Boundary-layer transition over a biomimetic fish scale array under adverse pressure gradient

The impact of a biomimetically inspired textured surface on the spatial evolution of a boundary layer under an imposed adverse pressure gradient (APG) is investigated through wall-resolved large eddy simulations and experiments. Overlapping scales, inspired by the fish scales found over the skin of European sea bass, have been fabricated on a plate. The study is performed at a transitional Reynolds number (based on the plate length and inlet freestream velocity) of 2×105, with a freestream turbulence of 1.02%. Under the imposed APG, a laminar separation bubble (LSB) evolves over the smooth surface, where the shear layer transition is driven via the Kelvin–Helmholtz instability. The textured surface suppresses the LSB observed over the smooth surface, promoting enhanced perturbations, while the onset of transition occurs earlier. The transition mechanism over the textured surface is significantly altered, with perturbation growth primarily driven by instability waves and the evolution of longitudinal streaks. The local linear stability analysis offers critical insight into the growth of these instability waves.