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.

Free-swimming jellyfish between 3.8 and 4.2 cm in diameter (n=8) were recorded in a glass filming vessel (30 × 10 × 25 cm) by a high-speed digital video camera (Fastcam 1024 PCI; Photron) at 1,000 frames per second. Only recordings of animals swimming upward were used in the analysis to eliminate the possibility of gravitational force aiding the forward motion of the animal between pulses.

Jellyfish were illuminated with a laser sheet (680 nm, 2W continuous wave; LaVision) oriented perpendicular to the camera’s optical axis to provide a distinctive body outline for image analysis and to ensure the animal remained in-plane, which ensures the accuracy of 2D estimates of position and velocity.  and checked for normality using a Shapiro–Wilks test. Data were subsequently tested using one-way ANOVA to determine if a significant difference existed between means.           

Fluid motion created by the jellyfish while swimming was quantified using 2D digital particle image velocimetry. Using the setup described above, the filtered seawater was seeded with 10 μm hollow glass beads (Dantec Dynamics). 

This dataset contains the raw particle image velocimetry data files collected on swimming Aurelia aurita jellyfish that were used in the manuscript entited "The most efficient metazoan swimmer creates a ‘virtual wall’ to enhance performance" and publshed in the Proceedings of the Royal Society B.