Experiments on the vortex wake of a swimming knifefish

TL;DR

This study investigates the vortex wake of a swimming knifefish, measuring vortex parameters to understand propulsion efficiency and inform bio-inspired robotic designs.

Contribution

It provides experimental data on vortex wake characteristics and establishes a relationship between spacing ratio and wave efficiency for the knifefish.

Findings

Average wave efficiency of 0.89 observed.

Measured ratio between vortex amplitude and wavelength.

Relationship between spacing ratio and wave efficiency established.

Abstract

The knifefish species propels itself by generating a reverse Karman street from an anal fin and without significantly moving its body. This unique feature makes this species' propulsion method highly efficient (Blake, 1983). It has been suggested that there is an optimal swimming range for fish based on the amplitude and frequency of the reverse K\'arm\'an street. Experiments have been performed to measure the ratio between the amplitude and wavelength of vortices in the wake of a knifefish. It is suggested that by optimizing the thrust created by the reverse Karman street the wave efficiency can be estimated for a given spacing ratio, and present observations have an average value of 0.89. The relationship established between spacing ratio and wave efficiency, in addition to the measured parameters, will be invaluable for bio-inspired designs based on the knifefish.