Flow pattern and propulsive role of edge vortices along the dorsal and ventral fin folds of fish larvae

Larvae of bony fish share a characteristic median fin fold that transforms into separate median fins as they grow into juveniles. They swim in the intermediate Reynolds number (Re) regime, using body- and caudal-fin undulation to propel themselves. The median fin fold was suggested to be an adaption for locomotion in the intermediate Reynolds regime but its fluid-dynamic role during swimming is still enigmatic. We studied cyclic swimming of larval fish by a combination of Particle Image Velocimetry (PIV) and three-dimensional fluid-dynamic computations. We focussed on the characteristic flow patterns along the upper and lower edges of the fin fold. The experimental and computational observations suggest that complex vortex structures occur around the upper and lower edges of the fin fold. Our computations show that thrust is generated posterior to the centre of mass, mainly in the region of the fin fold. For Re˜100, the fin fold helps to produce an effective flow separation that enhances drag-based thrust. Thrust contributions peak near the upper and lower edges of the fin fold, suggesting a direct relationship between the edge vortices and thrust generation. We conclude that propulsion in larval fish partly relies on high-intensity vortices along the upper and lower edges of the median fin fold, providing a functional explanation for the omnipresence of the fin fold in bony-fish larvae.