Three-dimensional hydrodynamic interactions inside a robotic fish school

Inspired by experiments with schooling shiners (Notemigonus crysoleucas), we fabricated a robotic, 4-fish school consisting of a flexible polymer having a refractive index similar to water, enabling unobstructed optical access and facilitating flow visualization inside a fish school. Each robotic fish has been actuated independently at a tailbeat frequency of 4-7 Hz to mimic live fish, over a range of Re (O[10^5]). We altered the tailbeat phase differences between neighboring fish and characterized the hydrodynamic interactions by measuring the flow field using time-resolved stereo Particle Image Velocimetry (PIV). Two different two-dimensional schooling configurations have been tested to assess the influence of different flow phenomena resulting from 1) a diamond arrangement, where inline swimming is aided by a strong channeling effect, and 2) a formation with two in-line fish rows placed side-by-side, which accentuates the inline swimming effect. Preliminary results suggest that out-of-plane flow generated from our two-dimensional schooling arrangement can be substantial, up to 25% of the freestream velocity. Our novel experimental approach allows, for the first time, the characterization of the hydrodynamics inside a school using actual fish-like 3D models at environmentally relevant high Re, which might be challenging to simulate using Computational Fluid Dynamics (CFD). Our work is poised to contribute valuable insights and inspire innovative arrangements for underwater vehicles.