  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 between 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 surface placed in a flow
channel. The results show an excellent agreement in the size and arrangement of
the streaky structures. 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.
