Data from: The most efficient metazoan swimmer creates a ‘virtual wall’ to enhance performance

It has been well documented that animals (and machines) swimming or flying
near a solid boundary get a boost in performance. This ground effect is
often modeled as an interaction between a mirrored pair of vortices
represented by a true vortex and an opposite sign ‘virtual vortex’ on the
other side of the wall. However, most animals do not swim near solid
surfaces and thus near body vortex-vortex interactions in open-water
swimmers have been poorly investigated. In this study we examine the most
energetically efficient metazoan swimmer known to date, the jellyfish
Aurelia aurita, to elucidate the role that vortex interactions can play in
animals that swim away from solid boundaries. We used high speed video
tracking, laser-based digital particle image velocimetry (dPIV) and an
algorithm for extracting pressure fields from flow velocity vectors to
quantify swimming performance and the effect of near body vortex-vortex
interactions. Here we show that a vortex ring (stopping vortex), created
underneath the animal during the previous swim cycle, is critical for
increasing propulsive performance. This well positioned stopping vortex
acts in the same way as a virtual vortex during wall-effect performance
enhancement, by helping converge fluid at the underside of the propulsive
surface and generating significantly higher pressures which result in
greater thrust. These findings advocate that jellyfish can generate a
wall-effect boost in open water by creating what amounts to a ‘virtual
wall’ between two real, opposite sign vortex rings. This explains the
significant propulsive advantage jellyfish possess over other metazoans
and represents important implications for bio-engineered propulsion
systems.