Quantitative analysis of the role of symmetry in biomimetic propulsive wakes

We address here the understanding of how animal propulsion is related to flow physics in biomimetic locomotion. It is known that the wake pattern observed in a cross-section behind swimming or flying animals is typically characterized by the presence of periodical vortex shedding. However, depending on species, propulsive wakes are distinguished by their spatial ordering: symmetric (squid-like) or asymmetric (fish-like), with respect to the motion axis. We conducted a very precise experiment to analyse the role of the wake topology in propulsion generation. Self-propulsion is achieved by the flapping motion of two identical pitching rigid foils, separated by a distance $d$. By keeping the momentum input unchanged, we compared both symmetric and asymmetric flapping modes. For the entire parameters range, the symmetric squid-like mode proves to be more efficient for thrust generation than the fish-like asymmetrical one. We show that this difference is due to a pressure effect related to the ability of each wake to produce, or not, significant mixing in the near wake region.