealed details of the complex fluid flow patterns around fish and in their wake. Eric Tytell, Emily Standen and George Lauder briefly review the current understanding of the fluid dynamics associated with fish swimming (p. 187), but also sound a note of caution. Pointing out the consistency found in the wakes of fish ranging from eels to mackerel, they warn that the simplified two dimensional PIV view could have obscured more complex fluid flows occurring in three dimensions. Expanding PIV techniques into the third dimension, the team discuss their work on the bluegill sunfish, brook trout and yellow perch,analysing the fishes' dorsal, anal and median fin motions during manoeuvres and free swimming. Identifying complex interactions between flow from different fins, the team say that `these data demonstrate that, while fish do move primarily in the horizontal plane, neither their bodies nor their motions can be accurately simplified in a two-dimensional representation'.
Scaling down from adult to larval fish, Ulrike Müller, Jos van den Boogaart and van Leeuwen point out that the sticky environment experienced by miniature swimmers is very different from the more fluid environment experienced by their adult counterparts(p. 196). Analysing fluid flow patterns as larval fish accelerated from stationary, swam steadily and came to a halt, the team found that tiny larval zebrafish swim with a wide body amplitude. They also found that the larvae are hugged by a relatively thick boundary layer, unlike the adult's relatively thin layer, and that the wake dies off quickly. When setting off, the larvae bend themselves into a C shape followed by a propulsive stroke, and when stopping they gradually reduce their tail beat frequency and amplitude.
While much has been learned from in vivo studies of swimmers,studies of robotic fins and wings can also teach us a great deal about the hydrodynamics of swimming. With the aim of developing robotic submarine vehicles, Promode Bandyopadhyay, David Beal and Alberico Menozzi, working at the Naval Undersea Warfare Center in Rhode Island, USA, have designed and built a robotic fin, loosely based on a penguin fin, which they can control while directly recording forces exerted on the moving limb(p. 206). Based on measurements from the moving fin, the team have developed a model that allows them to calculate the lift and drag exerted on a stiff, penguin-like fin, and to calculate optimised oscillation parameters for the fin during swimming. Surprisingly, the rigid fin's wake was remarkably similar to an eel's,shedding vortex rings alternately to the left and right (although the robotic fin's vortices die off more rapidly than the eel's). The team have also compared the performance of the rigid penguin-like fin with the sunfish's flexible pectoral fin and concluded that although the sunfish's fin confers greater hydrodynamic variation, penguin fin analogues may be better suited to propel biorobotic vehicles.