Jumping archer fish exhibit multiple modes of fin-fin interaction

The role of hydrodynamics in collective motions of fish schools and bioinspired underwater robots

Collective behaviour defines the lives of many animal species on the Earth. Underwater swarms span several orders of magnitude in size, from coral larvae and krill to tunas and dolphins. Agent-based algorithms have modelled collective movements of animal groups by use of social forces, which approximate the behaviour of individual animals. But details of how swarming individuals interact with the fluid environment are often under-examined. How do fluid forces shape aquatic swarms? How do fish use their flow-sensing capabilities to coordinate with their schooling mates? We propose viewing underwater collective behaviour from the framework of fluid stigmergy, which considers both physical interactions and information transfer in fluid environments. Understanding the role of hydrodynamics in aquatic collectives requires multi-disciplinary efforts across fluid mechanics, biology and biomimetic robotics. To facilitate future collaborations, we synthesize key studies in these fields.

Measuring the 3D wake of swimming snakes (Natrix tessellata) using volumetric particle image velocimetry

We describe a method for measuring the 3D vortical structures produced by an anguilliform swimmer using volumetric velocimetry. The wake of freely swimming dice snakes (Natrix tessellata) was quantified, revealing the creation of multiple vortices along the body of the snake due to its undulation. The 3D structure of the vortices generally consisted of paired vortex tubes, some of which were linked together to form a hairpin structure. The observations match predictions from computational fluid dynamic studies of other anguilliform swimmers. Quantitative measurements allowed us to study vortex circulation and size, and global kinetic energy of the flow, which varied with swimming speed, vortex topology and individual characteristics. Our findings provide a baseline for comparing wake structures of snakes with different morphologies and ecologies and investigating the energetic efficiency of anguilliform swimming.

Numerical modeling of a self-propelled dolphin jump out of water

A computational model is developed to investigate the jump of a self-propelled dolphin out of water. This model relies on the Navier-Stokes equations, where a fictitious domain approach with the volume penalization method is used for fluid-structure coupling, and the continuous surface force approach is used to model the water-air interface, the latter being tracked in a level-set framework. The dolphin's geometry is based on freely available data from the literature. While body deformation is imposed, the leading linear and angular displacements are computed from Newton's laws. Numerical simulations show that it is necessary to generate large propulsives forces to allow the jump out of water. When the dolphin is out of water, its trajectory follows a purely ballistic one.