Inspired by fish gills: vortex transition and mixing enhancement by pitching a perforated flexible panel.

Unsteady bio-mimetic propulsors and flexible vortex generators for mixing have primarily focused on fluid–structure interactions around panels with complete planforms. Porous, leaky structures, however, are common in nature. While bristled wings at low Re and static kirigami sheets have been studied, unsteady vortex dynamics around perforated panels remain underexplored. Inspired by fish gill, a system optimized for metabolic exchange, we designed a thermal dispenser integrating porosity, flexibility, and motion. Thin panels with/without perforations were pitched about the leading edge in a laminar water channel at Re = 590. 2D particle image velocimetry captured flow fields across actuation frequencies (0.64–1.14 Hz). Finite-time Lyapunov exponents revealed Lagrangian coherent structures indicating lateral entrainment and convective heat transport. Semi-empirical convection-diffusion simulations yielded temperature fields consistent with physical predictions and particle cloud dispersion. In summary, we described how perforations alter vortex structures by increasing lateral momentum and preventing leading-edge vortex. We found chord-wise flexibility sustains vortex transition and homogeneous mixing at high frequency, whereas rigid cases suffer. Our results motivate future exploration of fish-gill-inspired leaky hydrofoils for transport and propulsion.