Streak formation in flow over biomimetic fish scale arrays

The surface topology of the scale pattern from the European Sea Bass (Dicentrarchus labrax ) was measured using a digital microscope and geometrically reconstructed using Computer Assisted Design modelling. Numerical flow simulations and experiments with a physical model of the surface pattern in a flow channel mimic the flow over the fish surface with a laminar boundary layer. The scale array produces regular rows of alternating, streamwise low-speed and high-speed streaks inside the boundary layer close to the surface, with maximum velocity difference of about 9%. Low-velocity streaks are formed in the central region of the scales whereas the high-velocity streaks originated in the overlapping region be- tween the scales. Thus, those flow patterns are linked to the arrangement and the size of the overlapping scales within the array. Because of the velocity streaks, total drag reduction is found when the scale height is small relative to the boundary layer thickness, i.e. less than 10%. Flow simulations results were compared with surface oil-flow visualisations on the physical model of the biomimetic surface placed in a flow channel. The results show an excellent agreement in the size and arrangement of the streaky structures. The existence of streaks is also proven on sea bass (Dicentrarchus labrax ) and common carp (Cyprinus carpio) by surface flow visualisation. From comparison to recent literature about micro-roughness effects on laminar boundary layer flows it is hypothesized that the fish scales could delay transition which would further reduce the drag.