Impact of swimming frequency on robotically-controlled jellyfish swimming dynamics*

The fluid dynamics of jellyfish motion are of particular interest for engineers looking to address the energy demands of underwater propulsion due to their extremely low cost of transport compared to other swimming organisms. Previous work developed a method of externally stimulating Aurelia aurita jellyfish with a microcontroller that sends electrical impulses through electrodes embedded in the bell muscle. Our study explores the impact of the frequency of the bell muscle contractions on jellyfish propulsion by recording downward swimming in a 2.1m vertical tank. The jellyfish were equipped with positively-buoyant microcontrollers that varied stimulation frequency, and quantitative flow visualization was used to measure the wake dynamics. Models of jellyfish swimming in existing literature suggest that swimmers could be capable of varying their swimming speed by varying contraction frequency. Here, our results found that at low frequencies the contractions did not generate enough thrust for the jellyfish to act against the buoyancy of the controller, while at high frequencies the jellyfish muscles were not capable of contracting quickly enough to keep up with every impulse. However, for an intermediate range of frequencies jellyfish swimming speeds were found to be insensitive to the frequency of bell contraction. These results were compared with newly-developed models incorporating co-flow effect on vortex rings to seek an explanation for the observed insensitivity